Lecture 9: Ion Channel Function

Question 1

Where this is Going = 5

Answer 1

1 ➢Neural and muscle function depends on ion channels

2 ➢Opening voltage gated channels allows action
potentials

3 ➢Opening of voltage gate calcium channels allows release of neurotransmitters and contraction of muscle

4 ➢Opening of ligand gate ions channels allows reception of neurotransmitter signals.

5 ➢Sensory systems all rely on ion channels

Question 2

Ion Channels = 5

Answer 2

1 ➢Ion channels can be made up of SAME SUBUNITS (PROTEINS) or can be an assembly of different proteins

2 ➢ Allow LIPOPHOBIC ions to travel through lipid membrane

3 ➢Ion channels are SELECTIVE for specific ions

4 ➢Most ion channels are GATED, can be opened and
closed

5 ➢Movement of ions is PASSIVE

Question 3

Ion Channel Structure (PROTEINS) = 4

Answer 3

1. Primary structure – sequence of amino acids
2. Secondary structure – alpha helix
3. Tertiary structure – 3D folding of
   polypeptide
4. Quaternary structure – different polypeptides bonded together

Question 4

Ion Channel Structure =
Structure of membrane spanning protein 2

Answer 4

1. Membrane spanning -helices contain hydrophobic amino acid residues.
2. Connected by loops of hydrophilic residues.

Question 5

Ion Channel Structure = Typical structure of voltage-gated Na+ channel

Answer 5

1. 4 domains containing 6 x membrane spaning hydrophobic ALPHA- helices.
2. Aqueous pore formed by one of the ALPHA-helices.
3. Another ALPHA-helix (S4) contains the voltage sensor.

Question 6

Ion Channel Diversity

Answer 6

1. Due to availability many different subunits

TYPES

2. HETEROOLIGOMERS, HOMOOLIGOMERS, (IN SINGLE POLYPEPTIDE CHAIN) AND AUXILLARY SUBUNITS

Question 7

HETEROOLIGOMERS

Answer 7

Distinct subunits
(IN SINGLE POLYPEPTIDE CHAING)

Question 8

HOMOOLIGOMERS

Answer 8

single subunits(IN SINGLE POLYPEPTIDE CHAING)

Question 9

Ion Channel Selectivity =

Selectively permeable Selectivity determined by: 3

Answer 9

1 * Sizeofpore
2 * Electrical charges of chains of amino acids that enter the pore

3. EXAMPLE: Na+ channels Selectivity filter weakly binds Na+ ions.
   - charge stabilised by a hydrophilic amino acid

Question 10

UNDERSTANDING ION CHANNEL SELECTIVITY = 7

Answer 10

1 ➢ Selection by charge, negative charged throat repels anions makes the channel cation selective.

2 ➢ Selection by ion size, small throat will not let large ions through.

3 ➢ Easy to see how a channel can let Na+ but not the larger K+ or the anion Cl- through.

4 ➢ How do K+ channels keep the smaller cation Na+ out?

5 ➢ In solution ions have hydrogen bonds to water, the hydrated
shell.

6 ➢ Most hydrogen bonds to water replaced by bonds to channel amino acids.

7 ➢ Na+ is too small to hydrogen bond to both sides of the channel but too big to fit when bonded to water.

Question 11

UNDERSTANDING Ion Channels Gating

Answer 11

1. Patch-clamp setup:
   Records current flow from a single ion channel
2. All or nothing opening of variable duration, but constant amplitude

Question 12

Ion channel opening is not

Answer 12

determinant but stochastic (opening probability)

Question 13

GATING TYPES = 4

Answer 13

1. LIGAND GATED
2. VOLTAGE GATED
3. MECHANICALLY GATED
4. COVALENT MODIFICATION

Question 14

LIGAND GATES

Answer 14

Requires binding of a chemical e.g. a transmitter

Question 15

VOLTAGE GATES

Answer 15

Requires a voltage change across the membrane

Question 16

MECHANICAL GATES

Answer 16

Requires stretching or some displacement

Question 17

Ligand Gated channels = DIRECT 3

Answer 17

1. Channel opens in response to binding of the ligand to receptor.
2. The ligand is called an AGONIST
3. May be neurotransmitters or hormones, usually bind on extracellular side of channel.

Question 18

Antagonist

Answer 18

• can inhibit binding of endogenous ligand.

-LIGAND GATED CHANNELS

Question 19

Types of Ligands Gated Channels = Antagonist

Answer 19

1. Curare – blocks nicotinic ACh receptor.
2. Lidocaine - local anaesthetic, binds to domain IV of Na+ channel. Inhibits AP generation.
3. Tetrodotoxin (TTX; found in puffer fish) binds to voltage gated Na+ channels preventing AP generation. Fatal

Question 20

Ligand Gated Channels - INDIRECT -2

Answer 20

1. Channel opens in response to a second messenger signal activated by a neurotransmitter.
2. Second messenger (eg G-PROTEIN) acts on the intracellular side of the channel – couples the receptor to the ion channel

Question 21

Voltage Gated Channels

Answer 21

1. Change in membrane voltage can open and close some channels.
2. Detects changes in the electrical potential across the membrane.

outside (+), Inside (-) —–>. Change (transmembrane potential)—> outside (-), Inside (+)

Question 22

Inactivation Gate:

Opening vs Inactivation

Answer 22

1. Opening
   - Conformational change occurs in one region of the channel
2. Inactivation
   - Blocking particle swings into and out of channel mouth

Question 23

Multiple Confirmation States

3 states of Na+ channels

Answer 23

1. Resting - closed & activatable
2. Activated - open
3. Inactivated - closed and non-activatable

Question 24

what happens during resting stage in voltage gate

Answer 24

activation gate closed

inactivation gate open

Question 25

what happens during ACTIVATED stage in voltage gate

Answer 25

Activation gate open Inactivation gate open

Question 26

what happens during INACTIVATED stage in voltage gate

Answer 26

Activation gate open Inactivation gate closed

Question 27

Mechanically Gated Channels = 3

Answer 27

1. Activated by stretch or pressure. 2. The energy for gating may come from mechanical forces due to channel-cytoskeleton interactions 3. CLOSED ---STRETCH --> OPEN

Question 28

Mechanically Gated Channels: Example: inner ear hair cells = 5

Answer 28

1. Tip links STRETCH and open channels when stereocilia bend TOWARD tallest member 2. K+ enters; hair cell DEPOLARISES 3. Depolarisation Open voltage-gated Ca+2 channels 4. Ca+2 entry causes greater release of neurotransmitter. 5. More neurotransmitter leads to higher rate of action potential.

Question 29

Nicotinic Receptors in Muscle: 6

Answer 29

1. ➢Nicotinic receptors on skeletal muscle 2. ➢Ligand gated- opened by ACh from nerve 3. ➢5 subunits for the channel pore 4. ➢2 isoforms of subunit N1 and N2 5. ➢Selective for cations 6. ➢Opening depolarises muscle cell

Question 30

Voltage Gated Sodium Channels = 6

Answer 30

1. ➢Nav channel found in nerves 2. ➢Voltage gated- opened by depolarisation 3. ➢Single alpha subunits forms the pore 4. ➢Complete channel has an accessory b subunit 5. ➢9 alpha subunits (Nav 1.1- 1.9) and 3 b subunits (b1-3) 6. ➢Opening depolarises nerve causes action potential

Question 31

Summary = 8

Answer 31

1. ➢Ion channel properties are set by their protein structure 2. ➢Diversity of channels is very high (dozens of types of voltage gated sodium channels for example) 3. ➢Ion channels can be very selective 4. ➢Opening probability (gating) can be controlled by voltage, ligand binding or mechanical stretch (also covalent modification by enzymes) 5. ➢Many voltage gated channels open for only a short time before closing of an inactivation gate. 6. ➢Ligand gating allows detection of neurotransmitters 7. ➢Mechanical gating allows detection of stretch e.g. baroreceptors 8. ➢Voltage detection produces action potentials