title: Activation of partial reactions of the Ca2+-ATPase from human red cells by Mg2+ and ATP.
abstract: (1) At ATP concentrations up to 30 micrometer addition of 0.5 mM MgCl2 in the reaction mixture increases both the rate of formation and the steady-state level of the phosphoenzyme of the Ca2+-ATPase from human red cell membranes. Under these conditions Mg2+ has no effect on the rate of dephosphorylation, which remains slow. (2) In the presence of Mg2+ the rate of dephosphorylation is increased 5 to 10 times by high (1 mM) concentrations of ATP. (3) Provided Mg2+ has reacted with the phosphoenzyme, acceleration of dephosphorylation by ATP takes place in the absence of Mg2+. This suggests that the role of Mg2+ on dephosphorylation is to convert the phosphoenzyme into a form that, after combination with ATP, reacts rapidly with water. (4) The results are consistent with the idea that combination of ATP at a non-catalytic site is needed for rapid dephosphorylation of the Ca2+-ATPase.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000097:blood*; D004910:Erythrocyte Membrane / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010750:Phosphoproteins / Q000097:blood; D010766:Phosphorylation
chemical_list: D010750:Phosphoproteins; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D008274:Magnesium