Download Lecture 18 Membranes 1: Lipids and Lipid Bilayers

BIOC 460, Spring 2008
Lecture 18
Membranes 1: Lipids and
Lipid Bilayers
Subsequent 3 lectures:
– Membrane Proteins
– 2 lectures on Membrane Transport
Reading: Berg, Tymoczko & Stryer, 6th ed., Chapter 12,
pp. 326-335
Problems: Chapter 12, p. 150, #9
Key Concepts
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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)
–
Cholesterol
Membrane fluidity (vital to membrane function) depends on lipid
composition of bilayer.
–
fatty acid chainlength (more C atoms → more packing of tails, less
fluidity)
–
fatty acid numbers of double bonds (fewer double bonds → more
packing of tails, less fluidity)
–
cholesterol content ("buffers" fluidity)
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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 cerebrosides and 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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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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Biological Membranes
• 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 membrane lipids
• hydrophilic portion and hydrophobic portion
– hydrophilic portion = "head"; hydrophobic chain(s) = "tails"
• Consequence: Amphipathic lipids form micelles or bilayers, to bury their
hydrophobic tails so they're NOT exposed to H2O, but keep the hydrophilic
head groups in contact with H2O.
• Lipids with single hydrophobic tails can form micelles, but
• Membrane lipids almost all have 2 tails, and thus form bilayers.
– Bilayers curve around and seal edges → closed vesicles (liposomes).
• The hydrophobic effect provides the major driving force for the formation
of lipid bilayers.
“slice” through a micelle
Berg et al., Fig. 12-9
LEC 18, Membranes 1 - Lipids and Lipid
Bilayers
“slice” through a bilayer
Berg et al., Fig. 12-10
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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
“slice” through a liposome
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
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Lipid Components of Animal Cell Membranes
• 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
Membrane Assymmetry
– inner vs. outer "leaflets" [layers of bilayer] -- different lipid
compositions, different proteins or protein domains
– asymmetry maintained because of 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: on outer surface of membrane
– Glycolipids (have carbohydrate components) found only in outer
leaflet of plasma membranes.
– Glycoproteins:
Carbohydrate components
found only on outsides
of cells, even when protein
itself spans membrane.
• Overall lipid composition
related to environment (esp.
temperature) - lipid composition
regulates fluidity)
Berg et al., Fig. 12-30
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Fatty Acids
• Fatty acyl groups are components of membrane lipids, in ester or amide
linkages.
• longchain carboxylic acids, typically 14-24 C atoms
• C16 & 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
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)
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1. Glycerophospholipids (phosphoglycerides, glycerophosphatides)
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start with glycerol backbone (3 carbon tri-alcohol, CH2OH-CHOH-CH2OH)
– diacylglycerol (fatty acids esterified to the C1 and C2 OH groups on glycerol; R1
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)
• + another substituent also esterified to phosphate (any of several alcohols):
ethanolamine, choline, serine, glycerol, inositol, phosphatidyl glycerol
Berg et al.,
<-- Fig. 12-3
Berg et al.,Fig. 12-4
1. Glycerophospholipids, continued
Results of esterifying different alcohols to the phosphate on C3:
• phosphatidyl serine
• phosphatidyl choline (lecithin)
• phosphatidyl ethanolamine
• phosphatidyl inositol
• phosphatidyl glycerol
• diphosphatidyl glycerol (cardiolipin)
Berg et al., Fig. 12-5
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Phospholipase (PL) cleavage sites
Phospholipases catalyze hydrolysis of ester bonds in phospholipids.
PLA1 cleaves ester bond to C1 OH
PLA2 cleaves ester bond to C2 OH
PLC cleaves phosphate ester bond to C3 OH
PLD cleaves phosphate ester bond to other alcohol on C3 phosphate
(choline, ethanolamine, etc.)
• Phospholipase
specificities ------------->
• activity of phospholipases
important in signaling
pathways
–PLC generates 2 intracellular signaling molecules:
diacylglycerol (DAG) and
inositol phosphate (IP)
–PLA2 removes arachidonic
acid from membrane lipids
for COX enzymes to make
prostaglandins.
–Corticosteroid drugs like
Nelson & Cox,
prednisone inhibit PLA2.
Lehninger Principles
What effect would steroids
of Biochemistry, 4th
have on inflammation?
ed., Fig. 10-15
2. Sphingolipids
• backbone = sphingosine
• Similarity/differences with glycerol-based lipids (easier to see in figure on
next slide):
– C1 has an OH group (can be esterified to phosphate, or in a glycosidic
bond to carbohydrate)
– C2 has amino group (-NH3+) instead of -OH on glycerol → fatty acyl
group in amide linkage (not ester)
– C3 has -OH group that does NOT get derivatized, and instead of one H
atom on glycerol C3 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
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Structure comparison:
glycerophospholipid and sphingophospholipid
• 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
Sphingolipids
Phosphosphingolipids
Phosphosphingolipids:
Niemann-Pick types
phosphate esterified to
A&B: lack of enzyme
C1 OH.
to hydrolyze this bond
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
LEC 18, Membranes 1 - Lipids and Lipid
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Tay-Sachs:
lack of enzyme
to hydrolyze
this bond
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2. Sphingolipids, continued
• 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
3. Cholesterol
• structure:
4 fused hydrocarbon rings, 3 with 6 C's, 1 with 5 C's (“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)
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Other Lipids
(not structural components of membranes, but biologically important)
• eicosanoids
– paracrine hormones (locally acting)
– all synthesized starting from arachidonic acid (20-carbon fatty acid with 4
double bonds, removed by phospholipase A2 from position 2 of
membrane glycerophospholipids)
– 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)
MEMBRANE FLUIDITY -- controlled by lipid composition
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hydrocarbon chains: close packing, maximum interaction between
chains at low temperatures → rigid "gel"; the longer the chains and
the more saturated (fewer double bonds), the more ordered/rigid the
state of the lipid bilayer
Above transition temperature, lipid bilayer undergoes phase change
("melting") to more disorderly, FLUID state (chains not so closely
packed).
Transition temperature is lowered (so relative fluidity increases) by
fatty acid structures that reduce favorable packing interactions:
a) shorter hydrocarbon chainlength, and/or
b) more double bonds (which make "bends" in the chain)
Highly ordered packing of fatty acid side chains (stabilized by lots of close
van der Waals interactions) is disrupted by cis double bonds (kinks).
With more double bonds, membrane remains fluid at lower temperatures
(transition temp. is lowered).
Berg et al., Fig. 12-33
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Regulation of Membrane Fluidity
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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) "stiffens" membrane by packing
between unsaturated HC tails, but also disrupts close packing
between saturated tails, so broadens the transition sort of like a
fluidity "buffer", when temperature or fatty acid composition
changes.
Fluid bilayer
Rigid bilayer (“gel’)
Berg et al., Fig. 12-11
Lipid Bilayers -- formed spontaneously by phospholipids
• Single-tailed amphipathic lipids form micelles in H2O (spheres with
polar head groups out, exposed to H2O; nonpolar tails buried in center)
• "2-tailed" amphipathic lipids spontaneously form bilayers, burying the
tails between the 2 layers; 2 tails (e.g., phosphoglycerides and
sphingolipids) don’t fit in middle of a micelle -- surface with head groups not
large enough to bury double tails
• self-assembling and self-sealing -- form and grow spontaneously, and
close in on themselves spontanously, because a "hole" would expose the
lipid tails to the H2O.
• Bilayer structure stabilized by hydrophobic effect (the driving force for
their formation)
–hydration of polar/charged head groups
–van der Waals interactions (packing between atoms in hydrophobic core)
• Hydrophobic core of the membrane is like a nonpolar solvent.
– Permeability coefficients correlated with solubility in nonpolar solvent
relative to solubility in H2O.
– highly impermeable to ions and most polar molecules
– more permeable to nonpolar species
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