Two types of proton-modulated calcium binding in the sarcoplasmic reticulum Ca(2+)-ATPase. II. Characteristics of their calcium bindings.

The first part of this study reported the existence of two different conformations of chemically equivalent Ca(2+)-ATPase molecules from the sarcoplasmic reticulum, which were in pH-dependent equilibrium between E1 (high affinity state for calcium) and E2 (low affinity state for calcium) and predominantly in E2 independent of pH, respectively, before calcium binding (Nakamura, J., and Furokohri, T. (1994) J. Biol. Chem, 269, 30818-30821). Here, calcium bindings at the ATPase molecules were further studied by varying [Ca2+] and pH at 0 degree C. The molecule, which pre-existed in equilibrium between E1 and E2, noncooperatively (the Hill value (nH) approximately 1) bound calcium with an apparent calcium affinity (K0.5) of 2-6 microM independent of pH. nH of the pH profile of the pre-existing equilibrium was about 2, suggesting participation of two protons in the equilibrium. On the other hand, the molecule, which predominantly pre-existed in E2, cooperatively (nH approximately 2) bound calcium with K0.5, which was increased from about 7 microM to about 0.2 microM by increasing pH from 6.23 to 7.40. Analysis of pH dependency of the K0.5 showed participation of 1 proton/calcium in competition for the binding sites. Calcium bindings at the two ATPase molecules seem to be differently modulated by the proton(s).