The reaction catalyzed by pyruvate dehydrogenase
The pyruvate dehydrogenase multienzyme complex catalyzes the decarboxylation of pyruvate and the transfer of its acetyl group to coenzyme A yielding acetyl-CoA. The pyruvate dehydrogenase multienzyme complex contains three separate enzymes: E1, E2, and E3. Enzyme E1 decarboxylates pyruvate. Its thiamine pyrophosphate [TPP] cofactor reacts with pyruvate to form a hydroxyethyl intermediate, thereby releasing carbon dioxide. The hydroxyethyl group nucleophilically attacks the cyclic disulfide of the lipoamide prosthetic group of enzyme E2 and TPP is eliminated. In this process, the hydroxyethyl carbanion is oxidized to an acetyl group as the disulfide is reduced to yield dihydrolipoamide. E2 then catalyzes the transfer of the acetyl group to coenzyme A, to yield acetyl-CoA. Enzyme E3 of the complex then functions to oxidize the dihydrolipoamide of E2 back to lipoamide with the concomitant reduction of NAD+ to NADH.
Reaction 1
Citrate synthase catalyzes the condensation of acetyl-CoA and oxaloacetate to form citryl-CoA, which is then hydrolyzed to citrate and CoA.
Reaction 2
Aconitase catalyzes the dehydration of citrate to form the intermediate cis-aconitate followed by the hydration of cis-aconitate to form isocitrate.
Reaction 3
Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to a-ketoglutarate in a reaction that reduces NAD+ to NADH.
Reaction 4
The a-Ketoglutarate dehydrogenase multienzyme complex catalyzes the oxidative decarboxylation of a-ketoglutarate to form succinyl-CoA and the reduction of NAD+ to NADH, a series of reactions analogous to those carried out by the pyruvate dehydrogenase multienzyme complex. Note that we have now, in effect, lost both carbon atoms of the incoming acetyl-CoA as carbon dioxide, although these carbons actually came from the oxaloacetate in Reaction 1 rather than the incoming acetyl-CoA.
Reaction 5
Succinyl-CoA synthetase couples the cleavage of succinyl-CoA to the synthesis of a nucleoside triphosphate. Succinyl-CoA first reacts with inorganic phosphate to form succinyl-phosphate and free coenzyme A. The phosphoryl group is then transferred from succinyl-phosphate to a histidine residue of the enzyme, releasing succinate. Finally, the phosphoryl group on the enzyme is transferred to a nucleoside diphosphate, to form the corresponding nucleoside triphosphate. GTP is usually synthesized by mammalian enzymes; ATP is formed by the corresponding enzyme in plants and bacteria.
Reaction 6
Succinate dehydrogenase catalyzes the FAD-dependent dehydrogenation of succinate to form fumarate. The FAD is covalently linked to a histidine residue of the enzyme. The FADH2 is subsequently reoxidized to FAD by the mitochondrial electron transport chain of which succinate dehydrogenase is a component. Note that succinate dehydrogenase is the only citric acid cycle enzyme that is bound in the inner mitochondrial membrane; the other seven citric acid cycle enzymes are resident in the matrix.
Reaction 7
Fumarase catalyzes the hydration of fumarate to form malate in a reaction that has a carbanion transition state.
Reaction 8
Malate dehydrogenase catalyzes the oxidation of malate to oxaloacetate with the concomitant reduction of NAD+ to NADH.