Simple mechanism whereby the F1-ATPase motor rotates with near-perfect chemomechanical energy conversion

Significance ATP synthase produces most of the ATP in respiratory and photosynthetic cells. It is a rotary motor enzyme and its catalytic portion F1-ATPase hydrolyzes ATP to drive rotation of the central γ subunit. Efficiency of chemomechanical energy conversion by this motor is always near-perfect under different ATP hydrolysis energy (ΔGATP) conditions. However, the mechanism for the efficient conversion remains unknown. We measured the torque as a function of rotation angle under different ΔGATP conditions and estimated mechanical work. The torque profiles show three sawtooth-like repeats during a single ATP hydrolysis. When concentrations of ATP/ADP and Pi in the environment are changed, the height, and hence the area, of the sawtooth changes accordingly so that mechanical work can always match ΔGATP. F1-ATPase is a motor enzyme in which a central shaft γ subunit rotates 120° per ATP in the cylinder made of α3β3 subunits. During rotation, the chemical energy of ATP hydrolysis (ΔGATP) is converted almost entirely into mechanical work by an elusive mechanism. We measured the force for rotation (torque) under various ΔGATP conditions as a function of rotation angles of the γ subunit with quasi-static, single-molecule manipulation and estimated mechanical work (torque × traveled angle) from the area of the function. The torque functions show three sawtooth-like repeats of a steep jump and linear descent in one catalytic turnover, indicating a simple physical model in which the motor is driven by three springs aligned along a 120° rotation angle. Although the second spring is unaffected by ΔGATP, activation of the first spring (timing of the torque jump) delays at low [ATP] (or high [ADP]) and activation of the third spring delays at high [Pi]. These shifts decrease the size and area of the sawtooth (magnitude of the work). Thus, F1-ATPase responds to the change of ΔGATP by shifting the torque jump timing and uses ΔGATP for the mechanical work with near-perfect efficiency.

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