Carbony| -


(c) Ion movement through selectivity filter 0 ft* ft ft 9ft ft ft ftft ft

State 1

State 2

passage of K+ ions over Na+ by a factor of thousand. Calcium ions are too large to pass through a K+ channel with or without their bound water.

Recent x-ray crystallographic studies reveal that the channel contains K+ ions within the selectivity filter even when it is closed; without these ions the channel probably would collapse. These ions are thought to be present either at positions 1 and 3 or at 2 and 4, each boxed by eight carbonyl oxygen atoms (Figure 7-16b and c). K+ ions move simultaneously through the channel such that when the ion on the exoplasmic face that has been partially stripped of its water of hydration moves into position 1, the ion at position 2 jumps to position 3 and the one at position 4 exits the channel (Figure 7-16c).

Although the amino acid sequences of the P segment in Na+ and K+ channels differ somewhat, they are similar enough to suggest that the general structure of the ion-selectivity filters are comparable in both types of channels. Presumably the diameter of the filter in Na+ channels is small enough that it permits dehydrated Na+ ions to bind to the backbone carbonyl oxygens but excludes the larger K+ ions from entering.

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