Ion Channels Flashcards
Properties of ion channels
Ion selective
Gated either by voltage or ligand
Large protein family
Voltage gated ion channel example
Insulin release from beta cells
Ca2+ ion channel = depolarized membrane when ATP present = inhibits K+ channel
= open Ca2+ channel for triggering insulin
Ligand gated ion channel example
Acetylcholine receptor
Ligand is acetylcholine = Ach
When 2 ach bind —> M2 helices x5 twist = hydrophobic residues moved away from channel while polar residues are moved toward middles of channel
Components of Ach receptor
Hydrophobic leu side chains
Polar residues
5x M2 helices which each contribute residues
Ach pathway details
Ach receptor is extracellular, transmembrane receptor on neurons
Nearby neurons release Ach into synaptic cleft - binds to Ach receptor
Ach ion channel allows Na+ and Ca2+ in, K+ is pushed out
Cell is depolarized = too many pos ions disrupts negative mv of usual cell
Ca2+ channels open
Triggers exocytosis of Ach to next neuron
Acetylcholine structure reminders
CH3
Ester
Ch2
Ch2
N-(ch3)3
Ceccn = chicken = Ach Ach = Bawk bawk
7 carbons
2 oxygens
1 nitrogen
What enzyme maintains cell voltage?
Na+K+ ATPase
3 Na+ out for every 2 K+ in
=-60mV in cell
2 main factors ion flow depends on
Concentration gradient of given ion
Difference in voltage and interaction with ion charge
Open ca2+ channel
Vm drives both Na+ and ca2+ inwards
Follows concentration gradient
Open K+ channel
Concentration gradient drives K+ outwards despite Vm
Where are Ach receptors found?
Nerve endings
Neurons
?
K+ channel structure details
4 identical subunits = transmembrane = cone within cone (4x one cone, 4x another cone) = alpha helices
Each of 4 subunits = 2x alpha helices, 1x shorter helix
Shorter helix = negative residues into pore site to attract cations
First part of channel: large, K+ maintains its hydration cage
Channel then narrows = hydration sphere must be given up
= stabilized by carbonyl back bone which is just the right size for K+
K+ bigger than Na+ = na+ causes collapse = no passage
K+ ion channel carbonyl backbone details
Each k+ stabilized by 4 oxygens
4 total such binding sites
Only 2 occupied by 2 k+ at once
Must be single file through channel
Channel narrows towards outside cell
Protein cones narrow towards inside cell
Specific names for alpha helix subunits
4x kirb2 subunits
= inhibited by ATP, activated by PIP2
4x SUR1 subunits
=inhibited by tolbutamide, actives by diazoxide
What specific channel does PIP2 activate?
K+ ion channel
When pip2 to ip3 via PLC = no more activation of k+ channel = easier to depolarize membrane = ca2+ release
