A rotary molecular motor that can work at near 100% efficiency

A single molecule of F₁-ATPase is by itself a rotary motor in which a central γ-subunit rotates against a surrounding cylinder made of α₃β₃-subunits. Driven by the three βs that sequentially hydrolyse ATP, the motor rotates in discrete 120°steps, as demonstrated in video images of the movement of an actin filament bound, as a marker, to the central γ-subunit. Over a broad range of load (hydrodynamic friction against the rotating actin filament) and speed, the F₁ motor produces a constant torque of ca. 40 pN nm. The work done in a 120°step, or the work per ATP molecule, is thus ca. 80 pN nm. In cells, the free energy of ATP hydrolysis is ca. 90 pN nm per ATP molecule, suggesting that the F₁ motor can work at near 100% efficiency. We confirmed in vitro that F₁ indeed does ca. 80 pN nm of work under the condition where the free energy per ATP is 90 pN nm. The high efficiency may be related to the fully reversible nature of the F₁ motor: the ATP synthase, of which F₁ is a part, is considered to synthesize ATP from ADP and phosphate by reverse rotation of the F₁ motor. Possible mechanisms of F₁ rotation are discussed.