Tertiary structure and energy coupling in Ca2(+)-pump system.

Europium luminescence from europium bound to sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase indicates that there are two high affinity calcium binding sites. Furthermore, the two calcium ions at the binding sites are highly coordinated by the protein as the number of H2O molecules surrounding the Ca2+ ions are 3 and 0.5. In the presence of ATP, calcium ions are occluded even further down to 2 and zero H2O molecules, respectively. The Ca2+ - Ca2+ intersite distance is estimated to be 8-9 A and the average distance from the Ca2+ sites to CrATP is about 18 A. Digestion of the (Ca2+ + Mg2+)-ATPase at the T2 site (Arg 198) causes uncoupling of Ca2(+)-transport from ATPase activity while calcium occlusion due to E1-P formation remains unchanged. Further tryptic digestion beyond T2 and in the presence of ATP diminishes Ca2+ occlusion to zero while 50% of the ATPase hydrolytic activity remains. Tryptic digestion beyond T2 and in the absence of ATP diminishes ATPase hydrolytic activity to 50% of normal while Ca2+ occlusion remains intact. These data are consistent with a mechanism in which the functional enzyme must be in the dimeric form for occlusion and calcium uptake to occur, but each monomer can hydrolyze ATP.