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

Abstract

A single molecule of F ₁ –ATPase is by itself a rotary motor in which a central γ–subunit rotates against a surrounding cylinder made of α ₃ β ₃ –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 F 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 F ₁ motor can work at near 100% efficiency. We confirmed in vitro that F ₁ 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 F ₁ motor: the ATP synthase, of which F ₁ is a part, is considered to synthesize ATP from ADP and phosphate by reverse rotation of the F motor. Possible mechanisms of F ₁ rotation are discussed.