Equilibrium cooperative binding of calcium and protons by sarcoplasmic reticulum ATPase

The cooperative equilibrium binding of Ca2+ by sarcoplasmic reticulum ATPase, as modulated by pH, is analyzed by statistical mechanical treatment of a theoretical model. The model consists of four equivalent subunits, in the form of a square, with nearest-neighbor interactions. Each subunit has one site for binding of one Ca2+ or one proton, but not both. Binding of either ligand on a subunit induces a conformational change in the subunit that alters its interaction with its two neighbors. The model gives good agreement with experimental binding data. It should prove useful as a starting point in the analysis of steady-state ATPase activity as a function of Ca2+ and H+ concentrations.