Thermodynamic efficiency and mechanochemical coupling of F1-ATPase

F₁-ATPase is a nanosized biological energy transducer working as part of F_oF₁-ATP synthase. Its rotary machinery transduces energy between chemical free energy and mechanical work and plays a central role in the cellular energy transduction by synthesizing most ATP in virtually all organisms. However, information about its energetics is limited compared to that of the reaction scheme. Actually, fundamental questions such as how efficiently F₁-ATPase transduces free energy remain unanswered. Here, we demonstrated reversible rotations of isolated F₁-ATPase in discrete 120° steps by precisely controlling both the external torque and the chemical potential of ATP hydrolysis as a model system of F_oF₁-ATP synthase. We found that the maximum work performed by F₁-ATPase per 120° step is nearly equal to the thermodynamical maximum work that can be extracted from a single ATP hydrolysis under a broad range of conditions. Our results suggested a 100% free-energy transduction efficiency and a tight mechanochemical coupling of F₁-ATPase.