Binding-change mechanism for ATP synthase. The rotation of the γ subunit interconverts the three β subunits. The subunit in the T (tight) form interconverts ADP and Pi and ATP but does not allow ATP to be released. When the γ subunit is rotated by 120 degrees in a counterclockwise (CCW) direction, the T-form subunit is converted into the O form, allowing ATP release. ADP and Pi can then bind to the O-form subunit. An additional 120-degree rotation (not shown) traps these substrates in an L-form subunit.