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Lipid Chemistry Lecture 4 By Lipid chemistry Lecture 4 By. Dr. Naglaa F. Khedr 1 Lipids • Lipids are naturally occurring organic compounds from plants or animals that are insoluble in water and soluble in nonpolar organic solvents such as ether and chloroform. They are esters of fatty acids 2 Biological Importance of Lipids 1) Energy source. When oxidized, one gram of fat =9 kcal. 2) Energy storage (triglycerides are stored in fat cells called adipocytes). 3) Lipoproteins are cell membrane structural components and in the mitochondria, and also serve as lipid transporter in blood. 4) Hormones: like Steroidal hormones 5) Thermal Insulation: This blanket layer of tissue insulates the body from extreme temperatures and helps keep the internal climate under control. It pads our hands and buttocks and prevents friction by subcutaneous fat. 3 4) Dietary fat serves as a carrier of the lipid-soluble vitamins A, D, E, and K. Therefore a diet that is too low in fat (less than 20% of calories) can result in a deficiency of these four vitamins. 5) Protection: Fats serve as a shock absorber, or protective layer, for the vital organs (visceral fat surrounding vital organs like liver, kidney & ovary). 6) Non polar lipids act as electrical insulator. Lipids are structural component of myelin, the fatty insulating sheath surrounding each nerve fiber, enabling it to carry messages faster. 4 5 TYPES OF FATS 6 A- Fatty acids Aliphatic organic monocarboxylic acid R-(CH)n COOH • Human have an even number of carbon atoms (4-24 carbon) • Odd–carbon fatty acids in vegetables • Where R is saturated hydrocarbon, is considered to be hydrophobic tail. • COOH is hydrophilic head. 7 The presence of hydrophilic head together with hydrophobic tail in one molecule is called amphipathic character. 8 Fatty acids Fatty acids (FA) are differ from one another in: (1) Length of the hydrocarbon tails (short C4 – medium C6-C12, long C12-C20,very long >20 C) (2) Degree of unsaturation (double bond) (3) Position of the double bonds in the chain 9 According to saturation A. Saturated fatty acid : if the fatty acid has no double bond as: Palmitic acid (16 C) CH3-(CH2)14-COOH Stearic acid (18 C) CH3-(CH2)16-COOH N.B. Saturated fatty acids are solids at room temperature(butter fat) 10 B- Unsaturated fatty acid : if the fatty acid has double bond it is either: 1. Monounsaturated fatty acid : if the fatty acid has one double bond as: Cis (naturally occurring) Trans (by chemical hydrogenation of vegetable oils) Palmitoleic: 16:1;9 CH3-(CH2)5-CH=CH-(CH2)7-COOH Oleic: 18:1;9 CH3-(CH2)7-CH=CH-(CH2)7-COOH 11 2-Polyunsaturated fatty acid (PUFA) • Fatty acid has more than one double bond • Polyunsaturated: linoleic acid, linolenic acid and arachidonic acid. • The pattern of double bonds of PUFA is the divinylmethane pattern (-HC=CH-CH2-HC=CH). • The melting point of a fatty acid ↑↑ with chain length and ↓↓ with the degree of unsaturation. • Linoleic 18:2;9,12 CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7-COOH Note: Rancidity of fatty acids. • When lipid-rich foods are exposed too long to air oxygen, they may spoil and become rancid • Oxidative cleavage of double bonds in unsaturated fatty acids produce aldehydes and carboxylic acids (volatile) 12 Essential fatty acids: They are essential; that can not be synthesized in the human body and must be taken in adequate amounts in the diet. They are required for normal growth and metabolism. E.g. PUFA .Linolenic acid 18:3(ω-3): present in soybeans, walnuts, linseed oil, perilla, chia, and hemp .Linoleic acid 18:2(ω-6): present in poppy seed, safflower, sunflower, and corn oils 13 Importance of essential Fatty acids .Linolenic acid 18:3(ω-3): important for growth and development; its deficiency results in decreased vision and altered learning behavior and Autism. .Linoleic acid 18:2(ω-6): 1. Precursor of Arachidonic acid from which Eicosanoids are produced (prostaglandins, thromboxanes and leukotrienes) 2. Attach to cerebrosides in the skin forming (acylglucosylceramide) make skin impermeable to water. This function of linoleic acid may help to explain the red, scaly dermatitis associated with deficiency of essential FAs 3. Precursor of C22:6 ω-3 neuronal fatty acid14 Nomenclature of fatty acids • There are four common naming systems; three of them attempt to denote the chain length and the number and positions of any double bonds. 15 Trivial Names . Palmitic acid is found in palm oil. Oleic acid is a major constituent of olive oil (oleum). Stearic (from the Greek word meaning solid) acid is solid at room temperature. Arachidonic acid Spiders (arachnids) contain 16 Two Abbreviation Systems • The carboxyl-reference system indicates the number of carbons, the number of double bonds, and the positions of the double bonds, counting from the carboxyl carbon (which is numbered 1, as in the IUPAC system). • It differs from the IUPAC system in that it uses a number (e.g., 16) to denote chain length instead of a name derived from Greek (e.g., hexadecanoic acid). • Palmitic acid - (hexadecanoic acid) - 16:0 • Oleic acid - (9-octadecenoic acid)- 18:1 Δ9 17 ω - reference ω -6 The first carbon following the carboxyl carbon is the alpha (α) carbon. The second carbon following the carboxyl carbon is the beta (β) carbon. The last carbon in the chain, farthest from the carboxyl group, is the omega(ω)- indicates the number of carbons, the number of double bonds and the position of the double bond closest to the omega carbon, 18 19 Triglycerides • They are called neutral because they are uncharged due to absence of ionizable groups in it. • The neutral fats are the most abundant lipids in nature. • They constitute about 98% of the lipids of adipose tissue, 30% of plasma or liver lipids. • They are esters of glycerol with three fatty acids. • Glycerol (blue) and fatty acids (red). 20 triglycerides Triglycerides in animals • Triglycerides in Plants ingle bonds only in fatty S • with low melting points acids Saturated fatty acids which are liquid at room Solid at room temperature. temperature–high melting point • Triglycerides in Plants are Store energy referred to as oils insulation and buoyancy aid • Contain unsaturated fatty acid e.g. oleic acid. 21 Why fuel stored as fat not as carbohydrates Because • The carbon atoms of fatty acids are more reduced than those of sugars. So, oxidation of TGs yields more energy than that from carbohydrates. • TGs are hydrophobic and therefore unhydrated. So, the organism that carries fat as fuel does not have to carry the extra weight of water of hydration that is associated with stored polysaccharides (2g H2O/g polysaccharide). • In hibernating animals (e.g. bears) and warm-blooded polar animals (e.g. penguins), TGs stored under the skin serve, not only as energy stores, but as insulation against cold weather. 22 Phosphoacylglycerols (Phospholipids) • Phosphatidic acid is the precursor for synthesis of phosphoglycerides. • Phosphatidylcholine (lecithin), cardiolipin (diphosphatidylglycerol), and phosphatidylinositol 4,5- bisphosphate are important constituents of cell membrane. Glycerol + Ester of fatty acid +phosphate + polar alcohol •Polar Alcohol may be: •Ethanolamine •Choline •Serine •Inositol 23 24 25 Presence of Phospholipids Phospholipids are components of Cell Membranes -glycerophospholipids are main component of cell membranes (form a lipid bi-layer : hydrophobic tails and polar head groups exposed to water) e.g. phosphatidylcholine (lecithin) cell membrane 26 Types and functions of phospholipases Glycerophospholipid degradation occurs by phospholipases present in tissues (membrane bound or free), pancreatic juice, and venoms. • Phospholipases are specific for ester bonds in the glycerophospholipids: phospholipases A1, A2, C, and D. Phospholipase A – Phospholipase A1:cleaves the SN- 1 acyl chain. – Phospholipase A2:cleaves the SN- 2 acyl chain, releasing arachidonic acid. – Phospholipase C: cleaves before the phosphate. – Phospholipase D: cleaves after the phosphate. 27 Types and functions of phospholipases: • Phospholipases are of 4 types according to site of cleavage; PLA1, PLA2, PLC, and PLD. PLA2 has 3 functions: (1) Digests the dietary phosphoglycerides. (2) Catalyzes the release of arachidonic acid in response to signals for the synthesis of eicosanoids. (3) Provides the major repair mechanism for oxidatively- damaged membrane lipids. PLC: When activated by hormonal stimuli, hydrolyzes PIP2 to produce the second messengers DAG and IP3, which act for signal transmission. PLD is also involved in signal transduction. 28 Signal Transduction • The phospholipase C system which generates diacylglycerol and IP3 from phosphatidylinositol 4,5-bisphosphate. • Phospholipase D catalyzes the hydrolysis of phosphatidylcholine 29 Respiratory distress syndrome (RDS) • RDS of a premature infant is related to the deficiency in the synthesis of lung surfactant. The premature infant has not yet begun to produce adequate amounts of lung surfactant. • The constituents of lung surfactant are dipalmitoyl-lecithin (50-60%), phosphatidylglycerol, apoproteins (surfactant proteins), and cholesterol. • Lung surfactant contributes to a reduction in the surface tension within the air spaces (alveoli) of the lung, preventing their collapse (atelectasis) at the end of expiration phase of breathing. 31 •The risk of RDS can be assessed by analysis of amniotic fluid (surrounding the fetus), •Treatment: 1- mothers can be given corticosteroids to accelerate the maturation of their fetus's lungs. 2- But now a lifesaving treatment is available; a fine aerosol of an artificial surfactant is administered directly into the trachea. 32 D- steroids e.g. Cholesterol • Cholesterol is precursor for synthesis of important steroids: Bile acids, adrenocortical hormones, sex hormones, D vitamins, and cardiac glycosides. • Cholesterol is a major constituent of the plasma membrane and of plasma lipoproteins. • It affects membrane fluidity. It is often found as cholesteryl ester. ۳۳ Digestion of Dietary Lipids • Triglycerides are the major lipid in the human diet (~90%). • Limited digestion in the mouth (lingual lipase) and stomach (gastric lipase) because of the low solubility of the substrate.
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