Ca²⁺-stimulated ATPase: inactivation by Ca²⁺ and mechanism

Summary

The preincubation of the rat red blood cell membranes in the presence of low Ca²⁺ levels causes an irreversible inhibition of the Ca²⁺-stimulated ATPase activity. The inactivation is dependent on the Ca²⁺ concentration and the apparent Ki is identical to the Ca²⁺ concentration needed to reach the half-maximal activity of the enzyme. This fact and the energy of activation (E_a = 13.8 Kcal/mol) for the inhibition suggest that Ca²⁺ inactivates the Ca²⁺-stimulated ATPase by binding to the same site which it normally occupies to activate the enzyme. It is concluded that the Ca²⁺-stimulated ATPase is in a dynamic equilibrium between two states: a stable ATP-bound state and an unstable ATP-free state.