Transcript CHAPTER 6

Chapter 9
Membranes and Membrane
Transport
Biochemistry
by
Reginald Garrett and Charles Grisham
Garrett and Grisham, Biochemistry, Third Edition
Essential Question
• What are the properties and characteristics of
biological membranes that account for their
broad influence on cellular processes and
transport?
Garrett and Grisham, Biochemistry, Third Edition
Outline
• What Are the Chemical and Physical Properties of
Membranes?
• What Is the Structure and Chemistry of Membrane
Proteins?
• How Does Transport Occur Across Biological Membranes?
• What Is Passive Diffusion?
• How Does Facilitated Diffusion Occur?
• How Does Energy Input Drive Active Transport Processes?
• How Are Certain Transport Processes Driven by Light
Energy?
• How Are Amino Acid and Sugar Transport Driven by Ion
Gradients?
• How Are Specialized Membrane Pores Formed by Toxins?
• What Is the Structure and Function of Ionophore Antibiotics?
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9.1 – What Are the Chemical and
Physical Properties of Membranes?
Structures with many cell functions
•
•
•
•
•
•
•
Barrier to toxic molecules
Help accumulate nutrients
Carry out energy transduction
Facilitate cell motion
Assist in reproduction
Modulate signal transduction
Mediate cell-cell interactions
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Lipids Form Ordered Structures
Spontaneously in Water
Hydrophobic interactions all!
• Very few lipids exists as monomers.
• Monolayers arrange lipid tails in the air!
• Micelles bury the nonpolar tails in the
center of a spherical structure.
• Micelles reverse in nonpolar solvents.
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Lipids Form Ordered Structures
Spontaneously in Water
Hydrophobic interactions all!
• Lipid bilayers can form in several ways.
– unilamellar vesicles (liposomes)
– multilamellar vesicles (Bangosomes, from
Bangham)
 The nature and integrity of these vesicle
structures are very much dependent on the
lipid composition.
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The Fluid Mosaic Model Describes
Membrane Dynamics
S. J. Singer and G. L. Nicolson
• The phospholipid bilayer is a fluid matrix.
• The bilayer is a two-dimensional solvent.
• Lipids and proteins can undergo rotational and
lateral movement.
• Two classes of proteins:
– peripheral proteins (extrinsic proteins):
associate with membrane by ionic interactions
and H-bonds.
– integral proteins (intrinsic proteins): by
hydrophobic interactions, can only be
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dissociated
detergents
organic
solvents.
• Membrane bilayer thickness: 5 nm by X-ray
• The electron density of interiors of the
membranes (the hydrocarbon tails) are low, while
that of the outside edges (the polar head groups)
of the same membrane are high.
• Hydrocarbon chain orientation in the bilayer:
The tails tilt and bend and adopt a variety of
orientations. The portions of a lipid chain lie
nearly perpendicular near the membrane surface,
while the chains toward the middle of the bilayer
are not ordered.
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Motion in the Bilayer
Lipid chains can bend, tilt and rotate.
• Lipids and proteins can migrate ("diffuse") in the bilayer.
• Frye and Edidin proved that integral membrane proteins
can move laterally, using fluorescent-labelled antibodies.
(Figure 9.7)
• Lipid diffusion has been demonstrated by NMR and EPR
(electron paramagnetic resonance) and also by
fluorescence measurements.
• How fast? A few m/min, or <10 nm/sec for integral
proteins, which anchored to the cytoskeleton to maintain
cell’s shape. Several m/sec for lateral movement of
lipids. Transverse movement of lipids or proteins are
much slower (in days).
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Membranes are Asymmetric
Structures
Lateral Asymmetry of Lipids:
• Lipids can cluster in the plane of the membrane
- they are not uniformly distributed.
• Ex. Ca+2 induced clustering (phase separation)
of PS, PE, and PC. (Fig 9.8) --- regulate the
activity of membrane-bound enzymes.
• Intercalation of cholesterol can affect the
function of membrane proteins and enzymes.
• Distribution of lipids can be affected by proteins
--- preference of saturated FA chains or head
groups. Garrett and Grisham, Biochemistry, Third Edition
Lateral Asymmetry of Proteins:
• Proteins can associate and cluster in the
plane of the membrane - they are not
uniformly distributed in many cases.
• Some proteins form multisubunit complexes
to perform specific functions. Ex. The “purple
patches” bacteriorodopsinprotein (Fig 9.9).
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Membranes are Asymmetric
Structures
• The two sides of a membrane bilayer are different. Ex.
Membrane transport is for one direction. Hormone
interactions and immunological reactions are at the
outside surface of cells.
• Transverse asymmetry of proteins
– Mark Bretscher showed that N-terminus of
glycophorin is extracellular whereas C-terminus is
intracellular. (Fig 9.14)
• Transverse asymmetry of lipids
– In most cell membranes, the composition of the
outer monolayer is quite different from that of the
inner monolayer. (Fig 9.10)
– The carbohydrate groups of glycolipids and
glycoproteins always face outside of the membrane.
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Transverse asymmetry
Asymmetric lipid distributions led to difference
in total charge and the membrane potential
on the inner and outer surfaces. The
membrane potential modulates the activity
of certain ion channels and membrane
proteins.
• Lipid asymmetry due to two processes:
(A) asymmetric synthesis of phospholipid,
glycolipid, and cholesterol in ER and
Golgi system and flow by lipid transfer
proteins.
(B) energy-dependent transport --- flippases
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(Figand
9.11)
reduce
t Third
from
10 days to a
Flippases
A relatively new discovery!
• Lipids can be moved from one monolayer to
the other by flippase proteins.
• Some flippases operate passively and do not
require an energy source.
• Other flippases appear to operate actively and
require the energy of hydrolysis of ATP.
• Active flippases can generate membrane
asymmetries.
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Membranes Undergo Phase
Transitions
the "melting" of membrane lipids
• Below a certain transition temperature,
membrane lipids are rigid and tightly packed.
• Above the transition temperature, lipids are
more flexible and mobile.
• The transition temperature is characteristic of
the lipids in the membrane.
• Only pure lipid systems give sharp, well-defined
transition temperatures.
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Membrane phase transitions
Radical change in physical state occurring within narrow
range of transition (or melting) temperature.
Below Tm: Lipids close-pack: Lose lateral mobility and
rotational mobility of fatty acid chains.
Consequences: Membrane thickens and decreases
surface area.
Characteristics: (1) endothermic
(2) Tm increases with chain length and degree of
saturation and is influenced by nature of head group.
(3) Pure phospholipid bilayers show narrow
temperature range --- cooperative phase change.
(4) Native membranes show broad transition
influenced by protein and lipid composition.
(5) Pretransition (5-15 C below Tm) involves tilting.
(6) Increased volume (7) sensitive to interacting
cations and lipid-soluble agents. Cells adjust lipid
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composition
ofGrisham,
their membranes
to maintain
fluidity as
9.2 – What Is the Structure and
Chemistry of Membrane Proteins?
Functions: transport, receptor, etc.
Singer and Nicolson defined two classes.
• Integral (intrinsic) proteins
• Peripheral (extrinsic) proteins
•lipid-anchored proteins: covalent linkage
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Peripheral Proteins
• Peripheral proteins are not strongly bound
to the membrane.
• They can be dissociated with mild
detergent treatment or with high salt
concentrations.
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Integral Membrane Proteins Are
Firmly Anchored in the Membrane
• Integral proteins are strongly imbedded in the
bilayer, by -helices or -sheets due to
neutralization of H-bonds of N-H and C=O
peptide backbone.
• They can only be removed from the membrane
by denaturing the membrane (organic solvents
or strong detergents).
• Often transmembrane, but not necessarily.
• Glycophorin (Fig 9.14) and bacteriorhodopsin
(Fig 9.15) are examples.
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Glyophorin
A single-transmembrane-segment protein
• One transmembrane segment with globular
domains on either end.
• Transmembrane segment is -helical and
consists of 19 hydrophobic amino acids.
• Extracellular portion contains oligosaccharides
and these constitute the ABO and MN blood
group determinants, which also serve as
receptor for influenza virus.
• 40% protein and 60% carbohydrate by weight.
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Bacteriorhodopsin
A 7-transmembrane-segment (7-TMS) protein
• Found in purple patches of Halobacterium
halobium.
• Consists of 7 transmembrane helical
segments with short loops that interconnect
the helices back and forth across the
membrane. Very little exposed to aqueous
milieu.
• Note the symmetry of packing of bR (see
Figure 9.15)
• bR is a light-driven proton pump!
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Porins --- a -sheet motif for
membrane proteins
Found both in Gram-negative bacteria and in
mitochondrial outer membrane
• Porins are pore-forming proteins - 30-50 kD
• General or specific - exclusion limits 6006,000
• Most arrange in membrane as trimers.
• High homology between various porins.
• Porin from E. coli has 18-stranded beta
barrel that traverses the membrane to form
the pore (with eyelet!).
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Figure 9.16-17. The three-dimensional structure (left) and the
arrangement of the peptide chain (right) of maltoporin from E.
coli. Maltoporin participates in the entry of maltose and
maltodextrins into E. coli.
Polar and nonpolar residues alternate along the -strands, with
polar residues facing the cavity of the barrel and nonpolar
residues facing out, interacting with the hydrophobic lipid
milieu of the membrane.
Why Beta Sheets?
• Genetic economy
• -helix requires 21-25 residues per
transmembrane strand.
• -strand requires only 9-11 residues per
transmembrane strand.
• Thus, with -strands , a given amount of
genetic material can make a larger number
of transmembrane segments.
• -strands can present alternate polar and
nonpolar residues along the -strands.
Polar residues face the hydrophilic cavity of
the barrel and nonpolar residues face the
hydrophobic lipid bilayer.
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Treating allergies at the cell membrane
Antihistamine drugs
bind tightly to
histamine H1
receptors, without
eliciting the effect of
histamine.
Garrett and Grisham, Biochemistry, Third Edition
p.280
Lipid-Anchored Membrane
Proteins Are Switching Devices
A class of membrane proteins covalently bound to
lipid molecules
• The activity of the protein can be modulated by
attachment of the lipid.
• The reversible attachment of the lipid can serve as a
“switching device” for altering the affinity of a protein
to the membrane and controlling signal transduction
pathways.
• Four types have been found:
– Amide-linked myristoyl anchors
– Thioester-linked fatty acyl anchors
– Thioether-linked prenyl anchors
– Glycosyl
phosphatidylinositol
anchors
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Amide-Linked Myristoyl Anchors
•
•
•
•
Always myristic acid (14:0 fatty acid)
Always N-terminal
Always a Gly residue that links
Examples: cAMP-dependent protein
kinase, pp60src tyrosine kinase,
calcineurin B, alpha subunits of G
proteins, gag protein of HIV-1
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Thioester-linked Acyl Anchors
• Broader specificity for lipids - myristate,
palmitate, stearate, oleate all found.
• Broader specificity for amino acid links Cys, Ser, Thr all found.
• Examples: G-protein-coupled receptors,
surface glycoproteins of some viruses,
transferrin receptor triggers and signals.
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Thioether-linked Prenyl Anchors
• Prenylation refers to linking of "isoprene"based groups.
• Always Cys of CAAX (C=Cys, A=Aliphatic,
X=any residue)
• Isoprene groups include farnesyl (15carbon, three double bond) and
geranylgeranyl (20-carbon, four double
bond) groups
• Examples: yeast mating factors, p21ras and
nuclear lamins
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Glycosyl Phosphatidylinositol
Anchors
• GPI anchors are more elaborate than
others.
• Always attached to a C-terminal residue.
• Ethanolamine link to an oligosaccharide
linked in turn to inositol of PI.
• See Figure 9.20
• Examples: surface antigens, adhesion
molecules, cell surface hydrolases.
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9.3 How does transport Occur Across
Biological Membranes?
Cells must exchange materials with their environment.
Bring nutrients in and send waste products out of
cellular membrane and organelles.
• Concentration gradients must be maintained.
• Na+ and K+ gradients mediate transmission of nerve
impulses and normal functions of the brain, heart,
kidneys, and liver. Ca+2 controls muscle contractions
and hormonal responses.
• High [H+] in mucosal membrane of the stomach.
• Need transport proteins for water-soluble molecules.
Membrane transport: 3 types
• Passive diffusion
• Facilitated diffusion
• Active transport:
an energy
driven process
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9.4 – What is Passive Diffusion?
Thermodynamically favorable.
No special proteins needed.
• Entropically driven process: Molecules simply
moves down its concentration gradient - from
high [C] to low [C].
• ∆G = RTln([C2]/[C1]) for uncharged molecule
• ∆G = RTln([C2]/[C1]) + ZF∆ for charged molecules
R = 8.32 J/K·mol, T = K, Z = charge,
F = 96,485 J/V·mol, ∆ = electrical potential
Rate depends on concentration gradient and lipid solubility.
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9.5 – How Does Facilitated Diffusion Occur?
G negative, but proteins assist
• Solutes only move in the thermodynamically favored direction.
• But proteins may "facilitate" transport, increasing the rates of
transport.
• Understand plots in Figure 9.23
• Two important distinguishing features:
– Solute flows only in the entropically favored direction.
– Involves integral membrane protein
» Rate depends on concentration but is saturatable.
» Specificity and affinity due to protein/transported
molecule interaction.
Examples:
» Glucose transporter: RBC band 4.5: 55 kD protein
functions as trimer
» Anion transport system: RBC band 3: 95 kD
protein in Cl-, HCO3- exchange
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Vmax
Km
Figure 13.7, p.413
Figure 9.23 Passive diffusion and facilitated diffusion may be distinguished
graphically. The plots for facilitated diffusion are similar to plots of enzymecatalyzed processes (Chapter 13) and they display saturation behavior.
9.6 – How Does Energy Input Drive
Active Transport Processes?
Energy input drives transport
• Some transport must occur such that solutes flow against
thermodynamic potential.
• Energy input drives transport.
• Energy source and transport machinery are "coupled“.
• Energy source may be ATP, light or a concentration
gradient.
• Active transport: Energy driven process
• Primary active transport: Energy sources
– ATP hydrolysis (most common)
– Light energy
• Secondary active transport (Energy is ion gradient
formed by some other process.)
• Electrogenic transport: Active transport of ions and net
charge
both
occur. Third Edition
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and Grisham,
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The Sodium Pump
Na+,K+-ATPase
• Large protein - 120 kD and 35 kD subunits
• Maintains intracellular [Na+] low (10 mM) and
[K+] high (100 mM).
• Crucial for all organs, but especially for neural
tissue and the brain.
• 3 Na+ out, 2 K+ in per ATP hydrolyzed: Electrogenic.
• Ouabain: Cardiac glycoside that inhibits sodium
pump.
• Alpha subunit has 10 transmembrane helices
with large cytoplasmic domain.
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Na+,K+ Transport
• ATP hydrolysis occurs via an E-P intermediate.
• Mechanism involves two enzyme conformations
known as E1 and E2. Binding of Na+ ions to E1 is
followed by phosphorylation and release of ADP.
E2-P has low affinity for Na+ and high affinity for
K+. Na+ ions are transported and released and K+
ions are bound before dephosphorylation of the
enzyme.
• Cardiac glycosides inhibit by binding to the
extracellular surface of Na+,K+-ATPase in the E2-P
state. Garrett and Grisham, Biochemistry, Third Edition
E2P: low affinity for Na+
high affinity for K+
Figure 9.29 A mechanism for Na+,K+-ATPase . The model assumes
two principal conformations, E1 and E2. Binding of Na+ ions to E1 is
followed by phosphorylation and release of ADP. Na+ ions are
transported and released and K+ ions are bound before
dephosphorylation of the enzyme. Transport and release of K+ ions
complete the cycle.
Na+,K+ Transport
• Hypertension involves apparent inhibition of
sodium pump. Inhibition in cells lining blood
vessel walls results in Na+,Ca+2 accumulation
and narrowing the vessels.
• Studies show this inhibitor to be ouabain!
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Calcium Transport: Ca2+-ATPase
A process akin to Na+,K+ transport
• Calcium levels in resting muscle cytoplasm are
maintained low by Ca2+-ATPase - a Ca2+ pump.
• In muscles, [Ca+2]= 0.1 M in resting state; [Ca+2]=
10 M during contraction.
• Calcium is pumped into the sarcoplasmic reticulum
(SR) by a 110 kD protein that is very similar to the
-subunit of Na,K-ATPase.
• Aspartyl phosphate E-P intermediate is at Asp-351
and Ca2+-pump also fits the E1-E2 model.
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Ca2+-ATPase
Fig. 9-31, p.292
FIGURE 9.31 The structure of Ca2+-ATPase. The transmembrane (M) domain is
shown in red, the nucleotide (N) domain is shown in blue, the phosphorylation (P)
domain is purple, and the actuator (A) domain is green. The phosphorylation site,
Asp- 351, is yellow.
Two and
Ca2+Grisham,
ions in the
transmembrane
site are green.
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Cardiac Glycosides: Potent Drugs from Ancient Times
夾竹桃
Stimulation of the Na+-Ca+2 exchanger increased intracellular
[Ca+2] and stimulate muscle contraction to benefit patients with
p.293
heart problems.
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The Gastric H+,K+-ATPase
•
•
•
•
The enzyme that keeps the stomach at pH 0.8
The parietal cells of the gastric mucosa (lining
of the stomach) have an internal pH of 7.4
H+,K+-ATPase pumps H+ from the mucosal cells
into the stomach in exchange of K+, to maintain
a pH difference of 6.6 across a single plasma
membrane! ---a electronically neutral process.
K+ is pumped back together with Cl-.
Net transport of HCl into the interior of stomach.
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The Gastric H+,K+-ATPase
• This is the largest concentration gradient across
a membrane in eukaryotic organisms!
• 1 H+ out, 1 K+ in per ATP hydrolyzed.
• Gastric enzyme: ∆pH largest gradient known.
• H+,K+-ATPase is similar in many respects to
Na+,K+-ATPase and ER Ca+2-ATPase, forming
E-P intermediate.
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Osteoclast Proton Pumps
How your body takes your bones apart?
• 5% bone material undergoes ongoing remodeling
at any given time.
– osteoclasts tear down bone tissue
– osteoblasts build it back up
• Osteoclasts function by secreting acid into the
space between the osteoclast membrane and the
bone surface. Acid (pH 4) dissolves the Caphosphate (hydroxyapatitie) matrix of the bone.
• An ATP-driven proton pump in the membrane
does this!
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The ATPase that Transport peptides
and Drugs
• Animal cells have a transport system that
is designed to recognize foreign organic
molecules.
• This organic molecule pump recognizes a
broad variety of molecules and transports
them out of the cell using the hydrolytic
energy of ATP.
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The MultiDrug Resistance ATPase
Also known as the P-glycoprotein
• MDR ATPase is a member of a "superfamily" of
genes/proteins that appear to have arisen as a
"tandem repeat".
• It recognizes, binds, and transports a broad
group of structurally diverse molecules with
unknown mechanism.
• MDR ATPase defeats efforts of chemotherapy.
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9.7 – How Are Certain Transport
Processes Driven by Light Energy?
the Bacteriorhodopsin story
• Halobacterium halobium, the salt-loving bacterium
(optimum [NaCl]= 4.3M), carries out normal
respiration if O2 and substrates are plentiful.
• But when substrates are lacking, it can survive by
using bacteriorhodopsin and halorhodopsin to
capture light energy.
• Purple patches of H. halobium are 75% bR (the
only protein) and 25% lipid - a "2D crystal" of bR ideal for structural studies --- 7 transmembrane
helical segments (Figure 9.15).
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Figure 9.36 The Schiff base linkage between the retinal chromophore and
Lys216 gives rise to the purple color.
Figure 17.36 p. 571
Fig. 20-28, p. 662
Figure 9.37 The reaction cycle of bacteriorhodopsin. The intermediate states
are indicated by letters, with subscripts to indicate the absorption maxima of
the states. Also indicated for each state is the configuration of the retinal
chromophore (all-trans or 13-cis) and the protonation state of the Schiff base
(C=N: or C=N+H).
Bacteriorhodopsin Effects LightDriven Proton Transport
Protein opsin and retinal chromophore
• Retinal is bound to opsin via a Schiff base link.
• The Schiff base (at Lys-216) can be protonated,
and this site is one of the sites that participate in
H+ transport.
• Lys-216 is buried in the middle of the 7-TMS
structure of bR, and retinal lies mostly parallel to
the membrane and between the helices.
• Light absorption converts all-trans retinal to 13cis configuration - see Figure 9.37
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9.8 – How Are Amino Acid and Sugar
Transport Driven by Ion Gradients?
Transport processes driven by ion gradients
• Many amino acids and sugars are accumulated
by cells in transport processes driven by ion
gradients.
• Na+ or H+ coupled movement of amino acids
or sugars --- secondary active transport
systems.
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Secondary Active Transport
Na+ or H+ coupled movement of amino acids
or sugars
• Symport - ion and the amino acid or sugar are
transported in the same direction across the
membrane into the cell: e.g. proton symport
proteins and Na+-symport systems in E. coli.
• Antiport - ion and transported species move in
opposite directions: e.g. anion transporter of
erythrocytes.
• Several examples are described in Table 9.3
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9.9 – How Are Specialized
Membrane Pores Formed by Toxins?
• Lethal molecules produced by many
organisms.
• They insert themselves into the host cell
plasma membrane.
• They kill by collapsing ion gradients, facilitating
entry by toxic agents, or introducing a harmful
catalytic activity.
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Colicins
• Produced by E. coli
• Inhibit growth of other bacteria (even other
strains of E. coli)
• Single colicin molecule can kill a host!
• Three domains: translocation (T), receptorbinding (R), and channel-forming (C).
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210 Å
150 Å
Figure 9.38 The structure of colicin Ia. Colicin Ia, with a total length of 210
Å, spans the periplasmic space of a Gram-negative bacterium host, with
the R (receptor-binding) domain (blue) anchored to proteins in the outer
membrane and the C domain (violet) forming a channel in the inner
membrane. The T (translocation) domain is shown in red. The image on
the right shows details of the C domain, including helices 8 and 9 (green),
which are highly hydrophobic.
Clues to Channel Formation in Colicin
• C-domain: 10-helix bundle, with H8
and H9 forming a hydrophobic hairpin
• Other helices amphipathic (Fig. 9.38)
• H8 and H9 insert, with others splayed
on the membrane surface
• A transmembrane potential causes
the amphipathic helices to insert!
Figure 9.39 The umbrella model of membrane channel
protein insertion. Hydrophobic helices insert directly into the
core of the membrane, with amphipathic helices arrayed on
the surface like an open umbrella. A trigger signal (low pH or
a voltage gradient) draws some of the amphipathic helices
into and across Garrett
the membrane,
causing
the poreThird
to open.
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Other Pore-Forming Toxins
• -endotoxin also possesses a helix-bundle and may
work the same way.
• There are other mechanisms at work in other toxins.
• Hemolysin from Staphylococcus aureus forms a
symmetrical pore, allowing rapid Ca+2 influx into the
cells.
• Aerolysin may form a heptameric pore - with each
monomer providing 3 -strands to a membranespanning barrel. Each -strand consists of alternating
hydrophobic and polar residues, so that the pore has
polar residues in the center and nonpolar residues
facing the lipid bilayer.
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Amphiphilic Helices form
Transmembrane Ion Channels
• Many natural peptides form oligomeric
transmembrane channels.
• The peptides form amphiphilic -helices.
• Aggregates of these helices form channels that
have a hydrophobic outer surface and a polar
center.
• Melittin (bee venom), magainins (frogs) and
cecropin (from cecropia moths) are examples.
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Amphiphilic Helices form
Transmembrane Ion Channels
•
•
•
•
Melittin - bee venom toxin - 26 residues
Cecropin A - cecropia moths - 37 residues
Magainin 2 amide - frogs - 23 residues
See Figure 9.43 to appreciate helical wheel
presentation of the amphipathic helix.
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The Magainin Peptides
• Michael Zasloff noticed that incisions on
Xenopus laevis (African clawed frog)
healed without infection, even in bacteriafilled aquarium water.
• He deduced that the frogs produced a
substance that protected them from
infection!
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The Cecropins
• Produced by Hyalophora cecropia (the
cecropia moth - see Figure 9.44)
• Induced when the moth is challenged by
bacterial infections.
• These peptides are thought to form helical aggregates in membranes, creating
an ion channel in the center of the
aggregate.
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Gap Junctions Connect Cells in
Mammalian Cell Membranes
Vital connections for animal cells
•
•
•
•
Provide metabolic connections.
Provide a means of chemical transfer.
Provide a means of communication.
Permit large number of cells to act in
synchrony, such as in heart muscle.
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Gap Junctions Connect Cells in
Mammalian Cell Membranes
• Hexameric arrays of a single 32 kD protein
• Subunits are tilted with respect to central axis,
creating a pore of 1.82.0 nm diameter. Small
molecules (MW < 1.2 kD) can pass through, but
proteins and nucleic acids cannot.
• Pore in center can be opened or closed by the
tilting of the subunits, as response to protect
adjacent damaged or stressed cells.
• Gap junctions are sensitive to membrane
potentials, hormonal signals, pH changes, and
intracellular Ca+2 levels. Closed when [Ca+2] >
10-5M. (normal [Ca+2] < 10-7M)
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9.10 – What Is the Structure and
Function of Ionophore Antibiotics?
Mobile carrier or pore (channel)
• How to distinguish? Temperature!
• Pores will not be greatly affected by temperature,
so transport rates are approximately constant
over large temperature ranges.
• Carriers depend on the fluidity of the membrane,
so transport rates are highly sensitive to
temperature, especially near the phase transition
of the membrane lipids.
Garrett and Grisham, Biochemistry, Third Edition
Valinomycin Is a Mobile Carrier
Ionophone
A classic mobile carrier
• A depsipeptide - a molecule with both peptide
and ester bonds.
• Valinomycin is a dodecadepsipeptide.
• The structure places several carbonyl oxygens
in the center of the ring structure.
• Potassium and other ions coordinate the
oxygens.
• Valinomycin-potassium complex diffuses freely
and rapid across membranes.
Garrett and Grisham, Biochemistry, Third Edition
Selectivity of Valinomycin
Why?
• K+ and Rb+ bind tightly, but affinities for Na + and
Li + are about a thousand-fold lower.
• Radius of the ions is one consideration.
• Hydration is another - see page 304 for
solvation energies.
• It "costs more" energetically to desolvate Na+
and Li+ than K+.
Garrett and Grisham, Biochemistry, Third Edition
Gramicidin is a Channel-Forming
Ionophone
A classic channel ionophore
•
•
•
•
Linear 15-residue peptide - alternating D & L.
Structure in organic solvents is double helical.
Structure in water is end-to-end helical dimer.
Unusual helix - 6.3 residues per turn with a
central hole - 0.4 nm or 4 A diameter.
• H+ and all alkaline cations migrate through the
central pore. Transport 1x107 K+/sec. Ca+2 block
the channel.
Garrett and Grisham, Biochemistry, Third Edition