Poster Presentation 2014 International Biophysics Congress

The sodium potassium pump (Na⁺,K⁺-ATPase) shows a high selectivity for K⁺ over Na⁺ binding when in the E2P conformation, in which the ion transport sites have access to the extracellular medium. To understand the K⁺ extracellular selectivity in the presence of a high concentration of competing Na⁺ ions requires consideration of more than just the ion binding affinities. We compared equilibrium-based calculations of the extracellular occupation of the Na⁺,K⁺-ATPase transport sites by Na⁺ and K⁺ to fluxes through Na⁺ and K⁺ transport pathways based on a recently published kinetic model¹. The results show that, under physiological conditions, there is 99.7% selectivity for pumping of K⁺ from the extracellular medium into the cytoplasm relative to Na⁺, whereas equilibrium calculations alone predict only 20.0% selectivity for K⁺. Thus, kinetic effects are dominant in determining K⁺selectivity.