Lipids and Membranes

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• Lipids and Membranes

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Lipids

• Lipids are compounds that are soluble in
  non-polar organic solvents, but insoluble in
  water.
• Can be hydrophobic or amphipathic

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Major Lipid Classes

• Acyl-lipids - contain fatty acid groups as main
  non-polar group
• Isoprenoids made up of 5 carbon isoprene units

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Lipid Subclasses
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Function of major acyl-lipids

• Phospholipids membrane components
• Triacylglycerols storage fats and oils
• Waxes moisture barrier
• Eicosanoids signaling molecules (prostaglandin)
• Sphingomyelins membrane component (impt. in
  mylein sheaths)
• Glycospingolipids cell recognition (ABO blood
  group antigen)

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Function of major isoprenoid lipids

• Steroids (sterols) membrane component, hormones
• Lipid Vitamins Vitamin A, E, K
• Carotenoids - photosynthetic accessory pigments
• Chlorophyll major light harvesting pigment
• Plastoquinone/ubiquinone lipid soluble electron
  carriers
• Essential oils menthol

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Fatty acids

• Amphipathic molecule
• Polar carboxyl group
• Non-polar hydrocarbon tail
• Diverse structures (gt100 different types)
• Differ in chain length
• Differ in degree of unsaturation
• Differ in the position of double bonds
• Can contain oxygenated groups

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Fatty acid nomenclature

• Short hand nomenclature describes total number of
  carbons, number of double bonds and the position
  of the double bond(s) in the hydrocarbon tail.
• C181 D9 oleic acid, 18 carbon fatty acid with
  a double bond positioned at the ninth carbon
  counting from and including the carboxyl carbon
  (between carbons 9 and 10)

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Fatty acid nomenclature

• Omega (w) notation counts carbons from end of
  hydrocarbon chain.
• Omega 3 fatty acids advertised as health
  promoting
• Linoleate 183 D9,12,15 and 183w3,6,9

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Common saturated fatty acids
common name IUPAC name melting point (Co)
120 laurate dodeconoate 44
140 myristate tetradeconoate 52
160 palmitate hexadeconoate 63
180 stearate octadeconoate 70
200 arachidate eicosanoate 75
220 behenate docosanoate 81
240 lignocerate tetracosanate 84
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Common unsaturated fatty acids
common name IUPAC name melting point (Co)
160 palmitate hexadeconoate 63
161 D9 palmitoleate cis-D9-hexadeconoate -0.5
180 stearate octadeconoate 70
181 D9 oleate cis-D9- octadeconoate 13
182 D9,12 linoleate cis-D9,12- octadeconoate -9
183 D9,12,15 linolenate cis-D9,12,15- octadeconoate -17
200 arachidate eicosanoate 75
204 D5,8,11,14 arachindonate cis- D5,8,11,14-eicosatetraenoate -49
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Physical Properties of Fatty acids
180 181 183

• Saturated chains pack tightly and form more
  rigid, organized aggregates
• Unsaturated chains bend and pack in a less
  ordered way, with greater potential for motion

70o 13o -17o
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Melting points of fatty acids affect properties
of acyl-lipids

• Membrane fluidity determined by temperature and
  the degree of fatty acid unsaturation of
  phospholipids
• Certain bacteria can modulate fatty acid
  unsaturation in response to temperature
• Difference between fats and oils
• Cocoa butter perfect melt in your mouth fat
  made of triacylglycerol with 180-181-180 fatty
  acids
• Margarine is hydrogenated vegetable oil. Increase
  saturation of fatty acids. Introduces trans
  double bonds (thought to be harmful)

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Unusual fatty acids can function analogously to
unsaturated fatty acids
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• Major acyl-lipids
• Phospholipids membrane components
• Triacylglycerols storage fats and oils
• Waxes moisture barrier
• Eicosanoids signaling molecules (prostaglandin)
• Sphingomyelins membrane component (impt. in
  mylein sheaths)
• Glycospingolipids cell recognition (ABO blood
  group antigen)

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Phospholipids

• Phospholipids are built on glycerol back bone.
• Two fatty acid groups are attached through ester
  linkages to carbons one and two of glycerol.
• Unsaturated fatty acid often attached to carbon 2
• A phosphate group is attached to carbon three
• A polar head group is attached to the phosphate
  (designated as X in figure)

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Common membrane phospholipids
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Enzymes used to Dissect Phospholipid Structure
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Plasmalogens

• Plasmalogens have hydrocarbon at carbon 1
  attached thru vinyl ether linkage
• Polar head group could be ethanolamine or choline
• Important component of membranes in central
  nrevous system

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Sphingolipids

• Sphingolipids named from Sphinx due to mysterious
  role
• Abundant in eukaryotic membranes, but not found
  in bacteria
• Structural backbone made of sphingosine
• Unbranched 18 carbon alcohol with a trans double
  bond between C4 and C5
• Contains an amino group attached to C2 and
  hydroxyl groups on C1 and C3

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Ceramides

• Sphingosine with fatty acid attached to carbon 2
  by amide linkage
• Metabolic precursors to sphingolipids

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Sphingomyelin

• has phosphocholine group attached to C1 of
  ceramide.
• Resembles phosphatidylcholine
• Major component of myelin sheaths that surround
  nerve cells

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Cerebrosides

• contains one monosaccharide residue attached to
  C-1 of ceramide
• Glucose and galtactose are common
• Can have up to 3 more monosaccharide residues
  attached to sugar on C1
• Abundant in nerve tissue
• Up to 15 of myelin sheath made up of cerebrosides

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Gangliosides

• Gangliosides have oliosaccharide containing
  N-acetylneuraminic acid attached to C1 of
  ceramide
• Diverse class of sphingolipid due to variety of
  olgosaccharide species attached
• Oligosaccharide moiety present on extracellular
  surface of membranes
• ABO blood group antigens are gangliosides
• Impt in cell recognition, cell-cell communication

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Defects in sphingolipid metabolism lead to
disease state

• Tay-Saachs disease is a genetic defect in
  gangliosides degradation. Gangliosides accumulate
  in spleen and brain. Leads to retardation in
  development, paralysis, blindness, and early
  death.
• Niemann-Pick disease is a genetic defect in
  sphingomyelin degradation. Causes Sphingomyelin
  accumulation in brain, spleen and liver. Causes
  mental retardation. Children die by age 3 or 4.

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Triacylglycerols (TAG)

• Fats and oils
• Impt source of metabolic fuels
• Because more reduced than carbos, oxidation of
  TAG yields more energy (16 kJ/g carbo vs. 37 kJ/g
  TAG)
• Americans obtain between 20 and 30 of their
  calories from fats and oils. 70 of these
  calories come from vegetable oils
• Insulation subcutaneous fat is an important
  thermo insulator for marine mammals

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Olestra

• Olestra is sucrose with fatty acids esterified to
  OH groups
• digestive enzymes cannot cleave fatty acid groups
  from sucrose backbone
• Problem with Olestra is that it leaches fat
  soluble vitamins from the body

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isoprenoids

• Isoprenoids are derived from the condensation of
  5 carbon isoprene units
• Can combine head to head or head to tail
• Form molecules of 2 to gt20 isoprene units
• Form large array of different structures

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Terprenes
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Steroids

• Based on a core structure consisting of three
  6-membered rings and one 5-membered ring, all
  fused together
• Triterpenes 30 carbons
• Cholesterol is the most common steroid in animals
  and precursor for all other steroids in animals
• Steroid hormones serve many functions in animals
  - including salt balance, metabolic function and
  sexual function

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cholesterol

• Cholesterol impt membrane component
• Only synthesized by animals
• Accumulates in lipid deposits on walls of blood
  vessels plaques
• Plaque formation linked to cardiovascular disease

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Steroids
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Many steroids are derived from cholesterol
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Membranes

• Barrier to toxic molecules
• Help accumulate nutrients
• Carry out energy transduction
• Facilitate cell motion
• Modulate signal transduction
• Mediate cell-cell interactions

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The Fluid Mosaic Model

• 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)
• integral proteins (intrinsic proteins)

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The Fluid Mosaic Model
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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 this (for proteins), using
  fluorescent-labelled antibodies
• Lipid diffusion has been demonstrated by NMR and
  EPR (electron paramagnetic resonance) and also by
  fluorescence measurements
• Diffusion of lipids between lipid monolayers is
  difficult.

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fusion
After 40 minutes
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Flippases

• 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 are Asymmetric
In most cell membranes, the composition of the
outer monolayer is quite different from that of
the inner monolayer
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Membrane Phase Transitions

• 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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Phase Transitions

• Only pure lipid systems give sharp, well-defined
  transition temperatures
• Red pure phospholipid
• Blue phopholipid cholesterol

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Structure of Membrane Proteins

• Integral (intrinsic) proteins
• Peripheral (extrinsic) proteins
• Lipid-anchored proteins

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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

• Integral proteins are strongly imbedded in the
  bilayer
• They can only be removed from the membrane by
  denaturing the membrane (organic solvents, or
  strong detergents)
• Often transmembrane but not necessarily
• Glycophorin, bacteriorhodopsin are examples

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Seven membrane-spanning alpha helices, connected
by loops, form a bundle that spans the bilayer in
bacteriorhodopsin. The light harvesting
prosthetic group is shown in yellow. Bacteriorhodo
psin has loops at both the inner and outer
surface of the membrane. It displays a common
membrane-protein motif in that it uses alpha
helices to span the membrane.
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Lipid-Anchored Proteins

• 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
• Always N-terminal
• Always a Gly residue that links

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Thioester/ester-linked Acyl Anchors

• Broader specificity for lipids - myristate,
  palmitate, stearate, oleate all found
• Broader specificity for amino acid links - Cys,
  Ser, Thr all found

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Thioether-linked Prenyl Anchors

• Prenylation refers to linking of "isoprene"-based
  groups
• Always Cys of CAAX (CCys, AAliphatic, Xany
  residue)
• Isoprene groups include farnesyl (15-carbon,
  three double bond) and geranylgeranyl (20-carbon,
  four double bond) groups

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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

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Membrane transport

• Membranes are selectively permeable barriers
• Hydrophobic uncharged small molecules can freely
  diffuse across membranes.
• Membranes are impermeable to polar and charged
  molecules.
• Polar and charged molecules require transport
  proteins to cross membranes (translocators,
  permeases, carriers)

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Transport of non-polar molecules

• Non-polar gases, lipids, drugs etc
• Enter and leave cells through diffusion.
• Move from side with high concentration to side of
  lower concentration.
• Diffusion depends on concentration gradient.
• Diffusion down concentration gradient is
  spontaneous process (-DG).

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Transport of polar or charged compounds

• Involves three different types of integral
  membrane proteins
• Channels and Pores
• Passive transporters
• Active transporters
• Transporters differ in kinetic and energy
  requirements

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Channels and Pores

• Have central passage that allows molecules cross
  the membrane.
• Can cross in either direction by diffusing down
  concentration gradient.
• Solutes of appropriate size and charge can use
  same pore.
• Rate of diffusion is not saturable.
• No energy input required

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Porins

• Present in bacteria plasma membrane and outer
  membrane of mitochondria
• Weakly selective, act as sieves
• Permanently open
• 30-50 kD in size
• exclusion limits 600-6000
• Most arrange in membrane as trimers

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Passive Transport (Facilitated Diffusion)

• Solutes only move in the thermodynamically
  favored direction
• But proteins may "facilitate" transport,
  increasing the rates of transport
• Two important distinguishing features
• solute flows only in the favored direction
• transport displays saturation kinetics

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Three types of transporters

• Uniporter carries single molecule across
  membrane
• Symport cotransports two different molecules in
  same direction across membrane
• Antiport cotransports two different molecules
  in opposite directions across membrane.

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• Rate of diffusion is saturable.
• Ktr S when rate of transport is ½ maximun
  rate.
• Similar to M-M kinetics
• The lower the Ktr the higher the affinity for
  substrate.

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• Transporters undergo conformational change upon
  substrate binding
• Allows substrate to transverse membrane
• Once substrate is released, transported returns
  to origninal conformation.

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Active Transport Systems

• Some transport occur such that solutes flow
  against thermodynamic potential
• Energy input drives transport
• Energy source and transport machinery are
  "coupled"
• Like passive transport systems active
  transporters are saturable

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• Primary active transport
• Powered by direct source of energy(ATP, Light,
  concentration gradient)
• Secondary active transport
• Powered by ion concentration gradient.
• Transport of solute A is couple with the
  downhill transport of solute B.
• Solute B is concnetrated by primary active
  transport.

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Na-K ATPase

• Maintains intracellular Na low and K high
• Crucial for all organs, but especially for neural
  tissue and the brain
• ATP hydrolysis drives Na out and K in

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Na-K ATPase

• Na K concentration gradients are maintained
  by Na-K ATPase
• ATP driven antiportsystem.
• imports two K and exports three Na for every
  ATP hydrolyzed
• Each Na-K ATPase can hydrolyze 100 ATPs per
  minute (1/3 of total energy consumption of cell)
• Na K concentration gradients used for 2o
  active transport of glucose in the intestines

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1o active transport of Na
2o active transport of glucose
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Transduction of extracellular signals

• Cell Membranes have specific receptors that allow
  cell to respond to external chemical stimuli.
• Hormone molecules that are active at a
  distance. Produced in one cell, active in
  another.
• Neurotransmitters substances involved in the
  transmission of nerve impulse at synapses.
• Growth factors proteins that regulate cell
  proliferation and differentiation.

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• External stimuli(first messenger) (hormone,
  etc)
• Membrane receptor binds external stimuli
• Transducer membrane protein that passes signal
  to effector enzyme
• Effector enzyme generates an intracellular
  second messenger
• Second messenger small diffusible molecule that
  carrier signal to ultimate destination

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G-Proteins

• Signal transducers.
• Three subunits, (a,b, g) a and g anchored to
  membrane via fatty acid and prenyl group
• Catalyze hydrolysis of GTP to GDP.
• GDP bound form is inactive/GTP bound form active
• When hormone bound receptor complex interacts
  with G-protein, GDP leaves and GTP binds.
• Once GTP -gt GDP G-protein inactive
• GTP hydrolysis occurs slowly (kcat 3min-1) good
  timing mechanism

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Epinephrine signaling pathway

• Epinephrine regulation of glycogen degradation
• Fight or Flight response
• Ephinephrine primary messenger
• G-protein mediated response.
• G-protein activates Adenyl-cyclase to produce
  cAMP
• cAMP is the second messenger
• Activates protein kinase
• Activates glycogen phosphorylase

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Effect of Caffeine

• Caffeine inhibits cAMP phosphodiesterase ,
  prevents breakdown of cAMP.
• Prolongs and intensifies Epinephrine effect.

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Phosphatidylinositol (PI) Signaling Pathway

• G-protein mediated
• G-protein activates phospholipase C (PLC)
• PLC cleaves PI to form inositol-triphosphate
  (IP3) and diacylglycerol (DAG) both act as 2nd
  messengers
• IP3 stimulates Ca2 releases from ER
• DAG stimulates Protein kinase C

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