Taif University College of Medicine Preparatory Year Students Medical chemistry (2) Part II (Lipids) week 4 lectures 1435-36 Important reactions of lipids
Lectures outlines Definition and importance of lipids Classification of lipids Fatty acids structure and properties Important reactions of FA Glycerides Non-glyceride lipids Membranes Lipids
List the Important reactions of FA? Esterification Hydrolysis of esters Hydrogenation Oxidation and Rancidity Effect of radiation
Esterification
Esterification Fatty acids + Alcohol H + / Heat Ester + Water
Hydrolysis of esters Acid hydrolysis Base hydrolyses (Saponification)
Acid Hydrolysis Ester H + / Heat Acid + Alcohol
Hydrolysis of triglycerides TG In hydrolysis, Triacylglycerols (TAG) split into glycerol and three fatty acids Acid or enzyme catalyst is required
Base Hydrolysis Ester Saponification OH - Salt of the acid or soap + Alcohol Soap
Saponification of TG and Soap Saponification is the reaction of a fat with a strong base splits triacylglycerols into glycerol and the salts of fatty acids is the process of forming soaps (salts of fatty acids) with KOH gives softer soaps
Hydrolysis of Triglycerides Saponification
Learning Check What products are obtained from the complete hydrolysis of glyceryl trioleate? A. glycerol and three oleic acids B. glyceryl tristearate C. glycerol and three stearic acids
Hydrogenation
Hydrogenation When hydrogen adds to all the double bonds of glyceryl trioleate (triolein) using a nickel catalyst, the product is the saturated fat glyceryl tristearate (tristearin).
Learning Check What products are obtained from the complete hydrogenation of glyceryl trioleate? A. glycerol and three oleic acids B. glyceryltristearate C. glycerol and three stearic acids
Hydrogenation of Unsaturated Fats X? Carcinogenic
Learning Check Which of the following statements are true and which are false? A. There are more unsaturated fats in vegetable oils. B. Vegetable oils have higher melting points than fats. C. Hydrogenation of oils converts some cis double bonds to trans double bonds. D. Animal fats have more saturated fats.
Summary of Organic and Lipid Reactions
Oxidation
Oxidation Carbon carbon double bonds are subject to oxidation with molecular oxygen (from air) as the oxidizing agent. Such oxidation breaks the carbon carbon bonds, producing both aldehyde and carboxylic acid products. Un saturated FA Short chain aldehydes or ketones Short chain Fatty acids
Oxidation The short-chain aldehydes and carboxylic acids often have unpleasant odors and flavors. and fats and oils containing them are said to have become rancid. Rancidity is due to a combination of two reactions: Bacterial hydrolysis of ester bonds. Air oxidation of alkene double bonds.
Autoxidation Polyunsaturated Fatty Acids Free Radical Initiation H-abstraction Diene Conjugation O 2 uptake Lipid Peroxides Catalysts (Fe, Fe-O 2 ) Decomposition Polymerization (dark color, possibly toxic) Rancid off-flavor compounds such as ketones,alcohols, hydrocarbons, acids, epoxides
Rancidity In fats containing triacylglycerols with some lowmolecular-mass carboxylic acids, hydrolysis by airborne bacteria under moist, warm conditions is directly responsible for rancid odors and flavors. To avoid this unwanted oxidation process, commercially prepared foods always contain antioxidants substances that are more easily oxidized than the food: vitamins C vitamin E (naturally occurring antioxidants) BHA and BHT (synthetic oxidation inhibitors)
Consequences of Lipid Peroxidation Structural changes in membranes Alter fluidity and ion channels Alter membrane-bound signaling proteins Increase membrane permeability Form lipid oxidation products adducts/crosslinks with non lipids e.g., proteins and DNA Cause direct toxicity e.g., 4-hydroxynonenal, Malondialdehyde MDA DNA damage and mutagenesis
Antioxidant Defenses in Biological Systems Fat-soluble cellular membrane consists Vitamin E beta-carotene Coenzyme Q (10) Water soluble antioxidant scavengers Vitamin C Glutathione peroxidase, Superoxide dismutase Catalase
Effect of radiation
Effect of radiation cis biradical cis biradical trans Changing the configuration of the double bond
Lipids Classification
Glycerides Neutral- Glycerides Phospho- Glycerides
Neutral Glycerides
Glycerides Neutral glycerides Esters of Fatty acids with Glycerol
Esterification HO HO HO 3 H 2 O Ester Bond Triglyceride
Structure of the simple triacylglycerol The simple triacylglycerol produced from the triple esterification reaction between glycerol and three molecules of stearic acid (18:0 acid). Three molecules of water are a by-product of this reaction.
Structure of a mixed triacylglycerol Three different fatty acid residues are present
Triglycerides Triglycerides (TG) Simple Mixed Di-acid Tri-acid Tristearin Stearo, Stearo, Palmitin Palmito, Oleo, Stearin
Question Draw the structural formula of the triacylglycerol (TAG) produced from the reaction between glycerol and three molecules of oleic acid?
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