Elastic Coupling Power Stroke Mechanism Of The F-1-Atpase Molecular Motor

The angular velocity profile of the 120 degrees F-1-ATPase power stroke was resolved as a function of temperature from 16.3 to 44.6 degrees C using Delta mu(ATP) = -31.25 k(B)T at a time resolution of 10 mu s. Angular velocities during the first 60 degrees of the power stroke (phase 1) varied inversely with temperature, resulting in negative activation energies with a parabolic dependence. This is direct evidence that phase 1 rotation derives from elastic energy (spring constant,kappa = 50 k(B)T.rad(-2)). Phase 2 of the power stroke had an enthalpic component indicating that additional energy input occurred to enable the.-subunit to overcome energy stored by the spring after rotating beyond its 34 degrees equilibrium position. The correlation between the probability distribution of ATP binding to the empty catalytic site and the negative Ea values of the power stroke during phase 1 suggests that this additional energy is derived from the binding of ATP to the empty catalytic site. A second torsion spring (kappa = 150 k(B)T.rad(-2); equilibrium position, 90 degrees) was also evident that mitigated the enthalpic cost of phase 2 rotation. The maximum Delta G(+) was 22.6 k(B)T, and maximum efficiency was 72%. An elastic coupling mechanism is proposed that uses the coiled-coil domain of the gamma-subunit rotor as a torsion spring during phase 1, and then as a crankshaft driven by ATP-binding-dependent conformational changes during phase 2 to drive the power stroke.