title: The kinetics for the phosphoryl transfer steps of the sarcoplasmic reticulum calcium ATPase are the same with strontium and with calcium bound to the transport sites.
abstract: 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.
authors.affiliation: Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254-9110.
authors.forename: T
authors.initials: T
authors.lastname: Fujimori
authors.forename: W P
authors.initials: WP
authors.lastname: Jencks
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002413:Cations, Divalent; D006868:Hydrolysis; D007700:Kinetics; D010766:Phosphorylation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013324:Strontium / Q000378:metabolism*
chemical_list: D002413:Cations, Divalent; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium; D013324:Strontium