transport in a channel is usually gated; that
is, the channel is open or closed depending
on another factor. The controlling factors
are the binding of a specific ligand for
ligand-gated channels and the membrane
potential for voltage-gated channels.
Gating is very fast, with a channel open-
ing in microseconds for a short period of
a few milliseconds, thus providing a rapid
cellular response. Experimentally, ion flux
is measured electrically as changes in volt-
age or current using special devices called
patch clamps (Figure 18.7). This technique
is very sensitive, allowing the measure-
ment of small currents due to the flow of
ions through a channel. Pulling a small
region out of a membrane that contains the
channel using a fine glass pipette isolates
the channel. Once the electrical circuit
is complete, the flow of approximately
10,000 ions through an ion channel in
1 ms is detectable. The properties of gated channels are characterized
by the inclusion of the appropriate ligand or by setting the voltage at a
specific value.
The three-dimensional structures have been determined for several classes
of channel, including the mechanosensitive channel and the potassium
channel (Figure 18.8). The mechanosensitive channel opens in response
to a mechanical motion, namely the stretching of the cell membrane, to
respond to the associated change in osmolarity. Potassium channels conduct
K+ions across the cell membrane in many different cellular processes.
Some potassium channels are ligand-gated whereas others are voltage-
gated. Despite the differences in the response mechanism of these two
channels, the structure of these channels show remarkable similarity, with
the presence of a central channel surrounded by several transmembrane
helices, as was also found for transporters.
The diversity of mechanosensitive channels is reflected in the lack of any
identified sequence motifs associated with mechanosensitivity and the lack
of any homology between the structures of the MscL and MscS channels.
The structure of the MscS channel suggests a mechanism for the gating of
this channel (Figure 18.9). Two arginines are located on the periphery of
the protein at a turn between two transmembrane helices. Gating of the
protein may be correlated with the repositioning of these residues within
the membrane in response to changes in either the applied tension or
membrane depolarization. The shift of these helices would then be coupled
to changes in the interior of the protein that lead to gating of the channel.398 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY
ChannelMicropipette applied tightly
to plasma membranePatch of membrane
pulled from cell
Patch of membrane
placed in aqueous solution
MicropipetteElectronics to
hold transmembraneInwardcurrent
10pA 50msTime
Electrodes
Figure 18.7The patch-clamp technique for the
measurement of an individual ion channel.