Figure 21.6 A binding-change mechanism for ATP synthase. The rotation of the γ subunit interconverts the three β subunits. The subunit in the T (tight) form converts ADP and P_i into ATP but does not allow ATP to be released. When the γ subunit is rotated counterclockwise (CCW) 120 degrees, the T-form subunit is converted into the O form, allowing ATP release. New molecules of ADP and P_i can then bind to the O-form subunit. An additional 120-degree rotation (not shown) traps these substrates in an L-form subunit.