Calcium-dependent non-equivalent characteristics of calcium binding sites of the sarcoplasmic reticulum Ca2+-ATPase.

The Ca2+ binding sites of purified sarcoplasmic reticulum Ca2+-ATPase were filled with 0.1-20 microM Ca2+ to varying degrees of maximal Ca2+-binding capacity (1.9-2.3 mol/mol of the phosphorylatable enzyme) in the absence of ATP at pH 7.0 and 0 degree C. The exchange reaction of bound Ca2+ for unbound Ca2+ at equilibrium was studied by the nitrocellulose filtration method, and was compared with the dissociation reaction of the bound Ca2+ by EGTA. When about 90% of the Ca2+ sites were filled, half of the bound Ca2+ slowly exchanged at a rate about 20-30-times slower than the dissociation rate of about 0.3 s-1, i.e., the reaction was 'occluded'. The other half of the bound Ca2+ was not 'occluded', and rapidly exchanged at the same rate as the dissociation rate. The Ca2+ in both states was, however, equally dissociated by EGTA. The Ca2+ 'occlusion' was not as pronounced when 40-90% of the sites were filled. On the other hand, when less than 40% of the sites were filled, no Ca2+ 'occlusion' was observed, and the Ca2+ on the sites homogeneously exchanged. More than half of the unoccluded and exchangeable Ca2+ was converted to an 'occluded' state after saturation of the sites with Ca2+. Based on the fact that one Ca2+-ATPase polypeptide binds one Ca2+ (Verjovsky-Almeida, S. and Silva, J.L. (1981) J. Biol. Chem. 256, 2940-2944), these results suggest the existence of two types of interacting Ca2+-ATPase molecule which are kinetically distinguishable. It further suggests that the non-equivalence of the two sites, each of which is on one of the putative interacting molecules, is produced by Ca2+ saturation of the sites of the enzyme oligomer (2 X n-mer, n greater than 1) rather than by Ca2+ binding itself to the two putative sites. Ca2+ on one of the two sites may be 'occluded' in the enzyme. The Ca2+ 'occlusion' was relieved by ATP. The 'occluded' state seems to be involved in a process of the active Ca2+ transport in the sarcoplasmic reticulum.