In the evolution of the FoF1 family of proton-translocating membrane complexes, two reversals in function appear to have occurred, first changing it from an ATPase to an ATP synthase and then back again to an ATPase. Here we suggest that with each change in function, the ratio of protons transported per ATP hydrolyzed or synthesized (H+/ATP) was altered in order for the complex to better adapt to its new role. We propose that this was accomplished by gene duplication with partial loss in the number of functional catalytic sites (to increase H+/ATP) or functional proton channels (to decrease H+/ATP). This method of changing the H+/ATP ratio preserved overall structural features of the complex essential to energy coupling.
