A Requirement for Partial Dehydration of the K ⁺ Ion Governs Gating of the Shaker K ⁺ Channel: Quantum Calculations Show Complex Interactions of Ions, Water, Protons, and Protein Side Chains

There is a vast literature on the voltage gating of ion channels, with a fairly large fraction concerned with potassium channels, especially of the K _V 1 family, including Shaker . Experimental evidence derived from protein structure has been interpreted to give gating mechanisms that largely disregard water. We propose that the K ⁺ ion, in order to pass through the gating region and enter the cavity pore, must be largely dehydrated. Competitive interactions of single hydration shell water molecules at the gate, with counterions, protein, or other water molecules, can remove one water at a time; not every water molecule is removed. There are several such interactions for the ion hydration shell; for the ion to pass through the gating region, there must be enough such interactions to leave the ion with at most two hydrating water molecules, in which case the gate is open. Protein conformational changes are small, and mostly unimportant. The hypothesis has a second part: protons, previously shown to be candidate carriers of the gating current (Kariev and Green, JPC B, 2019, Membranes, 2022, 2024) are capable of reaching the gate; adding four protons to the gate prevents dehydration, leaving the ion with at least three hydrating water molecules, enough to block passage. Quantum calculations presented here support the dehydration part of the hypothesis; they also mostly support the second part, concerning the protons, but further work will be required to fully confirm this. The hypothesis explains the experimental finding that the P475D mutant is essentially constitutively open, while the P475S mutant, with a wider gate opening, is closed at all relevant potentials; the computations presented here show the mechanism for this in detail, further confirming the first part of the hypothesis, and largely but not completely confirming the second part, concerning protons, while showing where further work is needed. This mechanism can also qualitatively account for flicker noise and fluctuations, and their consequences.