L6: Ion Channels

Question 1

What are the four key structural requirements for an ion channel?

Answer 1

• A selectivity filter
• a sensor
• one or more gates
• a pore

Question 2

Describe the structure of the nACh Receptor

Answer 2

• Consists of 5 subunits (alpha, beta, delta, or gamma)
• Must have 2 alpha subunits
• Each subunit has 4 transmembrane domains
• N and C terminals are extracellular
• Pore is lined by the 2nd subunit

Question 3

Is the nACh receptor a selective or non-selective cation channel?

Answer 3

non-selective cation channel

Question 4

Describe the structure of Voltage-Dependent Sodium Channels

Answer 4

• Contains around 2000 amino acids
• Composed of 4 homologous domains (I-IV)
• Each domain has 6 transmembrane alpha-helices and a P loop
• The 4th alpha-helix acts as the voltage sensor

Question 5

How many transmembrane alpha-helices are there in each domain of the voltage-dependent sodium channel?

Answer 5

6 transmembrane alpha-helices

Question 6

Describe the structure of Voltage-Dependent Potassium Channels

Answer 6

• Made up of a single polypeptide
• Consists of 6 transmembrane alpha-helices and a P loop
• The 4th alpha-helix acts as the voltage sensor

Question 7

How many protein subunits come together to form the tetramer in the voltage-dependent potassium channel?

Answer 7

Four protein subunits

Question 8

Does the combination of subunits affect the function and electrophysiology of the potassium channel?

Answer 8

Yes, it alters its function and electrophysiology.

Question 9

What determines the ability of ions to flow through an ion channel?

Answer 9

• the ability of ions to flow through an ion channel is determined by:
• the selectivity filter
• gates
• the electrochemical gradient established by the electrochemical driving force

Question 10

What is the electrochemical driving force?

Answer 10

• the electrochemical driving force is the difference between the membrane potential (Em) and the equilibrium potential (Eeq) for a specific ion
• If driving force = positive = ion flows out of the cell; if it is negative, ion flows into the cell.

Question 11

When does the driving force become zero?

Answer 11

• at the reversal potential
• where electrochemical driving force = zero
• and there is no net movement of ions through the ion channel

Question 12

What are the equilibrium potentials for K+, Cl-, Na+, and Ca2+ ions?

Answer 12

K+ = -90mV
Cl- = -70mV
Na+ = +60mV
Ca2+ = +130mV

Question 13

What is the action potential threshold, and what happens when it is reached?

Answer 13

• approximately -55mV
• when the membrane potential reaches this threshold, voltage-dependent Na+ channels open, triggering an all-or-nothing action potential

Question 14

What is the role of voltage-dependent calcium channels in neurotransmitter release?

Answer 14

• open at around -30mV
• when they open Ca2+ enter the cell ↓ their electrochemical gradient
• this Ca2+ entry further depolarizes the cell and triggers the release of NT by exocytosis

Question 15

What determines the reversal potential of a non-selective cation channel?

Answer 15

• depends on the relative permeabilities of the channel to each ion
• if the channel is more permeable to Na+, the reversal potential will be closer to Na+
• if the channel is equally permeable to Na+ and K+, the reversal potential will be at the halfway point between Na+ and K+

Question 16

How is the nAch receptor structured?

Answer 16

• a pentameric structure composed of 5 subunits
• must be at least 2 alpha subunits in the receptor.

Question 17

Where are the C and N terminus of the nAch receptor found?

Answer 17

• C and N terminus of the nAch receptor are located outside of the cell

Question 18

How many subunits does the voltage-dependent Na+ channel have, and how many times does each subunit cross the cell membrane?

Answer 18

• one polypeptide made up of 4 domains, and each domain consists of 6 alpha helices, crossing the membrane 6 times

Question 19

How many subunits make up the voltage-dependent K+ channel, and where are the N and C terminus located?

Answer 19

• voltage-dependent K+ channel has 4 subunits, each made up of 6 alpha helices
• N and C terminus of this channel are located inside the cell