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

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).

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

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

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+.

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 acetyl-choline receptor.

Acetylcholine receptor  Pseudo five fold symmetry

Acetyl choline receptor - a ligand gated ion channel

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.

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 Causes –60 to + 30 mV in a ms K+ ions flow out restoring the membrane potential There must be specific ion channels

Potassium & Sodium Channels 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. Hydrophobic except S4 – positively charged

Structure of the potassium ion channel

Potassium ion channel

Path through the K+ channel Inside cell

Selectivity filter Thr-Val-Gly-Tyr-Gly

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 solvated by the protein

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

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

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