Potassium channels

Question 1

what are the ion channel properties required for an AP?

Answer 1

voltage dependence

ion selectivity

inactivation mechanism

Question 2

describe the structure of a K+ channel protein

Answer 2

4 subunits

1 K+ subunit resembles 1 domain of a Na+ channel protein

Question 3

why don’t smaller Na+ ions pass through K+ channels?

Answer 3

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+

Question 4

why was it so hard to purify potassium channels?

Answer 4

there isn’t a rich source like electric eels for Na+ channels

no specific toxins have a great effect on the channels

Question 5

which K+ channel gene was first cloned?

from which animal?

Answer 5

Shaker

Drosophila

Question 6

what did DrMacKinnon discover?

Answer 6

the crystal structure of a bacterial K+ channel from Strep. lividans called KcsV channel

Question 7

what did the crystal structure of the K+ channel show?

Answer 7

each subunit has:
an alpha helix 5 and 6 which span the membrane

and a P loop

Question 8

what did the crystal structure reveal about the diameters of the selectivity filter and the water-filled cavity?

Answer 8

selectivity filter
d = 3 A (angstroms)

water-filled cavity
d = 10 A

Question 9

which direction do K+ move?

Answer 9

from inside to outside the neurone

Question 10

why does the selectivity filter contain oxygens?

Answer 10

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

Question 11

how do K+ pass through filter without hinderance?

Answer 11

they swap a water molecule for the oxygens from the amino acids facing into the channel

Question 12

what is the signature sequence of a K+ channel?

where is it?

Answer 12

GYGVT
glycine
tyrosine
glycine
valine
threonine
(5 amino acids)

in the p loop

Question 13

what is the role of the signature sequence?

Answer 13

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

Question 14

why can’t sodium ions pass through selectivity filter?

Answer 14

unhydrated ion too small for a perfect fit

= energetically unfavourable

Question 15

oxygens are provided by which groups of GYGVT?

Answer 15

GYGV:
backbone carbonyl group

T:
hydroxyl group

Question 16

what does the joining of 2 amino acids result in?

Answer 16

a carbonyl group
+
an amino group

Question 17

how many K+ occupy the selectivity filter?

Answer 17

only 2 K+ at a given time

each is separated by a water molecule

Question 18

compare the selectivity filter in K+ and Na+ channels

Answer 18

wider diameter selectivity filter in Na+ channel
= 4.6A

opposite directions of flow

sodium channel doesn’t have GYGVT signature sequence

Question 19

what is the key residue for selectivity in a sodium channel?

Answer 19

glutamate residue

Question 20

in eukaryotic Na+ channels what are the single amino acids from each domain that determine selectivity for Na+?

Answer 20

domain 1
= D = aspartic acid

domain 2
= E = glutamic acid

domain 3
= K = lysine

domain 4
= A = alanine

Question 21

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?

Answer 21

attract/ coordinate sodium ions

excludes potassium ions

EEEE

Question 22

how does Na+ pass through the selectivity filter?

Answer 22

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

Question 23

why can’t K+ pass through the selectivity filter?

Answer 23

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

Question 24

in the Na+ selectivity filter, the side chains are at too great a distance to allow direct bonding to Na+, so what happens?

Answer 24

interactions occur via water molecules

Question 25

how does the K+ channel open?

Answer 25

conserved glycine in span 5 acts as a hinge

Question 26

describe K+ channel inactivation

Answer 26

‘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

Question 27

how is inactivation different to channel closing?

Answer 27

inactivation is the rapid occluding of a channel

before it slowly relaxes back to closed state

Question 28

compare mechanisms of inactivation of K+ and Na+ channels

Answer 28

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

Question 29

what are channelopathies?

what causes them?

what can this result in?

Answer 29

diseases associated with ion channel dysfunction

mutations in sodium or potassium channels
-> can produce hyper excitability of neurones

epilepsy