Molecular simulations and free-energy calculations suggest conformation-dependent anion binding to a cytoplasmic site as a mechanism for Na+/K+-ATPase ion selectivity

This is the authors' abstract. We don't add key points for this paper.

Na + /K + -ATPase transports Na + and K + ions across the cell membrane via an ion-binding site becoming alternatively accessible to the intra- and extracellular milieu by conformational transitions that confer marked changes in ion-binding stoichiometry and selectivity. To probe the mechanism of these changes, we used molecular simulation and free-energy perturbation approaches to identify probable protonation states of Na + - and K + -coordinating residues in E1P and E2P conformations of Na + /K + -ATPase. Analysis of these simulations revealed a molecular mechanism responsible for the change in protonation state: the conformation-dependent binding of an anion (a chloride ion in our simulations) to a previously unrecognized cytoplasmic site in the loop between transmembrane helices 8 and 9, which influences the electrostatic potential of the crucial Na + -coordinating residue Asp 926 . This mechanistic model is consistent with experimental observations and provides a molecular-level picture of how E1P to E2P enzyme conformational transitions are coupled to changes in ion-binding stoichiometry and selectivity.