Fig. 1.

Schematics of the electron transfer reaction coupled to proton translocation. In the reaction, an electron is tunneling over a long distance between two redox cofactors, O and R, and a coupled proton is transferred over a proton conducting channel. The initial and final states of the proton are two protonatable groups of the protein, U and P. Separately, both electron and proton transfer are uphill in energy, however, the coupled reaction is downhill. Figure reprinted with permission from Ref. 42. Copyright 2003 World Scientific.