Important reactions of lipids

Transcription

1 Taif University College of Medicine Preparatory Year Students Medical chemistry (2) Part II (Lipids) week 4 lectures Important reactions of lipids

2 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

3 List the Important reactions of FA? Esterification Hydrolysis of esters Hydrogenation Oxidation and Rancidity Effect of radiation

4 Esterification

5 Esterification Fatty acids + Alcohol H + / Heat Ester + Water

6 Hydrolysis of esters Acid hydrolysis Base hydrolyses (Saponification)

7 Acid Hydrolysis Ester H + / Heat Acid + Alcohol

8 Hydrolysis of triglycerides TG In hydrolysis, Triacylglycerols (TAG) split into glycerol and three fatty acids Acid or enzyme catalyst is required

9 Base Hydrolysis Ester Saponification OH - Salt of the acid or soap + Alcohol Soap

10 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

11 Hydrolysis of Triglycerides Saponification

12 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

13 Hydrogenation

14 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).

15 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

16 Hydrogenation of Unsaturated Fats X? Carcinogenic

17 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.

18 Summary of Organic and Lipid Reactions

19 Oxidation

20 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

21 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.

22 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

23 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)

24 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

25 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

26 Effect of radiation

27 Effect of radiation cis biradical cis biradical trans Changing the configuration of the double bond

28 Lipids Classification

29 Glycerides Neutral- Glycerides Phospho- Glycerides

30 Neutral Glycerides

31 Glycerides Neutral glycerides Esters of Fatty acids with Glycerol

32 Esterification HO HO HO 3 H 2 O Ester Bond Triglyceride

33 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.

34 Structure of a mixed triacylglycerol Three different fatty acid residues are present

35 Triglycerides Triglycerides (TG) Simple Mixed Di-acid Tri-acid Tristearin Stearo, Stearo, Palmitin Palmito, Oleo, Stearin

36 Question Draw the structural formula of the triacylglycerol (TAG) produced from the reaction between glycerol and three molecules of oleic acid?

37 Thank You