An Alternative Reaction Pathway of F1-ATPase Suggested by Rotation without 80°/40° Substeps of a Sluggish Mutant at Low ATP

F1-ATPase, a water-soluble portion of FoF1-ATP synthase, is a rotary motor driven by ATP hydrolysis. The central γ-subunit rotates in the α3β3 cylinder by repeating four stages of rotation: ATP-binding dwell, rapid 80° substep rotation, catalytic dwell, and rapid 40° substep rotation. In the catalytic dwell, at least two catalytic reactions occur—cleavage of the enzyme-bound ATP and presumably release of the hydrolyzed product(s) from the enzyme—but we found that a slow ATP cleavage mutant of F1-ATPase from thermophilic Bacillus PS3 rotates at low ATP concentration without substeps and the catalytic dwell. Analysis indicates that in this alternative reaction pathway the two catalytic reactions occur during the preceding long ATP-binding dwell. Thus, F1-ATPase can operate through (at least) two competing reaction pathways, not necessarily through a simple consecutive reaction.