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 ( 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 : 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 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 • •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

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 – A1:cleaves the SN- 1 acyl chain. – :cleaves the SN- 2 acyl chain, releasing arachidonic acid. – : cleaves before the phosphate. – : 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 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 of Dietary Lipids • Triglycerides are the major lipid in the human diet (~90%). • Limited digestion in the mouth (lingual ) and stomach () because of the low solubility of the substrate. • Cholecystokinin (CCK) is secreted by the intestine, CCK signals the gallbladder to contract and release bile acids, and the pancreas to release digestive . • Secretin hormone is also secreted in response to the low pH of the chyme entering the intestine. • Secretin causes the pancreas and the liver to release a watery solution rich in bicarbonate that neutralizes the pH. Digestion of Triglycerides: • Emulsification in intestine by bile salts →→ ↑↑ surface area of lipids available for pancreatic lipase and colipase to bind. • Hydrolysis by pancreatic lipase →→ two free fatty acids and 2- monoacylglycerol. • Function of colipase: bile salts coat the substrate and prevent the access of the . The colipase binds to the fat and to the lipase, thereby allow TG to enter of lipase

35 Emulsification and Digestion of Fats

36 Action of Cholesterol and PL A2

37 Absorption and Resynthesis of TG:

1. The products of enzymatic digestion form mixed micelles with bile acids in the intestinal lumen. 2. The micelles interact with the enterocyte (mucosal cell) membrane and allow diffusion of the lipid- soluble components into the cell. 3. The bile acids do not enter the enterocyte, but remain in the intestine, then reabsorbed and sent back to the liver by the enterohepatic circulation to be reused in fat digestion.

4. Short- and medium-chain fatty acids (C4 to C12) do not require bile salts for their absorption. They are absorbed directly into intestinal epithelial cells. 5. The intestinal epithelial cells will resynthesize TG from 2-monoacylglycerol and free fatty acids. 6. Each fatty acid is first activated to fatty acyl CoA consuming two moles of ATP. 7. Synthesis of one mole of TG consumes 4 moles of ATP. 8. The resynthesized TG are then packaged with a protein, apo B-48, phospholipids, and cholesterol esters into a soluble lipoprotein particle known as a . 39 Resynthesis of TG in the Intestinal cells

2

40 TRANSPORT OF DIETARY LIPIDS IN THE BLOOD Plasma Lipoproteins

• Lipoproteins are spherical aggregates consisting of a nonpolar core and a single surface layer of amphipathic lipids. • Hydrophobic core: TG and cholesteryl esters. • Single surface layer: amphipathic phospholipids and cholesterol. • Lipids of the surface layer are oriented so that their polar groups face outward to the aqueous medium. • The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein.

41 General Structure of Plasma Lipoprotein

42 Four major groups of lipoproteins: • (CMs), derived from intestinal absorption of TG and other lipids. • Very low density lipoproteins (VLDL), derived from liver for the export of TG. • Low-density lipoproteins (LDL) are the final stage in the catabolism of VLDL. • High-density lipoproteins (HDL), involved in VLDL and CM metabolism and also in cholesterol transport.

43

Major Lipids in Lipoproteins: • TG is the predominant lipid in CMs and VLDL • Cholesterol and phospholipids are the predominant lipids in LDL and HDL, respectively. • FFAs in plasma are carried by albumin.

45 Transport of Dietary Lipids in Blood:  The chylomicrons are secreted into the lymph and eventually end up in the circulation.  Fasting plasma level of TG is less than 150mg/dL.  After a fat-rich meal, plasma TG reach a peak within 3- 4 h (~200mg/dL), and returns to normal within 6-8 h.  Once in circulation, the newly released (“nascent”) chylomicrons interact with HDL and acquire two apoproteins from HDL, apoCII and apoE.  This converts the nascent chylomicron to a “mature” chylomicron.

46  The apoCII on the mature chylomicron activates the enzyme (LPL), which is located on the inner surface of the capillary endothelial cells of muscle and adipose tissue.  LPL digests the triacylglycerols of the chylomicrons and VLDL in the blood producing free fatty acids and glycerol.  The fatty acids are utilized either for energy production (in muscle) or fat storage (in adipocytes), whereas glycerol is metabolized in the liver.

47 48 As the chylomicron loses , its density increases and it becomes a chylomicron remnant, which is taken up by the liver via receptors that recognize apoE. In the liver, the chylomicron remnant is degraded by lysosomal enzymes into its component parts for reuse by cells. When the diet contains more fatty acids than are needed, the liver converts them to triacylglycerols, which are packaged with specific apolipoproteins into VLDL. The VLDL are transported in the blood to adipose tissues, where TG are stored within adipocytes.

49 50 Answer the following

1. Define essential and non essential fatty acids 2. Importance of fats for human body

51 52