Activation and inhibition of the Mg-Ca-ATPase from E. coli by Mg2+ and Ca2+

MgATP of CaATP is the substrate of the Mg-Ca-ATPase. At low Mg2+- or Ca2+-concentrations the ATPase is activated by Mg2+ or Ca2+, the activator being essential for activity. At higher Mg2+- or Ca2+-concentrations the Mg-Ca-ATPase is inhibited competitively. Thus the real Km is smaller than reported in the literature. H+ competes with Mg2+ or Ca2+ for the metal binding sites.