+ (L1), Molecule to malady

Question 1

Give the 7 key features that define ion channels

Answer 1

1. Selectivity amongst ions
2. Tight control over opening/closing (Gating)
3. Fine control or adjustment of gating (modulation)
4. Complex Structures (100s of AAs)
5. Accessory Proteins
6. A wide variety of structures which can be loosely arranged into distinct families based on secondary structure
7. Complex structure/function relationships

Question 2

General structure of ion channels

Answer 2

• One or more pore-forming subunits
• Often in association with accessory subunits
• General theme in their structure in which ion moves through central pore formed from 4 or 5 TM alpha-helices in a barrel shape
• Often, channel is tetrameric (e.g. in Kir channel family) or pentameric (e,g, Cys-loop receptor family)
• Voltage gated are composed of a single subunit containing 4 repeating domains
• Some Potassium channels are dimers

Question 3

What is the largest class of ion channels? Describe and give 3 examples:

Answer 3

Pore-Loop

• Pore-loop region loops back into membrane to form selectivity filter, determining permeativity of ion species
• Either tetramers or monomers w/ 4 similar domains
• Central core: 2 TM helices linked by pore loop
• e.g. hERG (human ether-a-go-go), Kir (potassium inward rectifier), glutamate receptor channels

Question 4

How does the tetrameric nature of most pore-loop channels affect ion-channel diversity?

Answer 4

• Closely related subunits can associate to form heteromeric channels with novel properties

Question 5

How is SUR distinguished from ion channels?

Answer 5

• Despite being a member of the ABC family of pumps, it is actually not a pump or a channel
• It is a channel regulator
• SUR: Sulphonylurea receptor

Question 6

Give an example of a large pore which discriminates exclusively on a size basis (exception to the rule)

Answer 6

• Gap-junction channels
• Function like molecular sieves

Question 7

Give an example of a cation selective channel

Neurotransmitters involved

Answer 7

• AChR
• Rings of negatively charge residues in pore exclude anions and facilitate cation flux

Question 8

Give 2 examples of anion-selective channels

Answer 8

• GABA
• Glycine receptors
• Ring of positively charge residues

Question 9

Bacterial KcsA
How did the resolution of this structure inform us about potassium channel selectivity? Describe the structure:

Hint: Jacob’s ladder

Answer 9

• K+ channels are 100-1000x more permeable to K+ than Na+ despite Na+ being smaller
• This selectivity is achieved by the narrow ‘selectivity filter’ at the narrowest part of the core
• All K+ channels have the same GYG sequence in this selectivity filter; the backbone of carbonyls in this GYG sequence lines the pore, allowing K+ to slip from one binding site to the next, at a distance larger apart than the size of an Na+ ion -> Sodium is too small to jump across this gap in the chain so cannot interact
• As such, in na+, the energy required to remove the ‘waters of hydration’ is greater than that gained by interacting with carbonyl oxygens

Question 10

How might the opening / closing of channels be influenced?

Answer 10

• Binding of intra/extracellular ligands
• Changes in membrane potential
• Changes in temperature
• Mechanical stress
• Biological reactions ie. Modulation (such as phosphorylation)

Question 11

How are most channels ‘closed’

Answer 11

• By a gate that acts as a physical barrier to ion movement
• In gap-junctions, the gate has an iris-like mechanism that contracts
• In pore-loop channels, the gate swings in a hinge-like motion which bends out to ‘open’ the channel
• IN some K^V channels, inactivation is achived by N-terminal blocker which swings round to plug the channel

Question 12

How does Ligand-gating work?

Answer 12

• The ligand must bind differentially to the open and closed conformation
• If it binds more tightly to the open state, it is a channel activator, if it binds more closely to the closed state, it is an inhibitor
• The ligand binding site is generally located between two adjacent subunits/domains (this is energetically favourable since these domain interfaces undergo larger conformational change upon ligand binding
• Many have multiple ligand-binding sites
• Flexible loops link the ligand binding domain to the pore

Question 13

How do accessory proteins influence channels?

Answer 13

• Accessory subunits specify the location and abundance of ion channels
• Modulate biophysical properties
• Finetune sensitivity to physiological ligands and pharmacological ligands
• Mixing and matching of accessory subunits with pore-forming subunits contributes greatly to diversity of ion channels

Question 14

Channelopathies general meaning

Answer 14

• Disorders resulting from mutations in ion-channel genes themselves

Question 15

Explain why LQT syndrome is an example of a genetically heterogenous channelopathy

Answer 15

• The same clinical phenotype is caused by mutations in different genes
• In LQT syndrome, there are at least 8 different possible genes that can produce it
• As a result, not all carriers of the same disease-causing mutation are affected / why they exhibit different phenotypes (on top of possible environmental influences)