Citric Acid Cycle Reactions

The citric acid cycle is a central metabolic pathway which generates NADH and FADH 2 for use in electron transport . It also produces GTP via substrate-level phosphorylation. Many metabolic processes use intermediates of the citric acid cycle in their pathways. The cyclic process is generally considered to "begin" with addition of acetyl-CoA to oxaloacetate to form citrate. Remember, however, that the pathway is cyclic.

Reaction Summary of the Citric Acid Cycle:

1. Acetyl-CoA + Oxaloacetate + H2O <=> Citrate + CoASH + H+ | Enzyme: Citrate Synthase
= -32.2 kJ/mol

2. Citrate <=> cis-Aconitate + H2O <=> Isocitrate | Enzyme: Aconitase
= +6.3 kJ/mol

3. Isocitrate + NAD+ <=> -Ketoglutarate + CO2 + NADH | Enzyme: Isocitrate Dehydrogenase
= -20.9 kJ/mol

4. -Ketoglutarate + NAD+ + CoASH <=> Succinyl-CoA + CO2 + NADH | Enzyme: Ketoglutarate Dehydrogenase Complex
= -33.5 kJ/mol

5. Succinyl-CoA + Pi + GDP <=> Succinate + GTP + CoASH | Enzyme: Succinyl-CoA Synthetase
= -2.9 kJ/mol

6. Succinate + FAD (enzyme bound) <=> Fumarate + FADH2 | Enzyme: Succinate Dehydrogenase
= 0 kJ/mol

7. Fumarate + H2O <=> L-Malate | Enzyme: Fumarate Hydratase
= -3.8 kJ/mol

8. L-Malate + NAD+ <=> Oxaloacetate + NADH + H+ | Enzyme: Malate Dehydrogenase
= +29.7 kJ/mol

See also: Table 14.1, Figure 14.3, Electron Transport

INTERNET LINK: Citric Acid Cycle