2'Membrane Transport

1
2. Membrane Transport
Adapted from http//en.wikipedia.org/wiki/Cell_b
iology
2
OUTLINE

• 2.1 Diffusion and passive transport
• 2.2 Energy considerations and patch clamp.
• 2.3 Channels and Ionophores.
• 2.4 Active transporters.
• 2.5 Co-transport systems and their couplings.
• 2.6 Endocytosis and exocytosis.
• 2.7 Revision exercise.

3
2.1 Diffusion and Passive transport (facilitated
diffusion)

• Different transport mechanisms are needed to
  accumulate nutrients and get rid of toxicants.
• Small, uncharged molecules can pass through a
  lipid bilayer by simple diffusion.
• Ficks Law of diffusion the net rate of
  diffusion is proportional to 1 the surface area
  of the membrane (A), 2 the concentration
  gradient across the membrane dc/dx, and 3 the
  diffusion coefficient (D).
• J - DA dc/dx, where J is the net rate of
  diffusion.

4
2.1.1 Ficks Law and permeability co efficiency
of molecules

• Diffusion efficiency is inversely proportional to
  molecular mass.
• Lipid solubility is also very important factor
  for membrane transport.
• The permeability of the plasma membrane to a
  particular molecule increases with its partition
  coefficient (ß) as measured by its ability to
  partition in olive oil versus water.
• E.g. diethylurea has 50-fold greater partition
  coefficient than urea can diffuse through the
  membrane 50 times faster than urea.

5
2.1.2 Permeability co efficiency of molecules

• dc/dx ßCo-Ci/ dx
• Co is concentration at outer face, Ci is
  concentration at inner face.
• Thus J -DA dc/dx
• -DA ßCo-Ci/ dx
• -Dß/dx A (Co-Ci)
• Hence J - PA(Co-Ci), where P is permeability
  coefficient Dß/dx

6
Simple diffusion

• Direction of diffusion is always from a higher to
  a lower concentration (down the gradient)
• As the concentration of the solute on one side is
  increased, there will be an increasing initial
  rate of diffusion until at equilibrium
  (concentrations at both side the same across the
  membrane).
• After that, there will be a continued exchange of
  solute.

down the gradient
equilibrium
exchange of solute
7
2.1.3 Facilitated diffusion

• Some molecules can go through the membrane faster
  than that as estimated by Ficks Law.
• E.g. The uptake of glucose into erythrocyte down
  the concentration gradient with the help of a
  membrane protein called permease.
• The velocity of this mode of transport is
  saturable.
• Permease is a family of multi-pass trans-membrane
  proteins to facilitate the diffusion of specific
  molecules across biological membrane.

8
Facilitated Diffusion with molecule specific
permease (passive transport) to facilitate the
transport down the gradient (not going against)
Rate of Glucose Uptake
Saturable
Transport velocity
Vmax
Facilitated diffusion
Half Vmax
Simple diffusion
Glucose transporter (permease)
P
Km
External concentration of glucose, mM
ADP
ATP
Transporters must have a specific binding site
for the solute, e.g. glucose. Once they get in
the cells, they are usually phosphorylated to go
into metabolic pathway and hence the outer
concentration of glucose is still higher than the
inside.
9
Water is a small molecule but membrane is
hydrophobic, how water moves across the plasma
membrane??
Structure of Aquaporin1, adapted from Ren et al.,
2001. PNAS 98(4)1398-1403.
Recently, water channels (aquaporins) are found
(2003 Nobel Price, Peter Agre and Rod Mackinnon)
for transport of water molecules through membrane.

• Overview of transport mechanisms

10
Passive VS active transports

• Permease and channels are passive transporters,
  transport of molecules follow the rules of
  diffusion.
• Active transports use ATP energy (require coupled
  input of energy) to maintain asymmetrical nature
  of the membrane at its two sides inside and
  outside. Membrane potential, e.g. chemical
  gradient and electric potentials, is created.
• They are both substrate specific, and saturable
  (saturation kinetics).
• They can be inhibited too by chemical inhibitors
  which bind to any sites on the transporters.

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2.2 Energy Considerations and patch clamp

• Direction of diffusion is always from a higher to
  a lower concentration (down the gradient).
• Passive transport velocity vary linearly with
  their concentration differences according to
  Ficks Law.
• Diffusion occurs spontaneously according to
  thermodynamic laws too.
• Real life is never this ideal, we need energy to
  move molecules against the concentration gradient
  (to concentrate solutes). In fact, life cannot
  exist without active transport.

12
Maintaining a typical cellular ion concentrations
by active transport (mammalian cells with enzymes
are in favor of low Na, high K and low Cl
concentrations).
13
Different transport strategies
Diffusion
Facilitated Diffusion, uniport
Channel, e.g. ion and water
Co-transport symport or antiport
ATP
Active transporter
ADP Pi
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2.2.1 Energy (Gibbs free energy, ?G)
considerations of passive and active transports

• ?GA G A in G A out
• ?GA RT ln A in/ A out
• ?G RT lg e Ci/ Co
• ?G 2.303 RT lg Ci/ Co,
• Ci is inside concentration, Co is outside
  concentration
• ?G negative, there is a release of free energy
  and the movement of solute will occur
  spontaneously.

15

• For transport of uncharged molecule from Co
  0.001 mM to Ci 0.1 mM,
• ?G 2.3 RT log (0.1/0.001)
• ?G 2.7 kcal/mol at 25 ºC.
• So if ATP energy (- 7.3 kcal/mol) is used, it
  can go across and against the gradient from 0.001
  mM to 0.1 mM.
• Active transport is essential to all living
  processes and maintain cellular homeostasis.

16
2.2.2 Concepts of active transport

• Active transport requires ATP hydrolysis directly
  or indirectly (secondary active transport) to
  move molecules across the membrane UP the
  concentration gradient.
• Active transporters are membrane proteins
  specifically bind and move the molecules across
  the membrane to a unique direction using ATP
  hydrolysis as an energy source. These are primary
  active transporters mainly for ions and
  chemicals.

17
Energy sources light, ATP, electrochemical
gradient (secondary active transport).
Light, hv
Electro-chemical gradient
e -
ATP
ADP Pi
Coupled Transporter (co-transporters)
Light-driven pump
ATP-driven pump (Active transporter)
Major function of active transporters moving
molecules against the electrochemical gradient.
18
Primary and secondary active transporters work
coordinately in animal cells
Na -driven symports For different solutes
solute
K
Na
Na
Na -K ATPase
ATP
ADP Pi
solute
Na
K 4 mM Na 145 mM
K
Na
lysosome
K 140 mM Na 12 mM
H
H ATPase
----

ADP Pi
ATP
H
They generate membrane potential, generate proton
gradient, maintain acidity, etc..
19
Secondary Active transport

• Secondary active transport involves the
  co-transport of a molecule with sodium as it
  moves down its electrochemical gradient.
• Na-K-ATPase creates Na gradient across the
  membrane to facilitate glucose-Na co-transporter
  to move glucose into the cell, this process is
  also known as secondary active transport.

20
Glucose uptake from lumen to the capillaries
using glucose-sodium symport and Na-K-ATPase.
Brush border cell
Glucose gets into the cell and simultaneously
transported by glucose transporter into the
capillaries.
Intestinal lumen
Capillaries
Glucose transporter permease
Glucose
Glucose
Glucose
Glucose-Na symport is driven by intracellular
Na levels using Na K -ATPase.
ATP
Na
Na
Na
K
Na-Glucose Symport
K
ADP Pi
Na-K-ATPase
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Summary ofDiffusion and Active Transport

• Hydrophobic molecules go across the membrane by
  diffusion.
• Other molecules need different strategies to go
  across the membrane by simple diffusion,
  facilitated diffusion, channel, and active
  transporter.
• Facilitated diffusion uses protein carriers
  specific for the molecule, e.g. sugar permeases.
• Channels are specific for a particular ions or
  water. They are passive transporters that the
  molecules go easily from high to low
  concentration.
• Active transporters use ATP energy to move
  molecules UP (against) the concentration gradient.

22
2.2.3 Electrochemical potentials

• Charged chemicals contribute electrical potential
  difference to membrane transport.
• For ions to go across the membrane, permeability
  is dependent on concentration gradient (?Gc) and
  electrical potential (?Gm). Hence, ?G ?Gc
  ?Gm.
• ?G 2.3 RT Log (C2/C1) Z F?V, Z is the charge
  of the molecule and F is Faraday constant.

23
2.2.4 Membrane potentials

• Equilibrium potential is the voltage necessary to
  achieve equilibrium between two sides of the
  membrane.
• Using Nernst equation Ek RT/ZF In i out/ i
  in,
• Ek is the equilibrium potential of an ion.
• For any ion with known concentrations, the Ek can
  be calculated.
• Membrane potential Vm Vi Vo.
• Vo is arbitrarily set as zero, Vm in the
  undisturbed resting cell is negatives.
• Resting potential is when Vr Ek.

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2.2.5 Polarization and depolarization
As in the cytoplasm, more negative charges
accumulate also active transport of
Na-K-ATPase removes 3 Na and put in 2 K,
keeping more K inside of the cells to maintain
the negative membrane potential.
Action potential in axon
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2.2.6 Patch-clamp Recording.

• An electrophysiological method to detect any ion
  channels in different cell types measuring
  aggregate current with an intracellular
  microelectrode in micropipette.
• When open, the channel has large conductance.
  Clamp means to use an electrode to maintain the
  membrane potential at a set value to record the
  ionic strength of a particular channel.
• Currents can be recorded when the micropipette
  is either attached or detached to the membrane.
• When detached, the membrane patch is removed and
  thus membrane potential can be measured in an all
  or nothing fashion.

26
The technique of patch-clamp recording enable us
to measure the ionic current and electrical
properties (membrane potential) of cell
membranes. Experiments can also be done by
adding specific ion channel blocks and chemicals
drugs could be tested too.
Adapted from Alberts et al., 1998. Essential Cell
Biology. An Introduction to Molecular Biology of
the Cell. Garland Pub.Inc.
27
2.3 Channels and Ionophores

• To facilitate transport of ions, channels and
  transporters can be used.
• Transporters are protein carriers of ions
  channels contain a hole (tunnel?) to let ions go
  through the plasma membrane selectively.
• There are some antibiotics secreted from
  microorganisms called Ionophores. There are two
  main types, and we learn from them the mechanisms
  of ions transport.

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2.3.1. Major Ion Channel Families

• Transmitter-gated ion channels
• Acetylcholine-gated cation channels (excitatory)
• Serotonin-gated cation channels (excitatory)
• Glutamate-gated cation channels (excitatory)
• GABA-gated chloride channels (inhibitory)
• Glycine-gated chloride channels (inhibitory)

• Voltage-gated cation channels
• sodium channel
• potassium channel
• calcium channel

29
The gate is specific for certain ions according
to their hydration energy to go through the
channel to be opened by activation of chemical
messenger such as acetylcholine.
Acetylcholine-gated cation channel (excitatory)
consists of five polypeptides each has a protein
with four trans-membrane domains.
g
d
acetylcholine binding site
a
a
channel
b
pore
lipid bilayer
4 nm
CYTOSOL
gate
Transmitter gated channel
30
Schematic diagrams of theasubunits of sodium
channel and potassium channel
COOH
NH2
NH2
COOH
Sodium channel consists of FOUR transmembrane
domain, each has SIX transmembrane ahelices, the
forth helice is believed to be the voltage sensor.
Potassium channel has ONE molecule of only SIX
transmembrane a helices
Voltage-gated channels
31
Proposed model of sodium channel (voltage gated)
with FOUR repeating units of transmembrane
domains each containing SIX helices all in one
protein molecule (a single polypeptide)
Na
voltage sensor
32
You could be intoxicated with sodium channel
blocking toxins !

• Tetrodotoxin in the liver and gonads of puffer
  fish.
• Saxitoxin from algae bio-accumulation from algae
  to bivalves such as oyster and mussels.
• Red-tide (algal bloom) could be dangerous. They
  have sodium channel blockers that are accumulated
  in mussels or oysters and consumed by human,
  paralytic shell-fish poisoning resulted.
• These sodium channel blockers caused suffocation
  because the nervous system controlling
  respiration is blocked. At low doses, paralytic
  effects observed in patients intoxicated with
  these toxins.

33
Sodium channel blockers saxitoxin from algae in
clams and tetradotoxin from puffer fish. Consumed
in high dose may lead to suffocation.
Adapted from Berg et al., 2002. Biochemistry.
Freeman Co. (Stryer)
Tetrodotoxins, TTXs, are found in gonads and
liver of puffer fish NOT in its spines. Puffer
fish sushi is regarded as a Japanese culinary
delicacy.
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2.3.2 Ionophores ( ion bearers)

• These are bacterial antibiotics kill microbial
  cells by disrupting transport processes or
  disturbing the ion gradients causing ion
  imbalance of the cells.
• They are useful experimental tools and weve
  learned a lot from ion transport mechanisms from
  them
• They are peptide ion channel on membranes or they
  create hydrophobic conditions and work like a
  donut, working as ionophore.

35
Schematic diagrams of ionophores
e.g. Gramicidin A
e.g. Valinomycin
Which mechanism moves molecule faster? Which is
affected by temperature?
36
Gramicidine A with a trans-membrane channel with
2 of 15-amino acid ßhelical structures getting
through the membrane to permit the passage of H
and alkali cations. It is from Bacillus brevis
and it can be blocked by Ca.
Adapted from Voet et al., 1999. Fundamentals of
Biochemistry. Wiley Sons.
37
Valinomycin is a peptide ring moving potassium
ions across the membrane.
Valinomycin is a peptide ionophore that binds K.
It is a cyclic depsipeptide with 3 repeating
units of L-Val-D-hydroxyisovalerate-D-Val-
L-Lactate.
Adapted from Voet et al., 1999. Fundamentals of
Biochemistry. Wiley Sons.
Adapted from Nelson and Cox, 2000. Lehninger
Principles of Biochemistry. Worth Pub.Co.
38
2.4 Active transporters

• P-type Na-K-ATPase, Ca2 and H pump, P means
  they have phosphorylation and they all sensitive
  to vanadate inhibition.
• V-type inner membrane ATPase to regulate H and
  adjust proton gradients, v means vacuole type for
  acidification of lysosomes, endosomes, golgi, and
  secretory vesicles.
• F-type ATP synthase to generate ATP energy from
  moving the proton across F means energy coupling
  factor. There are F1 and F0 subcomplexes F1
  generates ATP, F0 lets H go through the
  membrane.
• ABC transporters ATP-binding cassette protein
  for active transport of metal ions and chemicals.

39
Three types of ATPases P-, V- and F type.
1997 Nobel prize of Physiology and Medicine went
to Paul Boyer and John Walker for their work on
ATP synthase (F type), and Jens Skou for his work
on Na-K-ATPase, which uses one-third of the ATP
made by ATP synthase.
V-type ATPase
F-type ATPase
F1 complex
P-type ATPase
V1 complex
a
a
ß
ß
F0 complex
V0 complex
ATP synthase
40
2.4.1 Structures of P-type ATPases
Structure of Na -K -ATPase 2a and 2ß
subunits join together on cell membrane
ß
ß
Oligosaccharide chains
a
a
The asubunit has 12 transmembrane domains (TMDs)
COOH
Theßsubunit is glycosylated and has one TMD.
COOH
Adapted from Garrett and Grisham, 1995. Molecular
Aspects of Cell Biology. Sauders College Pub.
Phosphorylation site
NH2
NH2
41
Another P-type ATPase Ca2ATPase
Ca2 binding site
Ca2
Aspartate
COOH
ATP binding site
NH2
ATP
ADP
Other divalent ion transporters have similar
structure with this Ca2-ATPase and the asubunit
of Na-K-ATPase.
42
2.4.3 ABC (ATP-Binding Cassette) transporters 12
trans-membrane helices with 2 ATP binding sites,
drug or ligand- binding sites yet to be
identified.
Chloride channel the cystic fibrosis
transmembrane conductance regulator, CFTR, has an
extra R domain
P-glycoprotein
Oligosaccharide chains
NH2
NH2
COOH
COOH
R domain
ATP binding domains
ATP binding domains
43
Multiple drug resistance is related to
P-glycoprotein (P-gp) which is a chemical pump
using ATP energy to actively remove the
hydrophobic drugs out of the cells. The P-gp is a
membrane protein with two nucleotide (ATP)
binding site. After ATP hydrolysis, change in
protein structure facilitate the movement of
hydrophobic drugs.
Removal of cancer drugs by P-gp using ATP energy
N
ATP binding sites
C
Cancer drugs adriamycin, colchicine,
vinblastine, etc.
44
CL -
CFTR and Cystic Fibrosis
Agonist
MembraneReceptor
Genetic defects of CFTR leads to CF (Cystic
fibrosis). CF is the most common genetic diseases
in Caucasians (1/1000). Cell death in the lungs
epithelial due to lack of ion control, leading to
malfunction of cells with mucus obstruction of
gas exchanges and hence lethal to juveniles
having CF.
P
G
AC
ATP
ADP
PKAa
ATP
cAMP
PKAj
CFTR-Cl channel is an ATP- and cAMP dependent Cl
channel on epithelial cell membrane.
Adapted from Sperlakis (ed)., 1998. Cell
Physiology Source Book. Academic Press.
45
Summary of channels, ionophores, and active
transporters

• They move specific ions and chemicals across.
• Charges must be considered. Membrane potentials
  could be affected and the electrophysiological
  responses could be studied by using patch-clamp
  technique.
• Channels and ionophores (antibiotic peptides)
  allow ions to pass specifically according to the
  ions hydration energy.
• There are voltage-gated and chemical (ligand)
  gated channels also there are carrier and
  channel type of ionophores.
• Active transporters are classified by their
  protein structures, and relationships with ATP.
  There are F type, V type, P-type and the ATP
  binding cassettes (ABC) transporters.

46
2.5 Co-transport Systems

• Carbon dioxide metabolism in red blood cells with
  Band 3 protein (HCO3-/Cl --antiport)
• 2. Na-K-ATPase and cardiotonic steroids
  (ouabain).
• 3. Glutathione and amino acid transport
• 4. Anion antiports in parietal cells of stomach
  with HK-ATPase to transport HCl for
  acidification of the stomach lumen.

47
2.5.1 Carbon dioxide metabolism in red blood
cells with Band 3 protein (HCO3-/Cl --antiport)
In pulmonary capillaries low CO2 pressure, high
O2 pressure
CO2
O2
H20
H
Hemoglobin
Carbonic anhydrase
Adapted from Lodish et al., 2000. Molecular Cell
Biology. Freeman Co.
CO2 OH-
HCO3
Band 3 Protein
HCO3
Cl -
Band 3 protein is an antiport of HCO3 and Cl
ions, carbonic anhydrase convert HCO3 to CO2
which goes to the pulmonary capillaries.
48
In the systemic capillaries, CO2 gets into the
erythrocyte, in which carbonic anhydrase converts
CO2 to HCO3. Bicarbonate dissolves in blood
plasma.
In systemic capillaries high CO2 pressure, low
O2 pressure
CO2
O2
Hemoglobin
H
H20
Carbonic anhydrase
CO2 OH-
HCO3
Adapted from Lodish et al., 2000. Molecular Cell
Biology. Freeman Co.
HCO3
Cl -
Band 3 Protein
49
2.5.2 Na-K-ATPase Ca2 restoration in
cardiac muscle cells
Cardiotonic steroids (e.g. digitalis from fox
glove plant) inhibit the E2-P phase
(dephosphorylation) of Na-K - ATPase and lower
the Na gradient across the cardiac muscle cells.
Adapted from Stryer, 1998. Biochemistry 4th ed.
(Berg et al., 2002 5th ed.)
50
Enzymatic cycle of Na-K-ATPase
phosphorylation changes the conformation of the
asubunit to release 3 Na out to the cell
dephosphorylation changes the structure of the
asubunit to accept 2 K into the cell.
ADP
ATP
E1-P phase
E1- phase
E1-P phase
3 Na
Phosphate
E2- E1 phase
2 K
E2-P phase
E2- phase
Cardiotonic steroids ( e.g. digitalis from
fox glove plant and ouabain) inhibit the E2-P
phase (dephosphorylation) of Na-K -ATPase and
lower the Na gradient across the cardiac muscle
cells.
51
Ouabain is a useful drug to congestive heart
failure.
Ca 2
Ca-channel
Sarcoplasmic reticulum
By blocking the Na-K-ATPase, intracellular Na
remains high. As a result, the Na- Ca2 antiport
cannot remove Ca2 ions out from the cardiac
muscle cells. Therefore, ouabain can inhibit
Na-K-ATPase to restore the Ca2 ion levels and
thus maintain the contraction power of cardiac
muscle.
Ca-ATPase
Ca 2
Ca 2
Ca-Na Antiport
Na
Na
Na-K ATPase
Ttoponin C
K
Adapted from Sperlakis (ed)., 1998. Cell
Physiology Source Book. Academic Press.
52
2.5.3 Amino acid transport using ?glutamyl
transpeptidase and glutathione.
Glutathione
Amino acid
?Glutamyl transpeptidase
Cysteinylglycine
?Glutamyl Amino acid
Amino acid
53
2.5.4 Anion antiport in parietal cells of
stomach with H- K -ATPase to produce stomach
acid.
H -K -ATPase is an electroneutral antiport
with properties similar to that of Na -K
-ATPase. K is removed by K channel and
concurrently Cl channel removes Cl- to the same
direction. HCl is the overall transport product
in the stomach lumen.
Cl -channel
Anion antiport
Cl -
Cl -
Cl -
H
H
HCO3
HCO3
H-K-ATPase
ATP
Carbonic anhydrase
K
ADP Pi
H2O
CO2
K
CO2
K
OH -
K channel
Basolateral membrane
Apical membrane
54
2.5.5 Stomach acid and cimetidine

• Cimetidine resembles histamine and it inhibits
  the binding of histamine to its receptor.
• Because H -K-ATPase of the gastric mucosa is
  activated by histamine which stimulates a cell
  surface receptor via cyclic AMP to produce
  stomach acid, Cimetidine can help inhibiting
  stomach acid secretion indirectly.
• However, many ulcers are caused by infection of a
  bacterium Helicobacter pylori which thrives in
  the nutrient-rich gastric mucus.
• Antibiotics are useful drug to eliminate the
  infection for peptic ulcer. Cimetidine can still
  be used for heartburn or stomach reflux.

55
2.6. Endocytosis and exocytosis

• Membranes are dynamic fluid structures and thus
  fusion processes occur frequently.
• Peripheral membrane protein and receptors are
  involved to move the monolayer mechanically
  (coordinated and specifically regulated).
• During endocytosis, the extracellualr side is
  still the non-cytoplasmic side of the membrane.
• In exocytosis, adherence and joining of the
  cytoplasmic side of the monolayer and the
  non-cytoplasmic side become the extracellular
  side of the membrane.

56
2.6.1 Membrane Fusion Processes for cellular
uptake and secretion (or excretion)

• Endocytosis
• Receptor mediated endocytosis (highly specific),
  e.g. low density lipoprotein (LDL) uptake with
  LDL receptor and clathrin.
• Pinosytosis (non-specific) cellular drinking of
  0.1-0.2 µM.
• 3. Phagocytosis cellular ingestion (eating) of
  1-2 µM.

• Exocytosis
• 1. Constitutive pathway ( non specific from Golgi
  apparatus)
• 2. Regulated pathway, e.g. secretion of hormones
  (highly specific)

57
2.6.2 Exocytosis pathways constitutive and
regulated secretions.
Constitutive secretion
Plasma membrane protein
Cytosol
Hormone signal
Signalling Pathway
Secretory vesicles stored with secretory proteins
Golgi Apparatus
Regulated secretion
Adapted from Alberts et al., 1998. Essential Cell
Biology. An Introduction to Molecular Biology of
the Cell. Garland Pub. Inc.
58
2.6.3 Receptor-mediated endocytosis of LDL for
cholesterol metabolism.

• LDL receptor first binds with the B-100 protein
  on LDL.
• Endosome is formed with LDL inside the vesicle
  and after fused with an uncoupling vesicle
  (CURL), LDL is released from its receptor.
• The LDL receptor and clathrin are recycled back
  to the membrane surfaces, LDL degradation
  produces cholesterol, fatty acids and amino acids
  in the secondary lysosome.
• Familial hypercholesterolemia LDL receptor gene
  mutations leading to failure of LDL uptake and
  high blood cholesterol levels.

59
RECEPTOR-MEDIATED ENDOCYTOSIS OF LDL
LDL particle Cholesteryl esters in phospholipid
membrane layer
Uncoated vesicle
Clathrin Triskelions
Endosome LDL separated from receptor
ApoB-100 protein
Coated Pit
Lysosome
LDL receptor
LDL degradation
Reused LDL receptors
60

• Viruses, such as HIV virus or other RNA viruses,
  enter the cells when they recognize surface
  receptor on cell membrane of a specific
  cell-type.
• HIV virus enters T cells by recognizing CD4
  protein with the help of gp41 and gp120. Gp120
  facilitates endocytosis of the viral particles.
• Influenza virus uses its hemagglutination (HA)
  protein to recognize glycophorin A or sialic acid
  on erythrocytes to activate endocytosis for viral
  entry.

HIV Particle
CD4
gp41
T cell
gp120
Fusion cofactor
T cell
T cell
HIV infected T cell
Adapted from Nelson and Cox, 2000. Lehninger
Principles of Biochemistry. Worth Pub.Co.
61
Summary of Co-Transport and Membrane Processes

• Different membrane transport mechanisms work
  together coordinately to maintain membrane
  potential, acidity and absorb nutrients (e.g.
  glucose).
• Drugs might be used to affect specific target
  transporter, and deliver unique effect. E.g.
  Oubain inhibits Na-K-ATPase and affect heart rate
  by raising intracellular calcium indirectly.
• Endocytosis and Exosytosis are tightly controlled
  cellular processes.
• Unwanted viral particles can take advantages on
  binding to receptors to attack us.
• Genetic mutations in receptors could hamper the
  function of the receptor by avoiding it to go to
  the correct location of the cell.

62
Revision Exercises

• Compare and contrast the mechanisms and processes
  of endocytosis and exocytosis.
• Elaborate the enzymatic cycle of Na-K-ATPase
  and explain why ouabain can be used to cure
  congestive heart failure.
• Use LDL uptake in liver as an example to
  elaborate the mechanism of receptor-mediated
  endocytosis.
• Compare structures and functions of channels and
  transporters? Give examples to illustrate your
  answers.
• How ionophores transport ions, give examples to
  illustrate the mechanisms.

63
Self-study exercises (go find information from
websites if necessary)

• 6. How membrane potential is maintained?
• How do chemotherapeutic drugs get into cancer
  cells? What is the molecular basis of multiple
  drug resistance in cancer cells?
• List the main types of membrane ATPases for ion
  transports. Explain the molecular mechanism of
  actions of each type of ATPases for ion
  transports.
• Write a short essay on water channels
  aquaporins. Would special water (e.g. hydrogen or
  pwater) be needed for better absorption of water
  in our body?
• What is Cystic Fibrosis? How is this disease
  related to chloride channel? Why the CFTR
  Chloride channel is classified as ABC
  transporter?

64
Examples of open-Notes Examination Questions

• Professor You would like to study the toxic
  effects of divalent heavy metal ions on membrane
  proteins in fish gills. Which proteins or
  transporters would you suggest Professor You to
  study and why?
• What structural features of 18 carbon fatty acids
  can be related to their melting points. These
  fatty acids and their melting points are stearic
  acid, 69.6 C oleic acid, 13.4 C linoleic acid,
  -5 C, and linolenic acid, -11C.
• How can one differentiate cerebroside and
  sphingomyelin by doing chemical analyses?
• Explain how the asymmetric nature of lipid
  bilayer is maintained.
• The receptor for growth hormone in animal cells
  is an integral membrane protein of a relative
  molecular mass (Mr) of 95,000. This receptor
  contains one trans-membrane domain of
  alpha-helice to hold it up onto the membrane. You
  have identified the full-length amino acid
  sequence of this protein from a fish species, how
  can you predict the location (position) of these
  trans-membrane regions?

65
Useful links for self-learning
Cells II Cellular Organization http//www.emc.mar
icopa.edu/faculty/farabee/biobk/BioBookCELL2.html
The Cell Membrane http//en.wikipedia.org/wiki/C
ell_membrane Membrane Structure (UTMB)
http//cellbio.utmb.edu/cellbio/membrane.htm Lipid
s and membrane structure http//www.rpi.edu/dept/
bcbp/molbiochem/MBWeb/mb1/part2/lipid.htm Transpor
t In and Out of the Cells http//www.emc.maricopa
.edu/faculty/farabee/BIOBK/BioBooktransp.html Memb
rane Transport (Molecular Biochemistry
I) http//www.rpi.edu/dept/bcbp/molbiochem/MBWeb/
mb1/part2/carriers.htm Membrane
transport http//fajerpc.magnet.fsu.edu/Education
/2010/Lectures/12_Membrane_Transport.htm
66
Textbooks

• Alberts et al., 2002. Molecular Biology of the
  Cell. 4th ed., Chapter 10 (Membrane Structure,
  pp. 581-614) and Chapter 11 (Membrane Transport
  Of Small Molecules And The Electrical Properties
  Of Membranes, pp. 615-657).
• Alberts et al., 2004. Essential Cell Biology, 2nd
  ed., Chapter 11 (Membrane Transport, pp. 365-388)
  and Chapter 12 (Membrane Transport, pp.389-425).
• Nelson and Cox, 2005. Lehninger Principles of
  Biochemistry. 4th ed., Chapter 11 (Biological
  Membranes and Transport, pp369-420).
• Devlin, (ed.), 2002. Textbook of Biochemistry
  with Clinical Correlations. 5th ed., Chapter 12
  (Biological Membranes Structure and Membrane
  Transport, pp.493-533).
• Berg et al., 2002. Biochemistry. 5th ed., Chapter
  12 (Lipids and Cell Membranes, pp319-344) and
  Chapter 13 (Membrane Channels and Pumps,
  pp345-369).