Occluded bound calcium on the phosphorylated sarcoplasmic transport ATPase

The Ca2+ + Mg2+-activated ATPase of the sarcoplasmic reticulum is responsible for the active Ca2+ transport of this membrane system1, the key feature of which is the formation of an energy-rich phosphorylated transport enzyme (EP) and its conversion2. To understand the Ca2+-transport mechanism, it is essential to clarify the behaviour of this intermediate in relation to such ligands as ATP, ADP, Mg2+ and, particularly, Ca2+. Recent kinetic studies on the phosphate turnover of this system suggested a relatively slow rate of Ca2+ dissociation from the phosphorylated enzyme3,4, which possibly indicated Ca2+ binding in some occluded form with the intermediate. Here we report direct measurements of the binding and release of Ca2+ during phosphorylation of the sarcoplasmic transport enzyme. The results indicate an occlusion of the Ca2+ binding, accompanying an initial configurational change of the enzyme induced by the energy-rich phosphoryl transfer.