Calorimetric evidence for large entropy changes accompanying intermediate steps of the ATP-driven Ca2+ uptake by sarcoplasmic reticulum.

The hydrolysis of ATP catalyzed by sarcoplasmic reticulum vesicles prepared from rabbit skeletal muscle was studied by microcalorimetry in parallel with measuring the extent of hydrolysis and the level of the phosphorylated intermediate (E approximately P) of the Ca2+-ATPase in the reaction medium containing 100 mM MgCl2, 0.1 mM CaCl2, 20 mM 3-(N-morpholino)propanesulfonic acid/KOH (pH 7.1). During the first few turnovers of the enzyme catalytic cycle, when an appreciable amount of E approximately P is still present, the observed heat is much smaller than the heat expected from the amount of ATP hydrolyzed. Since the accompanying Ca2+ uptake, inclusive of Ca2+ binding to internal sites, is associated with very little heat change (Kodama, T., Kurebayashi, N., and Ogawa, Y. (1980) J. Biochem. (Tokyo) 88, 1259-1265), this result indicates that the formation of E approximately P is endothermic and hence driven by a large increase in entropy. The overall exothermicity of the ATP hydrolysis is brought about by the subsequent, strongly exothermic decomposition of E approximately P.