Two aspects of ion conduction by K1
channels have tantalized biophysicists for
the past quarter century. First, what is the
chemical basis of the impressive fidelity
with which the channel distinguishes between
K1 and Na1 ions, which are featureless
spheres of Pauling radius 1.33 Å and
0.95 Å, respectively? Second, how can K1
channels be so highly selective and at the
same time, apparently paradoxically, exhibit
a throughput rate approaching the diffusion
limit? The 104 margin by which K1 is
selected over Na1 implies strong energetic
interactions between K1 ions and the pore.
And yet strong energetic interactions seem
incongruent with throughput rates up to
108 ions per second. How can these two
essential features of the K1 channel pore be
reconciled?