Past exam of the short options paper shared by Parts A and B of the physics course of the University of Oxford / 2019 / S12 / 3 / a / Solution

ATP synthase has a membrane-embedded $F_o$ sector, a catalytic $F_1$ head and mechanical connections between them. The $F_o$ sector contains an ion-conducting subunit $a$ adjoining a rotating ring of $c$ subunits. A central shaft including the $\gamma$ subunit connects that ring to $F_1$. A peripheral stator holds the head fixed relative to the membrane. The $F_1$ head has three catalytic $\beta$ subunits alternating with three $\alpha$ subunits, surrounding the asymmetric central shaft; the middle panel of solution labels these relationships.

A transmembrane proton-motive force drives proton movement through offset half-channels and protonation changes around the ring, producing torque. Shaft rotation changes the conformations of the three catalytic sites, cycling their affinities for ADP, inorganic phosphate and ATP. In the binding-change mechanism, mechanical work particularly promotes release of tightly bound ATP; the threefold head couples a full revolution to three catalytic turnovers. The number of protons per revolution depends on the ring stoichiometry. Reversing the available free energy can reverse the motor: isolated $F_1$ hydrolyses ATP and rotates, and the coupled enzyme can pump protons. This structural separation of rotor, stator and catalytic head converts electrochemical free energy into chemical free energy.^[ref]