Energy transduction in ATP synthase

Mitochondria, bacteria and chloroplasts use the free energy stored in transmembrane ion gradients to manufacture ATP by the action of ATP synthase. This enzyme consists of two principal domains. The asymmetric membrane-spanning Fo portion contains the proton channel, and the soluble F1 portion contains three catalytic sites which cooperate in the synthetic reactions1. The flow of protons through Fo is thought to generate a torque which is transmitted to F1 by an asymmetric shaft, the coiled-coil γ-subunit. This acts as a rotating ‘cam’ within F1, sequentially releasing ATPs from the three active sites1,2,3,4,5. The free-energy difference across the inner membrane of mitochondria and bacteria is sufficient to produce three ATPs per twelve protons passing through the motor. It has been suggested that this protonmotive force biases the rotor's diffusion so that Fo constitutes a rotary motor turning the γ shaft6. Here we show that biased diffusion, augmented by electrostatic forces, does indeed generate sufficient torque to account for ATP production. Moreover, the motor's reversibility — supplying torque from ATP hydrolysis in F1 converts the motor into an efficient proton pump7 — can also be explained by our model.