Partial Reactions of the Ca2+-ATPase

The participation of an acid-stable phosphoenzyme as an intermediate in the reaction of hydrolysis of ATP has been extremely useful in the characterization of the mechanism of all cation-transport ATPases. Hydrolysis of ATP by the Ca²⁺-ATPase of the plasma membrane takes place following a sequence of elementary steps that involve cyclic conformational transitions and cyclic formation and hydrolysis of a phosphoenzyme. This mechanism is essentially similar to that of the other cation-transport ATPases. Phosphorylation is the only elementary step of the ATPase reaction that is absolutely dependent on Ca²⁺. In the phosphoenzyme of the Ca²⁺-ATPase of sarcoplasmic reticulum and of the-ATPase, the acylphosphate bond has been identified with aspartylphosphate. No chemical identification of the bond has yet been performed in the plasma membrane Ca²⁺-ATPase. The phosphatase activity of the Ca²⁺-pump requires Mg²⁺ and Ca²⁺, is strongly activated by K⁺ and Na⁺, and is absolutely dependent on the presence of ATP.