Biological Membranes and Transport
part II
? Duanqing Pei,Ph.D.
? 裴 端 卿
? Tsinghua University
? Cell phone,136-8301-7108
? peixx003@umn.edu
Biological Membranes and Transport
part II
? Passive transport,with the gradient
– Water transport—aquaporins
– Glucose transporters
– Co-transport of chloride and bicarbonate
? Active transport,against the gradient
– Transport ATPases
– Ion channels
Simple diffusion across permeable membranes
Electrochemical Potentials,chemical gradient/electric potential
Selectively permeable barriers
of living organisms,all
membranes—intracellular and
plasma
Simple diffusion—virtually
impermeable to polar and
charged species (too slow to be
meaningful); permeable for
certain gases- molecular
oxygen,nitrogen,methane and
water (huge concentration)
Passive transport (facilitated
diffusion)—passage of polar
compounds and ions aided by
membrane proteins that lowers
the activation energy Transporter or permeases
Aquaporins,transmembrane channels that allow rapid movement of
water across plasma membrane in various cells—erythrocytes,
proximal renal tubule cells.
Transmembrane topology of an aquaporin,AQP-1
Type III
A tetramer
Flow rate,5x108/sec,better than the best enzyme catalyase (4x107/s)
DG<15kJ/mol,continuous stream,not much interruption.
Glucose transporter of erythrocytes (GluT1),plasma conc=5 mM;
transporter—50,000 more efficient than simple diffusion
Helical Wheel Diagram:
Distribution of polar vs
nonpolar residues—
amphipathic helix (one
side hydrophobic,the
other hydrophilic)
hydrophobic
hydrophilic
Formation of a channel by
side-by-side association
of 5 or 6 amphipathic
helices
Hydrogen bonds
Kinetics,similar to Michaelis-Menten equation
Sout + T Sout, T Sin, T Sin + T
V0=
Kt~Km for affinity of transporter for substrate
k1
k-1
k2
k-2
k3
k-3
Vmax [S]out
Kt + [S]in
Kinetics:
Initial
velocity plot
Double-reciprocal
plot
Model of Glucose Transport into
erythrocytes by GluT1
1) Binding to T1 site
2) Conformational change from T1
to T2
3) Released
4) Return of conformation to T1
Kt for GluT1=1.5mM for D-glucose,
20-30 mM for D-mannose and D-
galactose,>3000mM for L-glucose
(fast,saturable and specific)
GluT2,transport glucose out of hepatocytes to
blood,Kt=66mM (>5mM of blood glucose
level)
GluT4,for muscle,adipocytes,can be stimulated
by insulin
Dynamic regulation of glucose concentration in blood,muscle
and adipose tissues
After each meal,glucose level increases beyond 5 mM,
triggering the production of insulin,which will stimulate the
uptake of glucose into muscles for storage in the form of
glycogen,or adipocyte (triacylglycerols);
Mechanism,insulin stimulate the cell surface expression of
GluT4
Type I diabetes mellitus,juvenile onset diabetes
no insulin due to the lack of beta cells in pancreas
therefore,low rate of glucose uptake into muscle and adipose
tissue—high glucose level >>5mM
Diabetics are defective in glucose uptake
Aquaporin-2 (AQP-2),Insulin and Water retention
In kidney,water is reabsorbed from the renel collecting duct via
AQP-2 and returned to the blood for circulation,
Mechanism,ADH (antidiuretic hormone) regulates this process
by mobilizing AQP-2 from intracellular vesicles to the apical
surface,enhanced the uptake of water,
In a rare form of genetic disorder,diabetes insipidus,a genetic
defect in AQP-2 is responsible,with impaired water
reabsorption by the kidney—excretion of copious volumes of
very diluted urine
Chloride and
Bicarbonate are co-
transported across the
erythrocyte membrane:
Anion exchanger
--HCO3- for Cl-
--12TM
--one million fold!!!
Co-transport
Passive or active transport
Active Transport,
1) Against electrochemical gradient
2) Thermodynamically unfavorable—
endergonic
3) Coupled to exergonic processes (sunlight,
oxidative reaction,breakdown of ATP,or flow
of some other chemical species down its
electrochemical gradient)
Forms of Active Transport,Primary and Secondary
coupled
Free-energy change for a chemical process converting S into P is
DG= DG’o + RTln[P]/[S]
R is gas constant at 8.315 J/mol.K
T is absolute temperature
For the transport of an uncharged solute from C1 to C2,no bonds
break or form,then DG’o = 0
DGt= RTln[C2]/[C1]
For a solute to be moved against a gradient of 10 fold
DGt= (8.315J/mol.K)(298K)(ln10/1)=5,705 J/mol=5.7kJ/mol
How about charged solutes?
DGt= RTln(C2/C1) + ZFDy
Z is the charge on the ion
F is the Faraday constant (96,480J/V.mol)
Dy is the transmembrane electrical potential (v,.05-.2v for e.cells)
For most cells,the ionic imbalance is more than 10 fold,therefore,
active transport is a major energy-consuming process
Transport ATPases
P-type ATPases
V-type ATPases
F-type ATPases
Multidrug Transporter
Inhibit P-type ATPases as a phosphate analog
P-type ATPases are made of two types of integral
membrane proteins,a and b subunits,The a
subunits contain the asp residue which is
phosphorylated during transport,
Na+K+ antiporter for Na+(out) K+(in) to maintain
low Na+ and high K+ inside the cell,create
transmembrane electrical potential
H+K+ATPase in parietal cells—acidification of
stomach contents
V-type ATPases have
peripheral V1 as well as
integral (Vo) domains
Not phosphorylated
cyclically,thus,can’t be
inhibited by vanadate
Function,proton pumps
for acidification of
vacuolar or lysosomal
compartments
F-type ATPases (energy coupling factor)
also have peripheral (F1,ATP synthesis
and hydrolysis) and integral (Fo)
domains
Catalyze the uphill transport of proton
driven by ATP hydrolysis
Or formation of ATP from ADP+Pi
driven by downhill proton flow in
mitochondria and chloroplast,
Reversibility of F-
type ATPases
Against the
gradient—
consume ATP;
Down the
gradient—
generate ATP
CFTR and MDR transporters:
~170 kDa protein with 12 transmembrane domains
MDR allows down gradient diffusion of Cl- without consumption of
ATP,however,it requires ATP to pump drugs out of cells.
CF,5% carriers in whites,die before 30 due to respiratory
insufficiency due to thickening of the mucus in lung(air flow and
Staphylococcus aureus,Pseudomonas aeruginosa)
CFTR—transport of Cl- is much enhanced when CFTR is
phosphorylated at multiple sites by cAMP dependent protein
kinases,Cl- channel to maintain low Cl- level outside of the
epithelial cells of the lung,a condition more favorable for the killing
and clearing of invading bacteria in the lung,
Structure of CFTR,Mutation at Phe 508
in 70% of CF patients
Nucleotide-
binding fold
(ATP) Phosphorylation
by cAMP
dependent kinase
bacteria
Epithelial
cells
Function,to maintain
Na+ and K+
imbalances across the
plasma membrane of
animal cells
Mechanism of
Na+K+ATPase
antiporter
One ATP:
two K+ in
Three Na+ out
A P-type ATPase catalyzes active co-transport of Na+ and K+
Mechanism of Na+K+ATPase antiporter (details not know,no
crystal structure yet)
Proposed model:
1) Formation of phosphoenzyme,ATP +EnzI---- ADP + P-EnzII
2) Hydrolysis of phosphoenzyme,P-EnzII EnzI + Pi
Net reaction is
ATP +H2O ADP + Pi
High affinity for
K+,low affinity
for Na+
Significance of Na+K+ ATPase antiporter
1) Maintain the transmembrane potential of -50-70 mV (inside
negative) due to the imbalance of 3 Na+ out and 2 K+ in,The
transmembrane potentials are important for electrical signaling
in neuron; Na+ gradient drives many uphill transport in animal
cells
2) 25% of energy consumption for a human at rest
Can it be a target for therapy?
Ouabain— a sterol
derivative,also a,arrow
poison” in Somalia,a
potent and specific
inhibitor of Na+K+ATPase
Ouabain and digitoxigenin are the
active ingredient of digitalis,an
extract from foxglove leaves that
treats congestive heart failure
Block Na+ efflux to increase cellular
Na+,activating a Na+ Ca2+
antiporter in cardiac muscle—
elevated cytosolic Ca2+ strengthens
the contraction of heart muscle
ATP-driven Ca2+ pumps maintain a low concentration of calcium in
the cytosol
Why? Inside the cell,inorganic phosphates (Pi and Pii) at millimolar
concentration—easily form precipitates if calcium is available
Solution,two P-type ATPase Ca2+ pumps--a plasma membrane Ca2+
pump and a ER one (in myocytes,Ca2+ is sequestered in a
specialized form of endoplasmic reticulum- the sarcoplasmic
reticulum); SERCA (sarcoplasmic and endoplasmic reticulum
calcium pumps---inhibited by tumor-promoting agent thapsigargin)
In sarcoplasmic membrane,80% of the proteins are SERCA~
100kDa with 10 transmembrane domain and a large cytosolic domain
containing a ATP binding site and an Asp phosphorylation site,
The mechanism of ATP hydrolysis and Ca pump is similar to the one
described for Na+K+ ATPases,i.e.,cyclic phosphorylations;
Phosphorylated pump has high affinity for Ca2+ which faces the
cytosol (low Ca2+),dephosphorylated pump has low affinity facing
the lumen or outside,
Secondary active transports driven by ion gradients established
by primary active transports—downhill flow of Na+ or H+
coupled to the uphill transport of ions,sugars,amino acids etc.
2Na+out +glucoseout---2Na+in + glucosein
2Na+out +glucoseout---2Na+in + glucosein
Where does the cell find the energy to mediate glucose uptake?
1) Na+ gradient (145 mM outside,12 mM inside)
2) Transmembrane potential,-50-70 mV
DG=RTln ([Na+]in/[Na+]out) + nFDE
(n=2,2 Na+ for every glucose),
DG= -25 kJ is good enough to maintain a gradient of glucose
across the membrane around 30,000 fold,
-25 kJ = RT ln([glucose]in/[glucose]out)
[glucose]in/[glucose]out=30,000
1:30,000
Consume
1 ATP
Lactose uptake in E.coli,primary active transport to pump H+ out and
establish the electrical potential coupled to oxidation of various fuels,
then the lactose is pump inside through the galactoside transporter
Poison by CN-
Equilibrium
Compounds that collapse the
ion gradient across cellular
membrane are poisons,those
specific for infectious
microorganisms are antibiotics
such as Valinomycin—K+
ionophore
Valinomycin,a
peptide ionophore
that binds and
shuttles K+ across
membranes
Monensin—a Na+
ionophore
So,they kills bugs
by disrupting
secondary transport
processes and
energy-conserving
reactions
K+
Hydrophobic coat
Ion Channels—moving inorganic ions across membranes
---vs transporter
1) Rate of flux—orders of magnitude greater,108-9
2) Not saturable—don’t approach max at high conc.
3) Gated—open or close in response to cellular events (ligand or
voltage gated)
---first known in neurons,now in lower organisms,intracellular
membranes
---determine the permeability to specific ion,with ion pumps,
determine the cytosolic concentration of ions and
transmembrane potentials
---in neuron,action potential (change in membrane potential)-
transmit signals from one end to another; in muscle,Ca2+
release causes muscle contraction
Structure of the K+ channel of
Streptomyces lividans
1) 8 transmembrane
helices—two from each of
the 4 identical subunits to
form a cone
2) Outer helices of the cone
line the surface interaction
with lipid bilayer,inner
helices form the channel,a
selective filter for K+
3) 10,000 times faster for K+
than Na+ (too small) at 108
ion/
Extracellular side
K+
Carbonyl oxygens
to interact with K+,
but not Na+
Water of hydration of K+ is
stripped,replaced by
carbonyl oxygen to allow
K+ to pass through the
channel
Acetylcholine Receptor,a ligand-gated ion channel
--essential for the passage of an electrical signal from a motor
neuron to a muscle fiber at the neuromuscular junction (signal the
muscle to contract)
--nicotinic acetylcholine receptor vs muscarinic receptor (as
defined by their sensitivity to nicotine or muscarine)
MOA,acetylcholine released
by motor neuron diffuses and
binds to the receptor on
myocyte,triggers a
conformational change that
opens the intrinsic channel that
allows Na+,Ca2+ and K+ to
move and depolarize the
membrane—muscle moves
The gating mechanism is known,but not the desensitization part
Leu Small and
polar
residue
Acetylcholine Receptor Family:
Channels or receptors for
g-aminobutyric acid (GABA) —anion channel for Cl- or
HCO3-
glycine—for Cl- or HCO3-
serotonin—cation specific
Second class of ligand-gated channels respond to intracellular ligands
such as cGMP in vertebrate eye,cGMP and cAMP in olfactory
neurons,ATP and IP3 in many other cell types---multipe subunits with
6 TMs
Voltage-gated channels,e.g.,neuronal Na+ channel
Characteristics,
in neuron,myocytes of heart and muscle
sense electrical gradients across the membrane
respond by openning or closing the channel
selective for Na+
high flux rate of > 107 ions/s
activated by reduction in transmembrane potential
rapid inactivation in milliseconds
inactivation—activation = neuronal signalling.
A single chain (1840aa) with 4 domains
Inactivation of Na+ channel
through a ball-and-chain
mechanism,A domain on the
cytosolic side (ball) moves
freely when the channel is
closed,When it opens,the
inner face of the channel
becomes available for the ball
to bind,blocking the channel,
The length of the tether
determines how long the
channel stays open,the longer
the tether,the longer the open
period,
MOA,voltage sensing by
helices 4 which move inward
to pull the gate open by
communicating the force to
the activation gate (helices 6s)
Puffer fish,spheroides rubripes,---voltage-gated Na+ channel of
neuron to block action potential
Saxitoxin—by,dinoflagellate Gonyaulax,red tides in ocean
coasts—functions as tetrodotoxin—v-gated Na+ channel in
neurons.
D-Tubocurarine,cobrotoxin,bungarotoxin are ingredients from
snake venoms,block acetylcholine receptor or prevent the
opening of its ion channel,
They block the signals from nerves to muscles,cause paralysis
and possibly death,
FhuA—iron transporter from
E.coli
cork
part II
? Duanqing Pei,Ph.D.
? 裴 端 卿
? Tsinghua University
? Cell phone,136-8301-7108
? peixx003@umn.edu
Biological Membranes and Transport
part II
? Passive transport,with the gradient
– Water transport—aquaporins
– Glucose transporters
– Co-transport of chloride and bicarbonate
? Active transport,against the gradient
– Transport ATPases
– Ion channels
Simple diffusion across permeable membranes
Electrochemical Potentials,chemical gradient/electric potential
Selectively permeable barriers
of living organisms,all
membranes—intracellular and
plasma
Simple diffusion—virtually
impermeable to polar and
charged species (too slow to be
meaningful); permeable for
certain gases- molecular
oxygen,nitrogen,methane and
water (huge concentration)
Passive transport (facilitated
diffusion)—passage of polar
compounds and ions aided by
membrane proteins that lowers
the activation energy Transporter or permeases
Aquaporins,transmembrane channels that allow rapid movement of
water across plasma membrane in various cells—erythrocytes,
proximal renal tubule cells.
Transmembrane topology of an aquaporin,AQP-1
Type III
A tetramer
Flow rate,5x108/sec,better than the best enzyme catalyase (4x107/s)
DG<15kJ/mol,continuous stream,not much interruption.
Glucose transporter of erythrocytes (GluT1),plasma conc=5 mM;
transporter—50,000 more efficient than simple diffusion
Helical Wheel Diagram:
Distribution of polar vs
nonpolar residues—
amphipathic helix (one
side hydrophobic,the
other hydrophilic)
hydrophobic
hydrophilic
Formation of a channel by
side-by-side association
of 5 or 6 amphipathic
helices
Hydrogen bonds
Kinetics,similar to Michaelis-Menten equation
Sout + T Sout, T Sin, T Sin + T
V0=
Kt~Km for affinity of transporter for substrate
k1
k-1
k2
k-2
k3
k-3
Vmax [S]out
Kt + [S]in
Kinetics:
Initial
velocity plot
Double-reciprocal
plot
Model of Glucose Transport into
erythrocytes by GluT1
1) Binding to T1 site
2) Conformational change from T1
to T2
3) Released
4) Return of conformation to T1
Kt for GluT1=1.5mM for D-glucose,
20-30 mM for D-mannose and D-
galactose,>3000mM for L-glucose
(fast,saturable and specific)
GluT2,transport glucose out of hepatocytes to
blood,Kt=66mM (>5mM of blood glucose
level)
GluT4,for muscle,adipocytes,can be stimulated
by insulin
Dynamic regulation of glucose concentration in blood,muscle
and adipose tissues
After each meal,glucose level increases beyond 5 mM,
triggering the production of insulin,which will stimulate the
uptake of glucose into muscles for storage in the form of
glycogen,or adipocyte (triacylglycerols);
Mechanism,insulin stimulate the cell surface expression of
GluT4
Type I diabetes mellitus,juvenile onset diabetes
no insulin due to the lack of beta cells in pancreas
therefore,low rate of glucose uptake into muscle and adipose
tissue—high glucose level >>5mM
Diabetics are defective in glucose uptake
Aquaporin-2 (AQP-2),Insulin and Water retention
In kidney,water is reabsorbed from the renel collecting duct via
AQP-2 and returned to the blood for circulation,
Mechanism,ADH (antidiuretic hormone) regulates this process
by mobilizing AQP-2 from intracellular vesicles to the apical
surface,enhanced the uptake of water,
In a rare form of genetic disorder,diabetes insipidus,a genetic
defect in AQP-2 is responsible,with impaired water
reabsorption by the kidney—excretion of copious volumes of
very diluted urine
Chloride and
Bicarbonate are co-
transported across the
erythrocyte membrane:
Anion exchanger
--HCO3- for Cl-
--12TM
--one million fold!!!
Co-transport
Passive or active transport
Active Transport,
1) Against electrochemical gradient
2) Thermodynamically unfavorable—
endergonic
3) Coupled to exergonic processes (sunlight,
oxidative reaction,breakdown of ATP,or flow
of some other chemical species down its
electrochemical gradient)
Forms of Active Transport,Primary and Secondary
coupled
Free-energy change for a chemical process converting S into P is
DG= DG’o + RTln[P]/[S]
R is gas constant at 8.315 J/mol.K
T is absolute temperature
For the transport of an uncharged solute from C1 to C2,no bonds
break or form,then DG’o = 0
DGt= RTln[C2]/[C1]
For a solute to be moved against a gradient of 10 fold
DGt= (8.315J/mol.K)(298K)(ln10/1)=5,705 J/mol=5.7kJ/mol
How about charged solutes?
DGt= RTln(C2/C1) + ZFDy
Z is the charge on the ion
F is the Faraday constant (96,480J/V.mol)
Dy is the transmembrane electrical potential (v,.05-.2v for e.cells)
For most cells,the ionic imbalance is more than 10 fold,therefore,
active transport is a major energy-consuming process
Transport ATPases
P-type ATPases
V-type ATPases
F-type ATPases
Multidrug Transporter
Inhibit P-type ATPases as a phosphate analog
P-type ATPases are made of two types of integral
membrane proteins,a and b subunits,The a
subunits contain the asp residue which is
phosphorylated during transport,
Na+K+ antiporter for Na+(out) K+(in) to maintain
low Na+ and high K+ inside the cell,create
transmembrane electrical potential
H+K+ATPase in parietal cells—acidification of
stomach contents
V-type ATPases have
peripheral V1 as well as
integral (Vo) domains
Not phosphorylated
cyclically,thus,can’t be
inhibited by vanadate
Function,proton pumps
for acidification of
vacuolar or lysosomal
compartments
F-type ATPases (energy coupling factor)
also have peripheral (F1,ATP synthesis
and hydrolysis) and integral (Fo)
domains
Catalyze the uphill transport of proton
driven by ATP hydrolysis
Or formation of ATP from ADP+Pi
driven by downhill proton flow in
mitochondria and chloroplast,
Reversibility of F-
type ATPases
Against the
gradient—
consume ATP;
Down the
gradient—
generate ATP
CFTR and MDR transporters:
~170 kDa protein with 12 transmembrane domains
MDR allows down gradient diffusion of Cl- without consumption of
ATP,however,it requires ATP to pump drugs out of cells.
CF,5% carriers in whites,die before 30 due to respiratory
insufficiency due to thickening of the mucus in lung(air flow and
Staphylococcus aureus,Pseudomonas aeruginosa)
CFTR—transport of Cl- is much enhanced when CFTR is
phosphorylated at multiple sites by cAMP dependent protein
kinases,Cl- channel to maintain low Cl- level outside of the
epithelial cells of the lung,a condition more favorable for the killing
and clearing of invading bacteria in the lung,
Structure of CFTR,Mutation at Phe 508
in 70% of CF patients
Nucleotide-
binding fold
(ATP) Phosphorylation
by cAMP
dependent kinase
bacteria
Epithelial
cells
Function,to maintain
Na+ and K+
imbalances across the
plasma membrane of
animal cells
Mechanism of
Na+K+ATPase
antiporter
One ATP:
two K+ in
Three Na+ out
A P-type ATPase catalyzes active co-transport of Na+ and K+
Mechanism of Na+K+ATPase antiporter (details not know,no
crystal structure yet)
Proposed model:
1) Formation of phosphoenzyme,ATP +EnzI---- ADP + P-EnzII
2) Hydrolysis of phosphoenzyme,P-EnzII EnzI + Pi
Net reaction is
ATP +H2O ADP + Pi
High affinity for
K+,low affinity
for Na+
Significance of Na+K+ ATPase antiporter
1) Maintain the transmembrane potential of -50-70 mV (inside
negative) due to the imbalance of 3 Na+ out and 2 K+ in,The
transmembrane potentials are important for electrical signaling
in neuron; Na+ gradient drives many uphill transport in animal
cells
2) 25% of energy consumption for a human at rest
Can it be a target for therapy?
Ouabain— a sterol
derivative,also a,arrow
poison” in Somalia,a
potent and specific
inhibitor of Na+K+ATPase
Ouabain and digitoxigenin are the
active ingredient of digitalis,an
extract from foxglove leaves that
treats congestive heart failure
Block Na+ efflux to increase cellular
Na+,activating a Na+ Ca2+
antiporter in cardiac muscle—
elevated cytosolic Ca2+ strengthens
the contraction of heart muscle
ATP-driven Ca2+ pumps maintain a low concentration of calcium in
the cytosol
Why? Inside the cell,inorganic phosphates (Pi and Pii) at millimolar
concentration—easily form precipitates if calcium is available
Solution,two P-type ATPase Ca2+ pumps--a plasma membrane Ca2+
pump and a ER one (in myocytes,Ca2+ is sequestered in a
specialized form of endoplasmic reticulum- the sarcoplasmic
reticulum); SERCA (sarcoplasmic and endoplasmic reticulum
calcium pumps---inhibited by tumor-promoting agent thapsigargin)
In sarcoplasmic membrane,80% of the proteins are SERCA~
100kDa with 10 transmembrane domain and a large cytosolic domain
containing a ATP binding site and an Asp phosphorylation site,
The mechanism of ATP hydrolysis and Ca pump is similar to the one
described for Na+K+ ATPases,i.e.,cyclic phosphorylations;
Phosphorylated pump has high affinity for Ca2+ which faces the
cytosol (low Ca2+),dephosphorylated pump has low affinity facing
the lumen or outside,
Secondary active transports driven by ion gradients established
by primary active transports—downhill flow of Na+ or H+
coupled to the uphill transport of ions,sugars,amino acids etc.
2Na+out +glucoseout---2Na+in + glucosein
2Na+out +glucoseout---2Na+in + glucosein
Where does the cell find the energy to mediate glucose uptake?
1) Na+ gradient (145 mM outside,12 mM inside)
2) Transmembrane potential,-50-70 mV
DG=RTln ([Na+]in/[Na+]out) + nFDE
(n=2,2 Na+ for every glucose),
DG= -25 kJ is good enough to maintain a gradient of glucose
across the membrane around 30,000 fold,
-25 kJ = RT ln([glucose]in/[glucose]out)
[glucose]in/[glucose]out=30,000
1:30,000
Consume
1 ATP
Lactose uptake in E.coli,primary active transport to pump H+ out and
establish the electrical potential coupled to oxidation of various fuels,
then the lactose is pump inside through the galactoside transporter
Poison by CN-
Equilibrium
Compounds that collapse the
ion gradient across cellular
membrane are poisons,those
specific for infectious
microorganisms are antibiotics
such as Valinomycin—K+
ionophore
Valinomycin,a
peptide ionophore
that binds and
shuttles K+ across
membranes
Monensin—a Na+
ionophore
So,they kills bugs
by disrupting
secondary transport
processes and
energy-conserving
reactions
K+
Hydrophobic coat
Ion Channels—moving inorganic ions across membranes
---vs transporter
1) Rate of flux—orders of magnitude greater,108-9
2) Not saturable—don’t approach max at high conc.
3) Gated—open or close in response to cellular events (ligand or
voltage gated)
---first known in neurons,now in lower organisms,intracellular
membranes
---determine the permeability to specific ion,with ion pumps,
determine the cytosolic concentration of ions and
transmembrane potentials
---in neuron,action potential (change in membrane potential)-
transmit signals from one end to another; in muscle,Ca2+
release causes muscle contraction
Structure of the K+ channel of
Streptomyces lividans
1) 8 transmembrane
helices—two from each of
the 4 identical subunits to
form a cone
2) Outer helices of the cone
line the surface interaction
with lipid bilayer,inner
helices form the channel,a
selective filter for K+
3) 10,000 times faster for K+
than Na+ (too small) at 108
ion/
Extracellular side
K+
Carbonyl oxygens
to interact with K+,
but not Na+
Water of hydration of K+ is
stripped,replaced by
carbonyl oxygen to allow
K+ to pass through the
channel
Acetylcholine Receptor,a ligand-gated ion channel
--essential for the passage of an electrical signal from a motor
neuron to a muscle fiber at the neuromuscular junction (signal the
muscle to contract)
--nicotinic acetylcholine receptor vs muscarinic receptor (as
defined by their sensitivity to nicotine or muscarine)
MOA,acetylcholine released
by motor neuron diffuses and
binds to the receptor on
myocyte,triggers a
conformational change that
opens the intrinsic channel that
allows Na+,Ca2+ and K+ to
move and depolarize the
membrane—muscle moves
The gating mechanism is known,but not the desensitization part
Leu Small and
polar
residue
Acetylcholine Receptor Family:
Channels or receptors for
g-aminobutyric acid (GABA) —anion channel for Cl- or
HCO3-
glycine—for Cl- or HCO3-
serotonin—cation specific
Second class of ligand-gated channels respond to intracellular ligands
such as cGMP in vertebrate eye,cGMP and cAMP in olfactory
neurons,ATP and IP3 in many other cell types---multipe subunits with
6 TMs
Voltage-gated channels,e.g.,neuronal Na+ channel
Characteristics,
in neuron,myocytes of heart and muscle
sense electrical gradients across the membrane
respond by openning or closing the channel
selective for Na+
high flux rate of > 107 ions/s
activated by reduction in transmembrane potential
rapid inactivation in milliseconds
inactivation—activation = neuronal signalling.
A single chain (1840aa) with 4 domains
Inactivation of Na+ channel
through a ball-and-chain
mechanism,A domain on the
cytosolic side (ball) moves
freely when the channel is
closed,When it opens,the
inner face of the channel
becomes available for the ball
to bind,blocking the channel,
The length of the tether
determines how long the
channel stays open,the longer
the tether,the longer the open
period,
MOA,voltage sensing by
helices 4 which move inward
to pull the gate open by
communicating the force to
the activation gate (helices 6s)
Puffer fish,spheroides rubripes,---voltage-gated Na+ channel of
neuron to block action potential
Saxitoxin—by,dinoflagellate Gonyaulax,red tides in ocean
coasts—functions as tetrodotoxin—v-gated Na+ channel in
neurons.
D-Tubocurarine,cobrotoxin,bungarotoxin are ingredients from
snake venoms,block acetylcholine receptor or prevent the
opening of its ion channel,
They block the signals from nerves to muscles,cause paralysis
and possibly death,
FhuA—iron transporter from
E.coli
cork
