PT  - JOURNAL ARTICLE
AU  - Asghar M. Razavi
AU  - Lucie Delemotte
AU  - Joshua R. Berlin
AU  - Vincenzo Carnevale
AU  - Vincent A. Voelz
TI  - Free energy calculations suggest a mechanism for Na+/K+-ATPase ion selectivity
AID  - 10.1101/106724
DP  - 2017 Jan 01
TA  - bioRxiv
PG  - 106724
4099  - http://biorxiv.org/content/early/2017/02/07/106724.short
4100  - http://biorxiv.org/content/early/2017/02/07/106724.full
AB  - Na+/K+-ATPase transports Na+ and K+ ions across the cell membrane via an ion binding site made 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 approaches to identify the protonation state of Na+ and K+ coordinating residues in E1P and E2P conformations. Further analysis of these simulations revealed a novel 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 D926. 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.MDmolecular dynamicsTMtransmembraneFEPfree energy perturbationPMFpotential of mean forceMBARMulti-state Bennett Acceptance RatioBARBennett Acceptance Ratio