How does the K1 channel structure
account for its prodigious ion selective
properties? When an ion enters the selectivity
filter, it evidently dehydrates (nearly
completely). To compensate for the
energetic cost of dehydration, the carbonyl
oxygen atoms must take the place of the
water oxygen atoms, come in very close
contact with the ion, and act like surrogate
water (20, 21). The structure reveals
that the selectivity filter is held open as if
to prevent it from accommodating a Na1
ion with its smaller radius. We propose
that a K1 ion fits in the filter precisely so
that the energetic costs and gains are well
balanced. The structure of the selectivity
filter with its molecular springs holding it
open prevents the carbonyl oxygen atoms
from approaching close enough to compensate
for the cost of dehydration of a
Na1 ion.