Lecture 9: Ion Channel Function Flashcards
Where this is Going = 5
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
Ion Channels = 5
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
Ion Channel Structure (PROTEINS) = 4
- Primary structure – sequence of amino acids
- Secondary structure – alpha helix
- Tertiary structure – 3D folding of
polypeptide - Quaternary structure – different polypeptides bonded together
Ion Channel Structure =
Structure of membrane spanning protein 2
- Membrane spanning -helices contain hydrophobic amino acid residues.
- Connected by loops of hydrophilic residues.
Ion Channel Structure = Typical structure of voltage-gated Na+ channel
- 4 domains containing 6 x membrane spaning hydrophobic ALPHA- helices.
- Aqueous pore formed by one of the ALPHA-helices.
- Another ALPHA-helix (S4) contains the voltage sensor.
Ion Channel Diversity
- Due to availability many different subunits
TYPES
- HETEROOLIGOMERS, HOMOOLIGOMERS, (IN SINGLE POLYPEPTIDE CHAIN) AND AUXILLARY SUBUNITS
HETEROOLIGOMERS
Distinct subunits
(IN SINGLE POLYPEPTIDE CHAING)
HOMOOLIGOMERS
single subunits(IN SINGLE POLYPEPTIDE CHAING)
Ion Channel Selectivity =
Selectively permeable Selectivity determined by: 3
1 * Sizeofpore
2 * Electrical charges of chains of amino acids that enter the pore
- EXAMPLE: Na+ channels Selectivity filter weakly binds Na+ ions.
- charge stabilised by a hydrophilic amino acid
UNDERSTANDING ION CHANNEL SELECTIVITY = 7
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.
UNDERSTANDING Ion Channels Gating
- Patch-clamp setup:
Records current flow from a single ion channel - All or nothing opening of variable duration, but constant amplitude
Ion channel opening is not
determinant but stochastic (opening probability)
GATING TYPES = 4
- LIGAND GATED
- VOLTAGE GATED
- MECHANICALLY GATED
- COVALENT MODIFICATION
LIGAND GATES
Requires binding of a chemical e.g. a transmitter
VOLTAGE GATES
Requires a voltage change across the membrane
MECHANICAL GATES
Requires stretching or some displacement
Ligand Gated channels = DIRECT 3
- Channel opens in response to binding of the ligand to receptor.
- The ligand is called an AGONIST
- May be neurotransmitters or hormones, usually bind on extracellular side of channel.
Antagonist
- can inhibit binding of endogenous ligand.
-LIGAND GATED CHANNELS
Types of Ligands Gated Channels = Antagonist
- Curare – blocks nicotinic ACh receptor.
- Lidocaine - local anaesthetic, binds to domain IV of Na+ channel. Inhibits AP generation.
- Tetrodotoxin (TTX; found in puffer fish) binds to voltage gated Na+ channels preventing AP generation. Fatal
Ligand Gated Channels - INDIRECT -2
- Channel opens in response to a second messenger signal activated by a neurotransmitter.
- Second messenger (eg G-PROTEIN) acts on the intracellular side of the channel – couples the receptor to the ion channel
Voltage Gated Channels
- Change in membrane voltage can open and close some channels.
- Detects changes in the electrical potential across the membrane.
outside (+), Inside (-) —–>. Change (transmembrane potential)—> outside (-), Inside (+)
Inactivation Gate:
Opening vs Inactivation
- Opening
- Conformational change occurs in one region of the channel - Inactivation
- Blocking particle swings into and out of channel mouth
Multiple Confirmation States
3 states of Na+ channels
- Resting - closed & activatable
- Activated - open
- Inactivated - closed and non-activatable
what happens during resting stage in voltage gate
activation gate closed
inactivation gate open
what happens during ACTIVATED stage in voltage gate
Activation gate open
Inactivation gate open
what happens during INACTIVATED stage in voltage gate
Activation gate open
Inactivation gate closed
Mechanically Gated Channels = 3
- Activated by stretch or pressure.
- The energy for gating may come from mechanical forces due to channel-cytoskeleton interactions
- CLOSED —STRETCH –> OPEN
Mechanically Gated Channels: Example: inner ear hair cells = 5
- Tip links STRETCH and open channels when stereocilia bend TOWARD tallest member
- K+ enters; hair cell DEPOLARISES
- Depolarisation Open voltage-gated Ca+2 channels
- Ca+2 entry causes greater release of neurotransmitter.
- More neurotransmitter leads to higher rate of action potential.
Nicotinic Receptors in Muscle: 6
- ➢Nicotinic receptors on skeletal muscle
- ➢Ligand gated- opened by ACh from nerve
- ➢5 subunits for the channel pore
- ➢2 isoforms of subunit N1 and N2
- ➢Selective for cations
- ➢Opening depolarises muscle cell
Voltage Gated Sodium Channels = 6
- ➢Nav channel found in nerves
- ➢Voltage gated- opened by depolarisation
- ➢Single alpha subunits forms the pore
- ➢Complete channel has an accessory b subunit
- ➢9 alpha subunits (Nav 1.1- 1.9) and 3 b subunits (b1-3)
- ➢Opening depolarises nerve causes action potential
Summary = 8
- ➢Ion channel properties are set by their protein structure
- ➢Diversity of channels is very high (dozens of types of voltage gated sodium channels for example)
- ➢Ion channels can be very selective
- ➢Opening probability (gating) can be controlled by voltage, ligand binding or mechanical stretch (also covalent modification by enzymes)
- ➢Many voltage gated channels open for only a short time before closing of an inactivation gate.
- ➢Ligand gating allows detection of neurotransmitters
- ➢Mechanical gating allows detection of stretch e.g. baroreceptors
- ➢Voltage detection produces action potentials
