The potassium channel is a voltage-gated channel that can have up to
six transmembrane helices. Only two are found for the bacterial channel
KcsA, which is thought to resemble the pore region. The features of the
remaining regions in eukaryotic channels are a voltage sensor, a tetramer-
ization domain, and a signaling domain.
The structures of the K+channel have
shown common features that reveal motifs
that facilitate the rapid and specific trans-
fer of K+ions. The channels are composed
predominantly of αhelices surrounding a
central channel. The channels have large
entrances for ions, allowing ions to enter
in a hydrated state and so not requiring
the energetically unfavorable removal of
water (Figure 18.10). The channel is
narrow but the width cannot be used to
explain the thousand-fold specificity for K+,
with an atomic radius of 1.33 Å, rather
than Na+, which has a smaller radius of
0.95 Å. Instead, the carbonyl atoms line the
channel to precisely mimic the arrange-
ment of water molecules surrounding a K+
ion. To facilitate rapid transfer, ions are not
moved individually through the channel.
Rather, the channel has K+ions located
throughout the channel. On entrance, a
K+ion pushes electrostatically against the
next and leads to the exit of another, in
400 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY
Periplasm
Depolarization
Arg-46
Arg-88
Arg-74
Tension
Open
Closed
Cytoplasm
Figure 18.9Model for the gating of mechanosensitive channels. Modified from Bass et al. 2002.
Selectivity filter sites Extracellular
sites
Direction of ion travel
Cavity
site
Pore axis
Internal
(a)
External
Outer configuration
Inner configuration
(b)
Figure 18.10A schematic diagram of the central
channel of the K+channel, with the domino process
of ion transfer. (a) There are seven main sites for ions
but (b) only half are occupied at any given time.
Modified from Miller (2001).