<P> The citric acid cycle is continuously supplied with new carbon in the form of acetyl - CoA, entering at step 0 below . </P> <Table> <Tr> <Th> </Th> <Th> Substrates </Th> <Th> Products </Th> <Th> Enzyme </Th> <Th> Reaction type </Th> <Th> </Th> </Tr> <Tr> <Td> 0 / 10 </Td> <Td> Oxaloacetate + Acetyl CoA + H O </Td> <Td> Citrate + CoA - SH </Td> <Td> Citrate synthase </Td> <Td> Aldol condensation </Td> <Td> irreversible, extends the 4C oxaloacetate to a 6C molecule </Td> </Tr> <Tr> <Td> </Td> <Td> Citrate </Td> <Td> cis - Aconitate + H O </Td> <Td> Aconitase </Td> <Td> Dehydration </Td> <Td> reversible isomerisation </Td> </Tr> <Tr> <Td> </Td> <Td> cis - Aconitate + H O </Td> <Td> Isocitrate </Td> <Td> Hydration </Td> </Tr> <Tr> <Td> </Td> <Td> Isocitrate + NAD </Td> <Td> Oxalosuccinate + NADH + H </Td> <Td> Isocitrate dehydrogenase </Td> <Td> Oxidation </Td> <Td> generates NADH (equivalent of 2.5 ATP) </Td> </Tr> <Tr> <Td> </Td> <Td> Oxalosuccinate </Td> <Td> α - Ketoglutarate + CO </Td> <Td> Decarboxylation </Td> <Td> rate - limiting, irreversible stage, generates a 5C molecule </Td> </Tr> <Tr> <Td> 5 </Td> <Td> α - Ketoglutarate + NAD + CoA - SH </Td> <Td> Succinyl - CoA + NADH + H + CO </Td> <Td> α - Ketoglutarate dehydrogenase </Td> <Td> Oxidative decarboxylation </Td> <Td> irreversible stage, generates NADH (equivalent of 2.5 ATP), regenerates the 4C chain (CoA excluded) </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Succinyl - CoA + GDP + P </Td> <Td> Succinate + CoA - SH + GTP </Td> <Td> Succinyl - CoA synthetase </Td> <Td> substrate - level phosphorylation </Td> <Td> or ADP → ATP instead of GDP → GTP, generates 1 ATP or equivalent . Condensation reaction of GDP + P and hydrolysis of Succinyl - CoA involve the H O needed for balanced equation . </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Succinate + ubiquinone (Q) </Td> <Td> Fumarate + ubiquinol (QH) </Td> <Td> Succinate dehydrogenase </Td> <Td> Oxidation </Td> <Td> uses FAD as a prosthetic group (FAD → FADH in the first step of the reaction) in the enzyme . These two electrons are later transferred to QH during Complex II of the ETC, where they generate the equivalent of 1.5 ATP </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Fumarate + H O </Td> <Td> L - Malate </Td> <Td> Fumarase </Td> <Td> Hydration </Td> <Td> Hydration of C-C double bond </Td> </Tr> <Tr> <Td> 9 </Td> <Td> L - Malate + NAD </Td> <Td> Oxaloacetate + NADH + H </Td> <Td> Malate dehydrogenase </Td> <Td> Oxidation </Td> <Td> reversible (in fact, equilibrium favors malate), generates NADH (equivalent of 2.5 ATP) </Td> </Tr> <Tr> <Td> 10 / 0 </Td> <Td> Oxaloacetate + Acetyl CoA + H O </Td> <Td> Citrate + CoA - SH </Td> <Td> Citrate synthase </Td> <Td> Aldol condensation </Td> <Td> This is the same as step 0 and restarts the cycle . The reaction is irreversible and extends the 4C oxaloacetate to a 6C molecule </Td> </Tr> </Table> <Tr> <Th> </Th> <Th> Substrates </Th> <Th> Products </Th> <Th> Enzyme </Th> <Th> Reaction type </Th> <Th> </Th> </Tr> <Tr> <Td> 0 / 10 </Td> <Td> Oxaloacetate + Acetyl CoA + H O </Td> <Td> Citrate + CoA - SH </Td> <Td> Citrate synthase </Td> <Td> Aldol condensation </Td> <Td> irreversible, extends the 4C oxaloacetate to a 6C molecule </Td> </Tr>

Which is the common substrate for mitochondria in the citric acid cycle