Stoichiometry of energy coupling by proton-translocating ATPases: A history of variability

One of the central energy-coupling reactions in living systems is the intra conversion of ATP with a transmembrane proton gradient, carried out by prot on-translocating F- and V-type ATPases/synthases. These reversible enzymes can hydrolyze ATP and pump protons, or can use the energy of a transmembran e proton gradient to synthesize ATP from ADP and inorganic phosphate. The s toichiometry of these processes (H+/ATP, or coupling ratio) has been studie d in many systems for many years, with no universally agreed upon solution. Recent discoveries concerning the structure of the ATPases, their assembly and the stoichiometry of their numerous subunits, particularly the proton- carrying proteolipid (subunit c) of the F-o and V-0 sectors, have shed new light on this question and raise the possibility of variable coupling ratio s modulated by variable proteolipid stoichiometries.