The enzyme F1-ATPase has been shown to be a rotary motor in which the central γ-subunit rotates inside the cylinder made of α3β3 subunits. At low ATP concentrations, the motor rotates in discrete 120° steps, consistent with sequential ATP hydrolysis on the three β-subunits. The mechanism of stepping is unknown. Here we show by high-speed imaging that the 120° step consists of roughly 90° and 30° substeps, each taking only a fraction of a millisecond. ATP binding drives the 90° substep, and the 30° substep is probably driven by release of a hydrolysis product. The two substeps are separated by two reactions of about 1 ms, which together occupy most of the ATP hydrolysis cycle. This scheme probably applies to rotation at full speed (∼130 revolutions per second at saturating ATP) down to occasional stepping at nanomolar ATP concentrations, and supports the binding-change model for ATP synthesis by reverse rotation of F1-ATPase.
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