Citric acid, also known as citrate, is formed in the Krebs cycle (also known as the citric acid cycle or TCA cycle) through the condensation of acetyl-CoA and oxaloacetate.
Here’s a step-by-step explanation of this process:
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Acetyl-CoA Formation: Acetyl-CoA is produced from the breakdown of carbohydrates, fats, or proteins. It is a two-carbon molecule derived from pyruvate (from glycolysis), fatty acids, or amino acids.
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Oxaloacetate: This is a four-carbon molecule that is regenerated in the Krebs cycle. It is one of the cycle's key intermediates and is formed from the breakdown of various metabolites, such as certain amino acids.
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Condensation Reaction: The cycle starts when acetyl-CoA (2 carbons) enters the cycle and condenses with oxaloacetate (4 carbons) to form citrate (6 carbons). This reaction is catalyzed by the enzyme citrate synthase.
- Reaction: Acetyl-CoA + Oxaloacetate → Citrate + CoA
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Citrate Formation: The resulting citrate is then further processed through a series of enzymatic reactions in the Krebs cycle. These reactions lead to the production of energy in the form of NADH, FADH2, and ATP, as well as the regeneration of oxaloacetate to continue the cycle.
In summary, citric acid is formed at the beginning of the Krebs cycle when acetyl-CoA combines with oxaloacetate in a reaction catalyzed by citrate synthase.