Membrane Protein Channels

Transcription

1 Membrane Protein Channels Potassium ions queuing up in the potassium channel Pumps: 1000 s -1 Channels: s -1

2 Pumps & Channels The lipid bilayer of biological membranes is intrinsically impermeable to ions and polar molecules. Permeability is conferred by two classes of membrane proteins, pumps and channels. Pumps use an energy source (ATP or light) to drive the thermodynamically uphill transport of ions or molecules. Channels, in contrast, enable downhill or passive transport (facilitated diffusion).

3 Pumps and channels Channel mediated Carrier mediated

4 Facilitated diffusion

5 Propagation of nerve impulses We shall examine three channels important in the propagation of nerve impulses: 1. ligand gated channel (for which the acetylcholine receptor is the model and which communicates the nerve impulse between certain neurons) 2. voltage gated Na + and K + channels, which conduct the nerve impulse down the axon of a neuron.

6 Ligand gated channels

7 Nerve communication across synapses Ligand gated channel Acetylcholine is a cholinergic neurotransmitter 50 nm synaptic cleft Synaptic vesicles have acetylcholine molecules

8 Nerve communication across synapses Synchronous export of 300 vesicles in response to a nerve impulse. Acetylcholine concentration increases from 10nM to 0.5mM in a ms. The binding of acetylcholine to the postsynaptic membrane changes its ionic permeabilities. The conductance of Na + and K + increases in 0.1 ms, leading to a large inward current of Na + and a smaller outward flux of K +.

9 Nerve communication across synapses The inward Na + current depolarises the plasma membrane and triggers an action potential. Acetylcholine opens a single type of cation channel, which is almost equally permeable to Na + and K +. This change in permeability is mediated by the acetylcholine receptor. Ligand-gated

10 Acetylcholine receptor 2a, b, g, d Pseudo five fold symmetry

11 Acetyl choline receptor - a ligand gated ion channel

12 Importance of amino acids lining the pore

13 Importance of amino acids lining the pore Large Non-polar Closed Open Small, polar

14 (M2) (M2) Flexible loops

15 Voltage gated channels

16 Voltage gated channels (K + & Na + channels) The nerve impulse is an electrical signal produced by the flow of ions across the plasma membrane of a neuron. The interior of the neuron has a high concentration of K + and a low concentration of Na +. These gradients are produced by a Na + -K + ATPase.

17 Action potential signals are send along neurons by transient depolarization & repolarization Depolarization beyond a threshold causes Na + ions to flow in leading to further depolarisation & more Na + influx K + ions flow out restoring the membrane potential Causes 60 to + 30 mv in a ms There must be specific ion channels

18 Potassium & Sodium Channels Structural similarity Hydrophobic except S4 positively charged (Arg, Lys) Protein purified on the basis that it could bind tetrodotoxin (from the puffer fish) binds Na + channels with K i ~ 1nM lethal dose for adult 10ng.

19 Structure of the potassium ion channel (tetramer) S5, S6

20 Potassium ion channel Six-transmembrane-helix voltage-gated (K v ) Channel opening apparatus Channel

21 View of a hypothetical K v -type K + channel

22 Ionised ions in solution have spheres of hydration

23 Path through the K+ channel Inside cell

24 Selectivity filter Carbonyl Thr-Val-Gly-Tyr-Gly (TVGYG)

25 Selectivity of K+ channel Ions with radius > 1.5Å are too big to fit through the channel of 3.0Å diameter Na + is not so well re-solvated by the protein

26 Energetic basis of ion selectivity Favourable interaction with Carbonyl groups

27

28 Model for potassium channel ion transport

29 Voltage gated requires substantial conformational change Model for activation - S1 to S4 form the voltage responsive paddles S4 Increased access

30 A channel can be inactivated within milliseconds of opening

31 The K + channel can be inactivated by occlusion of the pore Ball and chain model Trypsin cleaved channel (cytoplasmic tail) does not inactivate. Neither does a mutant lacking 42 N-terminal residues Adding back the peptide 1-20 restores inactivation

32 Ball and chain model for channel inactivation

33 Na + and K + channels work together to give the action potential Na + in first Then K + out

34

35 Reorientation of helix 6 opens channel T209