Final answer:
Succinyl-CoA synthetase, during its reaction cycle, involves a histidine residue being temporarily phosphorylated. Thus, the covalently modified enzyme during this process is correctly described by the option containing a phosphorylated histidine (C).
Step-by-step explanation:
The enzyme succinyl-CoA synthetase functions in the citric acid cycle, catalyzing a double-displacement reaction that forms either guanine triphosphate (GTP) or ATP from succinyl-CoA. A key step involves the formation of a high-energy phosphate bond.
During the reaction, a phosphate group replaces coenzyme A and a high-energy bond is formed. Succinyl-CoA synthetase goes through a phosphorylated intermediate stage.
Throughout the citric acid cycle, various covalent modifications of enzymes occur. However, for succinyl-CoA synthetase specifically, a phosphorylated histidine residue is an intermediate during the catalytic process. This covalent modification is part of the enzyme's mechanism where it forms a high-energy phosphate intermediate that can subsequently transfer the phosphate group to ADP, forming ATP, or to GDP, forming GTP.
Hence, the correct answer to the question "Which one of the following best describes the covalently modified enzyme?" is C) It contains a phosphorylated histidine.