The irreversible conversion of α-ketoglutarate to succinyl CoA is catalyzed by the α-ketoglutarate dehydrogenase complex, a protein aggregate of multiple copies of three enzymes (Fig. 9.5). The mechanism of this oxidative decarboxylation is very similar to that used for the conversion of pyruvate to acetyl CoA by the PDHC. The reaction releases the second CO2 and produces the second NADH of the cycle. The coenzymes required are TPP, lipoic acid, FAD, NAD+, and CoA. Each functions as part of the catalytic mechanism in a way analogous to that described for the PDHC (see p. 110). The large negative ∆G0 of the reaction favors formation of succinyl CoA, a high-energy thioester similar to acetyl CoA. The αketoglutarate dehydrogenase complex is inhibited by its products, NADH and succinyl CoA, and activated by Ca2+ . However, it is not regulated by phosphorylation/dephosphorylation reactions as described for the PDHC. [Note: α-Ketoglutarate is also produced by the oxidative deamination (see p. 252) and transamination of the amino acid glutamate (see p. 250).]
F. Succinyl coenzyme A cleavage
Succinate thiokinase (also called succinyl CoA synthetase, named for the reverse reaction) cleaves the high-energy thioester bond of succinyl CoA (see Fig. 9.5). This reaction is coupled to phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP). GTP and ATP are energetically interconvertible by the nucleoside diphosphate kinase reaction:
The generation of GTP by succinate thiokinase is another example of substrate-level phosphorylation (see p. 102). [Note: Succinyl CoA is also produced from propionyl CoA derived from the metabolism of fatty acids with an odd number of carbon atoms (see p. 193) and from the metabolism of several amino acids (see pp. 265–266). It can be converted to pyruvate for gluconeogenesis (see p. 118) or used in heme synthesis (see p. 278).]
G. Succinate oxidation
Succinate is oxidized to fumarate by succinate dehydrogenase, as its coenzyme FAD is reduced to FADH2 (see Fig. 9.5). Succinate dehydrogenase is the only enzyme of the TCA cycle that is embedded in the inner mitochondrial membrane. As such, it functions as Complex II of the ETC (see p. 75). [Note: FAD, rather than NAD+, is the electron acceptor because the reducing power of succinate is not sufficient to reduce NAD+.]
H. Fumarate hydration
Fumarate is hydrated to malate in a freely reversible reaction catalyzed by fumarase (fumarate hydratase, see Fig. 9.5). [Note: Fumarate is also produced by the urea cycle (see p. 255), in purine synthesis (see Fig. 22.7 on p. 294), and during catabolism of the amino acids phenylalanine and tyrosine (see p. 263).]
I. Malate oxidation
Malate is oxidized to OAA by malate dehydrogenase (Fig. 9.6). This reaction produces the third and final NADH of the cycle. The ∆G0 of the reaction is positive, but the reaction is driven in the direction of OAA by the highly exergonic citrate synthase reaction. [Note: OAA is also produced by the transamination of the amino acid aspartic acid (see p. 250).]
III. ENERGY PRODUCED BY THE CYCLE