In summary, the inner pore and cavity
lower electrostatic barriers without creating
deep energy wells. The structural and
chemical design of this part of the pore
ensure a low resistance pathway from the
cytoplasm to the selectivity filter, facilitating
a high throughput. Functional experiments
on K1 channels support this
conclusion. When TEA from the cytoplasm
migrates to its binding site at the
top of the cavity [.50% of the physical
distance across the membrane (Figs. 4 and
5)], it traverses only about 20% of the
transmembrane voltage difference (15).
Thus, 80% of the transmembrane voltage
is imposed across the relatively short selectivity
filter. The rate limiting steps for a
K1 ion traversing the channel are thereby
limited to this short distance. In effect,
the K1 channel has thinned the relevant
transmembrane diffusion distance to a
mere 12 Å.