CELLULAR BIOCHEMISTRY AND METABOLISM (CLS 331) Dr. Samah Kotb Nasr Eldeen Dr. Samah Kotb Nasr Eldeen 1 Dr Samah Kotb Lecturer of Biochemistry

DIGESTION OF LIPIDS 2 Dr Samah Kotb Lecturer of Biochemistry CHAPTER 5

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 Lipids are complex molecules composed of carbon, hydrogen, and oxygen.  Lipids are insoluble in water and soluble in nonpolar solvents.  Most lipids are non-polar and are hydrophobic because they contain hydrocarbon chains.

Lipid (TGL) Droplets In Adipose tissue A.The Functions of Fat in the Body: 1.Energy source. 2.Serves as an energy reserve.

3. Structural components 3. Cell Membrane Structural components

4. Thermal Insulator : Subcutaneous layer of fat reduces heat loss in cold weather (helps maintain body temperature).

1. Provide calories (9 kcal per gram). 2. Provide satiety. 3. Carry Lipid-soluble vitamins (A,D,E, and K) & essential fatty acids. 4. Contribute aroma & flavor.

Humans can grow more fat cells, but can never get rid of them!! As we gain and lose weight, they just swell and shrink. …Makes you think twice about those French fries!!!

 Length of the Carbon Chain  Long-chain, medium-chain, short-chain  Degree of Unsaturation  Saturated, Unsaturated (Mono, Poly)  Location of Double Bonds  Omega-3 Fatty Acid, Omega-6 Fatty Acid

Fat Saturation “full”, Saturation means “full”, so what could they be full of? H Full of H atoms! And wherever there is a chemical bond, there is stored energy.

CH3 (CH2)n COOH.  General Formula: CH3 (CH2)n COOH.  Each C being “saturated” with H. C–C  Contain only single C–C bonds.  long, straight chain. Properties of saturated fatty acids

 Closely packed.  Strong attractions between chains.  High melting points.  Solids at room temperature.

 Animal foods such as meat, poultry and full-fat dairy products.  Tropical oils such as palm and coconut.

 Contributes to cardiovascular disease. (atherosclerosis)

C=C  Contain one or more double C=C bonds.  The links made by double bonded C prevent the molecules from packing tightly together.  Few interactions between chains.

 Low melting points  Liquids at room temperature  Plant & fish fats  Vegetable oils

saturatedunsaturated

1. Monounsaturated FA 2. Polyunsaturated FA

1. Simple Lipids 2. Complex Lipids 3.Precursor & Derived Lipids Lipids

 Esters of fatty acids  Esters of fatty acids and alcohol. (trihydric or monohydric alcohol).

 Fats :esters of fatty acids with glycerol. Ex. triglycerides.  Oils : are fats in liquid state.

 Triglycerides are the main dietary fats in human body.  It is a naturally occurring ester of three fatty acids and glycerol that is the chief constituent of fats and oils.  Glycerol forms the “backbone” of the fat.

 Fat: is solid at room temperature. is prevalent in meats, whole milk, butter, and cheese.  Oil: Contain high proportion of unsaturated fatty acids and saturated fatty acids. is liquid at room temperature. is prevalent in plants such as olive and safflower. corn oil contains 86% unsaturated fatty acids and 14% saturated fatty acids

 Esters of long-chain fatty acids with higher molecular weight monohydric alcohols.  Has a weakly polar head group (ester linkage) and non- polar tails (hydrocarbon chain).  Fatty acids found in waxes are usually saturated ex. stearic acid.  Alcohols may be saturated or unsaturated and may include sterols ex. cholesterol.

Esters of fatty acids with various alcohols along with an additional group. a. Phospholipids: a. Phospholipids: Glycerophospholipids, Sphingophospholipids. b. Glycolipids c. Other Complex Lipids: c. Other Complex Lipids: Lipoproteins,aminolipids and sulfolipids. Simple lipid + Additional group = Complex Lipids

DIGESTIONOFLIPIDS Dr Samah Kotb 31

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DIGESTION & ABSORPTION OF LIPIDS  The major lipids in the diet are triacylglycerols.  Hydrolysis of triacylglycerols is initiated by gastric lipases that attack the ester bond, forming 1,2-diacylglycerols and free fatty acids, aiding emulsification. Dr Samah Kotb 33

Dr Samah Kotb 34  Pancreatic lipase is secreted into the small intestine and requires a further pancreatic protein, colipase, for activity.  It is specific for the primary ester links in triacylglycerols resulting in 2-monoacylglycerols and free fatty acids as the major end-products of luminal triacylglycerol digestion.

Triacylglycerols gastric lipases 1,2-diacylglycerols + free fatty acids 2-monoacylglycerols + free fatty acids Pancreatic lipasesmall intestine Stomach glycerol free fatty acids + hydrolysis Portal vein Secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct. Dr Samah Kotb 35

 Monoacylglycerols are hydrolyzed with difficulty to glycerol and free fatty acids.  Bile salts, formed in the liver and secreted in the bile, enable emulsification of the products of lipid digestion into micelles and liposomes together with phospholipids and cholesterol from the bile. Dr Samah Kotb 36

Dr Samah Kotb 37 Because the micelles are soluble, they allow the products of digestion, including the fat soluble vitamins, to be transported through the aqueous environment of the intestinal lumen and permit close contact with the brush border of the mucosal cells, allowing uptake into the epithelium, mainly of the jejunum.

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 The bile salts pass on to the ileum, where most are absorbed into the enterohepatic circulation.  Within the intestinal epithelium, 1- monoacylglycerols are hydrolyzed to fatty acids and glycerol and 2-monoacylglycerols are re- acylated to triacylglycerols via the monoacylglycerol pathway. Dr Samah Kotb 39

Dr Samah Kotb 40  Glycerol released in the intestinal lumen is not reutilized but passes into the portal vein; glycerol released within the epithelium is reutilized for triacylglycerol synthesis via the normal phosphatidic acid pathway.

Dr Samah Kotb 41  All long-chain fatty acids absorbed are converted to triacylglycerol in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct.

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