Chem*3560
Lecture 31: Ion selective channels
Ion selective channels allow specific ions to pass through a membrane bilayer at a very high rate.
They serve a very different purpose than simple transporters or pumps, so some definition of terms is
appropriate (Lehninger p.424-5).
A transporter (uniporter, symporter or antiporter) is a protein
with a defined binding site for substrate, exposed to one side
of the bilayer at any given time. Transport occurs when
conformational change of the transporter exposes the binding site
to the opposite side of the bilayer and allow release. Rate of
transport is dependent on the binding and change process, much
like enzyme catalysis, and the transporter obeys
Michaelis-Menten kinetics. Transport continues until the
substrate concentrations reach equilibrium with the
electrochemical gradient.
An Ion Pump is a protein that uses an energy yielding
reaction such as ATP hydrolysis to force ions across the
bilayer against the ion's electrochemical gradient. Transport
involves distinct substrate binding sites. However the ATP
hydrolysis biases the orientation of the binding sites so
that transport may proceed in a direction opposite to the
electrochemical gradient of the transport substrate.
e.g. Fo F1 ATPase , Na+/K + ATPase.
An ion selective channel or ion conductive
channel is more like a hole through the
membrane than an enzyme, and can pass many
ions through simultaneously. The ions do not bind
to specific substrate binding sites, and the channel
is not saturable, so that rate of passage is a linear
function of concentration. No change in protein
conformation is necessary for the passage
through the bilayer. Direction of movement is
the direction of the electrochemical gradient.
Unlike a simple hole, ion selective channels have:
1)
a selectivity filter that only allows matched ions to pass, e.g. so that Na+ and K+ can
be distinguished.
2)
a gate that opens and closes, either in response to a specific signalling molecule (ligand
gated channels) to the voltage levels of the membrane potential (voltage gated channels).
The selectivity filter provides ligands to match a specific ionic radius .
The conductivity channel is generally
large enough to accomodate hydrated
ions, and may be lined with oppositely
charged
side chains to favour
cations or anions as required. The
selectivity filter is a constriction point
where the hydration layer must be
stripped off. The channel provides
ligands to substitute for the H2 O
coordination shell, and these ligands
are spaced to match a specific ionic
radius. If the ion is too large, it clearly won't fit, e.g. K+ (radius 1.33 Å) in a Na+ (radius 0.95 Å)
specific channel. However, the smaller Na+ can also be excluded by a K+ selective channel, because it
fails to line up with all ligands at an appropriate distance. The energy of interaction with mismatched
ligands is too low to compensate for the energy needed for dehydration.
These ligands do not constitute a conventional substrate binding site, because the selected ion passes
through very rapidly. Ligand binding energy is an optimum match for hydration energy, so
dehydration/ligand binding and rehydration/ligand release represent almost zero energy change. The
matched ion therefore does not have to stay in and occupy the site. Mismatched ions do not pass
through because the energy of ligand binding is too low to allow dehydration.
Ion-selective channels allow for rapid perturbations of membrane potential
Membrane potential in the plasma membrane of animal cells is established by the asymmetrical
distribution of Na+ and K+ which is created and maintained by the
Na+/K + ATPase ion pump. Brief (milliseconds) opening and
closing of ion selective channels disrupts the membrane potential,
to create a voltage spike or action potential which is the basis
of transmission of nerve impulses. Gating is therefore
important to ensure opening occurs only on the required signal.
Rapid closure of the gate ensures that there is not too much
dissipation of the overall Na+ and K+ gradients (which would cost
ATP to regenerate).
Ligand gated channels
These channels open and close in response
to binding a specific ligand, e.g. the
neurotransmitter acetylcholine.
The acetylcholine receptor channel consists
of five subunits, each formed as a bundle of
four transmembrane helices.
In the resting state, the channel is closed off
by bulky leucine sidechains, one from each
subunit.
When acetylcholine binds, one helix in each
bundle twists so that the leucines rotate out
of the channel and their places are taken by
small polar amino acids that allow ions to pass (Lehninger p.426-427).
Voltage gated channels
Voltage gated channels open when the membrane
potential reaches certain values, and close again very
rapidly. A typical structure is seen in Lehninger, p.428.
The channel is made up of four bundles of 6 helices each.
One helix in each bundle (sensor helix) is highly positively
charged (not in contact with bilayer.) When the membrane
potential is –60 mV (inwards), the sensor helix is driven
downwards by electrostatic forces, towards the
negative side of the membrane, so the channel is closed.
When potential rises from –60 mV to zero, the helix can
move back up, and the channel opens to allow Na+ to
pass.
Inactivation occurs because of a small domain that is
loosely tethered to the cytoplasmic side. When the channel
opens, it exposes a binding site for the inactivation domain,
so after a few milliseconds in the open state, the
inactivation gate binds to block the channel, and the
channel ceases to conduct ions.