Which of the following is an enzyme required by the process of rephosphorylation, to create ATP

• A. Hexokinase
• B. NADH dehydrogenase
• C. Phosphofructokinase
• D. ATP Synthase

Answer: (D)

In oxidative phosphorylation, the energy from the proton gradient built by the electron transport chain is used to form ATP. ATP synthase is the enzyme that directly catalyzes the final step: ADP plus inorganic phosphate are joined to make ATP as protons flow through the enzyme. This mechanical rotation driven by the proton motive force powers the chemical synthesis of ATP, which is what we mean by rephosphorylation in this context. The other enzymes act earlier in metabolism—hexokinase adds a phosphate to glucose, and phosphofructokinase is a glycolysis regulator—while NADH dehydrogenase helps create the proton gradient but does not synthesize ATP itself. So the enzyme responsible for making ATP in this step is ATP synthase.