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Selectivity: How Biological Ion Channels Distinguish Among Different Ion Species

D. Gillespie, W. Nonner and R.S. Eisenberg
Rush University Medical Center, US

Keywords: ion channels, selectivity, excluded volume, electrostatics

Abstract:
Biological ion channels conduct ions (mainly Na+, K+, Ca2+, and Cl-) across membranes that are otherwise impermeable to charged particles. Following their chemical potential gradients, ions flow through the pore of a channel using much of the same physics as electrons and holes in semiconductors; the very successful Poisson-Nernst-Planck (PNP) model of ion conduction used the same Poisson/drift-diffusion equations used in semiconductor modeling. Although the current/voltage curves of different classes of ion channel show as much variation as the different classes of semiconductors, channels have some very unique properties due to evolutionary pressure and the availability of different charge carriers. Specifically, different ion channel classes have evolved to conduct specific ions, a characteristic called selectivity. By adding a model of the finite size of ions (ions are not point charges) to the Poisson/drift-diffusion equations, the physical mechanism of how ion channels distinguish among different ion species (e.g., between Na+ and K+) is becoming understood. Using the examples of Ca2+, Na+, and Cl- selective channels, selectivity is shown to be an interplay of electrostatics and excluded volume (two ions cannot occupy the same space). Because the structures of the various channel types are distinctly different, the weight given to each component (electrostatic or excluded volume) is varied, leading to very different selectivity properties among channels: the Ca2+ channel prefers Ca2+ over Na+ while the opposite is true for the Na+ channel; both of these channels prefer small ions, while the Cl- channel prefers large ions.

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