gating, selectivity, RMP

GHKv equation analysis, RMP determination, ion selectivity

pores

• non gated channel

• water filled ions and molecules

• low selectivity 

channels

• gated pore

• ions only

• sensor to regulate gating 

basic ion channel structure, what forms the pore 

• alpha or main subunit is an integral transmembrane mutlipass protein

• Two transmembrane passes in K+ channel alpha subunits to 24 passes in Na+ or Ca2+

• Each transmembrane spanning region contains a hydrophobic alpha helix

  • These form the ion conductive or pore region

K+ ion channel structure

• two transmembrane passes per subunit

• 8 in total

• tetramer

• charged region allows voltage sensor 

• most simple ion channel 

generalised gating

• transition between open and closed state

• twisting, tilting or bending of subunits and transmembrane spanning alpha helices

mechanisms of generalised gating

• ligand binding

• phosphorylation or dephosphorylation of amino acids

• voltage sensors - causes transmembrane regions to shift

• mechanical distortion through cytoskeleton tethering (eg cochlea by TRP channels)

mechanisms for inactivation

• C type 

• N type

localised C type inactivation

• short amino acid sequence in selectivity filter or pore wall

• filter changes conformation reducing ion transfer

particle N type

• intracellular tethered particle has affinity for binding to open intracellular side of channel

• long amino acid sequence

• ball and chain

• often found in potassium channels

evidence for high K+ permeability 

• hodgkin recorded frog intracellular skeletal muscle fibres RMP at different conc of K+

• K+ contributes to background ion permeability for RMP

> 10mM [K+] data fitted by Nernst

• <10mM [K+] deviates from Nernst → need GHK equation

• K+ channels have a relative selectivity for K+ over Na+, 100-fold

  • Ion channels are not absolute - they still support ion flow or current mediated by other ions