Thermodynamics of reverse electron transfer across site 1: ATP/2e- is greater than one

ATP-driven, rotenone-sensitive, reverse electron transfer from succinate to acetoacetate was measured in rat liver mitochondria in the presence of cyanide. In the approach to equilibrium, the absolute ratio of the free energy change of electron transfer to that of ATP hydrolysis exceeded 1, tending towards about 1 1/4. The data support an H+/2e- stoichiometry of 5 for Site 1 as predicted by a thirteen-proton model of chemiosmotic coupling.