Equilibrium cooperative binding of calcium and protons by sarcoplasmic reticulum ATPase.

Abstract

The cooperative equilibrium binding of Ca2+ by [rabbit] sarcoplasmic reticulum ATPase, as modulated by pH, was analyzed by statistical mechanical treatment of a theoretical model. The model consisted of 4 equivalent subunits, in the form of a square, with nearest-neighbor interactions. Each subunit had 1 site for binding of 1 Ca2+ or 1 proton, but not both. Binding of either ligand on a subunit induced a conformational change in the subunit that altered its interaction with its 2 neighbors. The model gave 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.