Final answer:
The covalently modified enzyme catalyzed by Succinyl CoA synthetase during the citric acid cycle contains a phosphorylated histidine. This modification is integral to the formation of a high-energy bond used for substrate-level phosphorylation to produce GTP or ATP.
Step-by-step explanation:
Succinyl CoA synthetase catalyzes a double-displacement reaction in which a covalently modified enzyme is involved. One of the key steps in the catalytic cycle involves a histidine residue that becomes temporarily phosphorylated. During the reaction where succinyl CoA is converted to succinate, a high-energy bond is formed. This is associated with the phosphorylation of a histidine residue on the enzyme itself, a process which is critical for the formation of either guanine triphosphate (GTP) or ATP through substrate-level phosphorylation.
The correct answer to the question of which covalently modified enzyme is best described by Succinyl CoA synthetase is c. It contains a phosphorylated histidine.
During the reaction, the phosphate group initially bonded to the succinyl group is transferred to the histidine residue on the enzyme before being used to generate GTP or ATP, depending on the isoenzyme form present in certain types of animal tissue.
The reaction sequence itself involves multiple enzyme co-factors and complex steps, taking place during the citric acid cycle and involving the intermediate succinyl-CoA.