Citric acid cycle

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Citric acid cycle
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Conversion of Pyruvate to acetyl-CoA

• Catalyzed by pyruvate dehydrogenase complex and
  coenzymes (PDC)
• Loss of pyruvate carboxyl group
• Activation of remaining 2 C unit
• Generation of CO2
• Generation of NADH
• Formation of Acetyl-CoA

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Pyruvate dehydrogenase complex

• Multi-enzyme assembly
• Pyruvate dehydrogenase (E1)
• Dihydrolipoamide acetyl-transferase (E2)
• Dihydrolipoamide dehydrogenase (E3)

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Coenzymes

• Thiamine pyrophosphate (TPP)
• Used for a-ketoacid decarboxylation
• Lipoic acid (lipoamide)
• Internal disulfide, attached covalently to lysine
• Flavin adenine dinucleotide (FAD)
• Electron acceptor
• NAD
• Coenzyme A( CoA)
• Acyl carrier

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Coenzyme A (hydrolysis)

• Less resonance stabilization (larger atomic size
  for S compared to O)
• Thioester is destabilized relative to ester
• ?G of hydrolysis is higher for thioester
• Acyl group is more readily transferred to other
  metabolites (as in Citric acid cycle)

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Citric acid Cycle

• Also known as Tricarboxylic acid or Krebs cycle
• Centre for energy production from sugar, fat and
  protein
• Proceeds in two phases
• Formation of citrate (Oxaloacetate acetyl-CoA)
  Citrate synthase
• Regeneration of Oxaloacetate

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