The E₂ Conformation of Na⁺/K⁺-ATPase During the Na⁺ ATPase Activity

Abstract

Two main reactions can lead to ATP hydrolysis by the Na⁺/K⁺-ATPase (EC 3.6.I.37) (see ref. 1). In the presence of K⁺(o), the (Na⁺/K⁺)-dependent activity associated to Na⁺(i)-K⁺(o) exchange. K⁺(o) accelerates the breakdown of the phosphoenzyme and in its transit across the membrane is trapped by the dephosphoenzyme in an “occluded” (E₂(K)) form. The spontaneous release of K⁺ into the cell is slow but that rate is markedly increased by ATP acting with low affinity in a non phosphorylating role. Without K⁺(o) there is what we know as Na⁺-ATPase activity where the return path, beginning with enzyme dephosphorylation is drastically altered. In the absence of Na⁺(o), dephosphorylation is spontaneous and goes with an “uncoupled” Na⁺ efflux. When there is Na⁺(o), this cation acts like K⁺ stimulating dephosphorylation and being transported inwardly in exchange for Na⁺(i). Little is known about the after dephosphorylation steps of the Na⁺-ATPase. Based on phosphorylation from Pi at 0° C Post et al. (2) suggested the existence of an E₂ form following dephosphorylation.