The sarcoplasmic reticulum Ca'+-ATPase catalyses a reversible calcium transport coupled to phosphate transfer between ATP and water. It has been proposed [Biochemistry (1980) 19, 4252-42611 that the reactivity of the acyl-phosphate bond is dependent on the water activity within the catalytic site. We have tested this hypothesis and found that the polarity in the free catalytic site is lower than that of water, a further and large decrease is observed when the enzyme is phosphorylated by Pi. Phosphorylation by ATP indicates that this polarity change is specifically associated with the formation of the ADP-insensitive phosphoenzyme.

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