Ion Channels 101

Question 1

What is the membrane potential?

Answer 1

Voltage difference across cell membrane

Maintained by ion pumps and influenced by ion distribution

Question 2

What does RMP stand for and what is its typical value in neurons?

Answer 2

Resting Membrane Potential, approximately -70 mV

It is influenced by the leak of potassium and fixed negative charges

Question 3

Define hyperpolarization

Answer 3

Membrane potential more negative than RMP

Question 4

Define depolarization

Answer 4

Membrane potential more positive than RMP

Question 5

What is an electrochemical gradient?

Answer 5

Combination of chemical gradient (due to concentration differences) and electrical gradient (due to membrane potential)

I.e. Na+ ions are driven into a cell by both their high extracellular concentration (chemical gradient) and their negative charge inside the cell (electrical gradient)

Influences direction and rate of ion movement

Question 6

What is current in the context of ion movement?

Answer 6

Flow of charged ions across the membrane driven by the electrochemical gradient

measured in amps (A) and represented by I

Question 7

What does conductance refer to?

Answer 7

How easily ions can flow through an ion channel, reciprocal of resistance

Denoted using G, e.g., GNa

Question 8

What are BK channels?

Answer 8

Big conductance potassium channels

Question 9

What is rectification in ion channels?

Answer 9

Channel conducts better in one direction than the other depending on membrane potential

Inward rectification: better at more negative potentials; Outward rectification: better at more positive potentials

Question 10

What is a carrier/transporter protein’s maximum transport rate?

Answer 10

Approximately 10,000 ions per second

Question 11

What is the maximum transport rate of Na+/K+ ATPase?

Answer 11

Approximately 1,000,000 ions per second

Question 12

Define selective permeability in ion channels

Answer 12

Ability to allow certain ions to pass while blocking others

Includes cations like K+, Na+, Ca2+ and anions like Cl-

Question 13

What types of gating mechanisms exist for ion channels?

Answer 13

• Second messenger
• Ligand-gated
• Voltage-gated
• G protein-gated

Question 14

What characterizes ion channels?

Answer 14

Transmembrane proteins that are selectively permeable and have controlled openings

Question 15

How are ion channels named?

Answer 15

Characterized by gating and ion selectivity

For ligand-gated channels, named after natural ligand (e.g., GABAA receptor)

Question 16

What are the typical ionic gradients of a mammalian cell?

Answer 16

Na+ high outside, K+ high inside, Cl- high outside, Ca2+ low inside

Question 17

What structure do voltage-gated potassium channels have?

Answer 17

Tetramer of four equivalent subunits = alpha subunits

Can be homotetramers or heterotetramers

Has 6 TM domains with the 4th one as a voltage centre and a membrane dipping domain (forms channel lining) between the 5th and 6th

Question 18

How many Kv channel genes are there in the human genome?

Answer 18

40 Kv channel genes

Question 19

What is significant about the evolution of KV channels?

Answer 19

KV channels appear early in evolution and are present in prokaryotes

Question 20

What is the function of voltage-gated sodium channels?

Answer 20

Depolarization phase of neuronal and skeletal muscle action potential

Question 21

What is the role of voltage-gated potassium channels?

Answer 21

Repolarization phase of action potentials and inhibitory responses to neurotransmitters

Question 22

What is the significance of voltage-gated calcium channels?

Answer 22

Depolarization phase of cardiac action potentials and vesicular release

Question 23

True or False: Voltage-gated channels are always open.

Answer 23

False

Most channels are closed most of the time.

Question 24

What therapeutic applications do calcium channel blockers have?

Answer 24

Used for hypertension and angina

Question 25

List the functional subtypes of calcium channels.

Answer 25

* L type (CaV 1.1-1.4) * P/Q type (CaV 2.1) * N type (CaV 2.2) * R type (CaV 2.3) * T type (CaV 3.1-3.3)

Question 26

What is the significance of the selectivity filter in ion channels?

Answer 26

Allows permeant ions to pass while blocking impermeant ions

Question 27

Fill in the blank: The flow of sodium into the cell is an ______.

Answer 27

inward current

Question 28

What percentage of human drug targets are ion channels?

Answer 28

11%

Question 29

What are carrier/transporter proteins?

Answer 29

Function by binding a substrate on one side of the membrane then a transporter binds to the substrate and undergoes conformational change = flips the substrate to the other side & releases it Can be passive or active transport E.g. Na+/K+ ATPase

Question 30

Describe how the ion channels have selective permeability

Answer 30

They have a selectivity filter which selects ions based on charge and size Rings of charge around the selectivity filter which has the opposite charge to the type of ion channels it’s selective for & it repels ions of the wrong charge

Question 31

Evolution of the CaV and NaV channels

Answer 31

- A single KV gene undergoes gene duplication - Forms a 2 pore channel which undergoes further gene duplication = gets mutated over time so is not identical to the original K+ channel - Results in 4 copies of the original gene strung together

Question 32

Structure of CaV and NaV channels

Answer 32

- Instead of having 4 separate subunits that come together, the alpha subunit folds so that the 4 pseudo-subunits form the channel - Pseudo-subunits relate back to the original voltage gated K+ channel - Alpha subunit is the pore forming subunit and consists of 4 copies of a KV channel-like structure strung together in a single polypeptide

Question 33

Translation and assembly of KV channels

Answer 33

- Potassium channel alpha subunit - 6 TM domains - 1 voltage sensing domain (4th TM domain) - 1 pore forming domain (between the 5th and 6th TM domain) 4 copies of a potassium channel subunit is translated from mRNA and come together to form a channel

Question 34

Translation and assembly of CaV and NaV channels

Answer 34

- Sodium/calcium channel alpha subunit is 4x bigger than that of the K+ channel - 4 pseudo-subunits in a single polypeptide chain with each subunit similar to a KV channel subunit and containing a voltage sensor & pore forming domain - A single sodium or calcium channel alpha subunit is translated from mRNA and the 4 pseudo-subunits fold to form a channel - The 4 pseudo-subunits act in a similar way to the separate subunits of the K+ channel

Question 35

Calcium channel subunits

Answer 35

10 different Ca2+ channel alpha subunits which are divided into 3 families - α CaV 1.1-1.4 - α CaV 2.1-2.3 - α CaV 3.1-3.3 4β 4α2δ 8γ subunits Native channel (natural state) = contains small accessory subunits that modify the function of the alpha subunit - Possibly = 1α:1β:1α2δ ?1γ

Question 36

Sodium channel subunits

Answer 36

9 different Na+ channel alpha subunits - α NaV 1.1-1.9 4β subunits Native channel possibly 1α:1/2β

Question 37

α2δ subunits

Answer 37

2 separate proteins formed from a single polypeptide Important as a drug target as it’s the site of action in some drugs used to treat epilepsy, anxiety and pain 1. Gene produces a single polypeptide 2. Disulfide bond formed in extracellular domain 3. Extracellular domain is cleaved to yield 2 linked peptides

Question 38

Calcium channel subtypes

Answer 38

Categorised into 5 main families **L-type** - Long lasting - Alpha subunits CaV 1.1-1.4 - Located in the heart, smooth muscle, CNS, retina and cochlear hair cells **P/Q type** - Purkinje - Alpha subunits CaV 2.1 - Located in neurons **N type** - Neuronal - Alpha subunit CaV 2.2 - Located in neurons **R type** - Residual - Alpha subunit CaV 2.3 - Located in neurons **T type** - Transient - Alpha subunits CaV 3.1-3.3 - Located in neurons, heart and smooth muscle

Question 39

What are the 3 main states of voltage gated ion channels?

Answer 39

1. Resting (at -70 mV) 2. Open 3. Inactivated Transition between the states is dependent on membrane potential

Question 40

Describe the state of the channel at resting membrane potential

Answer 40

- High Na+ outside the cell - Activation gate is closed to prevent Na+ crossing the membrane - Inactivation gate = ball and chain like structure

Question 41

Describe the state of the channel at threshold potential (-55 mV)

Answer 41

- Na+ channels start to open - Gating mechanism = opening of channel is caused by movement of the voltage sensor

Question 42

Describe the state of the channel when membrane potential is positive

Answer 42

- Channel inactivation - As more Na+ enters, the membrane potential becomes more positive - The inactivation gate swings up to block the inner mouth of the channel = Na+ can’t pass through - Inactivation in other channels such as Ca2+ and K+ channels is more complex

Question 43

Physiological significance of voltage gated ion channels

Answer 43

- NaV channels —> depolarisation phase of neuronal & skeletal muscle action potential - KV channels —> repolarisation phase of action potentials (2nd phase) and inhibitory responses to neurotransmitters - CaV channels —> depolarisation phase of cardiac action potentials (excitatory phase), vesicular release of neurotransmitters and excitation-contraction coupling for muscle contraction

Question 44

Therapeutic importance of voltage gated ion channels

Answer 44

- Hypertension —> Calcium channel blockers = work by relaxing smooth muscle in blood vessels, increasing their diameter - Pain —> Sodium channel blockers (e.g. in dental surgery) and calcium channel blockers = derived from snail toxin and is injected into the spinal cord to treat chronic pain - Disorders of cardiac rhythm —> Calcium, sodium and potassium channel blockers - Angina (O2 deprivation to heart muscle) —> Calcium channel blockers