Lipids_DC-111-.pdf

Biochemistry : DC-111 (1+1) General structures, Classification and functions of Lipids Instructor: Dr. Anju Boora Khatkar Assistant Professor (Dairy Chemistry)  Introduction Lipids: Heterogeneous, group of organi...

Biochemistry : DC-111 (1+1) General structures, Classification and functions of Lipids Instructor: Dr. Anju Boora Khatkar Assistant Professor (Dairy Chemistry)  Introduction Lipids: Heterogeneous, group of organic compounds that are insoluble in water and soluble in non-polar organic solvents. They are used as cell membrane components, energy storage molecules, insulation, and hormones. Major components of adipose tissue, and together with proteins and carbohydrates, constitute the principal structural components of all living cells Lipids includes fats, waxes, sterols, fat-soluble vitamins (A, D, E and K), triglycerides, diglycerides, monoglycerides, phospholipids and others.  Glycerol esters of fatty acids, which make up to 99% of the lipids of plant and animal origin, have been traditionally called fats and oils. Most membrane lipids (e.g. phospholipids) are amphipathic, (contain both hydrophilic & hydrophobic groups) with surface-active properties. Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers. Further, lipids are mostly small molecules. Source :  Plants and vegetables e.g.vegetable oils,cocoa butter, etc  Animal source (milk fat, lard, tallow, etc.)  Marine (whale oil, cod liver oil, etc.)  Properties of Lipids  Lipids may be either liquids or non-crystalline solids at room temperature.  Pure fats and oils are colorless.  They are energy-rich organic molecules  Insoluble in water  Soluble in organic solvents like alcohol, chloroform, acetone, benzene, etc.  Most lipids have no ionic charges  Solid triglycerols (Fats) have high proportions of saturated fatty acids.  Liquid triglycerols (Oils) have high proportions of unsaturated fatty acids Function:  Lipids play an important role in nutrition as well as physiological functions as they are  Rich energy source (9 kcal/g)  Source of essential fatty acids and fat soluble vitamins.  Some lipids-- amphiphilic in nature (contain both hydrophilic & hydrophobic groups) with surface- active properties  As a whole, fats enrich the nutritional quality and impart the desired body & texture, rich mouth feel to the food  It also contributes characteristic flavour to food and produces a feeling of satiety or loss of hunger.  Structure of Lipids Lipids are made of Carbon, Hydrogen and Oxygen, but have a much lower proportion of water than other molecules such as carbohydrate Unlike polysaccharides and proteins, lipids are not polymers—they lack a repeating monomeric unit. Lipids are made from two molecules: Glycerol and Fatty Acids. A glycerol molecule is made up of three carbon atoms with a hydroxyl group attached to it and hydrogen atoms occupying the remaining positions.  Fatty acids consist of an acid group at one end of the molecule and a hydrocarbon chain, which is usually denoted by the letter ‘R’. fatty acids may be saturated or unsaturated. A fatty acid is saturated if there exist no C=C bonds. Unsaturated fatty acids, on the other hand, contain C=C bonds. Monounsaturated fatty acids have one C=C bond, and polyunsaturated have more than one C=C bond.  Classification of lipids Simple lipid Fat waxes Complex/Compound lipid Phospholipid Glycolipid lipoprotein Other complex lipids like Sulpholipid Derived lipid Fatty acid Glycerol Sterols I. Simple lipids:  These lipids are composed of fatty acids and alcohol components, and include fats, oils and wax esters  They can be hydrolyzed to two different components, usually an alcohol and an acid  Fats and Oils: called triacylglycerols because they are esters composed of three fatty acids joined to glycerol, trihydroxy alcohol  The difference between fats and oil is on the basis of their physical states at room temperature.  It is customary to call a lipid a fat if it is solid at 25°C, and oil if it is a liquid at the same temperature.  These differences in melting points reflect differences in the degree of unsaturation of the constituent fatty acids Waxes Wax is an ester of long-chain alcohol (usually mono-hydroxy) and a fatty acid. These alcohols may be aliphatic or alicyclic. Cetyl alcohol is most commonly found in waxes. The acids and alcohols normally found in waxes have chains of the order of 12-34 carbon atoms in length. Waxes are used in the preparation of candles, lubricants, cosmotics, ointments, polishes etc Complex (or compound) lipids : These are esters of fatty acids with alcohols containing additional groups such as phosphate, nitrogenous base, carbohydrate, protein etc. They are further divided as follows (a) Phospholipids : They contain phosphoric acid and frequently a nitrogenous base. This is in addition to alcohol and fatty acids I. Glycerophospholipids : These phospholipids contain glycerol as the alcohol e.g., lecithin, cephalin. II. Sphingophospholipids : Sphingosine is the alcohol in this group of phospholipids e.g., sphingomyelin  Complex lipids They are lipids that contain additional substances, e.g., sulfur, phosphorus, amino group, carbohydrate, or proteins beside fatty acid and alcohol. Compound or conjugated lipids are classified into the following types according to the nature of the additional group: 1. Phospholipids 2. Glycolipids. 3. Sulfolipids 4. Amino lipids.  phospholipids Phospholipids or phosphatides are compound lipids, which contain phosphoric acid group in their structure. They are present in large amounts in the liver and brain as well as blood. Every animal and plant cell contains phospholipids. Phospholipids are composed of: 1. Fatty acids (a saturated and an unsaturated fatty acid). 2. Nitrogenous base (choline, serine or ethanolamine). 3. Phosphoric acid. 4. Fatty alcohols (glycerol, inositol or sphingosine). Phospholipid  Glycerophospholipids: Alcohol is glycerol  Sphingophospholipids: Alcohol is sphingosine.  Glycolipids Lipids containing a carbohydrate along with fatty acid, and alcohol Glycolipids are widely distributed in every tissue of the body particularly in nervous tissue such as brain. These lipids contain a fatty acid, carbohydrate and nitrogenous base. The alcohol is sphingosine, hence they are also called as glycosphingolipids. Glycerol and phosphate are absent e.g., cerebrosides, gangliosides. They contain ceramide (sphingosine+fattyacids) and one or more sugars Ceramide+Glucose------- Glucocerebrosides Ceramide + Galactose ----- Galactocerebrosides Ceramides are a family of lipid molecules, composed of sphingosine and a fatty acid  Sulpholipids Sulpholipids or sulfatides are formed when sulfate groups are attached to ceremide. For example Sulphated cerebrosides Sulphated globosides Sulphated gangliosides All these complex lipids are important components of membranes of nervous tissue.  Derived lipid These are substances derived from simple lipids and compound lipids by hydrolysis. This includes fatty acids, glycerol, sterols, fatty aldehydes, ketone bodies, lipid soluble vitamins and lipid soluble hormones. Neutral Lipids: acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids because they are uncharged  Fatty acid Fatty acid is a carboxylic acid with a long aliphatic tail (chain), which is either saturated or unsaturated Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28 Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as "free" fatty acids. Fatty acids are important sources of fuel because, when metabolized, they yield large quantities of ATP Types of fatty acids A) Saturated fatty acid Short chain fatty acid Medium chain fatty acid Long chain fatty acid B) Unsaturated fatty acid Monounsaturated fatty acid Polyunsaturated fatty acid  Short chain fatty acid Fatty acids with aliphatic tails of fewer than six carbons Common name Chemical structure C:D Butyric acid CH3(CH2)2COOH 4:0 Caproic acid CH3(CH2)4COOH 6:0  Medium chain fatty acids Common name Chemical structure C:D Caprylic acid CH3(CH2)6COOH 8:0 Capric acid CH3(CH2)8COOH 10:0 Lauric acid CH3(CH2)10COOH 12:0 Myristic acid CH3(CH2)12COOH 14:0 Long chain fatty acids Common name Chemical structure C:D Palmitic acid CH3(CH2)14COOH 16:0 Stearic acid CH3(CH2)16COOH 18:0 Arachidic acid CH3(CH2)18COOH 20:0  Unsaturated fatty acid Monounsaturated fatty acid: one double bond in their structure Common name Chemical structure Δx C:D Oleic acid CH3(CH2)7CH=CH(CH2)7COOH cis-Δ9 18:1 Polyunsaturated fatty acid: two or more double bonds in their structure, The double bonds are generally separated by at least one methylene group Common Chemical structure Δx C:D name Linoleic CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH cis,cis-Δ9,Δ12 18:2 acid Linolenic cis,cis,cis- CH3(CH2)CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH 18:3 acid Δ9,Δ12 , Δ15 Arachid cis,cis,cis,cis- CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH 20:4 onic acid Δ5Δ8,Δ11,Δ14  Shorter the chain of fatty acids lower is the melting temperature than those with longer chains. Unsaturated fatty acids have lower melting temperatures than saturated fatty acids of same chain length CIS CONFIGURATION Where a double bond exists, there is the possibility of either a cis or trans geometric isomerism, which significantly affects the molecule's molecular configuration. Adjacent hydrogen atoms are on the same side of the double bond Cis bonds limit the ability of fatty acids to be closely packed, and therefore could affect the melting temperature of the fat Cis-double bonds cause the fatty acid chain to bend, an effect that is more pronounced when more double bonds are there in a chain. This in turn plays an important role in the structure and function of cell membranes  TRANS CONFIGURATION Adjacent hydrogen atoms are bound to opposite sides of the double bond Most fatty acids in the trans configuration (trans fats) are not found in nature and are the result of human processing (e.g., hydrogenation) Triglyceride They are esters of glycerol with various fatty acids. Since the 3 hydroxyl groups of glycerol are esterified, the neutral fats are also called “Triglycerides”. Esterification of glycerol with one molecule of fatty acid gives monoglyceride, and that with 2 molecules gives diglyceride. O O HO C R1 CH2 OH H2C O C R1 O O HO C R2 + HO C H R2 C O C H O O CH2 OH 3 H 2O H2C O C R3 HO C R3 Glycerol Triglycerides Fatty acids (Triacylglycerol)  Types of triglycerides 1-Simple triglycerides: If the three fatty acids connected to glycerol are of the same type the triglyceride is called simple triglyceride, e.g., tripalmitin. 2-Mixed triglycerides: if they are of different types, it is called mixed triglycerides, e.g., stearo-diolein and palmito-oleo-stearin. Natural fats are mixtures of mixed triglycerides with a small amount of simple triglycerides.  Simple and Mixed Triglycerides O CH2 O C (CH2)14 CH3 O CH3 (CH2)14 C O C H O CH2 O C (CH2)14 CH3 Tripalmitin (simple triacylglycerol) O CH2 O C (CH2)16 CH3 O CH3 (CH2)7 CH CH (CH2)7 C O C H O CH2 O C (CH2)7 CH CH (CH2)7 CH3 1-Stearo-2,3-diolein (mixed triacylglycerol) O CH2 O C (CH2)14 CH3 O CH3 (CH2)7 CH CH (CH2)7 C O C H O CH2 O C (CH2)16 CH3 1-palmito-2-oleo-3-stearin (mixed triacylglycerol) Properties of Saturated Fatty Acids Contain only single C–C bonds Closely packed Strong attractions between chains High melting point Solids at room temperature Properties of Unsaturated Fatty Acids Contain one or more double C=C bonds Nonlinear chains do not allow molecules to pack closely Few interactions between chains Low melting points Liquids at room temperature  Properties of fatty acids 1. Hydrogenation 2. Halogenation 3. Melting Point 4. Salt formation 5. Ester formation 6. Oxidation  1. Hydrogenation Unsaturated compounds react with H2 Ni or Pt catalyst C=C bonds C–C bonds Hydrogenation of oils can lead to solidification and saturation Linolenic acid → Linoleic acid → Oleic acid → Stearic acid Unsaturated fatty acid  Product of hydrogenation Saturated fatty acid Oils Hydrogen, high pressure, nickel Hard fat (liquid) (margarine, solid) (with unsaturated (with saturated fatty acids, e.g., oleic) fatty acids, e.g., stearic) Hydrogenation converts double bonds in oils to single bonds. The solid products are used to make margarine and other hydrogenated items. Eg. Vanaspati  2. Halogenation Unsaturated fatty acid when treated with halogens under mild conditions, the unsaturated fatty acid can take up two halogen atoms at each double bond to form the halogenated form of fatty acid. The number of halogen atoms taken up will depend on number of double bonds and is an index of unsaturation. CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH Linoleic acid 2 I2 CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH I I I I Stearate-tetra-iodinate  3. Melting Point Saturated fatty acids Fit closely in regular pattern COOH COOH COOH Unsaturated fatty acids Cis Double bond: loose packing Each cis double bond causes a kink in the chain. H H C C cis double bond COOH  Melting point The short and medium chain fatty acids are liquids where as long chain fatty acids are solids at 250C. Higher melting point is due to close packing of straight chain fatty acid molecule. Unsaturated fatty acids have low melting point compared to saturated fatty acid with the same chain length because of more loose packing of cis- fatty acids. Fatty acid Melting point Stearic acid 690C Oleic acid 130C Linoleic acid -50C Linolenic acid -100C  4. Salt formation Hydrolysis with a strong base Triglycerides split into glycerol and the salts of fatty acids The Na & K salts of long chain fatty acids are called “soaps” KOH gives softer soaps RCOOH + NaOH RCOONa + H2O  Saponification O CH2 O C (CH2)16CH3 O CH O C (CH2)16CH 3 + 3 NaOH O CH2 O C (CH2)16CH3 CH2 OH O +- CH OH + 3 Na O C (CH2)14CH3 salts of fatty acids (soaps) CH2 OH  5. Ester formation Both saturated and unsaturated fatty acid form esters with alcohol especially with glycerol. Fatty acid can form mono-, di-, or tri- esters with alcoholic group of glycerol. O ester bonds CH2 OH HO C (CH2)14CH3 O O CH2 O C (CH2)14CH3 + H2O CH OH + HO C (CH2)14CH3 O O CH2 OH HO C (CH2)14CH3 CH O C (CH2)14CH3 + H2O glycerol palmitic acid (a fatty acid) O CH2 O C (CH2)14CH3 + H2O  6. Oxidation All fatty acids undergo oxidation in the body to give energy. β-oxidation is the major process by which acids are oxidized. Unsaturated fatty acid undergo oxidation due to highly reactive double bonds.  Reactions of triglycerides Hydrolysis Saponification Hydrogenation Drying Oxidation (Rancidity)  Hydrolysis Fats (triglycerides) are hydrolyzed into their constituents (fatty acid and Glycerol) by the action of super heated steam, acid, alkali or enzyme (e.g., lipase of pancreas). During their enzymatic and acid hydrolysis glycerol and free fatty acids are produced. O O CH2 O C R1 H2C OH R1 C OH O Lipase or Acid O R2 C O C H HO C H + R C OH 2 O O CH2 O C R3 3 H2O H2C OH R3 C OH Triacylglycerol Glycerol Free fatty acids  Saponification Alkaline hydrolysis produces glycerol and salts of fatty acids (soaps). Soaps cause emulsification of oily material this help easy washing of the fatty materials O O CH2 O C R1 H2C OH R1 C ONa O O R2 C O C H HO C H + R C ONa 2 O O CH2 O C R3 3 NaOH H2C OH R3 C ONa Triacylglycerol Glycerol Sodium salts of fatty acids (soap)  Halogenation Neutral fats containing unsaturated fatty acids have the ability of adding halogens (e.g., hydrogen or hydrogenation and iodine or iodination) at the double bonds. It is a very important property to determine the degree of unsaturation of the fat or oil that determines its biological value CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH Linoleic acid 2 I2 CH3 (CH2)4 CH CH CH2 CH CH (CH2)7 COOH I I I I Stearate-tetra-iodinate  Drying When highly unsaturated fatty oils are exposed to air. They undergo oxidation and polymerization to form a dry, hard and tough film. Such oils are called dry oils and the reaction is referred to as drying of oils.  Oxidation (Rancidity) Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste. Fats containing unsaturated fatty acids are more susceptible to rancidity. Rancidity occurs when fats and oils are exposed to air, moisture, light, bacteria etc. Hydrolytic rancidity occurs due to partial hydrolysis of triacylglycerols by bacterial enzymes. Oxidative rancidity is due to oxidation of unsaturated fatty acids. This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones etc. Rancid fats and oils are unsuitable for human consumption  Hazards of rancidity 1. The products of rancidity are toxic, i.e., causes food poisoning and cancer. 2. Rancidity destroys the fat-soluble vitamins (vitamins A, D, K and E). 3. Rancidity destroys the polyunsaturated essential fatty acids. 4. Rancidity causes economical loss because rancid fat is inedible.  Prevention of rancidity 1. Avoidance of the causes (exposure to light, oxygen, moisture, high temperature and bacteria or fungal contamination). By keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., good storage conditions). 2. Removal of catalysts such as lead and copper that catalyze rancidity. 3. Addition of anti-oxidants to prevent rancidity. They include BHA, BHT, tannins and hydroquinones. The most common natural antioxidant is vitamin E. Thank You