Equilibrium and kinetic studies of calcium transport and ATPase activity in sarcoplasmic reticulum.

A number of equilibrium and kinetic measurements are presented to characterize the partial reactions of the ATPase and transport cycle in sarcoplasmic reticulum vesicles. The cycle begins with calcium and nucleotide binding on sites available on the outer surface of the vesicles. A phosphorylated enzyme intermediate is then formed, and the calcium sites are subjected to a change in their orientation and their affinity for calcium. It is shown that steps involved in calcium release on the inner side of the vesicles are rate limiting for the cycle, and are followed by hydrolytic cleavage of the intermediate with release of inorganic phosphate and recycling of the enzyme.