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  • Title: Effect of divalent cation bound to the ATPase of sarcoplasmic reticulum. Activation of phosphoenzyme hydrolysis by Mg2+.
    Author: Shigekawa M, Wakabayashi S, Nakamura H.
    Journal: J Biol Chem; 1983 Dec 10; 258(23):14157-61. PubMed ID: 6227621.
    Abstract:
    In order to study the mechanism for activation of ATP hydrolysis by Mg2+, the stoichiometry of the high affinity calcium-binding sites with respect to each form of reaction intermediate of sarcoplasmic reticulum ATPase was determined at 0 degrees C and pH 7.0 in the presence and absence of added Mg2+ using the purified ATPase preparation. High affinity calcium binding to the enzyme-ATP complex and to ADP-sensitive (E1P) and ADP-insensitive (E2P) phosphoenzymes occurred with stoichiometric ratios of 2, 2, and 0, and 3, 3, and 1 in the presence and absence of added Mg2+, respectively. The results were interpreted to indicate that in addition to 2 mol of calcium bound to the transport sites of the ATPase, 1 mol of divalent cation, which is derived from the metal component of the substrate, the metal-ATP complex, remains bound to each mole of the enzyme at least until E2P is hydrolyzed. As activation of phosphoenzyme hydrolysis by Mg2+ was blocked by the low concentrations of Ca2+ used in the calcium binding experiments, it was concluded that it is the magnesium derived from MgATP that is responsible for rapid hydrolysis of the phosphoenzyme intermediate.
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