F1-ATPase is a rotary molecular motor that proceeds in 120° steps, each driven by ATP hydrolysis. How the chemical reactions that occur in three catalytic sites are coupled to mechanical rotation is the central question. Here, we show by high-speed imaging of rotation in single molecules of F1 that phosphate release drives the last 40° of the 120° step, and that the 40° rotation accompanies reduction of the affinity for phosphate. We also show, by single-molecule imaging of a fluorescent ATP analog Cy3-ATP while F1 is forced to rotate slowly, that release of Cy3-ADP occurs at ∼240° after it is bound as Cy3-ATP at 0°. This and other results suggest that the affinity for ADP also decreases with rotation, and thus ADP release contributes part of energy for rotation. Together with previous results, the coupling scheme is now basically complete.
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