Identification of Glu⁷⁷⁹ as an Essential Part of the Cation Binding Site of the Sodium Pump

Abstract

The chemical modification of purified renal Na⁺/K⁺-ATPase with the fluorescent carboxyl-selective reagent, 4-(diazomethyl)-7-(diethylamino)-coumarin (DEAC), results in enzyme inactivation via disruption of the monovalent cation binding sites and loss of K⁺ and Na⁺ binding capacity. Modification of 1 or 2 carboxyl residues in the α-subunit in a K⁺ or Na⁺-preventable manner leaves the ATP binding unaltered and the enzyme is still able to undergo E₁ ↔ E₂ transitions. These characteristics of the DEAC-modified enzyme are evidence that the modified residues are located at the cation binding site (1, 2). Furthermore, the specificity of DEAC to react with carboxyl groups supports the early idea that carboxyl residues, in or near the membrane, would be involved in the cation oclusion and transport by the Na⁺/K⁺-ATPase.