Hans A. Krebs first proposed a series of breakdown of pyruvate in the presence of oxygen, therefore the cycle is called Krebs Cycle. The cycle is also called citric acid cycle because formation of an important intermediate citric acid. krebs cycle takes place in the matrix of mitochondria.
Entry of Pyruvate in to Mitochondria
The pyruvate produced in the cytosol enters through impermeable inner membrane of mitochondria with the help of a trans-locater (pyruvate trans-locater) that catalyze exchange of pyruvate and OH” across the inner membrane.
Acetyl Coenzyme A (CO A) Formation
The pyruvate produced in glycolysis does not enter the Krebs cycle directly, rather it is decarboxylated in the presence of oxygen in a coordinated series of reactions catalyze by a multicomponent complex of several enzymes to a 2-C compound, acetyl Co A (CH3Co – CoA). The reaction requires participation of several coenzymes including NAD+ and coenzyme A (Co A). the enzyme catalyzing the reaction is pyruvic acid dehydrogenase. NADH2 is generated during the reaction. The overall reaction can be summarized as following:
Pyruvic acid + Co A + NAD+ ——————– Acetyl Co A + CO2 + NADH2
Biochemistry of Krebs Cycle
Krebs cycle involves the following steps:
- Formation of Citric Acid
The Krebs cycle starts with condensation of 2-C acetyl group (CH3CO) in acetyl coenzyme A with oxaloacetic acid (a 4-C acid produced during the cycle) and water to yield citric acid (a 6-C tricarboxylic acid). Enzyme citrate synthase catalyzes the steps.
- Isomerization of Citric Acid to Isocitrate
Citric acid undergoes isomerization to Isocitric acid by enzyme aconitase. A molecule of NADH2 is produced during this step.
- Oxidative Decarboxylation
Next two steps involve successive oxidative decarboxylation, each of which produces molecule of NADH2 and releases a molecule of CO2.
Isocitric acid is converted to alpha-ketogluteric acid by isocitrate dehydrogenase.
Alpha-ketogluteric acid produces succinyl Co A in the presence of Co A and alpha-ketogluetric acid dehrogenase.
- Regeneration of Oxaloacetic Acid
Up to this point three molecules of CO2 have been produced for each pyruvate, so complete oxidation of glucose has actually taken place. The remainder of Krebs cycle involves conversion of succinyl Co A to oxaloacetic acid to allow continued operation of the cycle. The following steps involved in the regeneration of oxaloacetic acid.
Steps Involved in Regeneration of Oxaloacetic Acid
- Succinyl Co A under the catalytic activity of succinyl Co A synthetase is converted to succinate. Coenzyme A is released and ATP is generated.
- The succinate is oxidized to fumarate (fumaric acid) by succinate dehydrogenase. The electron removed from succinate reduce FAD+ to FADH2.
- Fumaric acid is hydrated to produce malate (malic acid) by fumarase.
- The malate is oxidized by malate dehydrogenase to oxalacetic acid. A molecule of NADH2 is produced during this step.
The oxaloacetic acid is produced is now able to react another acetyl Co A and continue the process the cycle.
Products of Krebs Cycle
The stepwise oxidation of each pyruvate in the Krebs cycle give rise to the following products.
- Three molecules of CO2.
- Four NADH2 and one FADH2 molecules which store free energy released during these oxidations.
- One molecule of ATP is produced through substrate level phosphorylation.