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Nonaqueous Nature of the Environment Surrounding the Acyl‐phosphate of the Na+/K+‐ATPase Deduced from Nucleophilic Attack by Hydroxylamine on the Phosphate Group

The phosphate group is widely distributed in biosystems as a constituent moiety of ATP and plays a key role in processes responsible for decreases in entropy, such as active transport of ions mediated by Na+/Kt-ATPase. Investigation of the characteristics of the phosphate group incorporated into this enzyme is therefore essential for clarifying the molecular mechanism of Nat/IS+-ATPase as the functional unit of active ion transport. The present work represents a return to research on the nucleophilic substitution of the acyl-phosphate group of the phosphoenzyme (EP) of Na+/K+-ATPase by hydroxylamine.' We compared the reaction mechanism of the EP in the active state and that of acetylphosphate, which is a classic model compound for the phosphate bond in the EP of so-called P-type ATPases. The reaction mechanism of hydroxylamine with the phosphate group of the EP would be expected to reflect only the static chemical state of the acyl-phosphate bond in the active-site cleft of the enzyme, because hydroxylamine, unlike water, is not a physiological ligand.