Voltage-Gated Ion Channels

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Voltage-Gated Ion Channels

• Voltage-dependent ion channels play a
  fundamental role in the generation and
  propagation of the nerve impulse and in cell
  homeostasis.

• The voltage sensor is a region of the protein
  bearing charged amino acids that relocate upon
  changes in the membrane electric field.

Segments (S5 and S6) and the pore loop were
found to be responsible for ion conduction.
General architecture of voltage-gated channels
(Na and Ca2).The or - signs indicate
charges that have been implicated in voltage
sensing.
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Voltage-Gated Ion Channels
The Gate
The ion conduction through the pore may be
interrupted by closing a gate
Lipid bilayer
Closed states
voltage gated ion channel
field-effect transistor
Open states
voltage gated ion channel
field-effect transistor
Comparison between a field-effect transistor
(FET) and a voltage gated ion channel. The FET
transistor is represented as a p-channel device
to make a closer analogy to a cation selective
voltage-gated channel. In the FET, D is the
drain, S is the source, and G is the gate.
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Voltage-Gated Ion Channels
Ionic and gating currents in Shaker-IR K channel
Time course of ionic currents for pulses to the
indicated potentials starting and returning to
- 90 mV
The voltage dependence of the open probability
and the charge moved per channel.
Time course of the gating currents for the pulses
indicated.
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Voltage-Gated Ion Channels
Typical gating current trace
Gating current recorded at 10-kHz bandwidth.
When the bandwidth is increased to 200
kHz, the first surge of current is the
predominant amplitude
The time course of the charging of the membrane
capacitance for the experiment in (b).
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Voltage-Gated Ion Channels
Current fluctuations in gating currents
Top trace is the time course of a pulse to
10 mV middle trace is the variance and bottom
trace is the mean computed from several hundred
traces.
Top trace is the time course of a smaller pulse
to - 40 mV middle trace is the variance
lower trace is the mean computed from hundreds of
traces.
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Voltage-Gated Ion Channels
Three models of the voltage sensor
Helical screw model
Paddle model.
Transporter model.
Positively charged residues