Calcium and Proton Dependence of Sarcoplasmic Reticulum ATPase

The influence of Ca2+ and H+ concentrations on the sequential reactions of the ATPase cycle was studied by a series of pre-steady state and steady state experiments with sarcoplasmic reticulum vesicles. It is shown that H+ competition with calcium binding results in a reduced population of activated enzyme, which is manifested by a lower level of phosphorylated enzyme intermediate following addition of ATP. Further effects of Ca2+ and H+ are demonstrated on the progression of the phosphoenzyme through the reaction cycle and on the final hydrolytic cleavage of Pi. The overall dependence of steady state ATP flux on Ca2+ and H+ concentrations in leaky vesicles is expressed by a series of curves showing that as the H+ concentration is raised higher Ca2+ concentrations are required to obtain half-maximal ATP fluxes. At saturating Ca2+, maximal ATP fluxes are observed at an intermediate H+ concentration (pH 7.2), while lower levels are obtained as the H+ concentration is reduced (to pH 8) or increased (to pH 6). A preliminary model is then proposed based on the presence of two interacting domains permitting competitive binding of Ca2+ or H+, per each catalytic site undergoing phosphorylation by ATP. The model considers three main states and thirteen substates (depending on the occupancy of the binding sites in each state by Ca2+, H+, or neither) in the progression of the ATP cycle, coupled to transport of Ca2+ and counter transport of H+ in leaky vesicles. Considering the preliminary nature of the model and the experimental scatter, a rather satisfactory agreement is noted between a family of curves generated by theoretical analysis and the ATP flux curves obtained experimentally.