UNIT 5. LIPIDS

OUTLINE 5.1. Introduction. 5.2. Fatty acids. 5.3. Eicosanoids. 5.4. Triacylglycerols = Triglycerides. 5.5. Waxes. 5.6. Membrane lipids: glycerophospholipids and sphingolipids. 5.7. Isoprenoids (and steroids) 5.8. Lipoproteins.

5.1. INTRODUCTION: Organic molecules highly hydrophobic and water insoluble. Biological roles: - Energy reserve (i.e. triacylglicerols). - Structural component of the biological membranes (i.e. phospholipids). - Protection (i.e. wax). - Biochemical signals, cofactor or pigments Lipid Classification: - Fatty acids. - Phospholipids. - Eicosanoids. - Sphingolipids. - Triacylglicerols. - Isoprenoids. - Waxes. - Conjugated lipids.

Carboxylic acids with long-chain hydrocarbon side groups. 5.2. FATTY ACIDS: Carboxylic acids with long-chain hydrocarbon side groups. They can be branched and saturated, unsaturated or polyunsaturated (contain two or more double bonds). The double bonds tend to occur at every third carbon atom (not conjugated). Ionizated at physiological pH. Usually, they are in the free, uncomplexed state. conjugated Not conjugated

5.2. FATTY ACIDS: 5

Saturated fatty acid: Stearic acid Fatty acid double bonds almost always have the cis configuration. Polar group Unsaturated acid: Linoleic acid Saturated fatty acid: Stearic acid Hydrophobic chain

5.2. FATTY ACIDS: Properties:  Higher unsaturated grade: higher fluidity.  Higher unsaturated grade (same chain length): lower melting point.

5.3. EICOSANOIDS: They are derived from arachidonic acid, 20:4(5,8,11,14). Types:  Prostaglandins (they act as local hormones): they are involved in - The production of pain and fever - Inflammation processes - Increase of the body temperature - Regulation of blood pressure and blood coagulation. - Control of the Ionic transport. - Dream induction

5.3. EICOSANOIDS:  Thromboxanes (found within platelets): Stimulate vasoconstriction and platelet aggregation.  Leukotrienes (present within leukocytes): Involved in: - Oedema production. - Vasoconstriction. - Bronchi constriction - Anaphylactic processes.

5.4. TRIACYLGLICEROLS: Glycerol esterified with three fatty acids. Monoacylglicerols and diacylglicerols are metabolic intermediates. Major energy reserve (stored in anhydrous form). Types: Simple triacylglicerols (i.e. tristearoylglycerol(= tristearin); trioleoylglycerol (=triolein). Mixed triacylglicerols (i.e. 1-stearoyl, 2-linoleoyl, 3-palmitoyl glycerol). Highly hydrophobic. Adipocytes and adipose tissue. Thermal insulation (important for warm-blooded aquatic animals).

Chemical composition of 5.4. TRIACYLGLICEROLS: Chemical composition of three eatable fats. trioleina triestearina Provide about six times the metabolic energy of an equal weight of hydrated glycogen. They are hydrolysed by lipases or alkali (saponification): NaOH or KOH Soap G L Y C E R O Fatty acid Fatty acid + Na+ +

- Energy reserve (plancton). 5.5. WAXES: Esters of long-chain monohydroxylic alcohols (16-30 C) with long-chain fatty acids (14-36 C) (saturated or unsaturated) Properties: - Energy reserve (plancton). - Water-repellant surfaces (i.e. animals skin, leaves of certain plants, bird feathers). - Highly hydrophobic. - Several industrial uses (lotions, cosmetics, etc.). Triacontanylpalmitate, major component of the bee wax.

5.6. MEMBRANE LIPIDS: Amphipathic lipids

5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS: 1,2-diacylglycerol that has a phosphate group esterified at the carbon atom 3 of the glycerol backbone. The phosphate group is linked to a highly polar or charged group (X). Usually they contain a saturated fatty acid (C16 or C18)C1-linked and an unsaturated fatty acid (C16-C20) C2-linked. Amphipathic molecules. They are derived from phosphatidic acid. Glycerophopholipids backbone structure

5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS: (Lecitina) (Cefalina) Phosphatidylcholine

Plasmalogen (heart muscle) 5.6. MEMBRANE LIPIDS: GLYCEROPHOSPHOLIPIDS: Some phospholipids contain an alkyl group ether linked. choline Plasmalogen (heart muscle) Platelet activating factor: major mediator of hypersensivity, acute inflammatory reactions, allergic responses and anaphylactic shock.

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS One polar group and to unpolar chains (glycerol is not present) They contain the amino alcohol sphingosine, one fatty acid (long chain) and a polar group (alcohol or sugar) Polar group

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS Phospholipids Glycolipids

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS Cerebrosides and globosides (ceramide oligosaccharides) are neutral glycolipids. Gangliosides contain N-acetylneuraminic acid (sialic acid), with negative charge at pH 7.

5.6. MEMBRANE LIPIDS: SPHINGOLIPIDS The carbohydrate present in several sphingolipids define the human being blood groups ( A, B and 0).

5.7. ISOPRENOIDS: They contain isoprene units (5 C). They precursor for the synthesis is isopentenylpyrophosphate . The isoprenoids are grouped into terpenes and steriods.

5.7. ISOPRENOIDS: TERPENES Classification in the base of the number of isoprene units

5.7. ISOPRENOIDS: TERPENES Linear or cyclic structures. Present in plants, fungi and bacteria. Pigments, molecular signalling (hormones and pheromones) and defence agents. Plant oils belong to this groups (aromas and flavours). They are precursor for fat-soluble vitamins synthesis.

5.7. ISOPRENOIDS: TERPENES Fat-soluble vitamins: they cannot be synthesised by human beings (diet). They are dissolved as fats and oils. Vitamin A (retinol): - Hormone and main pigment involved in vision. - It is a carotenoid. Vitamin D: - It is derived from cholesterol. - It increases the concentration of Ca2+ in physiological serum Vitamin E (tocopherols): - Biological antioxidant, It avoid food degradation. - Prevent oxidation of the membrane lipids.

5.7. ISOPRENOIDS: TERPENES Vitamin K: - Blood clotting. Ubiquinones and plastoquinones: - Lipophilic electron transporters (redox reactions). Vitamin E Vitamin K1 Ubiquinone

5.7. ISOPRENOIDS: STERIODS: Membrane lipids in eukaryotic cells. They are derived from triterpenes containing 4 rings: phenathrene nucleus (3 rings containing 6 C), and a pentane ring (5 C): Cyclopentanoperhydrophenanthrene. They are classified on the base of the number and position of the double bonds, location of the radicals, etc.

5.7. ISOPRENOIDS: STERIODS: Cholesterol: Main steriod in animals (cellular membranes) It is amphipathic. It is the precursor in the synthesis of many steriods such as hormones, vitamin D, bile acids

5.7. ISOPRENOIDS: STERIODS: Bile acids: they act as detergents in gut. They are more soluble than cholesterol. Steroid hormones: they promote metabolism and gene expression changes. Types: - Glucocorticoids: i.e: cortisol: involved in proteins, lipids and carbohydrates metabolism. - Aldosterone and mineralocorticoids:, they regulate water and salt excretion (kidneys). - Androgens and estrogens: sexual development.

5.7. ISOPRENOIDS: STERIODS:

5.8. LIPOPROTEINS: Molecular complexes in blood plasma of mammals. They facilitate lipids transfer between tissues (triacylglycerols, phospholipids and cholesterol). Apolipoproteins or apoproteins are synthesised in the liver. They have different density and lipids/proteins ratio.

5.8. LIPOPROTEINS: - Chylomicrons: extremely low density. They transport triacylglycerols and cholesterol esters (food) from gut to adipose tissue and muscles. - Very-low-density lipoproteins (VLDL): they transfer lipids (triacylglycerols) to the tissues to be used for energy. - Low-density lipoproteins (LDL): they transport cholesterol and its esters to the tissues. - High-density lipoproteins (HDL): Rich in cholesterol but poor in triacyglycerol (they promote the excretion of the excess of cholesterol).

5.8. LIPOPROTEINS: