NEGATIVE OR POSITIVE COOPERATION IN CALCIUM-BINDING TO DETERGENT-SOLUBILIZED ATPASE OF THE SARCOPLASMIC-RETICULUM - ITS MODULATION BY A HIGH-CONCENTRATION OF ATP

Two different conformations of chemically equivalent Ca2+-ATPase molecules in the sarcoplasmic reticulum have been shown to non- and positive cooperatively bind two calcium ions, respectively (Nakamura, J. (1994) J. Biol. Chem. 269, 30822-30827). At pH 7.40, these ATPase molecules split into E(1) (high affinity state for calcium) and E(2) (low affinity state for calcium), respectively, before calcium binding. At this pH, calcium binding to the monomeric ATPase, solubilized with dodecyloctaethylenglycol monoether, was studied by examining Ca-45(2+) binding to the ATPase and calcium dependences of its phosphorylation, fluorescence intensity, ATP-hydrolysis at a low (5 mu M) concentration of ATP, and acetyl phosphate hydrolysis. The results suggest that the solubilized ATPase molecules predominantly preexist in E(2) and negative cooperatively (the Hill value (n(H)) = 0.5-0.6) bind 2 mol of calcium/mol of the ATPase with an apparent calcium affinity (K-0.5) of 3-5 mu M. The nonequivalences of calcium bindings at the membranous ATPase molecules seem to result from the intermolecular interaction of the molecules. A high concentration (5 mm) of ATP modulated the binding manner so that it became positively cooperative (n(H) similar to 2) and increased the K-0.5 to 0.1 mu M.