The coupling mechanism of sarcoplasmic reticulum ATPase is based on the reciprocal influence of calcium binding and phosphorylation domains. Cooperative calcium binding activates the enzyme, permitting utilization of ATP by transfer of its terminal phosphate to the enzyme. Occupancy of the phosphorylation domain then produces internalization and dissociation of the bound calcium. Hydrolytic cleavage of Pi completes the catalytic and transport cycle. Conversely, the phosphorylated enzyme intermediate can be formed with Pi in the absence of Ca2+. This intermediate is then destabilized by calcium binding, permitting formation of ATP by phosphoryl transfer to ADP.