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Ion permeation in K+ channels occurs by direct Coulomb knock-on

Science  17 Oct 2014:
Vol. 346, Issue 6207, pp. 352-355
DOI: 10.1126/science.1254840

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Abstract

Potassium channels selectively conduct K+ ions across cellular membranes with extraordinary efficiency. Their selectivity filter exhibits four binding sites with approximately equal electron density in crystal structures with high K+ concentrations, previously thought to reflect a superposition of alternating ion- and water-occupied states. Consequently, cotranslocation of ions with water has become a widely accepted ion conduction mechanism for potassium channels. By analyzing more than 1300 permeation events from molecular dynamics simulations at physiological voltages, we observed instead that permeation occurs via ion-ion contacts between neighboring K+ ions. Coulomb repulsion between adjacent ions is found to be the key to high-efficiency K+ conduction. Crystallographic data are consistent with directly neighboring K+ ions in the selectivity filter, and our model offers an intuitive explanation for the high throughput rates of K+ channels.

Ions knock each other across the membrane

Potassium channels play a key role in regulating a cell's membrane potential, which in turn affects diverse processes. The channels contain four potassium binding sites that are thought to be alternately occupied by potassium and water. Starting from high-resolution crystal structures, Köpfer et al. simulated over a thousand potassium ions crossing the channel (see the Perspective by Hummer). They found that ions are in direct contact rather than being separated by water as previously assumed. It seems that repulsion between the ions is the key to their efficient movement through the channel.

Science, this issue p. 352; see also p. 303

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