Solute Transport Across Membranes

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Solute Transport Across Membranes
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Learning Objectives Describe the movement of
electrically neutral and electrically charged
solutes across a permeable membrane. Describe the
function of transport proteins and explain how
they facilitate the passage of polar or charged
molecules through a lipid bilayer. Describe
active (including secondary active) and passive
transport. Describe the three general classes of
transport systems uniport, and the two
cotransport systems, symport and
antiport. Describe the two types of active
transport. Describe the kinetics of glucose
transport by GLUT1. Describe the
chloride-bicarbonate exchanger of the erythrocyte
membrane.
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Learning Objectives Describe the reaction
catalyzed by carbonic anhydrase. Define
electroneutral. Describe the NaK
ATPase. Describe the difference between GLUT1 (or
GLUT2) and the Na glucose symporter of the
intestine.
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Every living cell must acquire from its
surroundings the raw materials for biosynthesis
and for energy production. A few nonpolar
compounds can dissolve in the lipid bilayer, and
cross the membrane unassisted. But for polar or
charged compounds or ions a membrane protein is
essential for transmembrane movement.
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Summary of transport types
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Movement of solutes across a permeable membrane
Net movement of electrically neutral solutes is
toward the side of lower solute concentration
until equilibrium is achieved.
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Movement of solutes across a permeable membrane
Net movement of electrically charged solutes is
dictated by a combination of the electrical
potential (Vm) and the chemical concentration
difference across the membrane net ion movement
occurs until the electrochemical potential is
zero.
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Removal of the hydration shell is very
endergonic, and the energy of activation for
diffusion through the bilayer is very high.
A transport protein reduces the DG of activation
for diffusion. The protein forms noncovalent
interactions with the dehydrated solute to
replace the hydrogen bonding with water, and
provides a hydrophilic transmembrane passageway.
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Membrane proteins lower the activation energy for
transport of polar solutes and ions by providing
an alternative path through the lipid bilayer for
specific solutes.
This process is called facilitated diffusion or
passive transport. Passive transport is
energy-independent.
Similar to enzymes, transporters bind their
substrate with stereochemical specificity through
non-covalent weak interactions. The negative
free-energy change associated with these weak
interactions counterbalances the positive
free-energy that accompanies the loss of water of
hydration. This lowers the activation energy for
transmembrane passage.
Active transport is an energy-requiring transport
mechanism.
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Carriers and Channels
There are two broad categories of transporters
carriers and channels Carriers bind their
substrate with high stereospecificity
catalyze transport rates well below the limits of
diffusion are saturable in the same sense
as enzymes
Channels generally allow transmembrane movement
at rates several orders of magnitude
greater than carriers show less
specificity than carriers are usually not
saturable
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Three general classes of transport systems
(These may be either active or passive transport
systems)
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Two types of active transport
(often Na)
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Proposed structure of GLUT1
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A helical wheel diagram showing the distribution
of polar and non-polar residues on the surface of
a helical segment.
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Side-by-side association of four amphipathic
helices
Amphipathic containing both polar and nonpolar
domains
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Kinetics of glucose transport into erythrocytes
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Kinetics of glucose transport into erythrocytes
The kinetics of facilitated diffusion is
analogous to the kinetics of an enzyme catalyzed
reaction.
Kt is analogous to Km
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Model of glucose transport into erythrocytes by
GLUT1
The transporter exists in two conformations T1
and T2. Glucose in blood plasma binds to a
stereospecific site on T1. This lowers the
activation energy for a conformational change
from T1 to T2 effecting the transmembrane passage
of glucose.
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The Chloride-Bicarbonate Exchanger
Also called the anion exchange (AE) protein
increases the permeability of the erythrocyte
membrane to HCO3- more than a million fold. Waste
CO2 released from tissues into the blood enters
the erythrocyte where it is converted to
bicarbonate by carbonic anhydrase. The
bicarbonate reenters the blood for transport to
the lungs. Because bicarbonate is much more
soluble in blood plasma than CO2, this function
of the red blood cell increases the capacity of
the blood to carry carbon dioxide to the lungs.
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Chloride-bicarbonate exchanger of the erythrocyte
membrane
This is a cotransport system both bicarbonate
and chloride ion are involved. There is no net
transfer of charge the exchange is
electroneutral.
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Active Transport
In passive transport, the transported species
always moves down its electrochemical gradient,
and is not accumulated above its equilibrium
concentration. Active transport results in the
accumulation of solute above its equilibrium
level, is thermodynamically unfavorable, and
takes place only when coupled to an exergonic
process.
Primary active transport solute accumulation is
coupled directly to an exergonic chemical
reaction such as the conversion of ATP to ADP
Pi. Secondary active transport occurs when the
uphill (endergonic) transfer of a solute is
coupled to the downhill (exergonic) flow of
another solute that was originally pumped uphill
by primary active transport.
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P-Type ATPases
The family of active transporters called P-type
ATPases are ATP-driven cation exchangers that are
reversibly phosphorylated by ATP as part of the
transport cycle. Phosphorylation causes a
conformational change that is essential to moving
the cation across the membrane. All P-type
transporters have similar amino acid sequences,
especially near the aspartic residue that
undergoes phosphorylation.
All P-type transporters are inhibited by the
phosphate analogue vanadate.
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NaK ATPase
In virtually all animal cells, the concentration
of Na is lower inside the cell than in the
surrounding media and the concentration of K is
higher inside the cell than in the surrounding
media.
These concentration gradients are maintained by a
primary active transporter called the NaK
ATPase.
For each molecule of ATP converted to ADP Pi,
the transporter moves two K inward and three Na
outward. The NaK ATPase is an integral
membrane protein with two subunits (MW 50,000
and 110,000), both of which span the bilayer.
The crystal structure of this transporter has not
yet been determined.
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NaK ATPase
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NaK ATPase
This is a P-type ATPase.
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In animal cells, the NaK ATPase is primarily
responsible for setting and maintaining the
intracellular concentrations of Na and K and
for generating the transmembrane potential. The
electrical potential is central to signaling
between neurons, and the gradient of Na is used
to drive the uphill cotransport of solutes in
many cell types. About 25 of the total energy
consumption of a human at rest is invested in the
NaK ATPase.
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P-Type Ca2 Pumps
The cytosolic concentration of free Ca2 is
usually at 100nM or less. This is far lower
than the Ca2 level in blood plasma. The
concentration of inorganic phosphates (Pi and
PPi) inside the cell are in the mM range.
Inorganic phosphates combine with calcium to form
relatively insoluble calcium phosphates. Ca2 is
pumped out of the cell by a P-type ATPase, the
plasma membrane Ca2 pump. Another P-type Ca2
pump in the endoplasmic reticulum moves Ca2 into
the lumen of the ER. Muscle cells have a
specialized form of the endoplasmic reticulum
called the sarcoplasmic reticulum where Ca2 is
sequestered.
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Structure of the Ca2 pump of the sarcoplasmic
reticulum
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Lactose uptake in E. coli
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Structure of the lactose transporter (lactose
permease) from E. coli
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Glucose transport in intestinal epithelial cells
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Na glucose symporter
The intestinal glucose transporter takes up
glucose from the intestine in a process driven by
the downhill flow of Na. The energy required for
this process comes from two sources the greater
concentration of Na outside than inside, and the
transmembrane potential which is negative inside
and tends to draw the Na inward.
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K channel from Streptomyces lividans
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K binding sites in the selectivity pore or the
K channel
The carbonyl oxygens of the protein backbone are
positioned to fit the K but not the smaller
Na. The mutual repulsion of the K results in
only two of the sites occupied at a time both
green or both blue.