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 system, the key feature of which is the formation of an energy-rich phosphorylated transport enzyme (EP) and its conversion. 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 enzyme, 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.
D001665:Binding Sites; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004789:Enzyme Activation; D010766:Phosphorylation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism*
D000252:Calcium-Transporting ATPases; D002118:Calcium