Chemistry of Lipids
a. Simple lipids: Esters of fatty acid with alcohol.
• Fats or oils (triacylglycerol): Esters of fatty acids with glycerol
• Waxes: Esters of fatty acids with long-chain aliphatic alcohols other than glycerol, e.g.
cetyl alcohol. These are solid at room temperature.
b. Compound lipids: Esters of fatty acids with alcohol, carrying additional groups such as phosphate and nitrogenous group or carbohydrate. Compound lipids are of two types:
• Phospholipids: For example, glycerophospholipids, sphingophospholipids
• Glycolipids: For example, cerebrosides, sulfatides, gangliosides, globosides.
c. Complex lipids: For example, lipoproteins —very low density lipoprotein (VLDL), low density lipoprotein (LDL), high density lipoprotein (HDL) and chylomicrons.
d. Derived lipids: Obtained by hydrolysis of simple or compound lipids. For example, steroids (cholesterol), prostaglandins, leukotrienes, etc.
2. Give a brief account of functions of lipids.
• Source of energy
• Concentrated fuel reserve (adipose tissue)
• Constituent of membranes and regulate membrane permeability. For example, phospho- lipids, glycolipids, lipoproteins and sterols
• As compounds of inner mitochondrial membrane, lipids participate in electron transport chain • Phospholipids, glycolipids and sterols are constituents of plasma lipoprotein, which trans-
port fat
• Source of fat-soluble vitamins and bile acids
• Metabolic regulators—steroid hormones and prostaglandins • Protect internal organs, serve as insulating materials.
3. Define and classify fatty acids.
Definition: Fatty acids are monocarboxylic acids with hydrocarbon side chain. They are simplest form of lipids.
Classification
a. Depending on nature of hydrocarbon chain
• Saturated fatty acids: Do not contain double bonds in their hydrocarbon chain, e.g. palmitic and stearic acid
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Chemistry of Lipids
• Unsaturated fatty acids: Contain one (monounsaturated) or more (polyunsaturated) double bonds in their hydrocarbon chain, e.g. oleic acid, linoleic acid, linolenic acid, arachidonic acid.
b. Depending on number of carbon atoms
• Even-chain fatty acids: Natural lipids with even number of carbon atoms in their chain, e.g. palmitic acid (16), stearic acid (18)
• Odd-chain fatty acids: Propionic acid (3C) and valeric acid (5C).
c. Depending upon length of hydrocarbon chain of fatty acid
• Short-chain fatty acids: Chain contains six or less than six carbons, e.g. propionic acid, butyric acid, etc.
• Medium-chain fatty acids: Chain contains 8–14 carbons, e.g. caprylic acid, myristic acid, etc.
• Long-chain fatty acids: Chain contains 16–22 carbons, e.g. palmitic acid, stearic acid, etc.
d. Depending upon nutritional importance
• Essential fatty acids: Not synthesized in the body, therefore should be taken in the diet, e.g. linoleic acid, linolenic acid and arachidonic acid
• Non-essential fatty acids: These can be synthesized in the body, e.g. palmitic acid, stearic acid, etc.
4. Write a short note on essential fatty acids.
The essential fatty acids (EFA) are those which cannot be synthesized by the body, hence they should be supplied in the diet. Chemically, they are polyunsaturated fatty acids (PUFA).
Biochemical basis for essentiality: Linoleic and linolenic acids are essential fatty acids—
humans lack the enzymes that can introduce double bonds beyond carbon 9 of fatty acids.
Functions of essential fatty acids: It is required for the following:
• Membrane structure and function • Transport of cholesterol
• Formation of lipoproteins
• Synthesis of eicosanoids—prostaglandins, thromboxane, leukotrienes
• Protective effect against fatty liver.
Deficiency of EFA: The deficiency of EFA results in phrynoderma, hair loss and poor wound healing.
5. What are the types of rancidity? How can it be prevented?
Definition of rancidity: It is the deterioration of fats and oils resulting in unpleasant taste and smell.
Chemistry of Lipids
Types of rancidity
• Hydrolytic rancidity: Due to bacterial hydrolytic enzymes, which results in the formation of short- chain fatty acids. It can be prevented by heat deactivation, proper handling and storage • Oxidative rancidity: It is due to oxidation of fat by free radicals resulting in formation of
compounds such as dicarboxylic acids, aldehydes, ketones, etc.
Prevention: By antioxidants, e.g. tocopherol, hydroquinone, gallic acid and food preserva- tives like butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), etc.
6. Define iodine number. Mention its significance.
Definition: Iodine number is defined as number of grams of iodine absorbed by 100 g of fat. A higher iodine number indicates greater number of double bonds.
Significance: It measures average degree of unsaturation, so it is useful in detecting adul- teration of polyunsaturated fatty acid (PUFA) with saturated fatty acid. For example, iodine number of coconut oil is 6–10 and sunflower oil is 141–155.
7. Define and write the significance of saponification number.
Definition: The number of milligrams (mg) of potassium hydroxide (KOH) required to saponify 1 g of fat is saponification number.
Significance: It measures the average molecular weight of fat.
8. Define and classify phospholipids. Write its compositions and one example for each class.
Definition: Phospholipids are lipids containing phosphoric acid + fatty acids + alcohol
± nitrogenous base. They contain both polar and non-polar groups and are called am- phipathic lipids.
Classification
Glycerophospholipids: Contain glycerol as alcohol (Table 5.1).
Table 5.1: Composition of individual phospholipids
Name Composition Function
Lecithin Glycerol + fatty acids + phosphoric
acid + choline • Membrane component, nerve transmission
• Source of choline and methyl group Dipalmitoyl lecithin Glycerol + palmitic acid (2) + phos-
phoric acid + choline • Lung surfactant
• Deficiency causes respiratory distress syndrome
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Chemistry of Lipids
Name Composition Function
Cephalin Glycerol + fatty acids + phosphoric acid
+ ethanolamine • Structural component of membranes Plasmalogens Alcohol + fatty acids + phosphoric acid
+ nitrogenous base + glycerol • Structural component of membranes in brain and muscle
Platelet-activating factor
Contains alkyl linkage in first carbon of glycerol. Second carbon contains acetyl group. Third carbon is attached with phosphate group to choline.
• Mediator of hypersensitivity, acute inflam- matory reactions and anaphylactic shock
Cardiolipin Phosphatidic acid + glycerol + phos-
phatidic acid • Component of inner mitochondrial mem- brane has antigenic properties
Phosphatidylinositol Glycerol + fatty acids + phosphate +
inositol • Phosphatidylinositol 4, 5-bisphosphate is second messenger for hormone action
• Anchors the glycoproteins to membranes
Sphingophospholipids: Contain sphingosine as alcohol.
• For example, sphingomyelins—major component of membranes of nervous tissue • Composition: Ceramide (sphingosine + fatty acid) + phosphoric acid + choline • Niemann-Pick disease: Due to excessive accumulation of sphingomyelin as a result of deficiency of enzyme sphingomyelinase.
Functions of Phospholipids
• Constituent of cell membrane, myelin sheath
• Constituent of surfactant (dipalmitoyl lecithin) in lungs
• Second messenger (phosphatidylinositol)
• Source of arachidonic acid—for prostaglandin synthesis
• Components of lipoproteins (transport of lipids)
• Absorption of fat from the intestine (micelles and chylomicrons)
• Acts as lipotropic factor and prevents fatty liver
• Responsible for maintaining the conformation of electron transport chain in mitochondria.
9. Mention the composition of glycolipids and give examples.
Definition: Glycolipids are carbohydrates—lipid complexes having amphipathic nature.
They are the important components of cell membrane and nervous tissue.
Composition: Ceramide (sphingosine + fatty acid) + carbohydrate. Do not have phos- phoric acid.
Contd...
Chemistry of Lipids
Classification: Depending on the nature of attached carbohydrate chain.
Cerebrosides: Galactocerebroside (ceramide + galactose).
Sulfatides: Ceramide + galactose + sulfate.
Globosides: Ceramide + more than one hexose or hexosamine.
Gangliosides: Ceramide + oligosaccharides + NANA (N-acetylneuraminic acid).
10. What are prostaglandins? Mention their functions.
Definition: Unsaturated hydroxyl and ketosubstituted C20 fatty acids (Table 5.2).
Synthesis: Formed from arachidonic acid by cyclooxygenase pathway.
Significance: Act as local hormones and mediators of inflammation. Non-steroidal anti- inflammatory drugs (NSAIDs) like aspirin, are the inhibitors of cyclooxygenase.
Table 5.2: Functions of prostaglandins
Compounds Functions
PGE2 • Mediator of pain and fever
• Vasodilatation, relaxation of smooth muscle
PGF2α • Used for induction of labor
• Vasoconstriction
• Contraction of uterine smooth muscle
PGE1 • Decreases gastric acid secretion
PGI2 • Produced by endothelium of vessels
• Vasodilatation
• Inhibits platelet aggregation
TXA2 • Produced by platelets
• Platelet aggregation
• Vasoconstriction PG, prostaglandin; PGI, prostacyclin; TX, thromboxane.
11. Mention four biological functions of cholesterol.
Functions
• Constituent of cell membrane • Formation of bile salts (liver)
• Synthesis of steroid hormones (cortisol, aldosterone, testosterone, estrogen, progesterone)
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Chemistry of Lipids
12. What is the name of cholesterol ring? Mention the compounds derived from cholesterol.
• Name of the ring: Cyclopentanoperhydrophenanthrene ring
• Compounds derived from cholesterol: Vitamin D3, bile acids, testosterone, estrogens, cortisol and aldosterone.
Key Points Essential fatty acids: Linoleic, linolenic, arachidonic acid.
Trans fatty acids: They are found in partially hydrogenated vegetable oils and increase the risk of atherosclerosis and cancer.
Omega-6 fatty acids: Linoleic and arachidonic acid.
Omega-3 fatty acids: Linolenic acid. High levels of omega-6 fatty acids and low omega-3 fatty acids can lead to atherosclerosis and cancer.
Iodine number: Used for detection of adulteration of unsaturated fat with saturated fat.
Saponification number: Used for measure of average molecular weight of fat.
Palmitic acid: Most common saturated fatty acid in diet; metabolized in the body.
Rancidity: Deterioration of oils and fat resulting in an unpleasant smell.
Food preservatives: Butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT).
Dipalmitoyl lecithin: Lung surfactant. Deficiency causes respiratory distress syndrome.
Phosphatidylinositol 4, 5 bisphosphate: Acts as a second messenger in hormone action.
Niemann-Pick disease: Due to deficiency of sphingomyelinase.
Arachidonic acid: Acts as a source for synthesis of prostaglandins.
Choline: Lipotropic factor and prevents fatty liver.
PGE2: Used for dilatation of cervix during labor.
PGF2α: Induction of labor by uterine contraction.
Aspirin and NSAIDs: Inhibitors of cyclooxygenase; used as anti-inflammatory and analgesic agents.
Steroids: Block the enzyme phospholipase A2—prevent the release of arachidonic acid, hence act as anti-inflammatory agents.
LDL: Increases the risk of atherosclerosis.
HDL: Decreases the risk of atherosclerosis.
Cholesterol: Normal serum level is 140–200 mg/dL. Vitamin D3, bile acid, steroid hormone (cortisol, testosterone and estrogens) are derived from cholesterol.
Leukotrienes: Formed from arachidonic acid by lipo-oxygenase pathway. They mediate chemotaxis, allergy and inflammation. They are components of slow reacting substance of anaphylaxis.