Membrane lecture 3

Question 1

Biological roles of ion channels

Answer 1

• Axons have Na+ + K+ that are switched on/off

- Action potential activates Ca2+ channels → release Ca2+ → neurotransmitter fusion

Question 2

Key properties of channel

Answer 2

• TM protein that forms pore
• Some selectivity
• Filter that interacts w/ favoured ions only
• Switch btw open + closed w/ gate or ligand

Question 3

K+ channel topology

Answer 3

• Conserved core topology
• Central pore-forming region - M1, loop that goes in + out of membrane, M2
• 4 subunits come together, M2 form lining of core pore
• TVGYG

Question 4

Selectivity

Answer 4

• M2 close off channel + positions
• All carbonyl O point the same way
• As K+ enters, encounters 8O (S4), 8O(S3), 8O(S2), 8O(S1) and 4O(So)
• In solution, K+ surrounded by 8H20
• Replaced by 8O in protein = selective = no E lost
• Na+ is smaller so H20 have stronger interaction, x fit as well, more expensive to dehydrate

Question 5

Mechanism for K+ passing through the channel

Answer 5

1. K+-H20-K+ as if K+ occupied S1-4 would be unstable

2. All sites occupied w/ K+ + instability means ions move quickly

Question 6

Voltage gating

Answer 6

• TVGYG in filter
• Conserved Glycogen in inner helix → bend/hing in middle of helix, opens channel
• S4 has repeat of R (+ve charge)
• S1-4 move when change voltage
• Pulls S4 helix down when membrane changes voltage, pulls on S4/5 linker → opens channel