Sequential dissociation of Ca2+ from the calcium adenosinetriphosphatase of sarcoplasmic reticulum and the calcium requirement for its phosphorylation by ATP.

The kinetics for dissociation of the stable enzyme-calcium complex of the sarcoplasmic reticulum calcium ATPase, cE.Ca2, were followed by assay with simultaneous addition of [32P]ATP and EGTA, which gives 70% phosphorylation of cE.Ca2 with k = 300 s-1 (25 degrees C, pH 7.0, 5 mM MgSO4, 0.1 M KCl). The binding of ATP to cE.Ca2 is described by kATP = 1.0 X 10(7) M-1 s-1, k-ATP = 120 s-1, and Kdiss = 12 microM; ATP binding is partially rate limiting for phosphorylation at less than 100 microM ATP. The sequential dissociation of Ca2+ from cE.Ca2 is described by k-2 = 55-60 s-1 for the first, "outer" Ca2+, k-1 = 25-30 s-1 for the second, "inner" Ca2+, and K0.5 = 3.4 microM, n = 1.9 (from Kdiss = 7.4 X 10(-7) M for Ca.EGTA). Dissociation of the inner Ca2+ is inhibited by external Ca2+, with K0.5 = k-1/k2 = 0.7 microM. This confirms the conclusion that dissociation of the two Ca2+ ions is sequential. The ability of cE.Ca2 to catalyze phosphorylation by ATP disappears in the presence of EGTA with k = 50-55 s-1, the same as k-2 for dissociation of the outer Ca2+ ion. This result, and the absence of the induction period that would occur if both cE.Ca2 and cE.Ca1 were catalytically competent, shows that both Ca2+ ions are required for phosphorylation. This conclusion is confirmed by the stoichiometry of 1.4/0.7 = 2.0 for the ratio of Ca2+ internalized to phosphoenzyme formed after simultaneous addition of ATP and EGTA. Phosphorylation of cE.Ca2 in the presence of 45Ca gives 0.15, not 0.3, 45Ca internalized, which corresponds to exchange of only 1 Ca2+ and is in agreement with this conclusion. The requirements for binding of two Ca2+ for catalytic specificity toward ATP and loss of two Ca2+ from E approximately P.Ca2 for specificity toward water account for the stoichiometry of Ca2+ transport and provide a possible reason for the two steps in the phosphorylation of cE.Ca2.ATP.