12.4. Conversion of succinyl-CoA into succinate
Succinyl-CoA is a high- energy compound like acetyl-CoA. Succinyl CoA has a high negative ΔG°' of hydrolysis, and can, therefore, be coupled to the direct phosphorylation of GDP → GTP (which is equivalent to ATP); this reaction is fairly reversible. This is an example of substrate level phosphorylation. The energy released during the hydrolysis of thioester bond of Succinyl-S-CoA is accompanied by the phosphorylation of guanosine diphosphate (GDP) to guanosine triphosphate (GTP). The reaction is catalyzed by succinyl-CoA synthase (= succinic thiokinase).
The GTP readily donates its terminal phosphate group to ADP to form ATP by the action of Mg+2-dependent enzyme, nucleoside diphosphokinase present in the interspace membrane of mitochondria. This is a reversible reaction.
12.5. Dehydrogenation of succinate to fumarate
The oxidation of succinate to fumarate is the only dehydrogenation reaction in the citric acid cycle in which NAD+ does not participate. In this reaction the hydrogen is directly transferred from the succinate to flavoprotein enzyme and fumarate is formed. The succinate dehydrogenase is a flavoprotein located on the inner mitochondrial membrane. The enzyme contains the reducible prosthetic group flavin adenine dinucleotide (FAD) as the coenzyme. FAD functions as the hydrogen acceptor in this reaction, rather than NAD+. this is because the free energy change is insufficient to reduce NAD+.
In succinate dehydrogenase, the isoalloxazine ring of FAD is covalently linked to a histidine side chain of the enzyme. This is a reversible reaction.
Fumarate is hydrated to form L-malate in the presence of fumarate hydratase (formerly known as fumarase). This is a reversible reaction and involves hydration in malate formation and dehydration in fumarate formation.
Fumarate hydratase is highly specific and catalyzes trans addition and removal of H and OH and does not act on malate, the cis-isomer of fumarate.
Malate dehydrogenase is a good example of a reaction that has a net flow opposite to an unfavorable equilibrium. That is the oxidation of malate by NAD+ to produce oxaloacetate + NADH + H+ has a ΔG°' of + 7 kcal/mole.