Figure: Citric Acid Cycle

Biochemistry

• the citric acid cycle is the final common pathway for oxidation of fuel molecules
  • amino acids, fatty acids, & carbohydrates.
• most of the molecules enter the citric acid cycle as acetyl CoA formed via oxidative decarboxylation of pyruvate formed during glycolysis
• the citric acid cycle also provides intermediates for biosyntheses
• in eukaryotes, reactions of the citric acid cycle occur within the mitochondrial matrix.

• the 8 enzymatic reactions of the citric acid cycle produce 3 NADH & 1 FADH2 to feed 8 electrons into the electron transport chain

• citric acid cycle 1
  • enzyme: citrate synthase
  • substrates: oxaloacetate + acetyl-CoA
  • products: citrate + Coenzyme-A

• citric acid cycle 2
  • enzyme: aconitase-mt
  • substrate: citrate
  • product: isocitrate

• citric acid cycle 3
  • enzyme: isocitrate dehydrogenase
  • substrates: isocitrate + NAD+
  • products: alpha-ketoglutarate + NADH + CO2

• citric acid cycle 4
  • enzyme: alpha-ketoglutarate dehydrogenase
  • substrates: alpha-ketoglutarate + NAD+ + coenzyme-A
  • products: succinyl-CoA + CO2 + NADH

• citric acid cycle 5
  • enzyme: succinyl-CoA synthetase
  • substrates: succinyl-CoA + GDP + H2PO4-
  • products: succinate + coenzyme-A + GTP

• citric acid cycle 6
  • enzyme: succinate dehydrogenase
  • substrates: succinate + flavin adenine dinucleotide
  • products: fumarate + FADH2

• citric acid cycle 7
  • enzyme: fumarase
  • substrates: fumarate + H2O
  • product: malate

• citric acid cycle 8
  • enzyme: malate dehydrogenase
  • substrates: malate + NAD+
  • products: oxaloacetate + NADH