Ion Channels

Ion Channels

balanced and opposed by active transport

Ion flux

A balance of diffusion and electrostatic forces

Downwards current

Inward movement of protons

Upwards current

Outward movement of protons

Conductive Change

When Vm isn’t changing and the equilibrium potential of the ion (Eion) isn’t changing BUT the ionic current is changing

Sodium Channels

• Opens up once

• Fast and Short

• Close and don’t reopen

Potassium Channels

• Re open

• Slower and stays open

• Slow to inactivate

Tetramer

4 Proteins that associate together

• each have an identical alpha subunit that is transmembrane

• Each subunit: has 6 transmembrane segments (S1-S6)

Pore

K+

4 subunits come together

Segments 5 and 6 and linker domain (pore loop) contributes to the pore

Selectivity Filter

K+

In pore loop

4 binding sites: multiple K+ electrostaticly moves each faster through the binding sites

Narrow —→ only K+ can get through

Voltage Sensor

K+

Segment 4 is charged

When polarized: gets sucked inwards, which keeps the pore closed

When Vm is polarized, positively charged domain is repelled outwards and creates a conformational change that opens the pore

Na+ Channels

• one single protein

• 4 domains that mimic the K+ tetramer

• 24 transmembrane segments (6 per domain)

Pore

Na+

Partially dehydrates Na+ by stripping water molecules

Narrow: too small to allow a partially hydrated K+ through

Too big for a fully dehydrated K+ to pass through

Selectivity Filter

Na+

Lined with amino acids

Formed by the p-loop between segments V and VI on each domain

How Na+ is like K+

Segment S4 is charged and displaces outwards, opening pore

• Inactivation gate is linker between domains III and IV

  • physically blocks pore