Download Membranes 1: Lipids and Lipid Bilayers

BIOC 460, Summer 2011
Membranes 1: Lipids and
Lipid Bilayers
Reading: Berg, Tymoczko & Stryer, 6th ed., Chapter 12,
pp. 326-335
Problems: Chapter 12, p. 150, #9
This animation is excellent for its representation of dynamics of
membrane lipids as well as the proteins embedded with the lipid bilayer
and is highly recommended:
http://multimedia.mcb.harvard.edu/anim_innerlife.html
Excellent animations on lipids, membranes, and transport processes
http://telstar.ote.cmu.edu/biology/animation/index.html
Key Concepts
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Membranes Lipids
Major functions of lipids: energy storage, major membrane components
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Other functions: signals, electron carriers, emulsifying agents....
Membrane lipids (amphipathic) -- responsible for spontaneous
formation of lipid bilayers
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Glycerophospholipids: glycerol backbone + 2 fatty acyl "tails" in
ester linkage + a polar "head group” (a phosphate ester of another
alcohol like choline, ethanolamine, serine, inositol, etc.)
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Sphingolipids: sphingosine backbone (1 "tail") + fatty acid chain in
amide linkage (another "tail") + either carbohydrate (glycosidic bond to
sphingosine) or phosphate ester of another alcohol like choline or
ethanolamine (ester bond to sphingosine)
• glycosphingolipids (cerebrosides, gangliosides)
• phosphosphingolipids (sphingomyelins)
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Cholesterol
Membrane fluidity (vital to membrane function) depends on lipid
composition of bilayer.
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fatty acid chainlength (more C atoms → more packing of tails, less
fluidity)
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fatty acid numbers of double bonds (fewer double bonds → more
packing of tails, less fluidity)
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cholesterol content ("buffers" fluidity)
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Biological Membranes
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sheet-like structures, a few molecules thick, forming closed boundaries
(self-sealing)
– amphipathic lipids: polar "head" groups and nonpolar "tails”
• With 2 hydrophobic "tails", amphipathic lipids form bilayers
instead of micelles.
– Proteins carry out most of the specific functions.
– carbohydrates (covalently attached to lipids = glycolipids, or to
proteins = glycoproteins) - important in communication/recognition
noncovalent assembly (interactions between components) into a fluid
2-dimensional solution
– Proteins and lipids can diffuse rapidly in plane of membrane, but
– Proteins and lipids do not rotate across the membrane (no "flipflop" in orientation across membrane).
– asymmetric arrangement
• 2 sides (faces) different
• biosynthesized that way
• Components don’t "flip-flop" their orientation.
Membranes always synthesized by growth of preexisting membranes
Amphipathic nature of polar lipids
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Membranes Lipids
Polar hydrophilic “head” group and non-polar hydrophobic “tail”portion
Amphipathic lipids self-assemble forming micelles (detergents) or bilayers
(phospholipids).
hydrophobic tails are buried, NOT exposed to H 2O.
hydrophilic head groups in contact with H2O.
Detergents: single hydrophobic tails can form micelles
Membrane lipids: have 2 tails, spontaneously form bilayers.
The hydrophobic effect provides the major driving force for the formation of
lipid bilayers.
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Liposomes or Vesicles
• Membranes curve and
spontaneously form vesicles or
liposomes
• lipid vesicles, aqueous
compartments enclosed by a
lipid bilayer
• experimental tools for studying
membrane permeability
• vehicles for delivery to cells of
chemicals/drugs/DNA for gene
therapy
• possible to incorporate
proteins in membrane or inside
vesicle interior
Berg et al., Fig. 12-12
Membrane Functions
1) HIGHLY SELECTIVE PERMEABILITY BARRIERS
regulate molecular & ionic compositions of cells and intracellular organelles
a) channels and pumps (proteins that act as selective transport systems)
b) electrical polarization of membrane (inside of plasma membrane
negative, typically - 60 millivolts)
(maintain different ionic concentrations on opposite sides of membrane)
2) INFORMATION PROCESSING - biological communication
a) signal reception by specific protein receptors (BINDING)
b) transmission/transduction of signals (via protein conformational changes)
sometimes generation of signals, chemical or electrical, e.g.,nerve impulses
3) ENERGY CONVERSION - ordered arrays of enzymes and other proteins,
organization of reaction sequences
a) photosynthesis (light energy → chemical bond energy): inner
membranes of chloroplasts, and plasma membranes of some
prokaryotes
b) oxidative phosphorylation (oxidation of fuel molecules → chemical bond
energy "stored" in ATP): inner membranes of mitochondria, and plasma
membranes of prokaryotes
Membranes Lipids
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Lipid Components of Animal Cell Membranes
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LIPIDS (definition): water-insoluble biomolecules that are highly soluble
in organic solvents
– Biological functions:
• fuels (highly concentrated energy stores)
• signaling molecules
• membrane components
Membrane lipid functions:
– bilayer structure → compartments/permeability barriers
– provide environment for proteins to work
– electrical insulation (e.g., myelin sheath on myelinated nerve fibers,
but also maintenance of electrical potential in other cells)
Membrane lipid distribution:
functional significance of all the differences not really understood
– proportions of different lipids vary by
• type of membrane (plasma membrane vs. mitochondrial
membrane vs. nuclear membrane, etc.)
• type of cell
Membranes are Asymmetric!
– inner vs. outer "leaflets" [layers of bilayer] -- different lipid
compositions, different proteins or protein orientation
– asymmetry maintained by extremely slow rate of rotation of
components across membrane
– "flip-flop" essentially doesn’t occur except when catalyzed by
"flippases" (proteins involved in creating/maintaining lipid asymmetries
across membrane)
• Carbohydrate components: almost always on outer surface of membrane
– Glycolipids: lipid with attached carbohydrate found only in outer
leaflet of plasma membranes.
– Glycoproteins:
Carbohydrate component
found only on outsides
of cells, even when protein
itself spans membrane.
Berg et al., Fig. 12-30
Membranes Lipids
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Fatty Acids
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Fatty acyl groups are components of membrane lipids, in ester or amide
linkages.
longchain carboxylic acids, typically 14-24 C atoms
C16 and C18 most common (amphipathic) RCOO– with 0 - 4 double bonds,
usually cis
• palmitate (16-C
saturated F.A.)
• oleate (18-C
unsaturated F.A.,
with 1 cis double
bond. NOTE
"kink" in structure)
• F.A.s are
amphipathic
molecules.
Berg et al., Fig. 12-2
Membranes Lipids
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Main classes of membrane lipids (all amphipathic)
3 types of BACKBONE in membrane lipids
• Glycerol (glycerophospholipids), a 3-carbon tri-alcohol:
CH2OH-CHOH-CH2OH
• Sphingosine (sphingolipids, both sphingophospholipids and
sphingoglycolipids)
• Cholesterol (a steroid compound)
+
Cholesterol
(a steroid
compound)
1. Glycerophospholipids (phosphoglycerides, glycerophosphatides)
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glycerol backbone (3 carbon tri-alcohol, CH2OH-CHOH-CH2OH)
– diacylglycerol (fatty acids esterified to the C1 and C2 OH groups on glycerol; R 1
usually saturated, R2 usually unsaturated;
F.A.s usually 16-18 C's)
– C3 esterified to phosphate
That gives parent compound = phosphatidic acid (phosphatidate at pH 7)
Polar “head” group also esterified to phosphate (any of several alcohols):
ethanolamine, choline, serine, glycerol, inositol, phosphatidyl glycerol
Berg et al.,
<-- Fig. 12-3
Membranes Lipids
Berg et al.,Fig. 12-4
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1. Glycerophospholipids, continued
Results of esterifying different alcohols
• phosphatidyl serine
• phosphatidyl choline (lecithin)
• phosphatidyl ethanolamine
to the phosphate on C3:
• phosphatidyl inositol
• phosphatidyl glycerol
• diphosphatidyl glycerol (cardiolipin)
Berg et al., Fig. 12-5
Phospholipase (PL) cleavage sites
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Phospholipases catalyze hydrolysis of ester bonds in phospholipids.
PLA1: produces free FA(1)
PLA2: produces free FA(2)
PLC: produces diacylglycerol (DAG) and inositol phosphate (IP)
PLD: produces DAG-phosphate and free polar head group
• phospholipases important in
signaling pathways
–PLC: signaling molecules
DAG and IP
–PLA2: free arachidonic
acid to COX for
prostaglandin synthesis.
–Corticosteroid drugs like
prednisone inhibit PLA2.
What effect would steroids
have on inflammation?
Nelson & Cox,
Lehninger Principles
of Biochemistry, 4th
ed., Fig. 10-15
Membranes Lipids
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2. Sphingolipids
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backbone = sphingosine
Similarity/differences with glycerol-based lipids (easier to see in figure on
next slide):
– C1: -OH group (can be esterified to phosphate, or in a glycosidic bond to
carbohydrate)
– C2: amino group (-NH3+) → fatty acyl group in amide linkage
– C3: -OH group that does NOT get derivatized and has a long
hydrocarbon chain, with 1 double bond
– Ceramides have fatty acid in amide linkage to amino group of C2 in ALL
sphingolipids.
Nelson & Cox, Lehninger Principles of
Biochemistry, 4th ed., Fig. 12-6
Structure comparison:
glycerophospholipid and sphingophospholipid
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Note polar head groups and 2 nonpolar tails -- one of the tails on
sphingolipid is the long chain of the sphingosine backbone continuing from
C4-C18
Berg et al., Fig. 12-8
Membranes Lipids
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Sphingolipids
Phosphosphingolipids
Phosphosphingolipids:
Niemann-Pick types
A&B: lack of enzyme
phosphate esterified to
to hydrolyze this bond
C1 OH.
Glycosphingolipids
Sphingomyelins:
choline or
ethanolamine
esterified to C1
phosphate
Glycosphingolipids:
especially abundant in
nerve cell membranes;
carbohydrate(s) on
C1-OH instead of
phosphate group
Tay-Sachs:
lack of enzyme
to hydrolyze
this bond
2. Sphingolipids, continued
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gangliosides (complex oligosaccharides, branched sugar chains on C1 OH)
Degradation of lipids: specific enzymes required for each different bond
hydrolyzed
– Membrane lipids undergo constant metabolic turnover, rate of synthesis
and rate of breakdown being balanced.
– Genetic defects (deficiencies in specific enzymes) in
glycosphingolipid breakdown → abnormal accumulation of partially
degraded lipids, with toxic results (genetic diseases). example:
• Tay-Sachs disease -- lack of hexosaminidase A, needed to
hydrolyze glycosidic bond attaching terminal N-acetylgalactosamine
residue in ganglioside GM2 (previous slide); causes mental
retardation, blindness, muscular weakness, death by age 3-4
Electron micrograph of portion of a brain cell
from infant with Tay-Sachs disease, showing
abnormal ganglioside GM2 deposits in the
lysosomes
Niemann-Pick disease types A and B -- lack of
sphingomyelinase, enzyme needed to hydrolyze
phosphate ester linkage of phosphocholine to
ceramide; symptoms include enlarged liver and
spleen, mental retardation, early death
Nelson & Cox, Lehninger Principles of
Biochemistry, 4th ed., Box 10-2, Fig. 2
Membranes Lipids
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3. Cholesterol: The Good, The Bad, and The Ugly!
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structure:
4 fused hydrocarbon rings
(“steroid nucleus”)
planar, rigid, electrically neutral
amphipathic ("head" group =
OH)
mainly in plasma membranes of
animal cells; organelle
membranes generally have less;
rarely found in bacteria
functions: important membrane
constituent (influences fluidity)
precursor of bile acids
(emulsifiers)
precursor of hormones (steroid
hormones)
Other Lipids
(not structural components of membranes, but biologically important)
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Membranes Lipids
eicosanoids
– paracrine hormones (locally acting)
– all derived from arachidonic acid (20-carbon fatty acid with 4 double
bonds) removed by phospholipase A2
– prostaglandins: mediate fever, inflammation and pain, among other
functions
– thromboxanes (involved in blood clotting)
– leukotrienes (smooth muscle contraction, e.g., muscle lining airways to
lungs -- overproduction causes asthmatic attacks and is involved in
anaphylactic shock, potentially fatal allergic reaction)
“isoprenoid” lipids (all synthesized by condensation of
isoprene units (5 C unsaturated branched units)
– steroid hormones
– fat-soluble vitamins (A, D, E, and K)
– mobile electron carriers in membranes
• ubiquinone in mitochondrial membranes
• plastoquinone in chloroplast membranes
– sugar carriers (dolichols)
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MEMBRANE FLUIDITY: a very slippery topic
• hydrocarbon chains: close packing, maximum VDW interactions between
chains at low temperatures → rigid "gel state"
• Above a specific transition temperature (Tm) lipid bilayer "melts") to “fluid
state” (chains not so closely packed).
• Tm can be decreased by:
a) decreasing chainlength (decreasing VDW interactions)
b) increasing number of cis double bonds (bends in FA tail)
Both effects weaken VDW interactions, increasing fluidity!
Note packing of fatty acid side chains is disrupted by cis double bonds.
With more double bonds, membrane remains fluid at lower temperatures
(Tm is lowered).
Berg et al., Fig. 12-33
Why is Membrane Fluidity Important?
• Membranes of living cells must be fluid -- must have transition
temperatures below body temperature of the organism.
• Regulation of fluidity (especially in organisms that don’t rigorously
control their body temperature) by lipid composition:
1. fatty acid chainlength (shorter → more fluid)
2. number of double bonds (more d.b. → more fluid)
3. Cholesterol (animal cells) has a dual and contradictory role:
a) "stiffens" membrane by packing between unsaturated HC tails, but
b) disrupts close packing between saturated tails
Net: broadens the melting transition range sort of like a fluidity "buffer"
Fluid bilayer
Rigid bilayer (“gel’)
Berg et al., Fig. 12-11
Membranes Lipids
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Learning Objectives
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Terminology: micelle, lipid bilayer, amphipathic
List the biological roles and the molecular components of membranes.
With the structure of a lipid as an example, point out the features that
make a molecule amphipathic.
Explain why amphipathic membrane lipids form self-sealing bilayers in
aqueous environments, including the types of interactions stabilizing
the bilayer structure.
Write out the structure of a 16-carbon saturated fatty acid (i.e., no
double bonds), and describe the general properties of the fatty acyl
components of membrane lipids.
Be able to recognize the structures of phosphoglycerides,
phosphosphingolipids, glycosphingolipids, and cholesterol. What type
of lipids are gangliosides?
Briefly explain the consequences if an individual has a genetic
deficiency in any one specific enzyme involved in glycosphingolipid
degradation.
What bond in a glycerophospholipid is cleaved (hydrolyzed) by
phospholipase A1? A2? C? D?
Learning Objectives, continued
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Membranes Lipids
Discuss how living organisms regulate the fluidity of their membranes,
including in your discussion the effects on fluidity of temperature, fatty
acyl chainlength, and number of double bonds.
Discuss the concepts of lateral and transverse (“flip-flop”) diffusion of
membrane lipids and proteins, and the asymmetric distribution of
membrane components (especially carbohydrate portions) on the
extracellular and intracellular sides of the bilayer.
Describe the permeability properties of lipid bilayers.
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