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#ion channel

2 public questions tagged with this topic.

Membrane Transportation

Which ion channel is involved in action potential generation?

Action potential propagation in excitable cells depends on rapid voltage dependent changes in sodium and potassium conductances. Resting potential near minus 70 millivolts maintained by K+ leak and Na+/K+ ATPase. Upon synaptic depolarization reaching threshold about minus 55 millivolts, voltage sensor S4 helices of sodium channels containing repeated Arg-X-X motifs move outward within 100 microseconds, opening activation gate formed by S6 crossing. Sodium electrochemical gradient strong due to 145 millimolar outside, 12 millimolar inside and negative membrane potential drives inward current, depolarizing further to plus 30 millivolts regenerative upstroke. This Hodgkin Huxley sodium current underlies phase 0. Ligand gated nicotinic or glutamate receptors contribute to synaptic depolarization but require chemical transmitter and deactivate slower, not participating in axonal conduction. Aquaporins selectively conduct water via NPA motifs preventing proton conduction, GLUT transporters uniport glucose via rocker switch without ionic current. Subsequent activation of delayed rectifier Kv channels repolarizes. Hence voltage gated Na+ channel confers voltage sensitive excitation for action potential generation.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: Voltage-Gated Na+ Channels in Action Potentials.

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Membrane Transportation

The selectivity filter of an ion channel ensures:

Ion channels achieve selective conduction despite high throughput by incorporating a narrow selectivity filter that discriminates on basis of size, dehydration energy and electrostatic coordination. Pore domain S5, P-loop, S6 forms inverted teepee; P-loop folds as two short pore helices P1 and P2 scaffolding filter. In potassium channels carbonyl oxygens from TVGYG sequence replace hydration waters precisely matching potassium radius but requiring too large cavity for smaller sodium. In sodium channels DEKA motif and outer ring carboxylates create high field site preferring partially hydrated sodium with one water and lysine blocking divalents. Calcium channels use EEEE locus binding calcium with high affinity enabling double occupancy knock-off. Once optimal ion enters filter it is stabilized, non-matching ions face energetic barrier and are excluded. Gate opening controlled by voltage sensor S4 or ligand binding occurs elsewhere, filter itself remains rigid. No ATP hydrolysis occurs. Hence filter ensures only compatible ion type passes, establishing basis for electrical signaling, selective reabsorption and action potential generation.

Ref: Hille, Ion Channels of Excitable Membranes, Chapter 13: Selectivity Filter and Molecular Sieving.

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