Stoichiometry of a Cl(-)-translocating ATPase.

It has been determined that Mg2+ stimulates phosphorylation while Cl- stimulates dephosphorylation of the chloride pump during its reaction sequence. The stoichiometry of ATP hydrolyzed to Cl- transported during a single cycle of the reaction sequence was ascertained. Intracellular concentrations of ATP, ADP and inorganic phosphate were determined and, coupled with an estimate of the standard free energy of hydrolysis for ATP, the operant free energy for ATP hydrolysis was calculated. Because the operating free energy of the pump is approximately one-half the energy obtained from the total free energy of ATP hydrolysis, the only possible integral stoichiometries are one or, at the most, two net charges transported per cycle per ATP hydrolyzed.