Rotation of F1-ATPase:: How an ATP-driven molecular machine may work

F-1-ATPase is a rotary motor made of a single protein molecule. Its rotation is driven by free energy obtained by ATP hydrolysis. In vivo, another motor, F-0, presumably rotates the F-1 motor in the reverse direction, reversing also the chemical reaction in F-1 to let it synthesize ATP. Here we attempt to answer two related questions, How is free energy obtained by ATP hydrolysis converted to the mechanical work of rotation, and how is mechanical work done on F-1 converted to free energy to produce ATP? After summarizing single-molecule observations of F-1 rotation, we introduce a toy model and discuss its free-energy diagrams to possibly answer the above questions. We also discuss the efficiency of molecular motors in general.