Abstract
The sodium potassium pump (Na+,K+‐ATPase) shows a high selectivity for K+ over Na+ binding from the extracellular medium. To understand the K+ selectivity in the presence of a high concentration of competing Na+ ions requires consideration of more than just ion binding affinities. Here, equilibrium‐based calculations of the extracellular occupation of the Na+,K+‐ATPase transport sites by Na+ and K+ are compared to fluxes through Na+ and K+ transport pathways. The results show that, under physiological conditions, there is a 332‐fold selectivity for pumping of K+ from the extracellular medium into the cytoplasm relative to Na+, whereas equilibrium calculations alone predict only a 7.5‐fold selectivity for K+. Thus, kinetic effects make a major contribution to the determination of extracellular K+ selectivity.
Highlights
Selectivity for Na+ and K+ ions is crucial to animal physiology [1]
In the mechanism of the Na+,K+ATPase, K+ ions must bind from the extracellular medium, where the concentration of competing Na+ ions is much higher than that of K+
It is important to bear in mind that the Na+,K+-ATPase is not in equilibrium and its FEBS Open Bio (2018) a 2018 The Authors
Summary
Selectivity for Na+ and K+ ions is crucial to animal physiology [1]. Selective Na+ and K+ channels enable ion diffusion between the inside and outside of cells and are responsible for action potentials allowing nerve signal transmission and muscle contraction [1,2]. There are two contributions to ion-transporting selectivity: firstly, the equilibrium binding affinities of the enzyme for either Na+ or K+, and secondly, the kinetics of Na+- and K+- transporting pathways.
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