NUR8103LEC NOTES- Lipids
Lipids
➢ Heterogeneous class of biological molecules classified on the basis of common solubility
E.g. fats and oils
➢ Soluble in organic solvents such as diethyl ether, chloroform, methylene chloride, and
acetone but insoluble in water
➢ Amphipathic molecule
Molecule that has hydrophilic (water loving) and hydrophobic regions
Lipids
Open-chain
1. fatty acid
2. triacylgycerols
3. phosphoacylglycerols
4. sphingolipids
5. glycolipids
Ring- fused
1. cholesterol
2 .steroid hormoes
2. bile salts
Fatty Acids
➢ Unbranched chain of carboxylic acids
12–20 carbons long
➢ Derived from hydrolysis of animal fats, vegetable oils, or phosphodiacylglycerols of
biological membranes
➢ Amphipathic compounds
➢ Types
Saturated fatty acids
Unsaturated fatty acids
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Saturated fatty acids
-hydrocarbon chain single-bonded with carboxylic acids
-have even number of carbon atoms because these fatty acids are built from acetic acid
- solid at room temperature, high melting point because of close and regular packing of
hydrocarbon chain
-found in animal fats and resist oxidation
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Unsaturated fatty acids
-cis isomer predominates while trans isomer is rare
-cis double bond puts a kink in the long-chain hydrocarbon tail
-shape of a trans fatty acid is like that of a saturated fatty acid in its fully extended conformation
-double bonds are isolated by several singly bonded carbons
-have lower melting points than their saturated counterparts, liquids at room temperature
-greater the degree of unsaturation, lower the melting point
-essential fatty acids- linoleic (C18) and arachidonic (C20); polyunsaturated, not
synthesized in the body, must be obtained from the diet
-usually found in vegetable oils and can undergo oxidation producing short chain
aldehydes and carboxylic acids
- can undergo hydrogenation reaction producing saturated fatty acids
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Fatty Acid Nomenclature:
- named according to the number of C atoms and to the number and position of any
double bonds.
- systematic name: add ‘oic acid’ on the name of the parent hydrocarbon
- but fatty acids at physiological pH are ionized and name ending with ‘ate’ than
‘oic acid’ and represented as RCOO- shorthand notation: 18:0= fatty acid with 18 carbon and 0 double bonds
18:2= fatty acid with 18 carbon and 2 double bonds
cis, cis- Δ9, Δ12 … =first C=C is between C 9 and 10 and the
second C=C is between C12 and 13
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Omega-3 fatty acid
- polyunsaturated fatty acids found abundantly in marine animals: linolenic (18:3Δ9,12,15), eicosapentaenoic acid /EPA (20:5-Δ5,8,11,14,17), docosahexaenoic acid/ DHA (22:6Δ4,7,10,13,16,19)
- associated with low occurrence of atherosclerosis and heart attacks even in a
high-fat diet and high levels of cholesterol (HDL)
- lower the ability of platelets to form blood clots
- omega-3: double bond at C3 farthest from the -COOH group
- omega-6: polyunsaturated fatty acids found abundantly in vegetable oils such as
linoleic (C18) and arachidonic acids (C20)
Prostaglandins
➢ Derived from fatty acids
➢ First detected in seminal fluid, which is produced by the prostate gland
➢ Arachidonic acid
Metabolic precursor of all prostaglandins
Contains four double bonds, which are not conjugated
Have five-membered rings
Differ from one another in numbers and positions of double bonds and oxygen-containing
functional groups
Physiological effects of prostaglandins:
1. Activation of inflammatory response in joints
2. Production of fever
3. Production of pain
4. Induction of blood clotting
Triacylglycerols(triglycerides)
➢ formed by esterification of three fatty acids to glycerol
➢ ester groups are polar part and the tails are nonpolar part of the molecule
➢ stored in adipose tissues and give means of storing fatty acids
➢ lipases hydrolyze ester bonds when fatty acids are used by organisms
➢ act as concentrated stores of metabolic energy
➢ Acrolein test - presence of glycerol
➢ Br2- unsaturation test
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Saponification
➢ Reaction of glyceryl ester with NaOH or KOH to produce glycerol and respective Na or K
salts (soaps)
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Soaps form water-insoluble salts when used in hard water, which contains Ca2+, Mg2+, and
Fe3+
Glycerol is used in creams and in the manufacture of nitroglycerin
Phosphatidic acid
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Compound in which two fatty acids and phosphoric acid are esterified to the three hydroxyl
groups of glycerol
Phosphoric acid is triprotic in nature
One molecule can form ester bonds to glycerol and to some other alcohol to create
phosphatidyl esters
Phosphatidyl ester
Figure 5. Structure of phosphatidyl ester
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Structure
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Long, nonpolar, hydrophobic tails
Polar, highly hydrophilic head groups
Nature of fatty acids in a molecule varies widely
Amphipathic
Classified as phosphoacylglycerols
Classification depends on the second alcohol that is esterified to the phosphoric acid
Phosphoacylglycerol
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Polar head is charged
Phosphate group is ionized at neutral pH
Positively charged amino group is contributed by an amino alcohol esterified to the
phosphoric acid
Phospatidylserine- (+) in Ninhydrin test
Structures of phosphatidylcholine(lecithin); phosphatidylserine;
phosphatidylethanolamine(cephalin)
Waxes
➢ Complex mixtures of esters of long-chain carboxylic acids and long-chain alcohols
➢ Serve as protective coatings for plants and animals
Sphingolipids
➢ Contain sphingosine, a long-chain amino alcohol
➢ Found in plants and animals
➢ Abundant in the nervous system
➢ Sphingomyelin
Primary alcohol of sphingosine is esterified to phosphoric acid, which is esterified to
choline
Glycolipids
➢ Lipid bonded to a sugar
➢ Ceramides= parent compounds of glycolipids
➢ Cerebroside= compound formed-primary alcohol group of the ceramide and a sugar
residue (glucose or galactose)
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Gangliosides=glycolipids with a complex carbohydrate moiety that contains more than
three sugars (one is always a sialic acid )
=acidic glycosphingolipids because of their net negative charge at neutral pH
Steroids
➢ Lipids with a fused-ring structure
Three six-membered rings (the A, B, and C rings)
One five-membered ring (the D ring)
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Important steroids include sex hormones and cholesterol
Cholesterol: Occurs in cell membranes
Highly hydrophobic
Acts as a precursor of other steroids
Plays a role in the development of atherosclerosis
Bile Salts
➢ Synthesized in the liver from cholesterol and stored in the gall bladder
➢ Acts as emulsifying agents
➢ Gallstones: high levels of cholesterol in the gall bladder causing precipitation
➢ Jaundice:gallstones obstructing the bile duct, bile cannot be secreted and bile pigments
(bilirubin) enter the blood
Lipoproteins
➢ Lipids must be transported through the bloodstream for:
1. Energy storage
2. Synthesis of cell membranes, nerve tissues, hormones and bile salts
3. Carriers of insoluble lipids
➢ LDL (low density lipoproteins): bad cholesterol, deposit excess cholesterol in the arteries
(artherosclerosis) and a risk of myocardial infarction
➢ HDL (high density lipoproteins): good cholesterol, remove excess cholesterol from tissues,
carry them back to the liver, for the elimination as bile salts
➢ High-fat diet (saturated fatty acids) causes:
1. Reabsorption of cholesterol from bile salts
2. Liver to increase its synthesis of cholesterol
Lipoproteins
Origin
chylomicrons
VDL
Intestinal
mucosa
liver
LDL
liver
HDL
liver
Compounds
transported
Exogenous lipids
Targeted cells/
Tissues
adipose
Endogenous
lipids
adipose,
extrahepatic
tissues
Extrahepatic
tissues,
arteries(excess
cholesterol)
liver
Cholesterol and
other
endogenous
lipids
Cholesterol (as
products of
metabolism)
Function/s
Store energy
Store energy
Synthesis of cell
membrane,
hormones and
bile salts
Elimination as
bile salts
Biological Membranes
➢ Every cell has a cell (plasma) membrane
➢ Membrane-enclosed organelles (nuclei and mitochondria) present in eukayotes
➢ Lipid and protein components= molecular basis of membrane structure
➢ Separate cells from the external environment and transport specific substances into and
out of cells
➢ Membrane function is determined by interaction between lipid bilayers and membrane
proteins
➢ Contain many important enzymes whose function depends on the membrane environment
➢ Hydrophobic interaction= major force driving the formation of lipid bilayers is
➢ Differ from lipid bilayers as they contain proteins as well as lipids
Lipid bilayer
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Aggregate of a lipid molecule- polar head groups are in contact with water and the
hydrophobic parts are not
Polar surface contains charged groups
Hydrocarbon interior consists of saturated and unsaturated fatty acid chains and
cholesterol
Arrangement is held together by noncovalent interactions-van der Waals and hydrophobic
interactions
Bulkier molecules tend to occur in the outer layer
Smaller molecules tend to occur in the inner layer
HW- LIPIDS
1. Discuss why a saturated fatty acid like lauric acid has a good anti-oxidant property.
2. Why are the essential fatty acid associated with low incidence of heart disease? Cite
some clinical signs of essential fatty acid deficiency.
3. Explain how aspirin can block the synthesis of prostaglandins?
4. Discuss how soap can help in the fight against the coronavirus.
5. What structural features do a triacylglycerol and a phosphatidyl ethanolamine have in
common? How do the structures of these two types of lipids differ?
6. Refer to the lipid structure below.
A. Identify the lipid structure, give its role, function, and enumerate its hydrolysis products.
B. Explain which hydrolyzed component will test positive in the Acrolein test? Ninhydrin
test?
7. Explain which hydrolyzed component of the lipid structure below will test positive in the Br2
test.
8. What tests will identify lipids A and B?
A
B
9. Egg yolks contain a high amount of cholesterol, but they also contain a high amount of
lecithin. From a diet and health standpoint, how do these two molecules complement
each other?
10. Briefly discuss the structure of myelin and its role in the nervous system.
11. In terms of structure, how ischolesterol different from bile salts?
Cite causes of bile duct obstruction and symptoms.
12. Explain the cell membrane’s lipid bilayer. What are the lipids
cell membrane?
found
in
the