FIGURE 1. The binding-change model for F₀F₁ ATP synthase.

a, Looking up at F₁ from the membrane. Each blue or green area represents a pair of - and -subunits, in which the catalytic sites are interfacial but mostly on the -subunit. In step 1, the -subunit rotates through 120°, driving conformational changes in the three surrounding catalytic sites that alter their affinities (O, L or T, for open, loose or tight) for substrates and product. In step 2, ATP forms spontaneously from tightly bound ADP and inorganic phosphate (P_i). b, View from the side of F₀F₁. The a-subunit contains two partial channels. For a proton to traverse the membrane, it must move through one channel to the centre, bind to one of the c-subunits and then be carried to the other partial channel by rotation of the c-ring. The c-subunits are anchored to the -subunit (part of the rotor), whereas the a-subunit is anchored through b₂ (the stator) to the ₃₃ hexamer. Hence, rotation of the c-ring relative to the a-subunit in F₀ will drive the rotation of the -subunit relative to the ₃₃ hexamer in F₁. New results^{1, 15} show that the -subunit rotates in a clockwise direction when the engine generates ATP.