BioPHYSICAL chemistry

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).