Lipids, Membranes and Cellular Transport

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Lipids, Membranes and Cellular Transport

• Chapter 10

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Functions of Lipids
Energy Storage Heat Insulation Membranes
selectively permeable layer (60 100 Å thick),
separate cell from surroundings Function as
vitamins (vitamin E). Steroid hormones
(prostoglandins), Lipoproteins lipid transport
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Molecular Structure of Lipids
Amphipathic polar head group and nonpolar tail
(hydrocarbon) Fatty acids usually an even
number of carbon atoms Saturated carbons of the
tail are saturated with hydrogens Unsaturated
carbons of the tail contain one or more double
bonds, cis conformation favors puts a kink in
the tail Most hydrocarbon tails linear except
some branched fatty acids found in bacteria
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Triacylglycerides
Triesters fatty acids and glycerol Esterificatio
n with glycerol makes more water insoluable
Adipocytes - Fat storage cells filled by a fat
droplet Energy production oxidized to form
ATP Heat production oxidation to generate heat
(brown fat) Insulation thermal insulation,
layers under skin
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Soaps, Detergents and Waxes
Soaps fats hydrolyzed with alkalis (NaOH or
KOH) Ca and Mg ppt soaps destroy
emulsifying action Synthetic detergents - SDS
more soluable with Ca and Mg, gel
electrophoresis. - Triton X cell lysis Waxes
Fatty acid esterified to long chain alcohol More
hydrophobic water repellents
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Biological Membranes
Micelles single hydrocarbon tail Lipid bilayer
pack in parallel to form extended sheets -
glycerol phospholipids - sphingolipids -
glycosphingolipids - glycoglycerolipids
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Lipid Vesicles and Planar Bilayers
Studies of membranes can be performed by making
lipid vesicles or planar bilayers Suspending a
lipid (phosphatidyl choline), sonicating. Fused
vesicles uniform in size Trap ions or substance
studying inside the vesicles examine their
transport and regulation Effective for drug
delivery selective fusion with target cells
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Phospholipids
Derivatives of glycerol-3-phosphate R1 and R2 are
usually acyl side chains derived from fatty acids
(one saturated and one unsaturated) R3 group
provides the diversity among phospholipids
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Types of Phospholipids
Polar head groups many carrying some
charge Composition of membranes can vary for
specific functions RBC membrane 16-24 carbons
and 0-6 double bonds
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Sphingolipids
Build on a long chain amino acid
sphingosine Ceramide sphingolipids and a fatty
acid Modification of C1 hydroxyl leads to many
other sphingolipids
Sphingomyelin phosphocholine group is attached
to C-3 hydroxyl Glycolipids Saccharide group
attached Glycosphingolipids - cerebrosides,
gangliosides, important in brain and nerve tissue
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Cholesterol
Present in many membranes A steroid and precursor
sythesis of many steroids Weakly amphipathic
hydroxyl group Esterification of hydroxyl to
fatty acid hydrophobic Disrupts regularity of
membrane with bulky ring structure of
cholesterol in membrane will alter membrane
structure
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Membrane Structure
Within the phospholipid bilayer is a variety of
proteins Fluid mosiac model membrane is a
mosiac of lipids and proteins Peripheral membrane
proteins attached to surface of
membrane Integral membrane protein buried
within the membrane and exposed on either surface
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Motions in Membranes
Lateral diffusion can occur in a membrane How
rapidly diffusion occurs depends on temperature
and lipid composition
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Fluorescence Recovery After Photobleaching (FRAP)
Examine movement of selected molecules in a
membrane Average distance s traveled in time t
s (4Dt)1/2 D 1 µM 2 ?sec-1 for the average
membrane, travels 2 µM in 1 sec Some membrane
proteins very mobile rhodopsin in
photoreceptors D 0.4 µM 2 ?sec-1 , fibronectin
,D 10-4 µM2 ?sec-1 , anchored to cytoskeleton
and interacts with extracellular matrix
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Motion of Membranes
At low temperature hydrocarbon tails pack tightly
together Transition temperature (Tm) - At high
temperature regular order lost adopts fluid
structure Altering composition of the membrane
shifts the Tm shorter tails, introducing a cis
double bond, changing the head group Tm below
body temp
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Cholesterol and Membrane Fluidity
Cholesterol broadens Tm transition can stiffen
membrane above Tm and inhibit regularity in
structure below Tm Low concentration of
cholesterol fits into membrane but thickens
it High concentrations of cholesterol - forms
islands of cholesterol plaque formation in
circulatory system
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Asymmetry of membranes
Two individual layers leaflets , outer
(extracellular) inner (cytoplasm) Variability in
membrane fluidity and composition Difference in
charges groups contribute to membrane
potential Glyocoproteins on outer surface
identification of cells
Maintenance of assymetry dynamic steady state -
PE, PC, PI, PS and SP groups Growth and cellular
expansion Turnover and renewal
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Membrane Proteins
High proportion of hydrophobic residues in part
of the protein that is embedded in the
membrane Alpha-helical segments span the
membrane Transmembrane segments can be inferred
from hydrophobicity plot Protein content varies
in different cell types nerve cell (low
protein), bacterial cell wall and mitochondria
(high protein content)
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Erythrocyte Cell Membrane
Separation of integral and peripheral membrane
proteins Isolate membrane and run on SDS-gel Wash
away peripheral membrane proteins with pH and
ionic strength
Protein Skeleton treatment with Triton X
Spectrin - tetramers elongated Actin links
spectrin and band 4.1 Ankrin links spectrin to
cell membrane Band 4.1 Band 3 internal membrane
protein
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Integral Membrane Proteins
Band 3 anion carrier facilitates exchange of
HCO3- for Cl- Bringing HCO3- to transport CO2 and
transporting out Cl- to maintain charge 2 to 4
subunits with membrane domain creates a
channel N-terminal portion extends into the
cytoplasm makes several contacts (ankrin)
Glycophorin External CHO carrying domain sialic
acid (neg. charge) Single transmembrane
helix Cytosolic C-terminal domain linked to
band 4.1 stabilizing skeleton-membrane adhesion
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Membrane Biophysics
Bilayer lowest free energy is flat
(planar) Bending the Bilayer has free energy
cost Elastic energy per phospholipid molecule ½
k(?a)2, when ?a ltltltahead. Because ?a aheadd/R,
½ k(ahead/R)2 Double layer 2/ahead The bend
stiffness 2kd2 Free energy cost ½ k/R2 IF each
head group gets stretched in both directions
2k/R2 Tail length 1.3 nm, bending energy
0.8?10-19 J, 15 kT
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ahead ?a
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Lipid Rafts

• Plasma membrane microdomains.
• Membrane proteins may play a role.
• Lipidlipid interactions.
• Structures are transient on biological time-scale
• May enhance molecular interactions, such as those
  that occur in receptor-mediated signaling

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Lipid Rafts

• Small patches 10-100 nm in diameter
• Stabilized by membrane proteins
• Created or destroyed by vesicle movement
• Defined as insoluble in Triton X

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