title: Effects of Mg2+, anions and cations on the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum.
abstract: In a previous paper [Gould, East, Froud, McWhirter, Stefanova & Lee (1986) Biochem. J. 237, 217-227] we presented a kinetic model for the activity of the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum. Here we extend the model to account for the effects on ATPase activity of Mg2+, cations and anions. We find that Mg2+ concentrations in the millimolar range inhibit ATPase activity, which we attribute to competition between Mg2+ and MgATP for binding to the nucleotide-binding site on the E1 and E2 conformations of the ATPase and on the phosphorylated forms of the ATPase. Competition is also suggested between Mg2+ and MgADP for binding to the phosphorylated form of the ATPase. ATPase activity is increased by low concentrations of K+, Na+ and NH4+, but inhibited by higher concentrations. It is proposed that these effects follow from an increase in the rate of dephosphorylation but a decrease in the rate of the conformational transition E1'PCa2-E2'PCa2 with increasing cation concentration. Li+ and choline+ decrease ATPase activity. Anions also decrease ATPase activity, the effects of I- and SCN- being more marked than that of Cl-. These effects are attributed to binding at the nucleotide-binding site, with a decrease in binding affinity and an increase in 'off' rate constant for the nucleotide.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000838:Anions / Q000494:pharmacology*; D001665:Binding Sites; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002412:Cations / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008954:Models, Biological; D010766:Phosphorylation; D011485:Protein Binding; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D000838:Anions; D002412:Cations; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium