Ion Conduction Mechanisms in Potassium Channels Revealed by Permeation Cycles.

Potassium channels are responsible for the selective yet efficient permeation of potassium ions across cell membranes. Despite many available high-resolution structures of potassium channels, those conformations inform only on static information on the ion permeation processes. Here, we use molecular dynamics simulations and Markov state models to obtain dynamical details of ion permeation. The permeation cycles, expressed in terms of selectivity filter occupancy and representing ion permeation events, are illustrated. We show that the direct knock-on permeation represents the dominant permeation mechanism over a wide range of potassium concentrations, temperatures, and membrane voltages for the pore of MthK. Direct knock-on is also observed in other potassium channels with a highly conserved selectivity filter, demonstrating the robustness of the permeation mechanism. Lastly, we investigate the charge strength dependence of permeation cycles. Our results shed light on the underlying permeation details, which are valuable in studying conduction mechanisms in potassium channels.