How proton translocation across mitochondrial inner membranes drives the Fo rotor of ATP synthase.

It has been believed for some time that there are two major but alternative models for the selective transport of ions across membranes generally. On the one hand this transport is by way of transmembrane channels. These channels exist within macromolecular complexes which span the membrane and provide a hydrophilic pathway through which the ions can be translocated. Alternatively, carriers have been postulated which can dissolve in the lipid moiety of the membrane, are able to selectively co-ordinate ions, and then move from one side of the membrane to the other, before unloading the ion. Proton translocation across the inner mitochondrial membrane is intensely interesting, firstly because the process is tightly coupled to the synthesis of ATP, but additionally because the emerging picture of proton translocation incorporates features from both the classical mechanisms of ion transport. Thus there are two channels, one from either side of the membrane, both of which penetrate to the centre of the membrane. However neither of them individually spans the membrane, but they remain separated by a short distance in the plane of the membrane. Transport across this remaining gap involves a carrier that reversibly binds the ion. The mechanism for transport across this remaining region is not carrier-facilitated diffusion, nor any "flip flop" change of shape by the carrier. Rather it is an electrically driven rotation of the carrier, and the source of the electric field that drives this rotor is the transmembrane electric potential.