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Membrane Protein Channels
Potassium ions queuing
up in the potassium
channel
Pumps: 1000 s-1
Channels: 1000000 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).
Pumps and channels
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 model 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
10000 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.
• Ligand-gated
Acetylcholine receptor
2a, b, g, d
Pseudo five fold
symmetry
Acetyl choline receptor - a
ligand gated ion channel
Importance of amino acids lining
the pore
Importance of amino acids lining
the pore
Closed
Open
(M2)
(M2)
Flexible loops
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
K+ ions flow
out restoring
the
membrane
potential
Causes –60 to +
30 mV in a ms
There must be specific
ion channels
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 Ki ~ 1nM – lethal dose for
adult 10ng.
Structure of the potassium ion
channel (tetramer)
S5, S6
Potassium ion channel
Six-transmembrane-helix voltage-gated
(Kv)
View of a hypothetical Kv-type
K+ channel
Ionised ions in solution have
spheres of hydration
Path through the K+ channel
Inside cell
Selectivity filter
Thr-Val-Gly-Tyr-Gly (TVGYG)
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
Energetic basis of ion selectivity
Favourable interaction with
Carbonyl groups
Model for potassium
channel ion transport
Voltage gated requires substantial
conformational change
Model for activation - S1
to S4 form the voltage
responsive paddles
S4
Increased access
A channel can be inactivated
within milliseconds of opening
The 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 120 restores inactivation
Ball and chain model for channel
inactivation
Na+ and K+ channels work
together to give the action
potential
Na+ in first
Then K+ out
Reorientation of helix 6
opens channel
T209