LIPIDS. Biomolecules soluble in organic solvents but not water (hydrophobic). Important in: n Cell membranes. n Storage for energy.

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1 LIPIDS Biomolecules soluble i orgaic solvets but ot water (hydrophobic). Importat i: Cell membraes Storage for eergy Vitamis (Steroid) hormoes Classified ito: i. Simple lipids (oils/fats, wax) ii. Complex lipids (phospholipids) iii. Derived lipid (cholesterol, PG, vitami D) Subdivided ito family of lipids with specific features, icludig wax, steroids, glycolipids, ad phospholipids.

2 Classes of Lipids LIPID COMPOSITION Waxes Triglycerides Phospholipids a. Glycerophospholipids Fatty acid + log-chai alcohol Fatty acid + glycerol Fatty acid + glycerol + phosphate + amio alcohol (serie, ethaolamie, cholie, iositol) b. Sphigolipids Fatty acid + sphigosie + phosphate + amio alcohol c. Glycolipids Fatty acid + glycerol/sphigosie + moo or disaccharides Steroids Fused structure of 3 cyclohexaes ad oe cyclopetae

3 Structure of Fatty Acids

4 Digestio ad Absorptio of Lipids

5 Dietary Lipids Dietary lipids comprise triglycerides (90%), essetial fatty acids, cholesterol phospholipids, ad fat-soluble vitamis (A, D, E,K). Lipids must be processed to udertake their physiologic roles. Almost all of the dietary lipids are absorbed ad some are stored i adipose tissue. (Hydrophobic) Lipid digestio is mediated by hydrophilic ezymes eed for detergets/emulsifiers.

6 Bile Acids Amphipathic (cotais polar ad o-polar parts). Sythesized i the liver, stored i the gallbladder, ad secreted ito the itestie. Bile acids are mostly reabsorbed from the itestie (eterohepatic circulatio) Emulsify fats: Allow them to mix i a hydrophilic medium. Hydrophobic part of bile salt iteracts with lipids, hydrophilic part faces water. Bile salts act by icreasig surface area of the lipid droplet, which facilitates actio of lipiddegradig ezymes (lipases). Fat globule Hydro pholic + Hydrop hobic Bile acids Emulsificatio droplets Pacreatic lipase( )

7 Actio of Bile Acids Facilitate degradatio of triglycerides ito free fatty acids by lipases. Digestio ad absorptio of fats ad fat-soluble vitamis i the small itestie. Solubilizig cholesterol, hece prevetig its precipitatio ad depositio.

8 Triglyceride Digestio Mediated by lipases which degrade triglycerides ito mooacylglyerol (MAG) + free fatty acids (FFA). Short chai FFA are absorbed directly i stomach ad itestie to the blood. Medium ad log chai FFA eed emulsificatio by bile salts Compartmets: Mouth ad Stomach: Catalyzed by acid-stable lipases a ligual lipase secreted by Eber glads (back of the togue) agastric lipase secreted by gastric mucosa They removig short ad medium chai FA from TG, CE, PL Peristaltic cotractios create acid chyme which stimulates cholecystokii (CCK) release ito blood stream. Small Itesties: Major digestio occurs i the lume of duodeum /jejuum breakig TAG ito FFA, MAG & DAG Mediated by pacreatic ad itestial lipases, ad facilitated by actio of co-lipase protei (ehaces bidig of lipase to lipid-aqueous iterphase

9 Cholesterol Preset as free cholesterol (~90%) ad cholesterol ester (CE, ~ 10%). Cholesterol ester free cholesterol by pacreatic cholesteryl esterase [requires bile salts). I the ER of the eterocytes, free cholesterol reesterified with fatty acids by acylcoa:cholesterol acyl trasferrase (ACAT). The CE is packed ito chylomicros to be coveyed ito the lymph.

10 Phospholipid Degradatio Mediated by the pacreatic ezymes. Phospholipase A 2 : Proezyme activated by trypsi Requires bile salts ad calcium. Acts by removig fatty acid from C2 lysophospholipid Lysophospholipase: Removes the fatty acid from C1, resultig i glycerylphosphoryl base which may be absorbed, degraded, or excreted.

11 Absorptio of Lipids Products of lipid digestio (free fatty acids, free cholesterol, ad 2-mooacylglycerol) form stable mixed micelles whe combied with bile salts. Bile salts: Solubilize lipids i micelles. Facilitate the absorptio of solubilized lipids. Absorptio is at brush border of duodeal & jejual eterocytes (passive diffusio). Note: micelles are NOT absorbed Absorptio depeds o extet of water solubility (more vs. less) e.g. all FFA ad 2- MAG are absorbed, while oly 30% of dietary cholesterol is absorbed

12 Trasport of Lipids Fatty acids trasport: Short- ad medium-chai legth fatty acids ( 6 12C): No eed for micelles for absorptio pass without chage to portal blood. Log-chai fatty acids: trasported to edoplasmic reticulum for re-esterificatio ito triglycerides. Micelles: Solubilize other lipids (act as shuttle). Bile salts: No absorptio i duodeum or jejuum Reabsorbed i the termial ileum

13 Secretio of lipids I itestial cells, reassembled TAG, phospholipids ad cholesterol are packed i chylomicros (largest LP), which leaves itestie through lymphatic (lacteals), to thoracic duct to blood stream ad liver, the to adipose tissue ad muscles. Most lipids, predomiatly TAG are trasported i blood withi lipoproteis (LP) Lysophospholipids are re-acylated with FA to form the phospholipids o the surface of chylomicros. Fat absorptio is complete withi few hours of igestio Chylomicros ad VLDL are cleared from blood rapidly by fat, cardiac ad skeletal muscles cells, by lipoprotei lipase Factors that icrease liver TAG sythesis ad VLDL secretio icludes: high CHO diets, ethaol igestio, high isuli ad low glucago levels

14 Use of dietary lipids by tissues I blood, TAG cotaied i chylomicros are degraded to free fatty acids ad glycerol by lipoprotei lipase (LPL) (attached to the surface of edothelial cells i capillaries). Fate of FFA: trasported i the blood i associatio with serum albumi (secreted by liver ad fuctio i trasport of hydrophobic compouds) util they are take up by cells ad degrade by b- oxidatio. Fate of glycerol: take up from the blood ad phosphorylated by glycerol kiase to produce G3P (glycolysis, glucoeogeesis). Fate of the remaiig chylomicros: the chylomicros remats bid to receptors o the liver ad are edocytosed

15 Cotrol of lipid digestio i the small itestie Cholecystokii hormoe (CCK) - Secreted by itestial cell i lower duodeum &jejuum - Release stimulated by fatty acids i duodeum CCK acts o: i. Stimulates Gallbladder cotractio to release bile ii. Stimulates Pacreas to release ezymes iii. Decreases gastric motility Secreti: hormoe produced by itestial cells: - Acts i cocerts with CCK - It is secreted whe cells are exposed to the acidic cotets of the emptyig stomach. - It stimulates the exocrie portio of the pacreas to secrete bicarboate ito the pacreatic fluid (thus eutralizig the acidity of the itestial cotets).