| pubmed-article:1388154 | pubmed:abstractText | Rate constants for most of the steps of the reaction cycle of the sarcoplasmic reticulum calcium-ATPase are similar or identical with Ca2+ or Sr2+ as the transported ions in spite of the large differences in the size and affinity of Ca2+ and Sr2+ (5 mM MgCl2, 100 mM KCl, pH 7.0, 25 degrees C). Phosphorylation of cE.Sr2 and cE.Ca2 by ATP occurs with kp = 220-235 s-1, whereas phosphorylation of E.ATP+Ca2+ or Sr2+ is consistent with kb = 50-70 s-1. Hydrolysis of E approximately P.Sr2 and E approximately P.Ca2 occurs with kt = 20 s-1, and the addition of 7 mM ADP to E approximately P.Sr2 or to E approximately P.Ca2 gives a burst of approximately 43% dephosphorylation, followed by dephosphorylation with k = 46 s-1. However, one Sr2+ ion dissociates from cE.Sr2 and from cE.ATP.Sr2 with k congruent to 120 s-1, whereas one Ca2+ ion dissociates from cE.Ca2 with k = 38 s-1 and from cE.ATP.Ca2 with k = 80 s-1. | lld:pubmed |
