Cell Respiration and Oxidative Phosphorylation

The passage of protons through the F_O (membrane) portion of the molecule (driven by the electrochemical gradient of protons across the respiratory membrane) generates torque at the interface between the a and c subunits. This mean that the ring of 10 c subunits rotates relative to the a subunit. The γ subunit in the stalk rotates along with the ring of c subunits, while the ring of α and β subunits in F₁ is constrained to remain static with respect to the a subunit because of the interaction with the δ and b subunits (forming the peripheral stalk). So we have two portions of the molecule the "static" a-b₂-δ-α₃β₃ part and the rotating c₁₀-γ-ε part. The γ subunit rotates within the α₃β₃ part of F₁ and drives a three-step cycle of conformational changes (via intersubunit interactions) which are linked to the phosphorylation of ADP.

There is a useful animation here illustrating the three step catalytic cycle being driven by the rotation of the γ subunit.

(image from Aksimentiev et al. (2004)
Insights into the molecular mechanism of rotation in the F_O sector of ATP synthase. Biophys J. 86:1332-44)