title: Regulatory interaction between calmodulin and ATP on the red cell Ca2+ pump.
abstract: The interactions between calmodulin, ATP and Ca2+ on the red cell Ca2+ pump have been studied in membranes stripped of native calmodulin or rebound with purified red cell calmodulin. Calmodulin stimulates the maximal rate of (Ca2+ + Mg2+)-ATPase by 5-10-fold and the rate of Ca2+-dependent phosphorylation by at least 10-fold. In calmodulin-bound membranes ATP activates (Ca2+ + Mg2+)-ATPase along a biphasic concentration curve (Km1 approximately 1.4 microM, Km2 approximately 330 microM), but in stripped membranes the curve is essentially hyperbolic (Km approximately 7 microM). In calmodulin-bound membranes Ca2+ activates (Ca2+ + Mg2+)-ATPase at low concentrations (Km less than 0.28 microM) in stripped membranes the apparent Ca2+ affinities are at least 10-fold lower. The results suggest that calmodulin (and perhaps ATP) affect a conformational equilibrium between E2 and E1 forms of the Ca2+ pump protein.
mesh_terms: D000255:Adenosine Triphosphate / Q000097:blood / Q000494:pharmacology*; D001693:Biological Transport, Active; D002118:Calcium / Q000097:blood*; D002135:Calcium-Binding Proteins / Q000494:pharmacology*; D000252:Calcium-Transporting ATPases / Q000097:blood; D002147:Calmodulin / Q000097:blood / Q000494:pharmacology*; D004789:Enzyme Activation / Q000187:drug effects; D004910:Erythrocyte Membrane / Q000187:drug effects* / Q000378:metabolism; D004912:Erythrocytes / Q000187:drug effects*; D006801:Humans; D066298:In Vitro Techniques
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium