Principles of selectivity of sodium and potassium binding sites of the Na +/K +-ATPase. A corollary hypothesis

Abstract

The mechanisms whereby the sodium and potassium binding sites of heart sacrolemmal Na +/K +-ATPase (EC 3.6.1.3) distinguished between monovalent cations were investigated using methods of enzyme kinetics. The properties of the sodium binding sites were studied in the presence of 2,4,6-trinitrobenzenesulfonic acid in concentrations completely inhibiting the action of potassium on the enzyme. To test the selectivity of potassium binding sites, K +- p-nitrophenylphosphatase activity was employed as a model. The results suggest that the selectivity of Na +- and K +-binding sites of Na +/K +-ATPase may be due to two independent mechanisms: (i) The principle of key and lock (formation of coordination bounds); (ii) Optimal difference between solvatation energy (in the specific binding site) and hydration enthalpy of the respective cation.