Transcript Unit 1 PPT 7 (2ciii-iv Channels and transporters)

AH Biology: Unit 1
Cells and Proteins
Membrane Proteins:
Channel and Transport Proteins
Transport proteins
Transport proteins can be characterised
by the nature of the role they play.
Uniports simply transport a molecule from
one side of the membrane to another
down concentration or electrochemical
gradients
Active transport
• Symports are examples of coupled
transport where the transporting of one
solute results in the coupled transport
of another in the same direction, eg
glucose/Na+ symport.
• Antiports are an example of coupled
transport of two molecules being
transported in opposite directions, eg
Na+/K+ ATPase.
Uniport
Symport
Antiport
Transporter proteins
Transporter proteins
change shape
(conformation) in order to
transport molecules across
membranes.
This may be passive
(simply facilitate diffusion
down a gradient) or active
(transport against the
gradient).
This energy can come from:
• ATP, eg Na+/K+ ATPase
• electrochemical
gradients, eg glucose/Na+
symport
• light-driven pumps, eg
bacterial rhodopsin.
Passive transporters
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•
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The transporter exists
in two reversible states
which cycle from A to B
randomly.
•
Cycling is not
dependant on solute
binding.
•
If the solute
concentration is higher
on one side than the
other molecules are
transported in line with
their gradient.
GLUT 1, 3 and 4 proteins are located on the plasma membranes of cells and are
good examples of passive glucose transporters.
They have high-affinity binding sites for glucose and readily facilitate uptake of
glucose down its concentration gradient.
Na+/K+ ATPase
•
This transporter is mainly responsible for maintaining an Na+/K+ gradient across the
plasma membrane.
Na+/K+ ATPase
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The transporter has binding sites with high affinity for three Na+ ions.
•
When Na+ binds, ATP is reduced, transferring a phosphate to the transporter. The
resulting enforced conformational change opens the transporter to the extracellular
side, pumping the Na+ ions across the membrane as conformational change reduces
affinity for
Two binding sites with affinity for K+ are then exposed to the extracellular side.
•
Na+/K+ ATPase
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Two K+ ions bind from the extracellular
side.
This results in the transporter being
dephosphorylated and the phosphate
being released.
The transporter returns to its initial state.
The K+ is pumped into the cytoplasm.
The affinity for Na+ on the inside of the
membrane is restored
Na+/K+ ATPase
•
As both sodium and potassium are pumped against their concentration gradient this
system is an example of active transport driven by ATP hydrolysis.
•
As three Na+ are expelled for every two K+ this contributes to the membrane potential
of the cell, which has a higher net negative charge intracellularly.
•
Cells contain a large concentration of solutes plus a large number of negatively
charged molecules (eg DNA) which are balanced by other cations within the cell. This
creates a huge osmotic pull, driving water inwards by osmosis through the membrane
and via water channels, eg aquaporins.
•
Animal cells can counteract this osmotic pull to some degree by maintaining high
extracellular concentrations of ions such as Na+ and Cl–. The Na+/K+ ATPase helps
maintain osmolarity/tonicity by pumping out any sodium that leaks in down its high
concentration gradient. Negatively charged chloride is excluded by the membrane
potential.
•
The maintenance of ion gradients by this transporter is responsible for around 25% of
the basal metabolic rate of cells.
Glucose/Na+ symport
Glucose/Na+ symport
There are two classes of binding sites, one for Na+ and the other for glucose. Binding
of either molecule enhances the binding of the other. As this system is driven by the
Na+ gradient generated by the Na+/K+ ATPase it is described as secondary active
transport.
When all binding sites are filled a conformational change in the protein delivers both
molecules across the membrane. Later the sodium is pumped back out of the cell by
the Na+/K+ ATPase. Because the conformational change relies on both sets of sites
being filled or not the switch between states only happens if all sites are full or empty.
This transport protein exists in two states A and B.
Because of the much higher extracellular than intracellular Na+ levels, glucose is more
likely to bind to the molecule in the A state than the B state. More glucose and Na+
enter the cell by A–B transitions than are lost by the reverse. This is an example of
cooperative co-transport.
This net flow results in an accumulation of glucose against its concentration gradient.
The sodium ions flow down their electrochemical gradient while the glucose molecules
are pumped up their concentration gradient.
The Na+/glucose symport is used to actively transport glucose out of the intestine and
also out of the kidney tubules and back into the blood.
Channel proteins
Channel proteins exist as passive
facilitators of molecule movement.
Examples are:
• aquaporins (water channels)
• ligand-gated ion channels
• voltage-gated ion channels.
Aquaporins
Aquaporins are found in many disparate organisms, eg animals, plants and bacteria.
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The basic protein structure consists of tetramers each with a pore at its centre.
Hydrophilic amino acids line one side of the pore while hydrophobic amino acids form
the outside of the molecule embedded in the lipid bilayer.
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Aquaporins are not generally gated channels, but some plant species can exert
control over aquaporins in response to dehydration. This results in closure of the
channels.
•
Each subunit, eg in aquaporin 1, consists of six membrane-spanning domains
primarily consisting of alpha helices. Folding of the protein creates a pore through
which water can pass, although it does not transport ions. This selection is brought
about by producing a pore with hydrophobic amino acids on one side and
hydrophobic opposite. This narrowing of the pore results in the inability of any
hydrated ion to enter as it would be too large.
Aquaporins
Aquaporin Z
(AQPZ) showing
classic tetramer
structure and
alpha-helices in
subunits (shown
from above).
Structure of AQPZ
http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0000072#s3
Aquaporins
Two excellent animations showing water
transfer through aquaporins can be found
on this page.
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http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2003/animations.html
Aquaporins
Some roles of aquaporins in humans:
• Aquaporins 1, 2 and 3 are expressed in the kidney tissue, where
they are involved in water reabsorbtion from urine.
• AQP1 is involved with concentration of urine and is expressed in
the proximal kidney tubule cells.
• AQP2 is synthesised but not inserted into the apical plasma
membrane of collecting duct cells unless ADH stimulates them.
Water is then taken up from the lumen of the collecting duct.
• AQP3 is expressed on the basal membrane of collecting duct cells,
allowing water transport into the blood when water floods in through
the insertion of AQP2 into the apical membrane.
Aquaporins
Disease
• Mutations in AQP2 in humans which result in failed insertion of
channels on the apical membrane of collecting duct cells give rise to
insensitivity to ADH and produce a form of nephrogenic diabetes
insipidis.
• This disease is characterised by chronic urination, dehydration and
resultant K+ depletion.
Ligand-gated ion channels
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Ligand-gated channels are opened by a signal molecule binding to the
protein.
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The resultant conformational change triggers the opening of the
channel.
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Good examples of ligand-gated channels can be found at
neuromuscular junctions and at the synaptic junctions between nerve
cells.
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This enables a solution to the problem of transmitting an electrically
based nerve action potential across the gap between nerves or
neuromuscular junctions.
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The nerve action potential is transduced to a chemical signal that can
cross the synapse gap, giving rise to an action potential in the adjoining
nerve cell or triggering myofibril contraction in a muscle cell.
Ligand-gated ion channels
Voltage-gated ion channels
• Change in polarisation of a membrane
results in the opening or closure of
voltage-gated ion channels.
• Good examples are the Na+ and K+
voltage channels associated with
delivering a nerve impulse along a
nerve axon.
• The next few slides show a
diagrammatic representation of the
propagation of this depolarisation as a
result of the action of ligand- and
voltage-gated channels.
Propagation of a nerve impulse
Propagation of a nerve impulse
1
1.
Na+ channels open and Na+ flows into
the axon, causing depolarisation.
2.
Na+ channels close when the action
potential is reached. K+ channels
open, allowing efflux of K+ to
repolarise membrane.
3.
Axon becomes hyperpolarised. K+
channels close and the Na+/K+
ATPase pumps potassium back into
the axon and sodium out during the
refractory period. Sodium and
potassium also leak down
electrochemical gradients, aiding a
return to the resting state.
2
3
Graph by en:User:Chris 73, updated by en:User:Diberri, [GFDL (www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
Propagation of a nerve impulse
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A nerve axon is said to have a resting potential when it is not triggered. This is generated by the
action of systems such as Na+/K+ ATPase. This results in a net negative charge on the inside and
a net positive charge on the outside.
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The influx of Na+ into a nerve cell down its electrochemical gradient, as a result of the triggering of
its associated ligand-gated channel, results in a depolarisation of that local region of the plasma
membrane.
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If sufficient Na+ enters the cell the resultant depolarisation triggers the opening of voltage-gated
Na+ channels, increasing the depolarisation further and generating an action potential.
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The depolarisation spreads, triggering waves of opening and inactivation of the voltage-gated Na+
channels and propagating the action potential. These channels, once opened, eventually enter an
inactive state, preventing dissipation of the gradient or further triggering by neurotransmitter
ligands.
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K+ voltage-gated channels also react to the depolarisation but in a delayed fashion.
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They are triggered to release K+ back across the membrane to balance the Na+ influx.
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This helps to repolarise the membrane, triggering the resetting of the voltage-gated Na+ channels
to the closed state, which is receptive to neurotransmitters. On repolarisation the K+ channels also
close.
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The distribution of Na+ and K+ is re-established by Na+/K+ ATPase and by leakage down
electrochemical gradients. The action potential is therefore spread unidirectionally along the axon.
Propagation of a nerve impulse
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A good animation of the role of voltage-gated sodium channels in propagating the
action potential along an axon can be found on this page.
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http://www.youtube.com/watch?v=ifD1YG07fB8