BB14 Membrane Protein Channels

Question 0

Permeability is conferred by 2 classes of membrane proteins

Answer 0

• pumps

* channels

Question 1

The lipid bilayer of biological membranes is impermeable to

Answer 1

• ions

* polar molecules

Question 2

Pumps

Answer 2

• use an energy source (ATP or light) to drive thermodynamically uphill transport of ions or molecules

Question 3

Channels

Answer 3

• enable downhill or passive transport (facilitated diffusion)

Question 4

Ligand gated channel

Answer 4

• Acetylcholine – cholinergic neurotransmitter

* nerve communication across synapses

Question 5

Nerve communication across synapses

Answer 5

• 50nm synaptic cleft
• synaptic vessels have 10,000 acetylcholine molecules
• synchronous export of 300 vesicles in response to a nerve impulse
• acetylcholine concentration increases 10nM to 0.5mM

Question 6

Binding of acetylcholine to the postsynaptic membrane

Answer 6

• changes ionic permeabilities
• conductance of Na+ and K+ increases
• large inward current of Na+
• smaller outward flux of K+

Question 7

Inward Na+ current

Answer 7

• depolarizes the plasma membrane and triggers an action potential

Question 8

Acetylcholine opens a single type of cation channel

Answer 8

• almost equally permeable to Na+ and K+

* change in permeability is mediated by the acetyl-choline receptor

Question 9

Acetylcholine receptor

Answer 9

• extracellular domain, membrane-spanning segments, segments inside the cell
• 2α β γ δ
• pseudo 5-fold symmetry
• rotation of inner helices in response to acetylcholine

Question 10

Voltage-gated channels

Answer 10

K+ and Na+ channels
• nerve impulse is an electrical signal produced by the flow of ions across the plasma membrane of a neuron
• interior of neuron has a high concentration of K+ and low concentration of Na+
• these gradients produced by Na+ - K+ ATPase

Question 11

Action potential – signals are sent along neurons by transient

Answer 11

• depolarization – beyond threshold causes Na+ ions to flow in, leading to further of depolarization and more Na+ influx
• repolarization – K+ ions flow out, restoring the membrane potential
THERE MUST BE SPECIFIC ION CHANNELS – so Na first, then K

Question 12

Protein from potassium and sodium channels purified on the basis that it could

Answer 12

bind tetrodotoxin (from puffer fish)

Question 13

Structure of potassium ion channel

Answer 13

• 4 helical transmembrane subunits
• selectivity filter = Thr – Val – Gly – Tyr – Gly
• bigger ions (>1.5Å) can’t get through – too big
• smaller ions (Na+) can’t pass through – salvation, doesn’t interact with walls

Question 14

Selectivity filter solvates

Answer 14

K well

• carbonyl - oxygen around, gives better salvation than water

Question 15

Potassium’s energy

Answer 15

• resolvation within K+ channel site greater than desolvation energy

Question 16

Sodium’s energy

Answer 16

• desolvation energy less than resolvation energy

Question 17

Voltage gated channels require

Answer 17

• substantial conformational change

* S1 to S4 form the voltage responsive channels

Question 18

The voltage gated channel can be inactivated by

Answer 18

occlusion of the pore
• “ball and chain” model
• adding back peptides 1-20 restores inactivation

Question 19

Na+ and K+ channels work together to give the action potential

Answer 19

• Na+ in first

* then K+ out