Lipids Lecture Notes PDF

LIPIDS 040817231 (Introductory Biochemistry) By: Dalia Shaaban, PhD Lecturer of Biochemistry, Faculty of Science, Alexandria University Intended learning outcomes  What are Lipids ?  Biological Importance of Lipids  Class...

LIPIDS 040817231 (Introductory Biochemistry) By: Dalia Shaaban, PhD Lecturer of Biochemistry, Faculty of Science, Alexandria University Intended learning outcomes  What are Lipids ?  Biological Importance of Lipids  Classification of Lipids  Fatty acids A-Simple lipids 1.Neutral fats 2.Waxes  B-conjugated lipids 1.Phospholipids 2.Glycolipids 3.Lipoproteins  What are Lipids ?  The word lipid is derived from a greek word “lipos” which means FAT. These are heterogenous group of organic compounds which relatively insoluble in water and other polar solvents while soluble in nonpolar (organic) solvents (ether, chloroform, benzene, acetone).  They are esters of fatty acids with alcohol or substances capable of forming such esters. Unlike proteins, nucleic acids & polysaccharides, lipids are not polymers rather they are small molecules.  They are widely distributed in nature both in plants and in animals. In our body, lipids are found primarily in three compartments: plasma, adipose tissue & biological membranes.  Biological Importance of Lipids Energy reservoir: the chief storage form of energy. Dietary lipids supply the body with about 25% of daily needed calories.  Thermal insulator in the subcutaneous tissues, protecting body against external temperature (thermogenesis process). Structural components of biological membranes and nerves (glycolipids, phospholipids, sphingomyelins, cholesterol & lipoproteins) and maintains membrane fluidity.  Cell recognition & Tissue immunity.  Helps in fat digestion, absorption (Bile acids).  Acts as cellular metabolic regulators and modulators (Steroid hormones & Prostaglandins).  Provide body with fat-soluble vitamins (A,D,E & K) and essential fatty acids.  Components of the electron transport chain  Fixation and protection of internal organs. Classification of Lipids  Fattyacids Fatty Acids(FAs) (FAs)  Aliphatic monocarboxylic organic acids (R-COOH), they are mostly obtained from the natural fats and oils hydrolysis.  Fatty acids are amphipathic, hydrophilic COOH head with hydrophobic group ( hydrocarbon chain).  General formula CH3(CH2)n-2COOH. Accordingly, most of FAs have even number of C atoms(4-36), because they are synthesized from condensation of 2C (acetate) units.  The most commonly occurring FAs in animals have even numbers of C atoms of 12 -24 carbons, odd in rare conditions. Generally, straight unbranched chain with few exceptions have branched & heterocyclic chains.  According to number of C atoms in the chain length, they are classified into: Short, Medium & Long FAs.  According to degree of saturation (double bonds in the chain), FAs are either Saturated or Unsaturated.  Fatty Acids Nomenclature  Systematic name is derived from the name of its parent hydrocarbon.  Palmitic acid, (16: 0), n-Hexadecanoic Acid, CH3-(CH2)14-COOH (saturated FA).  Palmitoleic acid,(16:1 ), cis-Hexadecenoic acid, CH3-( CH2 )5CH = CH-(CH2)7 –COOH (unsaturated FA). Chain length : number of double bond  Numbering of fatty acid C atoms  Firstly, The carbon skeleton of FAs are numbered either from the COOH group C-system or from the terminal methyl group (termed omega (ꞷ) carbon- ꞷ- system).  The carbon atoms are numbered from the COOH end, which is known as C1  The C atom adjacent to the COO- group is known as C2 or α-C, and the next carbon as C3 or β-C and the next C 4 or γ-C,…etc.  The methyl C at the other end of FA is known as omega C )ω-C).  Saturated Fatty Acids (SFA)  Fatty acids with straight single bonded hydrocarbon chain (no double bond).  Nomenclature: Ex: Palmitic acid (16: 0), n-Hexadecanoic Acid, CH3-(CH2)14-COOH  the FA has 16 c atoms with no (zero) db,  n- represents the normal linear unbranched FA  ending suffix –anoic, is representing saturated FA.  Palmitic (16:0) & stearic acids (18:0) are the most prevalent SFA in animal cells.  UnSaturated UnSaturatedfattyacids(USFAs): Fatty Acids (USFAs)  Fatty acids with at least one double bond in their hydrocarbon chain.  In nearly all naturally USFAs, the db is in found in cis-configuration causing a rigid bend in the aliphatic chain. As a result, USFAs are loosely packed, easily disrupted by heat, accounting for low melting point.  Oleic & Elaidic acids are cis- & trans-isomers of octadecenoic acid [18:1(∆9 )].  Trans FAs have db but without bent structure, i.e, straight chain like saturated FA and share high melting point.  Trans FAs are very rare in nature, produced by fermentation in the rumen of dairy animal and obtained from diary products and meat. Also they are produced during hydrogenation of vegetable oils.  All enzymes in human body that can react with bent, cis FAs but not trans-isomer. Hence,  UnSaturated Fatty Acids (USFAs), Nomenclature CH3-( CH2 )5CH = CH-(CH2)7 –COOH  C-system: position of db, according to C1 is COOH-end, represented by the symbol ∆ followed by a superscript number [16:1: ∆9].  ω- or n-system: position of db, according to the terminal methyl group in the FA [16:1: ω-7]. Palmitoleic acid: cis-Hexadecenoic acid  the FA has 16 c atoms with (one) db at carbon 9 from the COOH end  cis- represents the as the 2 H atoms arranged in cis-configuration.  the ending suffix –enoic, is representing unsaturated FA.  UnSaturated Fatty Acids (USFAs)  There is a common pattern in the location of dbs.  In most monounsaturated FAs, the db is between C-9 and C-10 )∆9).  The other dbs of polyunsaturated FAs are generally ∆9, ∆12 and ∆15, i.e. separated by methylene group (not alternative), with exception for Arachidonate [20:4)∆5,8,11,14)].  Linoleic acid,C18 (18:2 ∆9,12,ω6(, CH3-(CH2)4-CH = CH-CH2-CH=CH-(CH2)7-COOH  Linolenic acid, C18 (18:3 ∆9,12.15,ω3( , CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH- (CH2)7-COOH  Arachidonic acid, C20(20:4∆5,8,11,14,ω6(, CH3-(CH2)4-CH=CH-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-(CH2)3COOH  Classification of USFAs , according to number of double bond A-Mono-unsaturated fatty acids (MUFAs): palmitoleic acid(C16:1: ∆9) and oleic acid (C18:1: ∆9). B-Polyunsaturated fatty acids(PUFAs): no. d) Example Carbon skeleton Importance Dienoic 2 linoleic acid 18:2(∆9,12)=ω6 precursor of arachidonic acid Trienoic 3 -linolenic 18:3(∆9,12,15)= precursor of important fatty acid ω3 acids found in membranes of the retina. Tetraenoic arachidonic 20:4 (∆5,8,11,14)= synthesis of eicosanoids(signalling 4 acid ω6 molecules play role in innate immunity)  Classification of USFAs A-Essential FAs (EFAs:)  They are not formed in the animal body, because the body doesn't contain enzymes that can form more than one double bond, so they should be taken in the diet.  PUFAs cannot be synthesized in our body as linoleic,  -linolenic acids. While arachidonic acid is semi-essential.  Sources: Vegetable oils such as corn oil, linseed oil, peanut oil, olive oil, cottonseed oil, soybean oil and many other plant oils, cod liver oil and animal fats.  Deficiency produces dermatitis, fatty liver and impaired growth and reproduction. B-Nonessential Fatty Acids  These include the rest of fatty acids because they are formed de novo in the animal body mainly from carbohydrates.  It is not essential to take them in diet.  Physical Properties of Fatty Acids  The length and degree of unsaturation of hydrocarbon chain determine the physical properties of FAs.  Short-chain FAs are slight soluble due to polarity of –COO- group.  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.  SFAs, packed in stable aggregates by many hydrophobic interactions in its usual extended conformation, so they have high melting point. While, USFAs (cis-) with one or several such kinks cannot pack together as tightly as fully SFAs, and their interactions with each other are therefore weaker, causing lower melting point  A. Simple lipids  Esters of fatty acids with various alcohols.  According to the types of alcohols, there are two main sub-groups : Neutral fats & Waxes. 1. Neutral fats\ Triacylglycerol \Triglyceride (TG)  Esters of three FAs with glycerol.  Glycerol, has the popular name glycerin, is a polyhydric alcohol (i.e., containing three OH groups). It is colorless, odorless and has a sweet taste. It is liquid and soluble in water.  A. Simple Lipids (1. Neutral fats; TG)  Neutral fats\TGs can be: Simple TG Mixed TG the 3 FAs connected to glycerol are of the same Glycerol is esterified by different type FAs e.g., tripalmitin e.g., stearo-diolein and palmito- oleo-stearin.  Natural fats are mixtures of mixed TG with a small amount of simple TG. O C H 2 O C ( C H 2 ) 1 4 C H 3 O C H 3 ( C H 2 ) 1 4 C O C H O C H 2 O C ( C H 2 ) 1 4 C H 3 T r i p a l m i t i n ( s i m p l e t r i a c y l g l y c e r o l ) O C H 2 O C ( C H 2 ) 1 6 C H 3 O C H 3 ( C H 2 ) 7 C H C H ( C H 2 ) 7 C O C H O C H 2 O C ( C H 2 ) 7 C H C H ( C H 2 ) 7 C H 3 1 - S t e a r o - 2 , 3 - d i o l e i n ( m i x e d t r i a c y l g l y c e r o l ) O C H 2 O C ( C H 2 ) 1 4 C H 3 O C H 3 ( C H 2 ) 7 C H C H ( C H 2 ) 7 C O C H O C H 2 O C ( C H 2 ) 1 6 C H 3 1 - p a l m i t o - 2 - o l e o - 3 - s t e a r i n ( m i x e d t r i a c y l g l y c e r o l )  A. Simple Lipids (1. Neutral fats; TG)  Neutral fats\TG are classified into two sub-groups: a) Oils:  liquid at RT due to it high USFAs content.  generally of plant origin (vegetable fats as: olive oil, sunflower oil & Coconut oil). b) Solid fats:  solid at RT due to its high content of long chains SFAs.  mainly of animal origin (butter, beef fat).  Oil hydrogenation(hardening), adding H at db of USFAs, is done under high pressure and is catalyzed by Ni/Cu and heat, as a base in margarine manufacturing. Oils Hydrogen, high pressure, nickel Hard fat (liquid) (margarine, solid) It is advisable not to saturate all dbs; otherwise margarine produced will be very hard, of very low biological value and difficult to digest.  A. Simple Lipids (Rancidity of Neutral fats) It is the development of bad odor & taste (bad flavor) of fats OR oils, due to its exposure to high temperature, oxygen, moisture or humidity, light and metals, causing its decomposition or chemical changes. Which may be: Hydrolytic Rancidity Oxidative Rancidity Hydrolysis of TG by lipase enzyme (which is also Oils which are rich in USFA are more liable to present in bacteria) especially in presence of high develop this type of rancidity. Oxidation of USFA temperature and moisture, causing release of short produces peroxides, lower chain FAs, fatty chain FAs which are volatile and have bad odor. aldehydes and ketones. They have bad odor and bitter taste. Protection against rancidity Addition of antioxidants e.g. vitamin E, phenols and quinones help to protect the USFA against oxidation and decrease the rate of development of rancidity.  A. Simple Lipids (Partial Hydrogenation of Neutral fats)  When lipid-rich foods are exposed too long to the oxygen in air, their USFAs may react leading to formation of aldehydes and ketones associated with oxidative rancidity. To improve the shelf-life of vegetable oils used in cooking, and to increase their stability at the high temperatures used in deep-frying, commercial vegetable oils are altered by partial hydrogenation.  This process converts many of the cis double bonds in the FAs to single bonds and increases the melting temperature of the oils so they are more nearly solid at RT. But as undesirable effect, some cis double bonds are converted to trans double bonds and Trans fats are formed.  Many fast foods & other prepared foods are deep-fried in partially hydrogenated vegetable oils and therefore contain high levels of trans FAs.  There is now strong evidence that dietary intake of trans fatty acids (trans fats) increases the incidence of cardiovascular disease. This may be due to their effect on levels of LDL (bad cholesterol) & HDL (good cholesterol).  A. Simple Lipids (Physical Properties of Neutral Fats)  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(precursor of vit.A).  They are insoluble in water but soluble in fat solvents.  Neutral nonpolar: Because the polar OHs of glycerol and the polar COO- of the fatty acids are bound in ester linkages, so TGs are very nonpolar insoluble in water.  Lower specific gravities than water (