Biological Membranes

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Chapter 12 Biological Membranes Transport
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Fluid Mosaic Model for Membrane Structure
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Amphipathic lipid aggregates that form in water
or Vesicle
Bilayers are noncovalent, cooperative structures
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Monolayer of oil molecules at an air-water
interface
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Membrane 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

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Higher the proportion of saturated fatty acid,
higher is the transition temperature.
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Sterol content of a membrane has 2 effects on
membrane fluidity
Below the transition temperature Insertion of
rigid planar sterol prevents highly
ordered packing of fatty acid side chains
Membrane fluidity
Above the transition temperature Rigid planar
sterol reduces the freedom of neighboring
fatty acid side chains
Membrane fluidity
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Cells regulate their lipid composition to achieve
a constant membrane fluidity under various growth
conditions
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Motion of Membrane Lipids
Lateral Diffusion
Transbilayer or flip-flop Diffusion
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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

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Demonstration of lateral diffusion of membrane
proteins
Membrane proteins, like membrane lipids, are free
to diffuse laterally in the plane of the bilayer
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Restricted motion of the erythrocyte
chloride-bicarbonate exchanger
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Asymmetric distribution of phospholipids between
the inner outer monolayers of erythrocyte
plasma membrane
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Structure of Membrane Proteins

• Singer Nicolson defined two classes
• Integral (intrinsic) proteins
• Peripheral (extrinsic) proteins
• We'll note a new one
• lipid-anchored proteins

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Peripheral Integral Proteins
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Glycophorin in the erythrocyte
Some membrane proteins span the lipid bilayer

• A single-transmembrane-segment protein
• One transmembrane segment with globular domains
  on either end
• Transmembrane segment is alpha helical and
  consists of 19 hydrophobic amino acids
• Extracellular portion contains oligosaccharides
  (and these constitute the ABO and MN blood group
  determinants)

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Lipid-linked membrane proteins
Covalently attached lipids anchor membrane
proteins to the lipid bilayer
A relative new class of membrane proteins 4
types have been found Amide-linked myristoyl
anchors Thioester-linked fatty acyl anchors
Thioether-linked prenyl anchors Glycosyl
phosphatidylinositol anchors
Glycosyl phosphatidylinositol (GPI) anchor
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Integral Membrane Proteins Held in the membrane
by hydrophobic interactions with lipids
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Bacteriorhodopsin, a membrane-spanning protein
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3-D structure of the photosynthetic reaction
center of purple bacterium
First integral membrane protein to have its
structure determined by X-ray diffraction methods
Prosthetic group (light-absorbing pigments)
Residues that are part of the trans-membrane
helices
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Hydropathy Plots
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Hydropathy Plots
1
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Porin FhuA, an integral membrane protein with
b-barrel structure
Not all integral membrane proteins are composed
of transmembrane a helices
Porin allows certain polar solutes to cross the
outer membrane of bacteria
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Porins

• Found both in Gram-negative bacteria and in
  mitochondrial outer membrane
• Porins are pore-forming proteins (30-50 kD)
• Most arrange in membrane as trimers
• High homology between various porins
• Porin from Rhodobacter capsulatus has
  16-stranded
• beta barrel that traverses the membrane to
  form the
• pore

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Why Beta Sheets?

• for membrane proteins??
• Genetic economy
• Alpha helix requires 21-25 residues per
  transmembrane strand
• Beta-strand requires only 9-11 residues per
  transmembrane strand
• Thus, with beta strands , a given amount of
  genetic material can make a larger number of
  trans-membrane segments

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Integral membrane proteins mediate cell-cell
interactions adhesion
4 examples of integral protein types that
function in cell-cell interaction
Serve as receptors signal transducers
Essential part of the blood-clotting process
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Gap Junctions

• 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
• (for example, synchronized contraction
  of heart muscle is
• brought about by flow of ions through
  gap junctions)

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Induces closure of gap junction central channel
Gap Junctions

• Hexameric arrays of a single 32 kD protein
• Subunits are tilted with respect to central axis
  
• Pore in center can be opened or closed by the
• tilting of the subunits, as response to
  stress

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Cont.
Chapter 12 Biological Membranes Transport
For chapter 12 Focus on the material covered in
lectures Will not be tested on materials covered
in Pages 424 - 429
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Membrane fusion is central to many biological
processes
Membranes undergo fusion without losing its
integrity
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Membrane fusion during viral entry into a host
cell
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Movements of solutes across a permeable membrane
Electrically neutral solutes
Electric gradient or membrane potential
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Energy changes accompanying passage of a
hydrophilic solute through the lipid bilayer of a
biological membrane
Energy of activation
Facilitated diffusion or passive transport
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Aquaporins form hydrophilic transmembrane
channels for the passage of water
Proposed structure of aquaporin channel (Formed
by 4 monomers)
Likely transmembrane topology of an aquaporin,
AQP-1
Monomer
Water flows through the channel in single file at
the rate of 5 X 108 molecules / second
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Glucose transporter of erythrocytes mediates
passive transport
Monomer
1
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Proposed structure of GluT1
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A helical wheel diagram
Shows the distribution of polar non-polar
residues on the surface of a helical segment
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Side-by-side association of 5 or 6 amphipathic
helices
Polar
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Model of glucose transport into erythrocytes by
GluT1
T1 T2 are 2 different conformations T1 has
glucose binding site on the outer surface of the
membrane T2, with the binding site on the inner
surface
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Three general classes of transport system

• Differ in of solutes
• transported
• 2) the direction in which each is transported

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Summary of transport types
X
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Types of transport Passive Transported species
always moves down its electrochemical gradient
and it is not accumulated above the equilibrium
point ATP not required Active Results in
accumulation of solute above the equilibrium
point ATP is required
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Three types of ion-transporting ATPase
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NaK ATPase

• In animal cells, this active transport system is
  responsible for
• maintaining intracellular
• Na and K concentrations
• for generating transmembrane
• electrical potential

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Postulated mechanism of Na and K transport by
the NaK ATPase
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Na, K Transport

1. Hypertension involves apparent inhibition of
   sodium pump. (Inhibition in cells lining blood)
2. Studies show this inhibitor to be ouabain!

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A defective ion channel causes cystic fibrosis
CF is the result of one amino acid change in the
protein CFTR, a chloride ion channel
Topology of cystic fibrosis transmembrane
conductance regulator, CFTR
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Ionophores
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Gramicidin

• 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
• Ions migrate through the central pore

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Valinomycin, a peptide ionophore that binds K

• 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

Carbonyl Oxygen Atom
K
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1) K has formed a stable complex with valinomycin
2) Carrier's hydrophobic periphery enables it to
enter move through the hydrophobic core of the
bilayer
3) Water ions compete with the carrier's carbonyl
atoms for K, and the ion eventually emerges into
the aqueous medium
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Selectivity of Valinomycin
K
Na

• K and Rb bind tightly, but affinities for Na
  Li are about a thousand-fold lower
• Radius of the ions is one consideration
• Hydration is another
• It "costs more" energetically to desolvate Na
  Li than K

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Porins are transmembrane channels for small
molecules
22-stranded b barrel (seen as a hollow pipe)
Cork domain (keeps the channel closed)
Binding of ferrichrome-iron on the outer surface
causes conformation change moves the
ferrichrome into the barrel
Structure of FhuA, an iron transporter from E.coli