<prompt> Which enzyme joins two substrates-LyaseLigaseIsomeraseSynthase  <response>: Ans. is b i.e., Ligase An enzyme which joins two substrates in an energy dependent process is called ligase Examples includeLigases, Synthetases, Carboxylases Enzymes are classified into 6 classes:i Oxidoreductasesii Transferasesiii Hydrolasesiv Lyasesv Isomerasesvi LigasesI. Oxidoreductases catalyse oxidation or reduction reactions of the primary substrate. Hence they are in turn classified as those which mediate: Oxidation reactions Reduction reactions Those which mediate oxidation reactions can be classified as those which mediate oxidation by: Removal of hydrogen ion or Addition of oxygen Those which cause REMOVAL OF HYDROGEN can again be classified depending upon the acceptor NADFADNADP as acceptor - DehydrogenasesExamples - Lactate Dehydrogenase, Pyruvate Dehydrogenase both remove hydrogen ion from Lactate and Pyruvate respectively and donate the hydrogen ions to NAD forming NADH and H Oxygen as acceptor - OxidasesExample - L Aminoacid oxidase removes hydrogen atoms from aminoacid and that is donated to Oxygen molecule forming hydrogen peroxide Those which add oxygen can be classified depending upon whether one or both the atoms are used asMonooxygenases Monooxygenase allows oxygen molecule to react with a substrate. However, it incorporates only one oxygen atom into the substrate. The substrate thereby gets converted into hydroxylated form. RH O2 - ROH H2O So the other name for monooxygenases is hydroxylases Example Phenylalanine hydroxylase, tyrosine hydroxylaseDioxygenases Dioxygenase allows oxygen molecule to react with a substrate. It incorporates both the oxygen atoms into the substrate. E.g., Homogentisate oxidase Those which mediate REDUCTION REACTIONS can be given various namesPeroxidases If hydrogen peroxide is getting reduced, it is called as peroxidase E.g., glutathione peroxidaseCatalases For reduction of hydrogen peroxide, if hydrogen peroxide is used as a source of hydrogen, it is catalase.Reductases A substance is reduced using reducing equivalentsII. TRANSFERASES transfer a group from one substrate to another. Depending upon the group that is transferred, they are classified as -Kinases Phosphotransferases They transfer phosphate group from one substrate to another Example hexokinase or glucokinase transfers phosphate group from ATP to glucose or any hexose respectivelyTransketolases Transfers ketone group from one substrate to anotherTransaldolase Transfers aldehyde group from one substrate to anotherAcyltransferase Transfers acyl group or fatty acid from one substrate to another Example Glycerol phosphate acyl transferase transfers acyl group or fatty acid from acyl CoA to glycerol phosphate forming triacylglycerolGlucosyltransferase - glycogen synthaseIII. HYDROLASES catalyse cleavage of a covalent linkage by addition of water These enzymes can cleave only ester phosphatepeptide ether glycosidic linkages Examples include:a Amidasese Phospholipasesb Peptidasesf Glycosidasesc Esterasesg Ribonucleasesd PhosphatasesIV. LYASES catalyse cleavage of a C-N C-C linkage without adding water Since all reactions are reversible, they can also synthesise the same linkage back Examples include:a Decarboxylasesd Aldolaseb Hydratasee Lyasesc Dehydratasef SynthasesV. ISOMERASES catalyse the conversion of one form of a substance to another form of substance without altering the molecular formula Examples include:a Isomerasesc Epimeraseb Racemased Mutase - shifts a group from one carbon atom to anotherVI. LIGASES catalyse the Synthesises a covalent linkage using ATP as a source of energy Examples includea Synthetasesb Carboxylases 