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Membranes and Membrane Lipids
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17.3
Membranes and Membrane Lipids
17.3 Membranes and Membrane Lipids
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The polar heads face into water, and the nonpolar tails stick up into the air. Bilayers
A double layer of lipids arranged so that nonpolar tails are found between an inner surface and outer surface consisting of hydrophilic heads. are double layers of lipids arranged so that the hydrophobic tails are sandwiched between an inner surface and an outer surface consisting of hydrophilic heads. The hydrophilic heads are in contact with water on either side of the bilayer, whereas the tails, sequestered inside the bilayer, are prevented from having contact with the water. Bilayers like this make up every cell membrane ( Figure 17.6 "Schematic Diagram of a Cell Membrane" ). Figure 17.5 Spontaneously Formed Polar Lipid Structures in Water: Monolayer, Micelle, and Bilayer
Figure 17.6 Schematic Diagram of a Cell Membrane
The membrane enclosing a typical animal cell is a phospholipid bilayer with embedded cholesterol and protein molecules. Short oligosaccharide chains are attached to the outer surface. In the bilayer interior, the hydrophobic tails (that is, the fatty acid portions of lipid molecules) interact by means of dispersion forces. The interactions are weakened by the presence of unsaturated fatty acids.
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Figure 17.5 Spontaneously Formed Polar Lipid Structures in Water: Monolayer, Micelle, and Bilayer
Figure 17.6 Schematic Diagram of a Cell Membrane
The membrane enclosing a typical animal cell is a phospholipid bilayer with embedded cholesterol and protein molecules. Short oligosaccharide chains are attached to the outer surface. In the bilayer interior, the hydrophobic tails (that is, the fatty acid portions of lipid molecules) interact by means of dispersion forces. The interactions are weakened by the presence of unsaturated fatty acids. As a result, the membrane components are free to mill about to some extent, and the membrane is described as fluid. The lipids found in cell membranes can be categorized in various ways. Phospholipids
A lipid containing phosphorus. are lipids containing phosphorus. Glycolipids
A sugar-containing lipid.
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As a result, the membrane components are free to mill about to some extent, and the membrane is described as fluid. The lipids found in cell membranes can be categorized in various ways. Phospholipids
A lipid containing phosphorus. are lipids containing phosphorus. Glycolipids
A sugar-containing lipid. are sugar-containing lipids. The latter are found exclusively on the outer surface of the cell membrane, acting as distinguishing surface markers for the cell and thus serving in cellular recognition and cell-to-cell communication. Sphingolipids
A lipid that contains the unsaturated amino alcohol sphingosine. are phospholipids or glycolipids that contain the unsaturated amino alcohol sphingosine rather than glycerol. Diagrammatic structures of representative membrane lipids are presented in Figure 17.7 "Component Structures of Some Important Membrane Lipids".
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are sugar-containing lipids. The latter are found exclusively on the outer surface of the cell membrane, acting as distinguishing surface markers for the cell and thus serving in cellular recognition and cell-to-cell communication. Sphingolipids
A lipid that contains the unsaturated amino alcohol sphingosine. are phospholipids or glycolipids that contain the unsaturated amino alcohol sphingosine rather than glycerol. Diagrammatic structures of representative membrane lipids are presented in Figure 17.7 "Component Structures of Some Important Membrane Lipids". Figure 17.7 Component Structures of Some Important Membrane Lipids
Phosphoglycerides (also known as glycerophospholipids) are the most abundant phospholipids in cell membranes. They consist of a glycerol unit with fatty acids attached to the first two carbon atoms, while a phosphoric acid unit, esterified with an alcohol molecule (usually an amino alcohol, as in part (a) of Figure 17.8 "Phosphoglycerides") is attached to the third carbon atom of glycerol (part (b) of Figure 17.8 "Phosphoglycerides" ). Notice that the phosphoglyceride molecule is identical to a triglyceride up to the phosphoric acid unit (part (b) of Figure 17.8 "Phosphoglycerides" ). Figure 17.8 Phosphoglycerides
(a) Amino alcohols are commonly found in phosphoglycerides, which are evident in its structural formula (b). There are two common types of phosphoglycerides.
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Figure 17.7 Component Structures of Some Important Membrane Lipids
Phosphoglycerides (also known as glycerophospholipids) are the most abundant phospholipids in cell membranes. They consist of a glycerol unit with fatty acids attached to the first two carbon atoms, while a phosphoric acid unit, esterified with an alcohol molecule (usually an amino alcohol, as in part (a) of Figure 17.8 "Phosphoglycerides") is attached to the third carbon atom of glycerol (part (b) of Figure 17.8 "Phosphoglycerides" ). Notice that the phosphoglyceride molecule is identical to a triglyceride up to the phosphoric acid unit (part (b) of Figure 17.8 "Phosphoglycerides" ). Figure 17.8 Phosphoglycerides
(a) Amino alcohols are commonly found in phosphoglycerides, which are evident in its structural formula (b). There are two common types of phosphoglycerides. Phosphoglycerides containing ethanolamine as the amino alcohol are called phosphatidylethanolamines or cephalins. Cephalins are found in brain tissue and nerves and also have a role in blood clotting. Phosphoglycerides containing choline as the amino alcohol unit are called phosphatidylcholines or lecithins. Lecithins occur in all living organisms. Like cephalins, they are important constituents of nerve and brain tissue.
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Phosphoglycerides containing ethanolamine as the amino alcohol are called phosphatidylethanolamines or cephalins. Cephalins are found in brain tissue and nerves and also have a role in blood clotting. Phosphoglycerides containing choline as the amino alcohol unit are called phosphatidylcholines or lecithins. Lecithins occur in all living organisms. Like cephalins, they are important constituents of nerve and brain tissue. Egg yolks are especially rich in lecithins. Commercial-grade lecithins isolated from soybeans are widely used in foods as emulsifying agents. An emulsifying agent is used to stabilize an emulsion
A dispersion of two liquids that do not normally mix. —a dispersion of two liquids that do not normally mix, such as oil and water. Many foods are emulsions.
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Egg yolks are especially rich in lecithins. Commercial-grade lecithins isolated from soybeans are widely used in foods as emulsifying agents. An emulsifying agent is used to stabilize an emulsion
A dispersion of two liquids that do not normally mix. —a dispersion of two liquids that do not normally mix, such as oil and water. Many foods are emulsions. Milk is an emulsion of butterfat in water. The emulsifying agent in milk is a protein called casein. Mayonnaise is an emulsion of salad oil in water, stabilized by lecithins present in egg yolk. Sphingomyelins
A sphingolipid that contains a fatty acid unit, a phosphoric acid unit, a sphingosine unit, and a choline unit. , the simplest sphingolipids, each contain a fatty acid, a phosphoric acid, sphingosine, and choline ( Figure 17.9 "Sphingolipids" ).
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Milk is an emulsion of butterfat in water. The emulsifying agent in milk is a protein called casein. Mayonnaise is an emulsion of salad oil in water, stabilized by lecithins present in egg yolk. Sphingomyelins
A sphingolipid that contains a fatty acid unit, a phosphoric acid unit, a sphingosine unit, and a choline unit. , the simplest sphingolipids, each contain a fatty acid, a phosphoric acid, sphingosine, and choline ( Figure 17.9 "Sphingolipids" ). Because they contain phosphoric acid, they are also classified as phospholipids. Sphingomyelins are important constituents of the myelin sheath surrounding the axon of a nerve cell. Multiple sclerosis is one of several diseases resulting from damage to the myelin sheath. Figure 17.9 Sphingolipids
(a) Sphingosine, an amino alcohol, is found in all sphingolipids. ( b) A sphingomyelin is also known as a phospholipid, as evidenced by the phosphoric acid unit in its structure.
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Because they contain phosphoric acid, they are also classified as phospholipids. Sphingomyelins are important constituents of the myelin sheath surrounding the axon of a nerve cell. Multiple sclerosis is one of several diseases resulting from damage to the myelin sheath. Figure 17.9 Sphingolipids
(a) Sphingosine, an amino alcohol, is found in all sphingolipids. ( b) A sphingomyelin is also known as a phospholipid, as evidenced by the phosphoric acid unit in its structure. Most animal cells contain sphingolipids called cerebrosides
A sphingolipid that contains a fatty acid unit, a sphingosine unit, and galactose or glucose. ( Figure 17.10 "Cerebrosides" ). Cerebrosides are composed of sphingosine, a fatty acid, and galactose or glucose. They therefore resemble sphingomyelins but have a sugar unit in place of the choline phosphate group. Cerebrosides are important constituents of the membranes of nerve and brain cells.
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Most animal cells contain sphingolipids called cerebrosides
A sphingolipid that contains a fatty acid unit, a sphingosine unit, and galactose or glucose. ( Figure 17.10 "Cerebrosides" ). Cerebrosides are composed of sphingosine, a fatty acid, and galactose or glucose. They therefore resemble sphingomyelins but have a sugar unit in place of the choline phosphate group. Cerebrosides are important constituents of the membranes of nerve and brain cells. Figure 17.10 Cerebrosides
Cerebrosides are sphingolipids that contain a sugar unit. The sphingolipids called gangliosides
A sphingolipid that contains a fatty acid unit, a sphingosine unit, and a complex oligosaccharide. are more complex, usually containing a branched chain of three to eight monosaccharides and/or substituted sugars. Because of considerable variation in their sugar components, about 130 varieties of gangliosides have been identified. Most cell-to-cell recognition and communication processes (e.g., blood group antigens) depend on differences in the sequences of sugars in these compounds.
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Figure 17.10 Cerebrosides
Cerebrosides are sphingolipids that contain a sugar unit. The sphingolipids called gangliosides
A sphingolipid that contains a fatty acid unit, a sphingosine unit, and a complex oligosaccharide. are more complex, usually containing a branched chain of three to eight monosaccharides and/or substituted sugars. Because of considerable variation in their sugar components, about 130 varieties of gangliosides have been identified. Most cell-to-cell recognition and communication processes (e.g., blood group antigens) depend on differences in the sequences of sugars in these compounds. Gangliosides are most prevalent in the outer membranes of nerve cells, although they also occur in smaller quantities in the outer membranes of most other cells. Because cerebrosides and gangliosides contain sugar groups, they are also classified as glycolipids. Membrane Proteins
If membranes were composed only of lipids, very few ions or polar molecules could pass through their hydrophobic “sandwich filling” to enter or leave any cell. However, certain charged and polar species do cross the membrane, aided by proteins that move about in the lipid bilayer. The two major classes of proteins in the cell membrane are integral proteins
A protein that spans the lipids bilayer of membranes.
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Gangliosides are most prevalent in the outer membranes of nerve cells, although they also occur in smaller quantities in the outer membranes of most other cells. Because cerebrosides and gangliosides contain sugar groups, they are also classified as glycolipids. Membrane Proteins
If membranes were composed only of lipids, very few ions or polar molecules could pass through their hydrophobic “sandwich filling” to enter or leave any cell. However, certain charged and polar species do cross the membrane, aided by proteins that move about in the lipid bilayer. The two major classes of proteins in the cell membrane are integral proteins
A protein that spans the lipids bilayer of membranes. , which span the hydrophobic interior of the bilayer, and peripheral proteins
A protein that is more loosely associated with the membrane surface. , which are more loosely associated with the surface of the lipid bilayer ( Figure 17.6 "Schematic Diagram of a Cell Membrane" ). Peripheral proteins may be attached to integral proteins, to the polar head groups of phospholipids, or to both by hydrogen bonding and electrostatic forces. Small ions and molecules soluble in water enter and leave the cell by way of channels through the integral proteins. Some proteins, called carrier proteins, facilitate the passage of certain molecules, such as hormones and neurotransmitters, by specific interactions between the protein and the molecule being transported.
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, which span the hydrophobic interior of the bilayer, and peripheral proteins
A protein that is more loosely associated with the membrane surface. , which are more loosely associated with the surface of the lipid bilayer ( Figure 17.6 "Schematic Diagram of a Cell Membrane" ). Peripheral proteins may be attached to integral proteins, to the polar head groups of phospholipids, or to both by hydrogen bonding and electrostatic forces. Small ions and molecules soluble in water enter and leave the cell by way of channels through the integral proteins. Some proteins, called carrier proteins, facilitate the passage of certain molecules, such as hormones and neurotransmitters, by specific interactions between the protein and the molecule being transported. Answers
a phosphate group
a saccharide unit (monosaccharide or more complex)
sphingosine
The dual character is critical for the formation of the lipid bilayer. The hydrophilic portions of the molecule are in contact with the aqueous env
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18.2
Reactions of Amino Acids
18.2 Reactions of Amino Acids
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Reactions of Amino Acids
18.2 Reactions of Amino Acids
Learning Objective
Explain how an amino acid can act as both an acid and a base. The structure of an amino acid allows it to act as both an acid and a base. An amino acid has this ability because at a certain pH value (different for each amino acid) nearly all the amino acid molecules exist as zwitterions. If acid is added to a solution containing the zwitterion, the carboxylate group captures a hydrogen (H +) ion, and the amino acid becomes positively charged. If base is added, ion removal of the H + ion from the amino group of the zwitterion produces a negatively charged amino acid. In both circumstances, the amino acid acts to maintain the pH of the system—that is, to remove the added acid (H +) or base (OH −) from solution. Example 1
Draw the structure for the anion formed when glycine (at neutral pH) reacts with a base. Draw the structure for the cation formed when glycine (at neutral pH) reacts with an acid. Solution
The base removes H + from the protonated amine group. The acid adds H + to the carboxylate group.
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Reactions of Amino Acids
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In both circumstances, the amino acid acts to maintain the pH of the system—that is, to remove the added acid (H +) or base (OH −) from solution. Example 1
Draw the structure for the anion formed when glycine (at neutral pH) reacts with a base. Draw the structure for the cation formed when glycine (at neutral pH) reacts with an acid. Solution
The base removes H + from the protonated amine group. The acid adds H + to the carboxylate group. Skill-Building Exercise
Draw the structure for the cation formed when valine (at neutral pH) reacts with an acid. Draw the structure for the anion formed when valine (at neutral pH) reacts with a base. The particular pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point
The pH at which a given amino acid exists in solution as a zwitterion. (pI). At its pI, the positive and negative charges on the amino acid balance, and the molecule as a whole is electrically neutral.
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18.2
Reactions of Amino Acids
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Skill-Building Exercise
Draw the structure for the cation formed when valine (at neutral pH) reacts with an acid. Draw the structure for the anion formed when valine (at neutral pH) reacts with a base. The particular pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point
The pH at which a given amino acid exists in solution as a zwitterion. (pI). At its pI, the positive and negative charges on the amino acid balance, and the molecule as a whole is electrically neutral. The amino acids whose side chains are always neutral have isoelectric points ranging from 5.0 to 6.5. The basic amino acids (which have positively charged side chains at neutral pH) have relatively high pIs. Acidic amino acids (which have negatively charged side chains at neutral pH) have quite low pIs ( Table 18.3 "pIs of Some Representative Amino Acids" ). Table 18.3 pIs of Some Representative Amino Acids
Amino Acid
Classification
pI
alanine
nonpolar
6.0
valine
nonpolar
6.0
serine
polar, uncharged
5.7
threonine
polar, uncharged
6.5
arginine
positively charged (basic)
10.8
histidine
positively charged (basic)
7.6
lysine
positively charged (basic)
9.8
aspartic acid
negatively charged (acidic)
3.0
glutamic acid
negatively charged (acidic)
3.2
Amino acids undergo reactions characteristic of carboxylic acids and amines. The reactivity of these functional groups is particularly important in linking amino acids together to form peptides and proteins, as you will see later in this chapter.
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18.2
Reactions of Amino Acids
18.2 Reactions of Amino Acids
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The amino acids whose side chains are always neutral have isoelectric points ranging from 5.0 to 6.5. The basic amino acids (which have positively charged side chains at neutral pH) have relatively high pIs. Acidic amino acids (which have negatively charged side chains at neutral pH) have quite low pIs ( Table 18.3 "pIs of Some Representative Amino Acids" ). Table 18.3 pIs of Some Representative Amino Acids
Amino Acid
Classification
pI
alanine
nonpolar
6.0
valine
nonpolar
6.0
serine
polar, uncharged
5.7
threonine
polar, uncharged
6.5
arginine
positively charged (basic)
10.8
histidine
positively charged (basic)
7.6
lysine
positively charged (basic)
9.8
aspartic acid
negatively charged (acidic)
3.0
glutamic acid
negatively charged (acidic)
3.2
Amino acids undergo reactions characteristic of carboxylic acids and amines. The reactivity of these functional groups is particularly important in linking amino acids together to form peptides and proteins, as you will see later in this chapter. Simple chemical tests that are used to detect amino acids take advantage of the reactivity of these functional groups. An example is the ninhydrin test in which the amine functional group of α-amino acids reacts with ninhydrin to form purple-colored compounds. Ninhydrin is used to detect fingerprints because it reacts with amino acids from the proteins in skin cells transferred to the surface by the individual leaving the fingerprint. Answers
an electrically neutral compound that contains both negatively and positively charged groups
the pH at which a given amino acid exists in solution as a zwitterion
Key Takeaways
Amino acids can act as both an acid and a base due to the presence of the amino and carboxyl functional groups. The pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point (pI).
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18.2
Reactions of Amino Acids
18.2 Reactions of Amino Acids
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Simple chemical tests that are used to detect amino acids take advantage of the reactivity of these functional groups. An example is the ninhydrin test in which the amine functional group of α-amino acids reacts with ninhydrin to form purple-colored compounds. Ninhydrin is used to detect fingerprints because it reacts with amino acids from the proteins in skin cells transferred to the surface by the individual leaving the fingerprint. Answers
an electrically neutral compound that contains both negatively and positively charged groups
the pH at which a given amino acid exists in solution as a zwitterion
Key Takeaways
Amino acids can act as both an acid and a base due to the presence of the amino and carboxyl functional groups. The pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point (pI). Exercises
Draw the structure of leucine and determine the charge on the molecule in a (n)
acidic solution (pH = 1). neutral solution (pH = 7). a basic solution (pH = 11)
Draw the structure of isoleucine and determine the charge on the molecule in a (n)
acidic solution (pH = 1). neutral solution (pH = 7). basic solution (pH = 11).
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18.2
Reactions of Amino Acids
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Exercises
Draw the structure of leucine and determine the charge on the molecule in a (n)
acidic solution (pH = 1). neutral solution (pH = 7). a basic solution (pH = 11)
Draw the structure of isoleucine and determine the charge on the molecule in a (n)
acidic solution (pH = 1). neutral solution (pH = 7). basic solution (pH = 11). Answer
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Enzymes
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18.5
Enzymes
18.5 Enzymes
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Enzymes
18.5 Enzymes
Learning Objectives
Explain the functions of enzymes. Explain how enzymes are classified and named. A catalyst
Any substance that increases the rate or speed of a chemical reaction without being changed or consumed in the reaction. is any substance that increases the rate or speed of a chemical reaction without being changed or consumed in the reaction. Enzymes
A biological catalyst. are biological catalysts, and nearly all of them are proteins. The reaction rates attained by enzymes are truly amazing. In their presence, reactions occur at rates that are a million (10 6) or more times faster than would be attainable in their absence. What is even more amazing is that enzymes perform this function at body temperature (~37°C) and physiological pH (pH ~7), rather than at the conditions that are typically necessary to increase reaction rates (high temperature or pressure, the use of strong oxidizing or reducing agents or strong acids or bases, or a combination of any of these). In addition, enzymes are highly specific in their action;
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are biological catalysts, and nearly all of them are proteins. The reaction rates attained by enzymes are truly amazing. In their presence, reactions occur at rates that are a million (10 6) or more times faster than would be attainable in their absence. What is even more amazing is that enzymes perform this function at body temperature (~37°C) and physiological pH (pH ~7), rather than at the conditions that are typically necessary to increase reaction rates (high temperature or pressure, the use of strong oxidizing or reducing agents or strong acids or bases, or a combination of any of these). In addition, enzymes are highly specific in their action; that is, each enzyme catalyzes only one type of reaction in only one compound or a group of structurally related compounds. The compound or compounds on which an enzyme acts are known as its substrates
A compound on which an enzyme acts. . Hundreds of enzymes have been purified and studied in an effort to understand how they work so effectively and with such specificity. The resulting knowledge has been used to design drugs that inhibit or activate particular enzymes.
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that is, each enzyme catalyzes only one type of reaction in only one compound or a group of structurally related compounds. The compound or compounds on which an enzyme acts are known as its substrates
A compound on which an enzyme acts. . Hundreds of enzymes have been purified and studied in an effort to understand how they work so effectively and with such specificity. The resulting knowledge has been used to design drugs that inhibit or activate particular enzymes. An example is the intensive research to improve the treatment of or find a cure for acquired immunodeficiency syndrome (AIDS). AIDS is caused by the human immunodeficiency virus (HIV). Researchers are studying the enzymes produced by this virus and are developing drugs intended to block the action of those enzymes without interfering with enzymes produced by the human body. Several of these drugs have now been approved for use by AIDS patients. The first enzymes to be discovered were named according to their source or method of discovery.
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An example is the intensive research to improve the treatment of or find a cure for acquired immunodeficiency syndrome (AIDS). AIDS is caused by the human immunodeficiency virus (HIV). Researchers are studying the enzymes produced by this virus and are developing drugs intended to block the action of those enzymes without interfering with enzymes produced by the human body. Several of these drugs have now been approved for use by AIDS patients. The first enzymes to be discovered were named according to their source or method of discovery. The enzyme pepsin, which aids in the hydrolysis of proteins, is found in the digestive juices of the stomach (Greek pepsis, meaning “digestion”). Papain, another enzyme that hydrolyzes protein (in fact, it is used in meat tenderizers), is isolated from papayas. As more enzymes were discovered, chemists recognized the need for a more systematic and chemically informative identification scheme. In the current numbering and naming scheme, under the oversight of the Nomenclature Commission of the International Union of Biochemistry, enzymes are arranged into six groups according to the general type of reaction they catalyze ( Table 18.5 "Classes of Enzymes" ), with subgroups and secondary subgroups that specify the reaction more precisely. Each enzyme is assigned a four-digit number, preceded by the prefix EC—for enzyme classification—that indicates its group, subgroup, and so forth.
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The enzyme pepsin, which aids in the hydrolysis of proteins, is found in the digestive juices of the stomach (Greek pepsis, meaning “digestion”). Papain, another enzyme that hydrolyzes protein (in fact, it is used in meat tenderizers), is isolated from papayas. As more enzymes were discovered, chemists recognized the need for a more systematic and chemically informative identification scheme. In the current numbering and naming scheme, under the oversight of the Nomenclature Commission of the International Union of Biochemistry, enzymes are arranged into six groups according to the general type of reaction they catalyze ( Table 18.5 "Classes of Enzymes" ), with subgroups and secondary subgroups that specify the reaction more precisely. Each enzyme is assigned a four-digit number, preceded by the prefix EC—for enzyme classification—that indicates its group, subgroup, and so forth. This is demonstrated in Table 18.6 "Assignment of an Enzyme Classification Number" for alcohol dehydrogenase. Each enzyme is also given a name consisting of the root of the name of its substrate or substrates and the - ase suffix. Thus urease is the enzyme that catalyzes the hydrolysis of urea. Table 18.5 Classes of Enzymes
Class
Type of Reaction Catalyzed
Examples
oxidoreductases
oxidation-reduction reactions
Dehydrogenases catalyze oxidation-reduction reactions involving hydrogen and reductases catalyze reactions in which a substrate is reduced. transferases
transfer reactions of groups, such as methyl, amino, and acetyl
Transaminases catalyze the transfer of amino group, and kinases catalyze the transfer of a phosphate group.
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This is demonstrated in Table 18.6 "Assignment of an Enzyme Classification Number" for alcohol dehydrogenase. Each enzyme is also given a name consisting of the root of the name of its substrate or substrates and the - ase suffix. Thus urease is the enzyme that catalyzes the hydrolysis of urea. Table 18.5 Classes of Enzymes
Class
Type of Reaction Catalyzed
Examples
oxidoreductases
oxidation-reduction reactions
Dehydrogenases catalyze oxidation-reduction reactions involving hydrogen and reductases catalyze reactions in which a substrate is reduced. transferases
transfer reactions of groups, such as methyl, amino, and acetyl
Transaminases catalyze the transfer of amino group, and kinases catalyze the transfer of a phosphate group. hydrolases
hydrolysis reactions
Lipases catalyze the hydrolysis of lipids, and proteases catalyze the hydrolysis of proteins
lyases
reactions in which groups are removed without hydrolysis or addition of groups to a double bond
Decarboxylases catalyze the removal of carboxyl groups. isomerases
reactions in which a compound is converted to its isomer
Isomerases may catalyze the conversion of an aldose to a ketose, and mutases catalyze reactions in which a functional group is transferred from one atom in a substrate to another. ligases
reactions in which new bonds are formed between carbon and another atom; energy is required
Synthetases catalyze reactions in which two smaller molecules are linked to form a larger one. Table 18.6 Assignment of an Enzyme Classification Number
Alcohol Dehydrogenase:
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hydrolases
hydrolysis reactions
Lipases catalyze the hydrolysis of lipids, and proteases catalyze the hydrolysis of proteins
lyases
reactions in which groups are removed without hydrolysis or addition of groups to a double bond
Decarboxylases catalyze the removal of carboxyl groups. isomerases
reactions in which a compound is converted to its isomer
Isomerases may catalyze the conversion of an aldose to a ketose, and mutases catalyze reactions in which a functional group is transferred from one atom in a substrate to another. ligases
reactions in which new bonds are formed between carbon and another atom; energy is required
Synthetases catalyze reactions in which two smaller molecules are linked to form a larger one. Table 18.6 Assignment of an Enzyme Classification Number
Alcohol Dehydrogenase: EC 1.1.1.1
The first digit indicates that this enzyme is an oxidoreductase; that is, an enzyme that catalyzes an oxidation-reduction reaction. The second digit indicates that this oxidoreductase catalyzes a reaction involving a primary or secondary alcohol. The third digit indicates that either the coenzyme NAD + or NADP + is required for this reaction. The fourth digit indicates that this was the first enzyme isolated, characterized, and named using this system of nomenclature.
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EC 1.1.1.1
The first digit indicates that this enzyme is an oxidoreductase; that is, an enzyme that catalyzes an oxidation-reduction reaction. The second digit indicates that this oxidoreductase catalyzes a reaction involving a primary or secondary alcohol. The third digit indicates that either the coenzyme NAD + or NADP + is required for this reaction. The fourth digit indicates that this was the first enzyme isolated, characterized, and named using this system of nomenclature. The systematic name for this enzyme is alcohol: NAD+ oxidoreductase, while the recommended or common name is alcohol dehydrogenase. Reaction catalyzed: Answers
sucrose
sucrase
Key Takeaways
An enzyme is a biological catalyst, a substance that increases the rate of a chemical reaction without being changed or consumed in the reaction. A systematic process is used to name and classify enzymes.
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The systematic name for this enzyme is alcohol: NAD+ oxidoreductase, while the recommended or common name is alcohol dehydrogenase. Reaction catalyzed: Answers
sucrose
sucrase
Key Takeaways
An enzyme is a biological catalyst, a substance that increases the rate of a chemical reaction without being changed or consumed in the reaction. A systematic process is used to name and classify enzymes. Exercises
Identify the substrate catalyzed by each enzyme. lactase
cellulase
peptidase
Identify the substrate catalyzed by each enzyme. lipase
amylase
maltase
Identify each type of enzyme. decarboxylase
protease
transaminase
Identify each type of enzyme. dehydrogenase
isomerase
lipase
Answers
lactose
cellulose
peptides
lyase
hydrolase
transferase
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Enzyme Action
18.6 Enzyme Action
Learning Objective
Describe the interaction between an enzyme and its substrate. Enzyme-catalyzed reactions occur in at least two steps. In the first step, an enzyme molecule (E) and the substrate molecule or molecules (S) collide and react to form an intermediate compound called the enzyme-substrate (E–S) complex. ( This step is reversible because the complex can break apart into the original substrate or substrates and the free enzyme.) Once the E–S complex forms, the enzyme is able to catalyze the formation of product (P), which is then released from the enzyme surface: S + E → E–S E–S → P + E
Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the substrate and transforms the substrate to product is called the active site
The location on an enzyme where a substrate binds and is transformed to product. of the enzyme ( Figure 18.10 "Substrate Binding to the Active Site of an Enzyme" ). It possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the structure of the substrate, so that the enzyme and substrate molecules fit together in much the same manner as a key fits into a tumbler lock.
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S + E → E–S E–S → P + E
Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the complex. The structural features or functional groups on the enzyme that participate in these interactions are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the substrate and transforms the substrate to product is called the active site
The location on an enzyme where a substrate binds and is transformed to product. of the enzyme ( Figure 18.10 "Substrate Binding to the Active Site of an Enzyme" ). It possesses a unique conformation (including correctly positioned bonding groups) that is complementary to the structure of the substrate, so that the enzyme and substrate molecules fit together in much the same manner as a key fits into a tumbler lock. In fact, an early model describing the formation of the enzyme-substrate complex was called the lock-and-key model
A model that portrays an enzyme as conformationally rigid and able to bond only to a substrate or substrates that exactly fit the active site. ( Figure 18.11 "The Lock-and-Key Model of Enzyme Action" ). This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. Figure 18.10 Substrate Binding to the Active Site of an Enzyme
The enzyme dihydrofolate reductase is shown with one of its substrates: NADP + (a) unbound and (b) bound.
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In fact, an early model describing the formation of the enzyme-substrate complex was called the lock-and-key model
A model that portrays an enzyme as conformationally rigid and able to bond only to a substrate or substrates that exactly fit the active site. ( Figure 18.11 "The Lock-and-Key Model of Enzyme Action" ). This model portrayed the enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. Figure 18.10 Substrate Binding to the Active Site of an Enzyme
The enzyme dihydrofolate reductase is shown with one of its substrates: NADP + (a) unbound and (b) bound. The NADP + (shown in red) binds to a pocket that is complementary to it in shape and ionic properties. Figure 18.11 The Lock-and-Key Model of Enzyme Action
(a) Because the substrate and the active site of the enzyme have complementary structures and bonding groups, they fit together as a key fits a lock. ( b) The catalytic reaction occurs while the two are bonded together in the enzyme-substrate complex. Working out the precise three-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. They discovered that the binding of a substrate often leads to a large conformational change in the enzyme, as well as to changes in the structure of the substrate or substrates.
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The NADP + (shown in red) binds to a pocket that is complementary to it in shape and ionic properties. Figure 18.11 The Lock-and-Key Model of Enzyme Action
(a) Because the substrate and the active site of the enzyme have complementary structures and bonding groups, they fit together as a key fits a lock. ( b) The catalytic reaction occurs while the two are bonded together in the enzyme-substrate complex. Working out the precise three-dimensional structures of numerous enzymes has enabled chemists to refine the original lock-and-key model of enzyme actions. They discovered that the binding of a substrate often leads to a large conformational change in the enzyme, as well as to changes in the structure of the substrate or substrates. The current theory, known as the induced-fit model
A model that says an enzyme can undergo a conformational change when it binds substrate molecules. , says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only after the substrate is bound, as shown for hexokinase in Figure 18.12 "The Induced-Fit Model of Enzyme Action". After catalysis, the enzyme resumes its original structure. Figure 18.12 The Induced-Fit Model of Enzyme Action
(a) The enzyme hexokinase without its substrate (glucose, shown in red) is bound to the active site. ( b) The enzyme conformation changes dramatically when the substrate binds to it, resulting in additional interactions between hexokinase and glucose.
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The current theory, known as the induced-fit model
A model that says an enzyme can undergo a conformational change when it binds substrate molecules. , says that enzymes can undergo a change in conformation when they bind substrate molecules, and the active site has a shape complementary to that of the substrate only after the substrate is bound, as shown for hexokinase in Figure 18.12 "The Induced-Fit Model of Enzyme Action". After catalysis, the enzyme resumes its original structure. Figure 18.12 The Induced-Fit Model of Enzyme Action
(a) The enzyme hexokinase without its substrate (glucose, shown in red) is bound to the active site. ( b) The enzyme conformation changes dramatically when the substrate binds to it, resulting in additional interactions between hexokinase and glucose. The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme’s active site. Amino acid side chains in or near the binding site can then act as acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The participating amino acids, which are usually widely separated in the primary sequence of the protein, are brought close together in the active site as a result of the folding and bending of the polypeptide chain or chains when the protein acquires its tertiary and quaternary structure. Binding to enzymes brings reactants close to each other and aligns them properly, which has the same effect as increasing the concentration of the reacting compounds. Example 1
What type of interaction would occur between an OH group present on a substrate molecule and a functional group in the active site of an enzyme?
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The structural changes that occur when an enzyme and a substrate join together bring specific parts of a substrate into alignment with specific parts of the enzyme’s active site. Amino acid side chains in or near the binding site can then act as acid or base catalysts, provide binding sites for the transfer of functional groups from one substrate to another or aid in the rearrangement of a substrate. The participating amino acids, which are usually widely separated in the primary sequence of the protein, are brought close together in the active site as a result of the folding and bending of the polypeptide chain or chains when the protein acquires its tertiary and quaternary structure. Binding to enzymes brings reactants close to each other and aligns them properly, which has the same effect as increasing the concentration of the reacting compounds. Example 1
What type of interaction would occur between an OH group present on a substrate molecule and a functional group in the active site of an enzyme? Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified. Solution
An OH group would most likely engage in hydrogen bonding with an appropriate functional group present in the active site of an enzyme. Several amino acid side chains would be able to engage in hydrogen bonding with an OH group. One example would be asparagine, which has an amide functional group. Skill-Building Exercise
What type of interaction would occur between an COO − group present on a substrate molecule and a functional group in the active site of an enzyme?
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Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified. Solution
An OH group would most likely engage in hydrogen bonding with an appropriate functional group present in the active site of an enzyme. Several amino acid side chains would be able to engage in hydrogen bonding with an OH group. One example would be asparagine, which has an amide functional group. Skill-Building Exercise
What type of interaction would occur between an COO − group present on a substrate molecule and a functional group in the active site of an enzyme? Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified. One characteristic that distinguishes an enzyme from all other types of catalysts is its substrate specificity. An inorganic acid such as sulfuric acid can be used to increase the reaction rates of many different reactions, such as the hydrolysis of disaccharides, polysaccharides, lipids, and proteins, with complete impartiality. In contrast, enzymes are much more specific. Some enzymes act on a single substrate, while other enzymes act on any of a group of related molecules containing a similar functional group or chemical bond.
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Suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you just identified. One characteristic that distinguishes an enzyme from all other types of catalysts is its substrate specificity. An inorganic acid such as sulfuric acid can be used to increase the reaction rates of many different reactions, such as the hydrolysis of disaccharides, polysaccharides, lipids, and proteins, with complete impartiality. In contrast, enzymes are much more specific. Some enzymes act on a single substrate, while other enzymes act on any of a group of related molecules containing a similar functional group or chemical bond. Some enzymes even distinguish between D- and L-stereoisomers, binding one stereoisomer but not the other. Urease, for example, is an enzyme that catalyzes the hydrolysis of a single substrate—urea—but not the closely related compounds methyl urea, thiourea, or biuret. The enzyme carboxypeptidase, on the other hand, is far less specific. It catalyzes the removal of nearly any amino acid from the carboxyl end of any peptide or protein. Enzyme specificity results from the uniqueness of the active site in each different enzyme because of the identity, charge, and spatial orientation of the functional groups located there.
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Some enzymes even distinguish between D- and L-stereoisomers, binding one stereoisomer but not the other. Urease, for example, is an enzyme that catalyzes the hydrolysis of a single substrate—urea—but not the closely related compounds methyl urea, thiourea, or biuret. The enzyme carboxypeptidase, on the other hand, is far less specific. It catalyzes the removal of nearly any amino acid from the carboxyl end of any peptide or protein. Enzyme specificity results from the uniqueness of the active site in each different enzyme because of the identity, charge, and spatial orientation of the functional groups located there. It regulates cell chemistry so that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell. Answers
The lock-and-key model portrays an enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. The induced fit model portrays the enzyme structure as more flexible and is complementary to the substrate only after the substrate is bound. Urease has the greater specificity because it can bind only to a single substrate.
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It regulates cell chemistry so that the proper reactions occur in the proper place at the proper time. Clearly, it is crucial to the proper functioning of the living cell. Answers
The lock-and-key model portrays an enzyme as conformationally rigid and able to bond only to substrates that exactly fit the active site. The induced fit model portrays the enzyme structure as more flexible and is complementary to the substrate only after the substrate is bound. Urease has the greater specificity because it can bind only to a single substrate. Carboxypeptidase, on the other hand, can catalyze the removal of nearly any amino acid from the carboxyl end of a peptide or protein. Key Takeaways
A substrate binds to a specific region on an enzyme known as the active site, where the substrate can be converted to product. The substrate binds to the enzyme primarily through hydrogen bonding and other electrostatic interactions. The induced-fit model says that an enzyme can undergo a conformational change when binding a substrate. Enzymes exhibit varying degrees of substrate specificity.
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Carboxypeptidase, on the other hand, can catalyze the removal of nearly any amino acid from the carboxyl end of a peptide or protein. Key Takeaways
A substrate binds to a specific region on an enzyme known as the active site, where the substrate can be converted to product. The substrate binds to the enzyme primarily through hydrogen bonding and other electrostatic interactions. The induced-fit model says that an enzyme can undergo a conformational change when binding a substrate. Enzymes exhibit varying degrees of substrate specificity. Exercises
What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme? COOH
NH 3+
OH
CH (CH 3) 2
What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme? SH
NH 2
C 6 H 5
COO −
For each functional group in Exercise 1, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified. For each functional group in Exercise 2, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified. Answers
hydrogen bonding
ionic bonding
hydrogen bonding
dispersion forces
The amino acid has a polar side chain capable of engaging in hydrogen bonding;
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18.6
Enzyme Action
18.6 Enzyme Action
Learning Objective
Example 1
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
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Exercises
What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme? COOH
NH 3+
OH
CH (CH 3) 2
What type of interaction would occur between each group present on a substrate molecule and a functional group of the active site in an enzyme? SH
NH 2
C 6 H 5
COO −
For each functional group in Exercise 1, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified. For each functional group in Exercise 2, suggest an amino acid whose side chain might be in the active site of an enzyme and form the type of interaction you identified. Answers
hydrogen bonding
ionic bonding
hydrogen bonding
dispersion forces
The amino acid has a polar side chain capable of engaging in hydrogen bonding; serine (answers will vary). The amino acid has a negatively charged side chain; aspartic acid (answers will vary). The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine (answers will vary).
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Enzyme Action
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18.6
Enzyme Action
18.6 Enzyme Action
Learning Objective
Example 1
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
serine (answers will vary). The amino acid has a negatively charged side chain; aspartic acid (answers will vary). The amino acid has a polar side chain capable of engaging in hydrogen bonding; asparagine (answers will vary). The amino acid has a nonpolar side chain; isoleucine (answers will vary).
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Describe how pH, temperature, and the concentration of an enzyme and its substrate influence enzyme activity. The single most important property of enzymes is the ability to increase the rates of reactions occurring in living organisms, a property known as catalytic activity. Because most enzymes are proteins, their activity is affected by factors that disrupt protein structure, as well as by factors that affect catalysts in general. Factors that disrupt protein structure, as we saw in Section 18.4 "Proteins", include temperature and pH; factors that affect catalysts in general include reactant or substrate concentration and catalyst or enzyme concentration. The activity of an enzyme can be measured by monitoring either the rate at which a substrate disappears or the rate at which a product forms. Concentration of Substrate
In the presence of a given amount of enzyme, the rate of an enzymatic reaction increases as the substrate concentration increases until a limiting rate is reached, after which further increase in the substrate concentration produces no significant change in the reaction rate (part (a) of Figure 18.13 "Concentration versus Reaction Rate" ). At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them. In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released (or been released without reacting).
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
The activity of an enzyme can be measured by monitoring either the rate at which a substrate disappears or the rate at which a product forms. Concentration of Substrate
In the presence of a given amount of enzyme, the rate of an enzymatic reaction increases as the substrate concentration increases until a limiting rate is reached, after which further increase in the substrate concentration produces no significant change in the reaction rate (part (a) of Figure 18.13 "Concentration versus Reaction Rate" ). At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them. In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released (or been released without reacting). Figure 18.13 Concentration versus Reaction Rate
(a) This graph shows the effect of substrate concentration on the rate of a reaction that is catalyzed by a fixed amount of enzyme. ( b) This graph shows the effect of enzyme concentration on the reaction rate at a constant level of substrate. Let’s consider an analogy. Ten taxis (enzyme molecules) are waiting at a taxi stand to take people (substrate) on a 10-minute trip to a concert hall, one passenger at a time. If only 5 people are present at the stand, the rate of their arrival at the concert hall is 5 people in 10 minutes.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
Figure 18.13 Concentration versus Reaction Rate
(a) This graph shows the effect of substrate concentration on the rate of a reaction that is catalyzed by a fixed amount of enzyme. ( b) This graph shows the effect of enzyme concentration on the reaction rate at a constant level of substrate. Let’s consider an analogy. Ten taxis (enzyme molecules) are waiting at a taxi stand to take people (substrate) on a 10-minute trip to a concert hall, one passenger at a time. If only 5 people are present at the stand, the rate of their arrival at the concert hall is 5 people in 10 minutes. If the number of people at the stand is increased to 10, the rate increases to 10 arrivals in 10 minutes. With 20 people at the stand, the rate would still be 10 arrivals in 10 minutes. The taxis have been “saturated.” If the taxis could carry 2 or 3 passengers each, the same principle would apply. The rate would simply be higher (20 or 30 people in 10 minutes) before it leveled off.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
If the number of people at the stand is increased to 10, the rate increases to 10 arrivals in 10 minutes. With 20 people at the stand, the rate would still be 10 arrivals in 10 minutes. The taxis have been “saturated.” If the taxis could carry 2 or 3 passengers each, the same principle would apply. The rate would simply be higher (20 or 30 people in 10 minutes) before it leveled off. Concentration of Enzyme
When the concentration of the enzyme is significantly lower than the concentration of the substrate (as when the number of taxis is far lower than the number of waiting passengers), the rate of an enzyme-catalyzed reaction is directly dependent on the enzyme concentration (part (b) of Figure 18.13 "Concentration versus Reaction Rate" ). This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased. Temperature
A general rule of thumb for most chemical reactions is that a temperature rise of 10°C approximately doubles the reaction rate. To some extent, this rule holds for all enzymatic reactions.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
Concentration of Enzyme
When the concentration of the enzyme is significantly lower than the concentration of the substrate (as when the number of taxis is far lower than the number of waiting passengers), the rate of an enzyme-catalyzed reaction is directly dependent on the enzyme concentration (part (b) of Figure 18.13 "Concentration versus Reaction Rate" ). This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased. Temperature
A general rule of thumb for most chemical reactions is that a temperature rise of 10°C approximately doubles the reaction rate. To some extent, this rule holds for all enzymatic reactions. After a certain point, however, an increase in temperature causes a decrease in the reaction rate, due to denaturation of the protein structure and disruption of the active site (part (a) of Figure 18.14 "Temperature and pH versus Concentration" ). For many proteins, denaturation occurs between 45°C and 55°C. Furthermore, even though an enzyme may appear to have a maximum reaction rate between 40°C and 50°C, most biochemical reactions are carried out at lower temperatures because enzymes are not stable at these higher temperatures and will denature after a few minutes. Figure 18.14 Temperature and pH versus Concentration
(a) This graph depicts the effect of temperature on the rate of a reaction that is catalyzed by a fixed amount of enzyme. ( b) This graph depicts the effect of pH on the rate of a reaction that is catalyzed by a fixed amount of enzyme.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
After a certain point, however, an increase in temperature causes a decrease in the reaction rate, due to denaturation of the protein structure and disruption of the active site (part (a) of Figure 18.14 "Temperature and pH versus Concentration" ). For many proteins, denaturation occurs between 45°C and 55°C. Furthermore, even though an enzyme may appear to have a maximum reaction rate between 40°C and 50°C, most biochemical reactions are carried out at lower temperatures because enzymes are not stable at these higher temperatures and will denature after a few minutes. Figure 18.14 Temperature and pH versus Concentration
(a) This graph depicts the effect of temperature on the rate of a reaction that is catalyzed by a fixed amount of enzyme. ( b) This graph depicts the effect of pH on the rate of a reaction that is catalyzed by a fixed amount of enzyme. At 0°C and 100°C, the rate of enzyme-catalyzed reactions is nearly zero. This fact has several practical applications. We sterilize objects by placing them in boiling water, which denatures the enzymes of any bacteria that may be in or on them. We preserve our food by refrigerating or freezing it, which slows enzyme activity. When animals go into hibernation in winter, their body temperature drops, decreasing the rates of their metabolic processes to levels that can be maintained by the amount of energy stored in the fat reserves in the animals’ tissues.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
At 0°C and 100°C, the rate of enzyme-catalyzed reactions is nearly zero. This fact has several practical applications. We sterilize objects by placing them in boiling water, which denatures the enzymes of any bacteria that may be in or on them. We preserve our food by refrigerating or freezing it, which slows enzyme activity. When animals go into hibernation in winter, their body temperature drops, decreasing the rates of their metabolic processes to levels that can be maintained by the amount of energy stored in the fat reserves in the animals’ tissues. Hydrogen Ion Concentration (pH)
Because most enzymes are proteins, they are sensitive to changes in the hydrogen ion concentration or pH. Enzymes may be denatured by extreme levels of hydrogen ions (whether high or low); any change in pH, even a small one, alters the degree of ionization of an enzyme’s acidic and basic side groups and the substrate components as well. Ionizable side groups located in the active site must have a certain charge for the enzyme to bind its substrate. Neutralization of even one of these charges alters an enzyme’s catalytic activity. An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
Hydrogen Ion Concentration (pH)
Because most enzymes are proteins, they are sensitive to changes in the hydrogen ion concentration or pH. Enzymes may be denatured by extreme levels of hydrogen ions (whether high or low); any change in pH, even a small one, alters the degree of ionization of an enzyme’s acidic and basic side groups and the substrate components as well. Ionizable side groups located in the active site must have a certain charge for the enzyme to bind its substrate. Neutralization of even one of these charges alters an enzyme’s catalytic activity. An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form. The median value of this pH range is called the optimum pH
The pH at which a particular enzyme exhibits maximum activity. of the enzyme (part (b) of Figure 18.14 "Temperature and pH versus Concentration" ). With the notable exception of gastric juice (the fluids secreted in the stomach), most body fluids have pH values between 6 and 8. Not surprisingly, most enzymes exhibit optimal activity in this pH range. However, a few enzymes have optimum pH values outside this range.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
The median value of this pH range is called the optimum pH
The pH at which a particular enzyme exhibits maximum activity. of the enzyme (part (b) of Figure 18.14 "Temperature and pH versus Concentration" ). With the notable exception of gastric juice (the fluids secreted in the stomach), most body fluids have pH values between 6 and 8. Not surprisingly, most enzymes exhibit optimal activity in this pH range. However, a few enzymes have optimum pH values outside this range. For example, the optimum pH for pepsin, an enzyme that is active in the stomach, is 2.0. Answers
If the concentration of the substrate is low, increasing its concentration will increase the rate of the reaction. An increase in the amount of enzyme will increase the rate of the reaction (provided sufficient substrate is present). Key Takeaways
Initially, an increase in substrate concentration leads to an increase in the rate of an enzyme-catalyzed reaction. As the enzyme molecules become saturated with substrate, this increase in reaction rate levels off.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
For example, the optimum pH for pepsin, an enzyme that is active in the stomach, is 2.0. Answers
If the concentration of the substrate is low, increasing its concentration will increase the rate of the reaction. An increase in the amount of enzyme will increase the rate of the reaction (provided sufficient substrate is present). Key Takeaways
Initially, an increase in substrate concentration leads to an increase in the rate of an enzyme-catalyzed reaction. As the enzyme molecules become saturated with substrate, this increase in reaction rate levels off. The rate of an enzyme-catalyzed reaction increases with an increase in the concentration of an enzyme. At low temperatures, an increase in temperature increases the rate of an enzyme-catalyzed reaction. At higher temperatures, the protein is denatured, and the rate of the reaction dramatically decreases. An enzyme has an optimum pH range in which it exhibits maximum activity. Exercises
In non-enzyme-catalyzed reactions, the reaction rate increases as the concentration of reactant is increased.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
The rate of an enzyme-catalyzed reaction increases with an increase in the concentration of an enzyme. At low temperatures, an increase in temperature increases the rate of an enzyme-catalyzed reaction. At higher temperatures, the protein is denatured, and the rate of the reaction dramatically decreases. An enzyme has an optimum pH range in which it exhibits maximum activity. Exercises
In non-enzyme-catalyzed reactions, the reaction rate increases as the concentration of reactant is increased. In an enzyme-catalyzed reaction, the reaction rate initially increases as the substrate concentration is increased but then begins to level off, so that the increase in reaction rate becomes less and less as the substrate concentration increases. Explain this difference. Why do enzymes become inactive at very high temperatures? An enzyme has an optimum pH of 7.4. What is most likely to happen to the activity of the enzyme if the pH drops to 6.3?
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
In an enzyme-catalyzed reaction, the reaction rate initially increases as the substrate concentration is increased but then begins to level off, so that the increase in reaction rate becomes less and less as the substrate concentration increases. Explain this difference. Why do enzymes become inactive at very high temperatures? An enzyme has an optimum pH of 7.4. What is most likely to happen to the activity of the enzyme if the pH drops to 6.3? Explain. An enzyme has an optimum pH of 7.2. What is most likely to happen to the activity of the enzyme if the pH increases to 8.5? Explain. Answers
In an enzyme-catalyzed reaction, the substrate binds to the enzyme to form an enzyme-substrate complex.
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Enzyme Activity
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18.7
Enzyme Activity
18.7 Enzyme Activity
Learning Objective
Concentration of Substrate
Concentration of Enzyme
Temperature
Hydrogen Ion Concentration (pH)
Answers
Key Takeaways
Exercises
Answers
|
Explain. An enzyme has an optimum pH of 7.2. What is most likely to happen to the activity of the enzyme if the pH increases to 8.5? Explain. Answers
In an enzyme-catalyzed reaction, the substrate binds to the enzyme to form an enzyme-substrate complex. If more substrate is present than enzyme, all of the enzyme binding sites will have substrate bound, and further increases in substrate concentration cannot increase the rate. The activity will decrease; a pH of 6.3 is more acidic than 7.4, and one or more key groups in the active site may bind a hydrogen ion, changing the charge on that group.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Explain why vitamins are necessary in the diet. Many enzymes are simple proteins consisting entirely of one or more amino acid chains. Other enzymes contain a nonprotein component called a cofactor
A nonprotein component of an enzyme that is necessary for an enzyme’s proper functioning. that is necessary for the enzyme’s proper functioning. There are two types of cofactors: inorganic ions [e.g., zinc or Cu (I) ions] and organic molecules known as coenzymes
A cofactor that is an organic molecule. . Most coenzymes are vitamins or are derived from vitamins. Vitamins
An organic compound that is essential in very small amounts for the maintenance of normal metabolism. are organic compounds that are essential in very small (trace) amounts for the maintenance of normal metabolism.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
inorganic ions [e.g., zinc or Cu (I) ions] and organic molecules known as coenzymes
A cofactor that is an organic molecule. . Most coenzymes are vitamins or are derived from vitamins. Vitamins
An organic compound that is essential in very small amounts for the maintenance of normal metabolism. are organic compounds that are essential in very small (trace) amounts for the maintenance of normal metabolism. They generally cannot be synthesized at adequate levels by the body and must be obtained from the diet. The absence or shortage of a vitamin may result in a vitamin-deficiency disease. In the first half of the 20th century, a major focus of biochemistry was the identification, isolation, and characterization of vitamins. Despite accumulating evidence that people needed more than just carbohydrates, fats, and proteins in their diets for normal growth and health, it was not until the early 1900s that research established the need for trace nutrients in the diet. Because organisms differ in their synthetic abilities, a substance that is a vitamin for one species may not be so for another.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
They generally cannot be synthesized at adequate levels by the body and must be obtained from the diet. The absence or shortage of a vitamin may result in a vitamin-deficiency disease. In the first half of the 20th century, a major focus of biochemistry was the identification, isolation, and characterization of vitamins. Despite accumulating evidence that people needed more than just carbohydrates, fats, and proteins in their diets for normal growth and health, it was not until the early 1900s that research established the need for trace nutrients in the diet. Because organisms differ in their synthetic abilities, a substance that is a vitamin for one species may not be so for another. Over the past 100 years, scientists have identified and isolated 13 vitamins required in the human diet and have divided them into two broad categories: the fat-soluble vitamins, which include vitamins A, D, E, and K, and the water-soluble vitamins, which are the B complex vitamins and vitamin C. All fat-soluble vitamins contain a high proportion of hydrocarbon structural components. There are one or two oxygen atoms present, but the compounds as a whole are nonpolar. In contrast, water-soluble vitamins contain large numbers of electronegative oxygen and nitrogen atoms, which can engage in hydrogen bonding with water. Most water-soluble vitamins act as coenzymes or are required for the synthesis of coenzymes.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
Over the past 100 years, scientists have identified and isolated 13 vitamins required in the human diet and have divided them into two broad categories: the fat-soluble vitamins, which include vitamins A, D, E, and K, and the water-soluble vitamins, which are the B complex vitamins and vitamin C. All fat-soluble vitamins contain a high proportion of hydrocarbon structural components. There are one or two oxygen atoms present, but the compounds as a whole are nonpolar. In contrast, water-soluble vitamins contain large numbers of electronegative oxygen and nitrogen atoms, which can engage in hydrogen bonding with water. Most water-soluble vitamins act as coenzymes or are required for the synthesis of coenzymes. The fat-soluble vitamins are important for a variety of physiological functions. The key vitamins and their functions are found in Table 18.8 "Fat-Soluble Vitamins and Physiological Functions" and Table 18.9 "Water-Soluble Vitamins and Physiological Functions". Table 18.8 Fat-Soluble Vitamins and Physiological Functions
Vitamin
Physiological Function
Effect of Deficiency
vitamin A (retinol)
formation of vision pigments; differentiation of epithelial cells
night blindness; continued deficiency leads to total blindness
vitamin D (cholecalciferol)
increases the body’s ability to absorb calcium and phosphorus
osteomalacia (softening of the bones);
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
The fat-soluble vitamins are important for a variety of physiological functions. The key vitamins and their functions are found in Table 18.8 "Fat-Soluble Vitamins and Physiological Functions" and Table 18.9 "Water-Soluble Vitamins and Physiological Functions". Table 18.8 Fat-Soluble Vitamins and Physiological Functions
Vitamin
Physiological Function
Effect of Deficiency
vitamin A (retinol)
formation of vision pigments; differentiation of epithelial cells
night blindness; continued deficiency leads to total blindness
vitamin D (cholecalciferol)
increases the body’s ability to absorb calcium and phosphorus
osteomalacia (softening of the bones); known as rickets in children
vitamin E (tocopherol)
fat-soluble antioxidant
damage to cell membranes
vitamin K (phylloquinone)
formation of prothrombin, a key enzyme in the blood-clotting process
increases the time required for blood to clot
Table 18.9 Water-Soluble Vitamins and Physiological Functions
Vitamin
Coenzyme
Coenzyme Function
Deficiency Disease
vitamin B 1 (thiamine)
thiamine pyrophosphate
decarboxylation reactions
beri-beri
vitamin B 2 (riboflavin)
flavin mononucleotide or flavin adenine dinucleotide
oxidation-reduction reactions involving two hydrogen atoms
—
vitamin B 3 (niacin)
nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate
oxidation-reduction reactions involving the hydride ion (H −)
pellagra
vitamin B 6 (pyridoxine)
pyridoxal phosphate
variety of reactions including the transfer of amino groups
—
vitamin B 12 (cyanocobalamin)
methylcobalamin or deoxyadenoxylcobalamin
intramolecular rearrangement reactions
pernicious anemia
biotin
biotin
carboxylation reactions
—
folic acid
tetrahydrofolate
carrier of one-carbon units such as the formyl group
anemia
pantothenic Acid
coenzyme A
carrier of acyl groups
—
vitamin C (ascorbic acid)
none
antioxidant; formation of collagen, a protein found in tendons, ligaments, and bone
scurvy
Vitamins C and E, as well as the provitamin β-carotene can act as antioxidants in the body. Antioxidants
A substance that prevents oxidation. prevent damage from free radicals, which are molecules that are highly reactive because they have unpaired electrons. Free radicals are formed not only through metabolic reactions involving oxygen but also by such environmental factors as radiation and pollution.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
known as rickets in children
vitamin E (tocopherol)
fat-soluble antioxidant
damage to cell membranes
vitamin K (phylloquinone)
formation of prothrombin, a key enzyme in the blood-clotting process
increases the time required for blood to clot
Table 18.9 Water-Soluble Vitamins and Physiological Functions
Vitamin
Coenzyme
Coenzyme Function
Deficiency Disease
vitamin B 1 (thiamine)
thiamine pyrophosphate
decarboxylation reactions
beri-beri
vitamin B 2 (riboflavin)
flavin mononucleotide or flavin adenine dinucleotide
oxidation-reduction reactions involving two hydrogen atoms
—
vitamin B 3 (niacin)
nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate
oxidation-reduction reactions involving the hydride ion (H −)
pellagra
vitamin B 6 (pyridoxine)
pyridoxal phosphate
variety of reactions including the transfer of amino groups
—
vitamin B 12 (cyanocobalamin)
methylcobalamin or deoxyadenoxylcobalamin
intramolecular rearrangement reactions
pernicious anemia
biotin
biotin
carboxylation reactions
—
folic acid
tetrahydrofolate
carrier of one-carbon units such as the formyl group
anemia
pantothenic Acid
coenzyme A
carrier of acyl groups
—
vitamin C (ascorbic acid)
none
antioxidant; formation of collagen, a protein found in tendons, ligaments, and bone
scurvy
Vitamins C and E, as well as the provitamin β-carotene can act as antioxidants in the body. Antioxidants
A substance that prevents oxidation. prevent damage from free radicals, which are molecules that are highly reactive because they have unpaired electrons. Free radicals are formed not only through metabolic reactions involving oxygen but also by such environmental factors as radiation and pollution. Note
β-carotene is known as a provitamin because it can be converted to vitamin A in the body. Free radicals react most commonly react with lipoproteins and unsaturated fatty acids in cell membranes, removing an electron from those molecules and thus generating a new free radical. The process becomes a chain reaction that finally leads to the oxidative degradation of the affected compounds. Antioxidants react with free radicals to stop these chain reactions by forming a more stable molecule or, in the case of vitamin E, a free radical that is much less reactive. ( Vitamin E is converted back to its original form through interaction with vitamin C.)
Answers
A coenzyme is one type of cofactor.
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
Note
β-carotene is known as a provitamin because it can be converted to vitamin A in the body. Free radicals react most commonly react with lipoproteins and unsaturated fatty acids in cell membranes, removing an electron from those molecules and thus generating a new free radical. The process becomes a chain reaction that finally leads to the oxidative degradation of the affected compounds. Antioxidants react with free radicals to stop these chain reactions by forming a more stable molecule or, in the case of vitamin E, a free radical that is much less reactive. ( Vitamin E is converted back to its original form through interaction with vitamin C.)
Answers
A coenzyme is one type of cofactor. Coenzymes are organic molecules required by some enzymes for activity. A cofactor can be either a coenzyme or an inorganic ion. Coenzymes are synthesized from vitamins. Key Takeaways
Vitamins are organic compounds that are essential in very small amounts for the maintenance of normal metabolism. Vitamins are divided into two broad categories:
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
Coenzymes are organic molecules required by some enzymes for activity. A cofactor can be either a coenzyme or an inorganic ion. Coenzymes are synthesized from vitamins. Key Takeaways
Vitamins are organic compounds that are essential in very small amounts for the maintenance of normal metabolism. Vitamins are divided into two broad categories: fat-soluble vitamins and water-soluble vitamins. Most water-soluble vitamins are needed for the formation of coenzymes, which are organic molecules needed by some enzymes for catalytic activity. Exercises
Identify each vitamin as water soluble or fat soluble. vitamin D
vitamin C
vitamin B 12
Identify each vitamin as water soluble or fat soluble. niacin
cholecalciferol
biotin
What vitamin is needed to form each coenzyme?
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Enzyme Cofactors and Vitamins
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18.9
Enzyme Cofactors and Vitamins
18.9 Enzyme Cofactors and Vitamins
Learning Objective
Note
Answers
Key Takeaways
Exercises
Answers
|
fat-soluble vitamins and water-soluble vitamins. Most water-soluble vitamins are needed for the formation of coenzymes, which are organic molecules needed by some enzymes for catalytic activity. Exercises
Identify each vitamin as water soluble or fat soluble. vitamin D
vitamin C
vitamin B 12
Identify each vitamin as water soluble or fat soluble. niacin
cholecalciferol
biotin
What vitamin is needed to form each coenzyme? pyridoxal phosphate
flavin adenine dinucleotide
coenzyme A
nicotinamide adenine dinucleotide
What coenzyme is formed from each vitamin? niacin
thiamine
cyanocobalamin
pantothenic acid
What is the function of each vitamin or coenzyme? flavin adenine dinucleotide
vitamin A
biotin
What is the function of each vitamin or coenzyme? vitamin K
pyridoxal phosphate
tetrahydrofolate
Answers
fat soluble
water soluble
water soluble
vitamin B 6 or pyridoxine
vitamin B 2 or riboflavin
pantothenic acid
vitamin B 3 or niacin
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Describe how a new copy of DNA is synthesized. Describe how RNA is synthesized from DNA. Identify the different types of RNA and the function of each type of RNA. We previously stated that deoxyribonucleic acid (DNA) stores genetic information, while ribonucleic acid (RNA) is responsible for transmitting or expressing genetic information by directing the synthesis of thousands of proteins found in living organisms. But how do the nucleic acids perform these functions? Three processes are required: ( 1) replication, in which new copies of DNA are made; ( 2) transcription, in which a segment of DNA is used to produce RNA; and (3) translation, in which the information in RNA is translated into a protein sequence. ( For more information on protein sequences, see Section 19.4 "Protein Synthesis and the Genetic Code" .)
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Three processes are required: ( 1) replication, in which new copies of DNA are made; ( 2) transcription, in which a segment of DNA is used to produce RNA; and (3) translation, in which the information in RNA is translated into a protein sequence. ( For more information on protein sequences, see Section 19.4 "Protein Synthesis and the Genetic Code" .) Replication
New cells are continuously forming in the body through the process of cell division. For this to happen, the DNA in a dividing cell must be copied in a process known as replication
The process in which the DNA in a dividing cell is copied. . The complementary base pairing of the double helix provides a ready model for how genetic replication occurs. If the two chains of the double helix are pulled apart, disrupting the hydrogen bonding between base pairs, each chain can act as a template, or pattern, for the synthesis of a new complementary DNA chain.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Replication
New cells are continuously forming in the body through the process of cell division. For this to happen, the DNA in a dividing cell must be copied in a process known as replication
The process in which the DNA in a dividing cell is copied. . The complementary base pairing of the double helix provides a ready model for how genetic replication occurs. If the two chains of the double helix are pulled apart, disrupting the hydrogen bonding between base pairs, each chain can act as a template, or pattern, for the synthesis of a new complementary DNA chain. The nucleus contains all the necessary enzymes, proteins, and nucleotides required for this synthesis. A short segment of DNA is “unzipped,” so that the two strands in the segment are separated to serve as templates for new DNA. DNA polymerase, an enzyme, recognizes each base in a template strand and matches it to the complementary base in a free nucleotide. The enzyme then catalyzes the formation of an ester bond between the 5′ phosphate group of the nucleotide and the 3′ OH end of the new, growing DNA chain. In this way, each strand of the original DNA molecule is used to produce a duplicate of its former partner ( Figure 19.9 "A Schematic Diagram of DNA Replication" ).
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
The nucleus contains all the necessary enzymes, proteins, and nucleotides required for this synthesis. A short segment of DNA is “unzipped,” so that the two strands in the segment are separated to serve as templates for new DNA. DNA polymerase, an enzyme, recognizes each base in a template strand and matches it to the complementary base in a free nucleotide. The enzyme then catalyzes the formation of an ester bond between the 5′ phosphate group of the nucleotide and the 3′ OH end of the new, growing DNA chain. In this way, each strand of the original DNA molecule is used to produce a duplicate of its former partner ( Figure 19.9 "A Schematic Diagram of DNA Replication" ). Whatever information was encoded in the original DNA double helix is now contained in each replicate helix. When the cell divides, each daughter cell gets one of these replicates and thus all of the information that was originally possessed by the parent cell. Figure 19.9 A Schematic Diagram of DNA Replication
DNA replication occurs by the sequential unzipping of segments of the double helix. Each new nucleotide is brought into position by DNA polymerase and is added to the growing strand by the formation of a phosphate ester bond. Thus, two double helixes form from one, and each consists of one old strand and one new strand, an outcome called semiconservative replications. (
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Whatever information was encoded in the original DNA double helix is now contained in each replicate helix. When the cell divides, each daughter cell gets one of these replicates and thus all of the information that was originally possessed by the parent cell. Figure 19.9 A Schematic Diagram of DNA Replication
DNA replication occurs by the sequential unzipping of segments of the double helix. Each new nucleotide is brought into position by DNA polymerase and is added to the growing strand by the formation of a phosphate ester bond. Thus, two double helixes form from one, and each consists of one old strand and one new strand, an outcome called semiconservative replications. ( This representation is simplified; many more proteins are involved in replication.) Example 1
A segment of one strand from a DNA molecule has the sequence 5′‑TCCATGAGTTGA‑3′. What is the sequence of nucleotides in the opposite, or complementary, DNA chain? Solution
Knowing that the two strands are antiparallel and that T base pairs with A, while C base pairs with G, the sequence of the complementary strand will be 3′‑AGGTACTCAACT‑5′ (can also be written as TCAACTCATGGA). Skill-Building Exercise
A segment of one strand from a DNA molecule has the sequence 5′‑CCAGTGAATTGCCTAT‑3′. What is the sequence of nucleotides in the opposite, or complementary, DNA chain?
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
This representation is simplified; many more proteins are involved in replication.) Example 1
A segment of one strand from a DNA molecule has the sequence 5′‑TCCATGAGTTGA‑3′. What is the sequence of nucleotides in the opposite, or complementary, DNA chain? Solution
Knowing that the two strands are antiparallel and that T base pairs with A, while C base pairs with G, the sequence of the complementary strand will be 3′‑AGGTACTCAACT‑5′ (can also be written as TCAACTCATGGA). Skill-Building Exercise
A segment of one strand from a DNA molecule has the sequence 5′‑CCAGTGAATTGCCTAT‑3′. What is the sequence of nucleotides in the opposite, or complementary, DNA chain? What do we mean when we say information is encoded in the DNA molecule? An organism’s DNA can be compared to a book containing directions for assembling a model airplane or for knitting a sweater. Letters of the alphabet are arranged into words, and these words direct the individual to perform certain operations with specific materials. If all the directions are followed correctly, a model airplane or sweater is produced. In DNA, the particular sequences of nucleotides along the chains encode the directions for building an organism.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
What do we mean when we say information is encoded in the DNA molecule? An organism’s DNA can be compared to a book containing directions for assembling a model airplane or for knitting a sweater. Letters of the alphabet are arranged into words, and these words direct the individual to perform certain operations with specific materials. If all the directions are followed correctly, a model airplane or sweater is produced. In DNA, the particular sequences of nucleotides along the chains encode the directions for building an organism. Just as saw means one thing in English and was means another, the sequence of bases CGT means one thing, and TGC means something different. Although there are only four letters—the four nucleotides—in the genetic code of DNA, their sequencing along the DNA strands can vary so widely that information storage is essentially unlimited. Transcription
For the hereditary information in DNA to be useful, it must be “expressed,” that is, used to direct the growth and functioning of an organism. The first step in the processes that constitute DNA expression is the synthesis of RNA, by a template mechanism that is in many ways analogous to DNA replication. Because the RNA that is synthesized is a complementary copy of information contained in DNA, RNA synthesis is referred to as transcription
The process in which RNA is synthesized from a DNA template.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Just as saw means one thing in English and was means another, the sequence of bases CGT means one thing, and TGC means something different. Although there are only four letters—the four nucleotides—in the genetic code of DNA, their sequencing along the DNA strands can vary so widely that information storage is essentially unlimited. Transcription
For the hereditary information in DNA to be useful, it must be “expressed,” that is, used to direct the growth and functioning of an organism. The first step in the processes that constitute DNA expression is the synthesis of RNA, by a template mechanism that is in many ways analogous to DNA replication. Because the RNA that is synthesized is a complementary copy of information contained in DNA, RNA synthesis is referred to as transcription
The process in which RNA is synthesized from a DNA template. . There are three key differences between replication and transcription: ( 1) RNA molecules are much shorter than DNA molecules; only a portion of one DNA strand is copied or transcribed to make an RNA molecule. ( 2) RNA is built from ribonucleotides rather than deoxyribonucleotides. (
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
. There are three key differences between replication and transcription: ( 1) RNA molecules are much shorter than DNA molecules; only a portion of one DNA strand is copied or transcribed to make an RNA molecule. ( 2) RNA is built from ribonucleotides rather than deoxyribonucleotides. ( 3) The newly synthesized RNA strand does not remain associated with the DNA sequence it was transcribed from. The DNA sequence that is transcribed to make RNA is called the template strand, while the complementary sequence on the other DNA strand is called the coding or informational strand. To initiate RNA synthesis, the two DNA strands unwind at specific sites along the DNA molecule. Ribonucleotides are attracted to the uncoiling region of the DNA molecule, beginning at the 3′ end of the template strand, according to the rules of base pairing. Thymine in DNA calls for adenine in RNA, cytosine specifies guanine, guanine calls for cytosine, and adenine requires uracil.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
3) The newly synthesized RNA strand does not remain associated with the DNA sequence it was transcribed from. The DNA sequence that is transcribed to make RNA is called the template strand, while the complementary sequence on the other DNA strand is called the coding or informational strand. To initiate RNA synthesis, the two DNA strands unwind at specific sites along the DNA molecule. Ribonucleotides are attracted to the uncoiling region of the DNA molecule, beginning at the 3′ end of the template strand, according to the rules of base pairing. Thymine in DNA calls for adenine in RNA, cytosine specifies guanine, guanine calls for cytosine, and adenine requires uracil. RNA polymerase—an enzyme—binds the complementary ribonucleotide and catalyzes the formation of the ester linkage between ribonucleotides, a reaction very similar to that catalyzed by DNA polymerase ( Figure 19.10 "A Schematic Diagram of RNA Transcription from a DNA Template" ). Synthesis of the RNA strand takes place in the 5′ to 3′ direction, antiparallel to the template strand. Only a short segment of the RNA molecule is hydrogen-bonded to the template strand at any time during transcription. When transcription is completed, the RNA is released, and the DNA helix reforms. The nucleotide sequence of the RNA strand formed during transcription is identical to that of the corresponding coding strand of the DNA, except that U replaces T.
Figure 19.10 A Schematic Diagram of RNA Transcription from a DNA Template
The representation of RNA polymerase is proportionately much smaller than the actual molecule, which encompasses about 50 nucleotides at a time.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
RNA polymerase—an enzyme—binds the complementary ribonucleotide and catalyzes the formation of the ester linkage between ribonucleotides, a reaction very similar to that catalyzed by DNA polymerase ( Figure 19.10 "A Schematic Diagram of RNA Transcription from a DNA Template" ). Synthesis of the RNA strand takes place in the 5′ to 3′ direction, antiparallel to the template strand. Only a short segment of the RNA molecule is hydrogen-bonded to the template strand at any time during transcription. When transcription is completed, the RNA is released, and the DNA helix reforms. The nucleotide sequence of the RNA strand formed during transcription is identical to that of the corresponding coding strand of the DNA, except that U replaces T.
Figure 19.10 A Schematic Diagram of RNA Transcription from a DNA Template
The representation of RNA polymerase is proportionately much smaller than the actual molecule, which encompasses about 50 nucleotides at a time. Example 2
A portion of the template strand of a gene has the sequence 5′‑TCCATGAGTTGA‑3′. What is the sequence of nucleotides in the RNA that is formed from this template? Solution
Four things must be remembered in answering this question: ( 1) the DNA strand and the RNA strand being synthesized are antiparallel; ( 2) RNA is synthesized in a 5′ to 3′ direction, so transcription begins at the 3′ end of the template strand; ( 3) ribonucleotides are used in place of deoxyribonucleotides;
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Example 2
A portion of the template strand of a gene has the sequence 5′‑TCCATGAGTTGA‑3′. What is the sequence of nucleotides in the RNA that is formed from this template? Solution
Four things must be remembered in answering this question: ( 1) the DNA strand and the RNA strand being synthesized are antiparallel; ( 2) RNA is synthesized in a 5′ to 3′ direction, so transcription begins at the 3′ end of the template strand; ( 3) ribonucleotides are used in place of deoxyribonucleotides; and (4) thymine (T) base pairs with adenine (A), A base pairs with uracil (U; in RNA), and cytosine (C) base pairs with guanine (G). The sequence is determined to be 3′‑AGGUACUCAACU‑5′ (can also be written as 5′‑UCAACUCAUGGA‑3′). Skill-Building Exercise
A portion of the template strand of a gene has the sequence 5′‑CCAGTGAATTGCCTAT‑3′. What is the sequence of nucleotides in the RNA that is formed from this template? Three types of RNA are formed during transcription:
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
and (4) thymine (T) base pairs with adenine (A), A base pairs with uracil (U; in RNA), and cytosine (C) base pairs with guanine (G). The sequence is determined to be 3′‑AGGUACUCAACU‑5′ (can also be written as 5′‑UCAACUCAUGGA‑3′). Skill-Building Exercise
A portion of the template strand of a gene has the sequence 5′‑CCAGTGAATTGCCTAT‑3′. What is the sequence of nucleotides in the RNA that is formed from this template? Three types of RNA are formed during transcription: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). These three types of RNA differ in function, size, and percentage of the total cell RNA ( Table 19.2 "Properties of Cellular RNA in " ). mRNA makes up only a small percent of the total amount of RNA within the cell, primarily because each molecule of mRNA exists for a relatively short time; it is continuously being degraded and resynthesized. The molecular dimensions of the mRNA molecule vary according to the amount of genetic information a given molecule contains.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). These three types of RNA differ in function, size, and percentage of the total cell RNA ( Table 19.2 "Properties of Cellular RNA in " ). mRNA makes up only a small percent of the total amount of RNA within the cell, primarily because each molecule of mRNA exists for a relatively short time; it is continuously being degraded and resynthesized. The molecular dimensions of the mRNA molecule vary according to the amount of genetic information a given molecule contains. After transcription, which takes place in the nucleus, the mRNA passes into the cytoplasm, carrying the genetic message from DNA to the ribosomes, the sites of protein synthesis. In Section 19.5 "Mutations and Genetic Diseases", we shall see how mRNA directly determines the sequence of amino acids during protein synthesis. Table 19.2 Properties of Cellular RNA in Escherichia coli
Type
Function
Approximate Number of Nucleotides
Percentage of Total Cell RNA
mRNA
codes for proteins
100–6,000
~3
rRNA
component of ribosomes
120–2900
83
tRNA
adapter molecule that brings the amino acid to the ribosome
75–90
14
Ribosomes
A cellular substructure where proteins are synthesized. are cellular substructures where proteins are synthesized. They contain about 65% rRNA and 35% protein, held together by numerous noncovalent interactions, such as hydrogen bonding, in an overall structure consisting of two globular particles of unequal size.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
After transcription, which takes place in the nucleus, the mRNA passes into the cytoplasm, carrying the genetic message from DNA to the ribosomes, the sites of protein synthesis. In Section 19.5 "Mutations and Genetic Diseases", we shall see how mRNA directly determines the sequence of amino acids during protein synthesis. Table 19.2 Properties of Cellular RNA in Escherichia coli
Type
Function
Approximate Number of Nucleotides
Percentage of Total Cell RNA
mRNA
codes for proteins
100–6,000
~3
rRNA
component of ribosomes
120–2900
83
tRNA
adapter molecule that brings the amino acid to the ribosome
75–90
14
Ribosomes
A cellular substructure where proteins are synthesized. are cellular substructures where proteins are synthesized. They contain about 65% rRNA and 35% protein, held together by numerous noncovalent interactions, such as hydrogen bonding, in an overall structure consisting of two globular particles of unequal size. Molecules of tRNA, which bring amino acids (one at a time) to the ribosomes for the construction of proteins, differ from one another in the kinds of amino acid each is specifically designed to carry. A set of three nucleotides, known as a codon
A set of three nucleotides on the mRNA that specifies a particular amino acid. , on the mRNA determines which kind of tRNA will add its amino acid to the growing chain. ( For more information on sequences, see Section 19.4 "Protein Synthesis and the Genetic Code" .) Each of the 20 amino acids found in proteins has at least one corresponding kind of tRNA, and most amino acids have more than one.
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Replication and Expression of Genetic Information
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19.3
Replication and Expression of Genetic Information
19.3 Replication and Expression of Genetic Information
Learning Objectives
Replication
Example 1
Skill-Building Exercise
Transcription
Example 2
Skill-Building Exercise
Answers
Key Takeaways
Exercises
Answers
|
Molecules of tRNA, which bring amino acids (one at a time) to the ribosomes for the construction of proteins, differ from one another in the kinds of amino acid each is specifically designed to carry. A set of three nucleotides, known as a codon
A set of three nucleotides on the mRNA that specifies a particular amino acid. , on the mRNA determines which kind of tRNA will add its amino acid to the growing chain. ( For more information on sequences, see Section 19.4 "Protein Synthesis and the Genetic Code" .) Each of the 20 amino acids found in proteins has at least one corresponding kind of tRNA, and most amino acids have more than one. The two-dimensional structure of a tRNA molecule has
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
|
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Describe the importance of ATP as a source of energy in living organisms. Adenosine triphosphate (ATP), a nucleotide composed of adenine, ribose, and three phosphate groups, is perhaps the most important of the so-called energy-rich compounds in a cell. Its concentration in the cell varies from 0.5 to 2.5 mg/mL of cell fluid. Energy-rich compounds are substances having particular structural features that lead to a release of energy after hydrolysis. As a result, these compounds are able to supply energy for biochemical processes that require energy. The structural feature important in ATP is the phosphoric acid anhydride, or pyrophosphate, linkage: The pyrophosphate bond, symbolized by a squiggle (~), is hydrolyzed when ATP is converted to adenosine diphosphate (ADP). In this hydrolysis reaction, the products contain less energy than the reactants; there is a release of energy (> 7 kcal/mol). One reason for the amount of energy released is that hydrolysis relieves the electron-electron repulsions experienced by the negatively charged phosphate groups when they are bonded to each other ( Figure 20.3 "Hydrolysis of ATP to Form ADP" ).
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
|
The structural feature important in ATP is the phosphoric acid anhydride, or pyrophosphate, linkage: The pyrophosphate bond, symbolized by a squiggle (~), is hydrolyzed when ATP is converted to adenosine diphosphate (ADP). In this hydrolysis reaction, the products contain less energy than the reactants; there is a release of energy (> 7 kcal/mol). One reason for the amount of energy released is that hydrolysis relieves the electron-electron repulsions experienced by the negatively charged phosphate groups when they are bonded to each other ( Figure 20.3 "Hydrolysis of ATP to Form ADP" ). Figure 20.3 Hydrolysis of ATP to Form ADP
Energy is released because the products (ADP and phosphate ion) have less energy than the reactants [ATP and water (H 2 O)]. The general equation for ATP hydrolysis is as follows: ATP + H2O → ADP + Pi + 7.4 kcal/mol
If the hydrolysis of ATP releases energy, its synthesis (from ADP) requires energy. In the cell, ATP is produced by those processes that supply energy to the organism (absorption of radiant energy from the sun in green plants and breakdown of food in animals), and it is hydrolyzed by those processes that require energy (the syntheses of carbohydrates, lipids, proteins; the transmission of nerve impulses;
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
|
Figure 20.3 Hydrolysis of ATP to Form ADP
Energy is released because the products (ADP and phosphate ion) have less energy than the reactants [ATP and water (H 2 O)]. The general equation for ATP hydrolysis is as follows: ATP + H2O → ADP + Pi + 7.4 kcal/mol
If the hydrolysis of ATP releases energy, its synthesis (from ADP) requires energy. In the cell, ATP is produced by those processes that supply energy to the organism (absorption of radiant energy from the sun in green plants and breakdown of food in animals), and it is hydrolyzed by those processes that require energy (the syntheses of carbohydrates, lipids, proteins; the transmission of nerve impulses; muscle contractions). In fact, ATP is the principal medium of energy exchange in biological systems. Many scientists call it the energy currency of cells. Note
P i is the symbol for the inorganic phosphate anions H 2 PO 4− and HPO 42−.
ATP is not the only high-energy compound needed for metabolism. Several others are listed in Table 20.1 "Energy Released by Hydrolysis of Some Phosphate Compounds".
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
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muscle contractions). In fact, ATP is the principal medium of energy exchange in biological systems. Many scientists call it the energy currency of cells. Note
P i is the symbol for the inorganic phosphate anions H 2 PO 4− and HPO 42−.
ATP is not the only high-energy compound needed for metabolism. Several others are listed in Table 20.1 "Energy Released by Hydrolysis of Some Phosphate Compounds". Notice, however, that the energy released when ATP is hydrolyzed is approximately midway between those of the high-energy and the low-energy phosphate compounds. This means that the hydrolysis of ATP can provide energy for the phosphorylation of the compounds below it in the table. For example, the hydrolysis of ATP provides sufficient energy for the phosphorylation of glucose to form glucose 1-phosphate. By the same token, the hydrolysis of compounds, such as creatine phosphate, that appear above ATP in the table can provide the energy needed to resynthesize ATP from ADP. Table 20.1 Energy Released by Hydrolysis of Some Phosphate Compounds
Type
Example
Energy Released (kcal/mol)
acyl phosphate
1,3-bisphosphoglycerate (BPG)
−11.8
acetyl phosphate
−11.3
guanidine phosphates
creatine phosphate
−10.3
arginine phosphate
−9.1
pyrophosphates
PP i* → 2P i
−7.8
ATP → AMP + PP i
−7.7
ATP → ADP + P i
−7.5
ADP → AMP + P i
−7.5
sugar phosphates
glucose 1-phosphate
−5.0
fructose 6-phosphate
−3.8
AMP → adenosine + P i
−3.4
glucose 6-phosphate
−3.3
glycerol 3-phosphate
−2.2
*PPi is the pyrophosphate ion.
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
|
Notice, however, that the energy released when ATP is hydrolyzed is approximately midway between those of the high-energy and the low-energy phosphate compounds. This means that the hydrolysis of ATP can provide energy for the phosphorylation of the compounds below it in the table. For example, the hydrolysis of ATP provides sufficient energy for the phosphorylation of glucose to form glucose 1-phosphate. By the same token, the hydrolysis of compounds, such as creatine phosphate, that appear above ATP in the table can provide the energy needed to resynthesize ATP from ADP. Table 20.1 Energy Released by Hydrolysis of Some Phosphate Compounds
Type
Example
Energy Released (kcal/mol)
acyl phosphate
1,3-bisphosphoglycerate (BPG)
−11.8
acetyl phosphate
−11.3
guanidine phosphates
creatine phosphate
−10.3
arginine phosphate
−9.1
pyrophosphates
PP i* → 2P i
−7.8
ATP → AMP + PP i
−7.7
ATP → ADP + P i
−7.5
ADP → AMP + P i
−7.5
sugar phosphates
glucose 1-phosphate
−5.0
fructose 6-phosphate
−3.8
AMP → adenosine + P i
−3.4
glucose 6-phosphate
−3.3
glycerol 3-phosphate
−2.2
*PPi is the pyrophosphate ion. Answer
ATP is the principal molecule involved in energy exchange reactions in biological systems. Key Takeaway
The hydrolysis of ATP releases energy that can be used for cellular processes that require energy. Exercises
How do ATP and ADP differ in structure? Why does the hydrolysis of ATP to ADP involve the release of energy? Identify whether each compound would be classified as a high-energy phosphate compound.
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ATP—the Universal Energy Currency
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20.1
ATP—the Universal Energy Currency
20.1 ATP—the Universal Energy Currency
Learning Objective
Note
Answer
Key Takeaway
Exercises
Answers
|
Answer
ATP is the principal molecule involved in energy exchange reactions in biological systems. Key Takeaway
The hydrolysis of ATP releases energy that can be used for cellular processes that require energy. Exercises
How do ATP and ADP differ in structure? Why does the hydrolysis of ATP to ADP involve the release of energy? Identify whether each compound would be classified as a high-energy phosphate compound. ATP
glucose 6-phosphate
creatine phosphate
Identify whether each compound would be classified as a high-energy phosphate compound. ADP
AMP
glucose 1-phosphate
Answers
ATP has a triphosphate group attached, while ADP has only a diphosphate group attached. yes
no
yes
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
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Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Describe how carbohydrates, fats, and proteins are broken down during digestion. We have said that animals obtain chemical energy from the food—carbohydrates, fats, and proteins—they eat through reactions defined collectively as catabolism. We can think of catabolism as occurring in three stages ( Figure 20.4 "Energy Conversions" ). In stage I, carbohydrates, fats, and proteins are broken down into their individual monomer units: carbohydrates into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. One part of stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer units—which occurs in the mouth, stomach, and small intestine—and is referred to as digestion
The breakdown of food molecules by hydrolysis reactions into the individual monomer units in the mouth, stomach, and small intestine. . In stage II, these monomer units (or building blocks) are further broken down through different reaction pathways, one of which produces ATP, to form a common end product that can then be used in stage III to produce even more ATP. In this chapter, we will look at each stage of catabolism—as an overview and in detail. Figure 20.4 Energy Conversions
The conversion of food into cellular energy (as ATP) occurs in three stages.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
One part of stage I of catabolism is the breakdown of food molecules by hydrolysis reactions into the individual monomer units—which occurs in the mouth, stomach, and small intestine—and is referred to as digestion
The breakdown of food molecules by hydrolysis reactions into the individual monomer units in the mouth, stomach, and small intestine. . In stage II, these monomer units (or building blocks) are further broken down through different reaction pathways, one of which produces ATP, to form a common end product that can then be used in stage III to produce even more ATP. In this chapter, we will look at each stage of catabolism—as an overview and in detail. Figure 20.4 Energy Conversions
The conversion of food into cellular energy (as ATP) occurs in three stages. Digestion of Carbohydrates
Carbohydrate digestion begins in the mouth ( Figure 20.5 "The Principal Events and Sites of Carbohydrate Digestion" ), where salivary α-amylase attacks the α-glycosidic linkages in starch, the main carbohydrate ingested by humans. Cleavage of the glycosidic linkages produces a mixture of dextrins, maltose, and glucose. ( For more information about carbohydrates, see Chapter 16 "Carbohydrates" .) The α-amylase mixed into the food remains active as the food passes through the esophagus, but it is rapidly inactivated in the acidic environment of the stomach. Figure 20.5 The Principal Events and Sites of Carbohydrate Digestion
The primary site of carbohydrate digestion is the small intestine.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
Digestion of Carbohydrates
Carbohydrate digestion begins in the mouth ( Figure 20.5 "The Principal Events and Sites of Carbohydrate Digestion" ), where salivary α-amylase attacks the α-glycosidic linkages in starch, the main carbohydrate ingested by humans. Cleavage of the glycosidic linkages produces a mixture of dextrins, maltose, and glucose. ( For more information about carbohydrates, see Chapter 16 "Carbohydrates" .) The α-amylase mixed into the food remains active as the food passes through the esophagus, but it is rapidly inactivated in the acidic environment of the stomach. Figure 20.5 The Principal Events and Sites of Carbohydrate Digestion
The primary site of carbohydrate digestion is the small intestine. The secretion of α-amylase in the small intestine converts any remaining starch molecules, as well as the dextrins, to maltose. Maltose is then cleaved into two glucose molecules by maltase. Disaccharides such as sucrose and lactose are not digested until they reach the small intestine, where they are acted on by sucrase and lactase, respectively. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
The secretion of α-amylase in the small intestine converts any remaining starch molecules, as well as the dextrins, to maltose. Maltose is then cleaved into two glucose molecules by maltase. Disaccharides such as sucrose and lactose are not digested until they reach the small intestine, where they are acted on by sucrase and lactase, respectively. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream. Digestion of Proteins
Protein digestion begins in the stomach ( Figure 20.6 "The Principal Events and Sites of Protein Digestion" ), where the action of gastric juice hydrolyzes about 10% of the peptide bonds. Gastric juice
A mixture of water, inorganic ions, hydrochloric acid, and various enzymes and proteins found in the stomach. is a mixture of water (more than 99%), inorganic ions, hydrochloric acid, and various enzymes and other proteins. Note
The pain of a gastric ulcer is at least partially due to irritation of the ulcerated tissue by acidic gastric juice.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
These are absorbed through the wall of the small intestine into the bloodstream. Digestion of Proteins
Protein digestion begins in the stomach ( Figure 20.6 "The Principal Events and Sites of Protein Digestion" ), where the action of gastric juice hydrolyzes about 10% of the peptide bonds. Gastric juice
A mixture of water, inorganic ions, hydrochloric acid, and various enzymes and proteins found in the stomach. is a mixture of water (more than 99%), inorganic ions, hydrochloric acid, and various enzymes and other proteins. Note
The pain of a gastric ulcer is at least partially due to irritation of the ulcerated tissue by acidic gastric juice. Figure 20.6 The Principal Events and Sites of Protein Digestion
The hydrochloric acid (HCl) in gastric juice is secreted by glands in the stomach lining. The pH of freshly secreted gastric juice is about 1.0, but the contents of the stomach may raise the pH to between 1.5 and 2.5. HCl helps to denature food proteins; that is, it unfolds the protein molecules to expose their chains to more efficient enzyme action. The principal digestive component of gastric juice is pepsinogen, an inactive enzyme produced in cells located in the stomach wall.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
Figure 20.6 The Principal Events and Sites of Protein Digestion
The hydrochloric acid (HCl) in gastric juice is secreted by glands in the stomach lining. The pH of freshly secreted gastric juice is about 1.0, but the contents of the stomach may raise the pH to between 1.5 and 2.5. HCl helps to denature food proteins; that is, it unfolds the protein molecules to expose their chains to more efficient enzyme action. The principal digestive component of gastric juice is pepsinogen, an inactive enzyme produced in cells located in the stomach wall. When food enters the stomach after a period of fasting, pepsinogen is converted to its active form—pepsin—in a series of steps initiated by the drop in pH. Pepsin catalyzes the hydrolysis of peptide linkages within protein molecules. It has a fairly broad specificity but acts preferentially on linkages involving the aromatic amino acids tryptophan, tyrosine, and phenylalanine, as well as methionine and leucine. Protein digestion is completed in the small intestine. Pancreatic juice, carried from the pancreas via the pancreatic duct, contains inactive enzymes such as trypsinogen and chymotrypsinogen. They are activated in the small intestine as follows ( Figure 20.7 "Activation of Some Pancreatic Enzymes in the Small Intestine" ): The intestinal mucosal cells secrete the proteolytic enzyme enteropeptidase, which converts trypsinogen to trypsin;
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
When food enters the stomach after a period of fasting, pepsinogen is converted to its active form—pepsin—in a series of steps initiated by the drop in pH. Pepsin catalyzes the hydrolysis of peptide linkages within protein molecules. It has a fairly broad specificity but acts preferentially on linkages involving the aromatic amino acids tryptophan, tyrosine, and phenylalanine, as well as methionine and leucine. Protein digestion is completed in the small intestine. Pancreatic juice, carried from the pancreas via the pancreatic duct, contains inactive enzymes such as trypsinogen and chymotrypsinogen. They are activated in the small intestine as follows ( Figure 20.7 "Activation of Some Pancreatic Enzymes in the Small Intestine" ): The intestinal mucosal cells secrete the proteolytic enzyme enteropeptidase, which converts trypsinogen to trypsin; trypsin then activates chymotrypsinogen to chymotrypsin (and also completes the activation of trypsinogen). Both of these active enzymes catalyze the hydrolysis of peptide bonds in protein chains. Chymotrypsin preferentially attacks peptide bonds involving the carboxyl groups of the aromatic amino acids (phenylalanine, tryptophan, and tyrosine). Trypsin attacks peptide bonds involving the carboxyl groups of the basic amino acids (lysine and arginine). Pancreatic juice also contains procarboxypeptidase, which is cleaved by trypsin to carboxypeptidase.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
trypsin then activates chymotrypsinogen to chymotrypsin (and also completes the activation of trypsinogen). Both of these active enzymes catalyze the hydrolysis of peptide bonds in protein chains. Chymotrypsin preferentially attacks peptide bonds involving the carboxyl groups of the aromatic amino acids (phenylalanine, tryptophan, and tyrosine). Trypsin attacks peptide bonds involving the carboxyl groups of the basic amino acids (lysine and arginine). Pancreatic juice also contains procarboxypeptidase, which is cleaved by trypsin to carboxypeptidase. The latter is an enzyme that catalyzes the hydrolysis of peptide linkages at the free carboxyl end of the peptide chain, resulting in the stepwise liberation of free amino acids from the carboxyl end of the polypeptide. Figure 20.7 Activation of Some Pancreatic Enzymes in the Small Intestine
Aminopeptidases in the intestinal juice remove amino acids from the N-terminal end of peptides and proteins possessing a free amino group. Figure 20.8 "Hydrolysis of a Peptide by Several Peptidases" illustrates the specificity of these protein-digesting enzymes. The amino acids that are released by protein digestion are absorbed across the intestinal wall into the circulatory system, where they can be used for protein synthesis. Figure 20.8 Hydrolysis of a Peptide by Several Peptidases
This diagram illustrates where in a peptide the different peptidases we have discussed would catalyze hydrolysis the peptide bonds.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
The latter is an enzyme that catalyzes the hydrolysis of peptide linkages at the free carboxyl end of the peptide chain, resulting in the stepwise liberation of free amino acids from the carboxyl end of the polypeptide. Figure 20.7 Activation of Some Pancreatic Enzymes in the Small Intestine
Aminopeptidases in the intestinal juice remove amino acids from the N-terminal end of peptides and proteins possessing a free amino group. Figure 20.8 "Hydrolysis of a Peptide by Several Peptidases" illustrates the specificity of these protein-digesting enzymes. The amino acids that are released by protein digestion are absorbed across the intestinal wall into the circulatory system, where they can be used for protein synthesis. Figure 20.8 Hydrolysis of a Peptide by Several Peptidases
This diagram illustrates where in a peptide the different peptidases we have discussed would catalyze hydrolysis the peptide bonds. Digestion of Lipids
Lipid digestion begins in the upper portion of the small intestine ( Figure 20.9 "The Principal Events and Sites of Lipid (Primarily Triglyceride) Digestion" ). A hormone secreted in this region stimulates the gallbladder to discharge bile into the duodenum. The principal constituents of bile are the bile salts, which emulsify large, water-insoluble lipid droplets, disrupting some of the hydrophobic interactions holding the lipid molecules together and suspending the resulting smaller globules (micelles) in the aqueous digestive medium. ( For more information on bile salts, see Chapter 17 "Lipids", Section 17.4 "Steroids" .) These changes greatly increase the surface area of the lipid particles, allowing for more intimate contact with the lipases and thus rapid digestion of the fats.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
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Digestion of Lipids
Lipid digestion begins in the upper portion of the small intestine ( Figure 20.9 "The Principal Events and Sites of Lipid (Primarily Triglyceride) Digestion" ). A hormone secreted in this region stimulates the gallbladder to discharge bile into the duodenum. The principal constituents of bile are the bile salts, which emulsify large, water-insoluble lipid droplets, disrupting some of the hydrophobic interactions holding the lipid molecules together and suspending the resulting smaller globules (micelles) in the aqueous digestive medium. ( For more information on bile salts, see Chapter 17 "Lipids", Section 17.4 "Steroids" .) These changes greatly increase the surface area of the lipid particles, allowing for more intimate contact with the lipases and thus rapid digestion of the fats. Another hormone promotes the secretion of pancreatic juice, which contains these enzymes. Figure 20.9 The Principal Events and Sites of Lipid (Primarily Triglyceride) Digestion
The lipases in pancreatic juice catalyze the digestion of triglycerides first to diglycerides and then to 2‑monoglycerides and fatty acids: The monoglycerides and fatty acids cross the intestinal lining into the bloodstream, where they are resynthesized into triglycerides and transported as lipoprotein complexes known as chylomicrons. Phospholipids and cholesteryl esters undergo similar hydrolysis in the small intestine, and their component molecules are also absorbed through the intestinal lining. The further metabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in stages II and III of catabolism.
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
Another hormone promotes the secretion of pancreatic juice, which contains these enzymes. Figure 20.9 The Principal Events and Sites of Lipid (Primarily Triglyceride) Digestion
The lipases in pancreatic juice catalyze the digestion of triglycerides first to diglycerides and then to 2‑monoglycerides and fatty acids: The monoglycerides and fatty acids cross the intestinal lining into the bloodstream, where they are resynthesized into triglycerides and transported as lipoprotein complexes known as chylomicrons. Phospholipids and cholesteryl esters undergo similar hydrolysis in the small intestine, and their component molecules are also absorbed through the intestinal lining. The further metabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in stages II and III of catabolism. Answers
Pepsinogen is an inactive form of pepsin; pepsin is the active form of the enzyme. Both enzymes catalyze the hydrolysis of peptide bonds. Chymotrypsin catalyzes the hydrolysis of peptide bonds following aromatic amino acids, while trypsin catalyzes the hydrolysis of peptide bonds following lysine and arginine. Aminopeptidase catalyzes the hydrolysis of amino acids from the N-terminal end of a protein, while carboxypeptidase catalyzes the hydrolysis of amino acids from the C-terminal end of a protein.
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http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s23-02-stage-i-of-catabolism.html
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Stage I of Catabolism
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20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
Answers
Pepsinogen is an inactive form of pepsin; pepsin is the active form of the enzyme. Both enzymes catalyze the hydrolysis of peptide bonds. Chymotrypsin catalyzes the hydrolysis of peptide bonds following aromatic amino acids, while trypsin catalyzes the hydrolysis of peptide bonds following lysine and arginine. Aminopeptidase catalyzes the hydrolysis of amino acids from the N-terminal end of a protein, while carboxypeptidase catalyzes the hydrolysis of amino acids from the C-terminal end of a protein. glucose, fructose, and galactose
monoglycerides and fatty acids
amino acids
the small intestine
Key Takeaways
During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids. Most of the digestion reactions occur in the small intestine. Exercises
What are the products of digestion (or stage I of catabolism)? What is the general type of reaction used in digestion? Give the site of action and the function of each enzyme.
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http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s23-02-stage-i-of-catabolism.html
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Stage I of Catabolism
|
20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
glucose, fructose, and galactose
monoglycerides and fatty acids
amino acids
the small intestine
Key Takeaways
During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids. Most of the digestion reactions occur in the small intestine. Exercises
What are the products of digestion (or stage I of catabolism)? What is the general type of reaction used in digestion? Give the site of action and the function of each enzyme. chymotrypsin
lactase
pepsin
maltase
Give the site of action and the function of each enzyme. α-amylase
trypsin
sucrase
aminopeptidase
What is the meaning of the following statement? “ Bile salts act to emulsify lipids in the small intestine.” Why is emulsification important? Using chemical equations, describe the chemical changes that triglycerides undergo during digestion.
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http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s23-02-stage-i-of-catabolism.html
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Stage I of Catabolism
|
20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
chymotrypsin
lactase
pepsin
maltase
Give the site of action and the function of each enzyme. α-amylase
trypsin
sucrase
aminopeptidase
What is the meaning of the following statement? “ Bile salts act to emulsify lipids in the small intestine.” Why is emulsification important? Using chemical equations, describe the chemical changes that triglycerides undergo during digestion. What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment? gly-ala-phe-thr-leu
ala-ile-tyr-ser-arg
val-trp-arg-leu-cys
What are the expected products from the enzymatic action of trypsin on each amino acid segment? leu-thr-glu-lys-ala
phe-arg-ala-leu-val
ala-arg-glu-trp-lys
Answers
proteins: amino acids; carbohydrates:
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http://2012books.lardbucket.org/books/introduction-to-chemistry-general-organic-and-biological/s23-02-stage-i-of-catabolism.html
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Stage I of Catabolism
|
20.2
Stage I of Catabolism
20.2 Stage I of Catabolism
Learning Objective
Digestion of Carbohydrates
Digestion of Proteins
Note
Digestion of Lipids
Answers
Key Takeaways
Exercises
Answers
|
What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment? gly-ala-phe-thr-leu
ala-ile-tyr-ser-arg
val-trp-arg-leu-cys
What are the expected products from the enzymatic action of trypsin on each amino acid segment? leu-thr-glu-lys-ala
phe-arg-ala-leu-val
ala-arg-glu-trp-lys
Answers
proteins: amino acids; carbohydrates: monosaccharides; fats: fatty acids and glycerol
Chymotrypsin is found in the small intestine and catalyzes the hydrolysis of peptide bonds following aromatic amino acids. Lactase is found in the small intestine and catalyzes the hydrolysis of lactose. Pepsin is found in the stomach and catalyzes the hydrolysis of peptide bonds, primarily those that occur after aromatic amino acids.
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