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Unformatted text preview: mimics the embrace of the water molecules that surround the ion in solution (Fig.16). As far as the K+ ion is concerned, being hydrated or being in the filter are energetically equal, and thus the two states are in equilibrium. Thus as water is stripped from the K+ ion, it just slides into a binding site and can move through the binding site as readily as it diffuses in solution. But dehydrated Na+ ions are smaller than K+ ions, so why can't the smaller Na+ ions pass through the selectivity filter? The answer is that Na+ ions do not fit well in the sites of the selectivity filter that stabilize K+ ions, and thus the hydrated Na+ ion is far more stable than a dehydrated Na+ that entered the filter. The probability of the two states greatly favors hydration, as shown by the arrows in the right panel of Fig. 16. Let us explore this in a bit more detail. Imagine that the selectivity filer is open, and thus the external face of the selectivity filter is open to the extracellular fluid where there is a high concentration of Na+ that should drive Na+ into and through the filter. However, the dimensions of the segments fit K+ ions stripped of their surrounding water perfectly. Hydrated Na+ ions are too large to enter the pore. Dehydrated Na+ ions, although smaller than dehydrated K+ ions, fit poorly into the filter, and thus are far more likely to flip back to a hydrated state. The precise fit of K+ ions in the filter sites is the key feature that endows the K+ channel with its ion selectivity. H H H H H H Figure 16. Hydrated K+ and K+ ions are H H shown in the top panels. The dehydrated ions are shown in the bottom panel. Notice that the dehydrated K+ ion is larger than the dehydrated Na+ ion, but that the dehydrated Na+ ion fits poorly in the binding sites of the selectivity filter, while the K+ ion fits perfectly. H H H H H H H H 16...
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This note was uploaded on 09/17/2009 for the course BIO 365R taught by Professor Draper during the Spring '08 term at University of Texas at Austin.
- Spring '08