Kinetics of the Ca2+H+, and Mg2+ interaction with the ion-binding sites of the SR Ca-ATPase

Electrochromic styryl dyes were used to investigate mutually antagonistic effects of Ca2+ and H+ on binding of the other ion in the E-1 and P-E-2 states of the SR Ca-ATPase. On the cytoplasmic side of the protein in the absence of Mg2+ a strictly competitive binding sequence, H2E1 reversible arrow HE1 reversible arrow E-1 reversible arrow CaE1 reversible arrow Ca2E1, was found with two Ca2+ ions bound cooperatively. The apparent equilibrium dissociation constants were in the order of K-1/2(2 Ca) = 34 nM, K-1/2(H) = 1 nM and K-1/2(H-2) = 1.32 muM. Up to 2 Mg2+ ions were also able to enter the binding sites electrogenically and to compete with the transported substrate ions (K-1/2(Mg) = 165 muM, K-1/2(Mg-2) = 7.4 mM). In the P-E-2 state, with binding sites facing the lumen of the sarcoplasmatic reticulum, the measured concentration dependence of Ca2+ and H+ binding could be described satisfactorily only with a branched reaction scheme in which a mixed state, P-E2CaH, exists. From numerical simulations, equilibrium dissociation constants could be determined for Ca2+ (0.4 mM and 25 mM) and H+ (2 muM and 10 muM). These simulations reproduced all observed antagonistic concentration dependences. The comparison of the dielectric ion binding in the E-1 and P-E-2 conformations indicates that the transition between both conformations is accompanied by a shift of their (dielectric) position.