Final answer:
Succinyl CoA synthetase (C) is the citric acid cycle enzyme that catalyzes substrate-level phosphorylation, converting succinyl CoA to succinate while producing GTP, which can be converted to ATP. The correct option is
Step-by-step explanation:
The citric acid cycle enzyme that catalyzes substrate-level phosphorylation is succinyl CoA synthetase (C). This process occurs when succinyl CoA is converted into succinate.
In this step, the high-energy thioester bond in succinyl CoA is used to phosphorylate GDP to GTP, which can then be converted to ATP. The importance of this process is underscored by the fact that it is one of the few steps in the citric acid cycle that directly yields a molecule of ATP (or GTP).
Enzymes such as isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are involved in oxidative decarboxylation and are heavily regulated by ATP, NADH, and other factors, but they do not directly participate in substrate-level phosphorylation.
Succinate dehydrogenase and malate dehydrogenase are also part of the cycle but play no role in the phosphorylation of ADP to ATP. Overall, succinyl CoA synthetase is distinct in its role of generating directly usable energy in the form of GTP/ATP.