1. Lipids and Membranes

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

3. Major Lipid Classes
Acyl-lipids - contain fatty acid groups as main non-polar group
Isoprenoids – made up of 5 carbon isoprene units

4. Lipid Subclasses

5. 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)

6. 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

7. Fatty acids Amphipathic molecule Polar carboxyl group
Non-polar hydrocarbon tail
Diverse structures (>100 different types)
Differ in chain length
Differ in degree of unsaturation
Differ in the position of double bonds
Can contain oxygenated groups

8. 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.
C18:1 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)

9. Fatty acid nomenclature
Omega (w) notation – counts carbons from end of hydrocarbon chain.
Omega 3 fatty acids advertised as health promoting
Linoleate = 18:3 D9,12,15 and 18:3w3,6,9

10. Common saturated fatty acids
common name
IUPAC name
melting point (Co)
12:0
laurate
dodeconoate 44
14:0
myristate
tetradeconoate 52
16:0
palmitate
hexadeconoate 63
18:0
stearate
octadeconoate 70
20:0
arachidate
eicosanoate 75
22:0
behenate
docosanoate 81
24:0
lignocerate
tetracosanate 84

11. Common unsaturated fatty acids
common name
IUPAC name
melting point
(Co)
16:0
palmitate
hexadeconoate 63
16:1 D9
palmitoleate
cis-D9-hexadeconoate
-0.5
18:0
stearate
octadeconoate 70
18:1 D9
oleate
cis-D9- octadeconoate 13
18:2 D9,12
linoleate
cis-D9,12- octadeconoate -9
18:3 D9,12,15
linolenate
cis-D9,12,15- octadeconoate
-17
20:0
arachidate
eicosanoate 75
20:4 D5,8,11,14
arachindonate
cis- D5,8,11,14-eicosatetraenoate
-49

12. Physical Properties of Fatty acids
18: : :3
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 o o

13. 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 18:0-18:1-18:0 fatty acids
Margarine is hydrogenated vegetable oil. Increase saturation of fatty acids. Introduces trans double bonds (thought to be harmful)

14. Unusual fatty acids can function analogously to unsaturated fatty acids

15. 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)

16. 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)

18. Common membrane phospholipids

19. Enzymes used to Dissect Phospholipid Structure

20. 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

21. 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

22. Ceramides
Sphingosine with fatty acid attached to carbon 2 by amide linkage
Metabolic precursors to sphingolipids

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

24. 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

25. 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

26. 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.

27. 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

28. 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

29. 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 >20 isoprene units
Form large array of different structures

30. Terprenes

31. 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

32. 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

33. Steroids

34. Many steroids are derived from cholesterol

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

36. 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)

37. The Fluid Mosaic Model

38. 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.

39. fusion
After 40 minutes

40. 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

42. Membranes are Asymmetric
In most cell membranes, the composition of the outer monolayer is quite different from that of the inner monolayer

43. 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

44. Phase Transitions
Only pure lipid systems give sharp, well-defined transition temperatures
Red = pure phospholipid
Blue = phopholipid + cholesterol

45. Structure of Membrane Proteins
Integral (intrinsic) proteins
Peripheral (extrinsic) proteins
Lipid-anchored proteins

46. Peripheral Proteins
Peripheral proteins are not strongly bound to the membrane
They can be dissociated with mild detergent treatment or with high salt concentrations

47. 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

48. 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.
Bacteriorhodopsin 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.

49. Lipid-Anchored Proteins
Four types have been found:
Amide-linked myristoyl anchors
Thioester-linked fatty acyl anchors
Thioether-linked prenyl anchors
Glycosyl phosphatidylinositol anchors

50. Amide-Linked Myristoyl Anchors
Always myristic acid
Always N-terminal
Always a Gly residue that links

51. 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

53. Thioether-linked Prenyl Anchors
Prenylation refers to linking of "isoprene"-based groups
Always Cys of CAAX (C=Cys, A=Aliphatic, X=any residue)
Isoprene groups include farnesyl (15-carbon, three double bond) and geranylgeranyl (20-carbon, four double bond) groups

55. 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

57. 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)

58. 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).

60. 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

61. 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

62. 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
Most arrange in membrane as trimers

63. 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

64. 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.

65. Saturation Kinetics of transport
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.

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

67. 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

68. 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.

69. 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

70. 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

71. 1o active transport of Na+
glucose

72. 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.

73. 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

74. 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 -> GDP G-protein inactive
GTP hydrolysis occurs slowly (kcat= 3min-1) good timing mechanism

75. 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

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

78. 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