30+-+Ion+Channels - BME 418 Quantitative Cell Biology Alan...

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BME 418, Quantitative Cell Biology Alan J. Hunt Lecture #30: Properties of ion channels Electrically activity in cells requires ion-selective membrane channels. The conductivity of these channels may be static, or it may be gated by ligand binding, or by the voltage across the membrane as seen during action potential propagation.
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BME 418, Quantitative Cell Biology Alan J. Hunt Given that action potentials and many other cellular processes depend on the ion channels with selective permeability, how is selective permeability achieved? Furthermore, a channel must not only be selective, but it must have high throughput. For example, potassium channels may allow millions of potassium ions to pass per second. This is at odds with the usual chemical perspective that high specificity is achieved through high affinity. One simple explanation is that the size and the charge of the channels provide selectivity. Thus a potassium channel would be just large enough to allow potassium ions through, and would be negatively charged to repel chloride ions. But this does not explain all aspects of selective permeability; for example, why many potassium channels (such as those responsible for voltage-gated potassium currents during an action potential) relatively impermeable to smaller sodium ions? Current theory holds that ion selectivity depends not just on the interactions between ions and the channels, but also on the interactions between ions and water. Polar water molecules will tend to orient their dipoles to interact favorably with ions. This “hydration shell” is constantly changing, and cannot be though of as a fixed structure, even on a time scale as short as 1 ns.
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BME 418, Quantitative Cell Biology Alan J. Hunt The effects of the hydration shell are apparent from the diffusion of ions through water. Diffusion of uncharged molecules is predicted fairly well by the Stokes-Einstein relation, even when the molecules are smaller than H2O. However, charged molecules diffuse more slowly than predicted. They are slowed by the additional drag from the dynamic sphere of hydration.
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BME 418, Quantitative Cell Biology Alan J. Hunt The historical view. Potassium channels, key controllers of resting and action potentials ( A ), were thought to be "long" pores, wide at the ends with a narrow selectivity filter ( B ) that was a good fit for hydrated ions ( C and E ) and dehydrated K + ( D ), but a poor fit for Na + ( F ). (From Armstrong, "The vision of a pore" Science. 1998 Apr 3;280(5360):56-7) Selectivity could be achieved if a channel's diameter was small enough to force dehydration of an ion as it passes through a channel. If the channel’s geometry is set so that charges in the channel can readily substitute for the water dipoles, permeability for the ion will be increased. This explains how channels can exist that selectively pass one ion, but not a smaller ion with the same charge (e.g. potassium, but not sodium). When the larger ion is moves into the selectivity filter it is dehydrated, but will make
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This note was uploaded on 03/03/2009 for the course BIOMEDE 418 taught by Professor Hunt during the Winter '08 term at University of Michigan.

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30+-+Ion+Channels - BME 418 Quantitative Cell Biology Alan...

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