As explained in Chapters 4 and 5, the ability of living organisms to perform the various kinds of work whereby they subsist and proliferate rests overwhelmingly on the coupling between downhill electron transfer and the assembly of ATP from ADP and inorganic phosphate. We have seen in Chapter 6 how this coupling can be accomplished by thioester-dependent, substrate-level phosphorylation mechanisms. Although of immense qualitative importance, these processes account for only a minimal fraction of the ATP produced in most organisms. By far the greater part of the ATP used in the biosphere is assembled by carrier-level phosphorylations operating by way of protonmotive force, yet another of life's amazing singularities.

Anatomy of a Protonmotive Coupling Engine

Protonmotive machineries are obligatorily housed in the matrix of a membrane impermeable to protons. As shown schematically in Figure 11.1, they consist essentially of two reversible proton pumps oriented in the same direction, one driven by the transfer of electrons between two carriers, and the other by ATP hydrolysis. By forcing protons from one side of the membrane to the other, the pumps create a proton potential that tends to oppose the further translocation of protons. If the potential built by one pump exceeds the maximum potential attainable by the other pump, it will cause the weaker pump to function in reverse, thus accounting for coupling between the two systems.