Figure 1.

The binding change mechanism for F_oF₁ ATP synthases. This figure was adapted from ref. 8 and modified. (a) Looking up at F₁ from the membrane. In step 1, the asymmetric γ subunit rotates 120° clockwise driving conformational changes in the three catalytic sites that alter their affinities for substrates and product. In this illustration, the catalytic sites remain stationary. In step 2, ATP forms spontaneously from tightly bound ADP and P_i. For additional details and alternative views see refs. 7, 9, and 10. (b) View from the side of F_oF₁. The a-subunit contains two partial channels, each in contact with a different side of the membrane. In order for a H⁺ to traverse the membrane it moves through one channel to the center of the membrane, binds to one of the c-subunits (at Asp-61), and then is carried to the other partial channel by rotation of the c-subunit complex. The c-subunits are anchored to γ (11), whereas the a-subunit is anchored through subunits b and δ to the periphery of the α₃β₃ hexamer (12, 13). Hence the rotation of c-subunits relative to the a-subunit in F_o will drive the rotation of γ relative to the α₃β₃ hexamer in F₁.