Week 4 - Ion Channels

Question 1

How is the selectivity of an ion channel determined?

Answer 1

• size of pore
• amino acids lining pore

Question 2

What are the components of a subunit in a ligand-gated ion channel?

Answer 2

• M2 alpha helix
• in its lining
• charged amino acids
• negative for cation specific
• p-loop
• extends into pore as selectivity filter

Question 3

How is a ligand-gated ion channel activated?

Answer 3

• ligand binds
• conformational change
• M2 helices twist
• twisting opens channel

Question 4

What is the voltage-sensing region?

Answer 4

• contains positive amino acids
• depolarisation occurs
• membrane interior becomes positive
• region shifts towards outer surface
• channel is opened

Question 5

How can a voltage-gated ion channel be inactivated?

Answer 5

N-type
- rapid
- ball and chain
- small group of positive amino acids in intracellular domain
- positive so inactivation can occur during prolonged depolarisation
- bind to activated channel
- pore blocked
C-type
- amino acids rearrange
- at external mouth of pore

Question 6

What is the Nernst equation?

Answer 6

E = RT/zF ln C_o/C_i

• E (or V) = membrane potential
• R = gas constant (1.98 cal/molK)
• T = temperature (K)
• z = ion charge
• F = faraday’s constant (2.3x10⁴ cal/molV)
• ln = natural log base e
• C = concentration (outside & inside)
• relates ion gradient to electrochemical potential

Question 7

What is the GHK equation?

Answer 7

V_m = RT/F ln(p_K[K⁺]_o + p… / p_K[K⁺]_i + p…

• Goldman-Hodgkin-Katz
• ions relative permeability determines membrane permeability