Which enzyme uses the proton gradient to synthesize ATP in mitochondria?

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

Answer: (A)

ATP synthesis in mitochondria is driven by chemiosmosis, where a proton gradient powers ATP production. The enzyme that uses this gradient to make ATP is ATP synthase. As protons flow back into the mitochondrial matrix through the F0 portion of ATP synthase, the rotary motion drives the F1 catalytic sites to combine ADP and inorganic phosphate into ATP. This rotary mechanism is how the energy from the proton-motive force is converted into chemical energy (ATP).

Other enzymes listed don’t perform this energy conversion. Some are glycolytic enzymes in the cytosol, helping to metabolize glucose but not using a proton gradient to generate ATP. The NADH dehydrogenase (a component of the electron transport chain) helps build the gradient by pumping protons but does not itself synthesize ATP.