Potassium channels Flashcards
what are the ion channel properties required for an AP?
voltage dependence
ion selectivity
inactivation mechanism
describe the structure of a K+ channel protein
4 subunits
1 K+ subunit resembles 1 domain of a Na+ channel protein
why don’t smaller Na+ ions pass through K+ channels?
in an aqueous solution, ions have hydration shells
smaller Na+ has more tightly bound shells of water molecules than K+
-> Na+ has bigger diameter hydrations shell than K+
why was it so hard to purify potassium channels?
there isn’t a rich source like electric eels for Na+ channels
no specific toxins have a great effect on the channels
which K+ channel gene was first cloned?
from which animal?
Shaker
Drosophila
what did DrMacKinnon discover?
the crystal structure of a bacterial K+ channel from Strep. lividans called KcsV channel
what did the crystal structure of the K+ channel show?
each subunit has:
an alpha helix 5 and 6 which span the membrane
and a P loop
what did the crystal structure reveal about the diameters of the selectivity filter and the water-filled cavity?
selectivity filter
d = 3 A (angstroms)
water-filled cavity
d = 10 A
which direction do K+ move?
from inside to outside the neurone
why does the selectivity filter contain oxygens?
these mimic the O2 molecules that would otherwise surround the K+ in a hydration shell
mimic in terms of geometry (8 oxygens form a cage) and the distances
how do K+ pass through filter without hinderance?
they swap a water molecule for the oxygens from the amino acids facing into the channel
what is the signature sequence of a K+ channel?
where is it?
GYGVT glycine tyrosine glycine valine threonine (5 amino acids)
in the p loop
what is the role of the signature sequence?
the geometry provides the oxygens
that are perfectly aligned in the selectivity filter
to neutralise the +ve K ions
-> allows them to pass through filter
why can’t sodium ions pass through selectivity filter?
unhydrated ion too small for a perfect fit
= energetically unfavourable
oxygens are provided by which groups of GYGVT?
GYGV:
backbone carbonyl group
T:
hydroxyl group
what does the joining of 2 amino acids result in?
a carbonyl group
+
an amino group
how many K+ occupy the selectivity filter?
only 2 K+ at a given time
each is separated by a water molecule
compare the selectivity filter in K+ and Na+ channels
wider diameter selectivity filter in Na+ channel
= 4.6A
opposite directions of flow
sodium channel doesn’t have GYGVT signature sequence
what is the key residue for selectivity in a sodium channel?
glutamate residue
in eukaryotic Na+ channels what are the single amino acids from each domain that determine selectivity for Na+?
domain 1
= D = aspartic acid
domain 2
= E = glutamic acid
domain 3
= K = lysine
domain 4
= A = alanine
what is the role of acidic residues D and E in eukaryotic Na+ channels?
what about lysine (K)?
what is the sequence i prokaryotic Na+ channels?
attract/ coordinate sodium ions
excludes potassium ions
EEEE
how does Na+ pass through the selectivity filter?
in a partially hydrated state
negative charge on glutamate at the extracellular side of the selectivity filter substitutes perfectly for 1 water
- leaving others in place
moves from extracellular to intracellular side
why can’t K+ pass through the selectivity filter?
lysine is a +vely charged amino acid
- > repels a K+ ion because of its large size
- > stops K+ going in opposite direction to Na+, up the filter
in the Na+ selectivity filter, the side chains are at too great a distance to allow direct bonding to Na+, so what happens?
interactions occur via water molecules
how does the K+ channel open?
conserved glycine in span 5 acts as a hinge
describe K+ channel inactivation
‘ball and chain’
N-type inactivation:
uses N-terminus of each K+ channel subunit
OR
N-terminus of an axillary beta subunit to block channel
(this has 4 subunits)
C-type inactivation:
slow unknown mechanism of inactivation
how is inactivation different to channel closing?
inactivation is the rapid occluding of a channel
before it slowly relaxes back to closed state
compare mechanisms of inactivation of K+ and Na+ channels
K+ channel:
‘ball and chain’
provided by N terminus of each subunit of channel tetramer
(or in some cases by N terminus of B subunit tetramer)
Na+ channel:
‘hinged lid’
provided by motif in domain 3-4 cytoplasmic linker
what are channelopathies?
what causes them?
what can this result in?
diseases associated with ion channel dysfunction
mutations in sodium or potassium channels
-> can produce hyper excitability of neurones
epilepsy
