Lecture 3: Lipids PDF

040817203 (Structure and Function of Biomolecules) LIPIDS lecture 3. By: Aliaa A. Masoud, PHD Lecturer of Biochemistry, Faculty of Science, Alexandria University Agenda  Physical & chemical Properties of Fatty...

040817203 (Structure and Function of Biomolecules) LIPIDS lecture 3. By: Aliaa A. Masoud, PHD Lecturer of Biochemistry, Faculty of Science, Alexandria University Agenda  Physical & chemical Properties of Fatty Acids  Class I. Simple lipids A. Neutral fats\ Tri acyl glycerols (Triglycerides: TGs) B. Waxes  Physical Properties of Fatty Acids  Depend on: The length & degree of unsaturation of hydrocarbon chain of FAs 1- Solubility Polarity of FAs decreases with :  increasing the length of the non polar hydrocarbon chain accounts for poor solubility of FA.  Fewer double bonds in its chain.  Short-chain FAs are slight soluble due to polarity of –COO- group  Lauric acid (12:0) is more soluble than palmitic (16:0)  α-linolenic acid (18:3) is more soluble than oleic (18:1). 2- Amphipathic Character At physiological pH, FAs (un-dissociated acidic form; poorly water soluble) are ionized into FA anion (relatively hydrophilic form). pH7 - + H+ 3-Physical State Stearic acid Streate  liquid: Less than 8C.  Solid ( greasy, waxy): more than 12 C  Oily : USFAs  Physical Properties of Fatty Acids 4- Melting point  SFAs packed in stable aggregates (have extended conformation, so they have high melting point.  USFAs (cis-) with one or several such kinks, poor ordered arrays, causing lower melting point, (required less energy to disorder the array of its molecules).  MP of Lauric acid (12:0) is 44, Palmitic (16:0) is 63  MP of Palmitoleic (16:1) is -0.5, Arachidonic acid (20:4) is -49.5. 5- Absorption Spectra  Naturally occurring FAs do not absorb light in UV/Visible region  Conjugated double bonds show an appreciable absorption at 230-260 nm  Conjugated linoleic acid (CLA) is a PUFA, refers to a mixture of positional and geometric isomers of linoleic acid (LA) with conjugated double bonds.  Chemical Properties of Fatty Acids  Depend on : the presence of COOH gp. & degree of unsaturation. 1- Saponification: formation of salts (soap) with alkali SOAP  Na &K Salts are soluble and act as good emulsifying agents for cleaning.  Ca & Mg Salts (heavier than Na & K) are curdy precipitate soap (scum): water- insoluble.  Heavy-metal soaps are used in lubricating greases, gel thickeners, and paints. 2- Detergent Formation: Reduction of COOH gp of FAs to Alkyl alcohols, which sulfated\sulfonated, forming detergents. ANIONIC DETERGENT  Both soaps & detergents act as emulsifying agents because their molecules are soluble in oil or grease and water. They consist of two parts, hydrophilic part that is soluble in water. hydrophobic part that is soluble in oil or grease. They have are effective cleansing properties. Cleaning action of soap & detergent  Chemical Properties of Fatty Acids 3-Fatty Aldehydes Synthesis:  Produced by the reduction of FAs in presence of catalyst (enzymatic or chemical as iron particles)  They have a high commercial value as natural flavoring agents and fragrance compounds in food industry, pharmaceuticals, cosmetics. (α-DOXs): α-Dioxygenases are biocatalysts (plant enzymes involved in the FAs α-oxidation. Fatty acids Fatty aldehyde 4-Ester Formation:  OH- form carboxylic acid (FA) pulls H+ from an alcohol, forming esters by condensation.  Most important acyl esters are with glycerol forming oils & fats.  Chemical Properties of Fatty Acids 4- Hydrogenation:  Saturation of USFAs with H in presence of catalyst (Pt, Ni, or Cu). 5- Halogenation:  Addition of halogen (F2>Cl2>Br2>I2), under mild condition, to C=C double bond site.  It is important property to determine the degree of unsaturation of FA & its biological activity. 6- Oxidation: a. Chemical Oxidation  During the oxidation of USFAs, in presence of oxidants, C=C bonds get cleaved Oxidative cleavage  Producing mono- & di-carboxylic acids. Odd mono-acid Oxidative cleavage  Valuable materials in polyester, lubricants & nylon industries a and as a precursor to active pharmaceutical, cosmetic ingredients & additives Di-acid  Chemical Properties of Fatty Acids b. Natural oxidation: Oxidative Rancidity  Exposure of USFAs into O2 is responsible to: oil rancidity & lipid peroxidation.  Oxidation of db of USFAs form peroxides, aldehydes of unpleasant odor and taste.  Accelerated by factors such as exposure to light, heat, and air.  The higher the degree of unsaturation of FA, the higher exposure to oxidation Lipid Peroxidation l  Class I. Simple lipids  Esters of FAs with various alcohols.  According to the types of alcohols, there are two main sub-groups. A. Neutral fats\ Tri acyl glycerols (Triglycerides: TGs)  The most common type of simple lipids.  They consist of glycerol molecules linked to FAs chains through ester bonds.  Called storage lipids.  Class I. Simple lipids Glycerol  has the popular name glycerin, colorless, odorless and has a sweet taste..  is a polyhydric alcohol (i.e., containing three OH groups).  It is liquid and H2O-soluble.  is achiral compound( with no chiral center). D-isomer L-isomer I. Simple lipids, A. Neutral Fats Naturally occurring fats & oils are mixture of TGs, containing a variety of FAs with chain length & degree of saturation. Esters of glycerol with 3 FAs, resulting in loss of –ve charge and formation of neutral fat. Sub classified into: Simple TGs Mixed TGs same kind of FA in the 3 positions in TG 2 or more different FAs. Ex: Tripalmitin Dioleopalmitin  Fat extraction Rendering: animal tissue Pressing: Seeds are crushed by Solvent extraction: containing fat are chopped & steel rollers. residues after pressing heated dry/steam until melts. are soaked in organic solvents. I. Simple lipids, A. Neutral Fats Animal Fats Plant Fats Physical Properties of Neutral Fats According to FA content ( chain length & degree of unsaturation) 1. Pure, freshly prepared TGs are colorless, odorless and tasteless. The yellow color of fats and oils is due to the presence of certain pigments e.g. carotenoids. 2. Fats have low heat conductivity, so subcutaneous fatty tissue acts as heat insulator. 3. Solubility  MGs& DGs are polar ( water soluble) and can form micelles  TGs are neutral nonpolar, Because the polar OHs of glycerol and the polar COO- of the FAs are bound in ester linkages, so TGs are insoluble in water but soluble in fat solvents.  TGs have lower specific gravities than water (