Ion Channel Gating and Permeation

How ion channels open and close in response to stimuli (gating) and how, when open, they conduct specific ions rapidly through a narrow pore (permeation).

Definition

Gating is the conformational switching of a channel between open and closed states; permeation is the movement of ions through the open channel, governed by its selectivity filter and electrochemical driving force.

Scope

This topic covers the two defining behaviours of an ion channel: gating, the stochastic switching between conducting and non-conducting states driven by voltage, ligands, or mechanical force; and permeation, the selective, high-throughput passage of ions through the open pore. It treats single-channel observations, the structural basis of selectivity, and the physical models of conduction, while membrane-potential dynamics and pumping are handled in adjacent topics.

Core questions

• What stimuli open and close channels, and why does gating appear stochastic at the single-channel level?
• How does a channel select strongly for one ion species while conducting it quickly?
• What is the structural basis of the selectivity filter?
• How do single-channel currents reveal channel behaviour?

Key theories

Selectivity by precise coordination

The potassium channel structure shows a filter whose backbone carbonyl oxygens mimic the hydration shell of the favoured ion, so that ion is conducted while smaller ions, which the filter cannot coordinate as well, are excluded.

Stochastic single-channel gating

Patch-clamp recording shows that individual channels jump abruptly between discrete open and closed levels, so macroscopic currents are the statistical sum of many stochastic single-channel transitions.

Mechanisms

Gating couples a stimulus—membrane voltage acting on charged voltage sensors, ligand binding, or membrane tension—to a conformational change that opens or closes the conduction pathway, and because thermal energy drives these transitions they appear as random openings and closings of fixed amplitude. Permeation through the open pore is fast because a selectivity filter replaces the ion's hydration shell with precisely placed coordinating atoms, lowering the energy barrier for the favoured ion while excluding others; the net flux is set by the electrochemical driving force across the membrane.

Clinical relevance

Mutations that alter gating or permeation cause channelopathies, and many drugs and toxins act by binding channels, so the mechanisms here are educational background for that pharmacology and pathophysiology rather than clinical advice.

History

Macroscopic conductances inferred by Hodgkin and Huxley were resolved into discrete events by Neher and Sakmann's patch-clamp recordings in the 1970s, and MacKinnon's atomic structures of the potassium channel in 1998 finally explained selectivity and conduction in structural terms.

Key figures

• Bertil Hille
• Erwin Neher
• Bert Sakmann
• Roderick MacKinnon

Seminal works

• doyle1998
• neher1976

Frequently asked questions

What is the difference between gating and permeation?

Gating is whether the channel is open or closed; permeation is how ions flow when it is open. A channel must first gate open before any permeation can occur.

How can a channel tell potassium from sodium?

Its selectivity filter coordinates the larger potassium ion almost as well as water does, but cannot fit the smaller sodium ion as snugly, so potassium passes far more readily.