Lipids: An Introduction

Lipids: Introduction

• Naturally occurring organic compounds, lipids are insoluble in water but soluble in organic solvents.
• Lipids are composed of a carbon skeleton with hydrogen and oxygen substitutions.
• Hydrocarbon chains (-CH2- CH2- CH2-) in the structure of lipids make them hydrophobic.
• Fatty acids are water insoluble, saturated or unsaturated, long chain hydrocarbons, featuring a carboxyl group at the end of the chain.

Introduction Cont…

• Lipids are essential dietary components because of their high energy value.
• Lipids are an important source of fat-soluble vitamins in addition to essential fatty acids.
• Fats are thermal insulators when stored in adipose tissue in subcutaneous tissues and around certain organs.
• Lipoproteins are important cellular constituents found in the cell membrane, in the mitochondria, and serve the function of transporting lipids in the blood.
• Lipids are organic compounds mainly formed from alcohol and fatty acids combined together by ester linkage.

Occurrence

• Lipids occur in humans, animals, plants, and microorganisms.
• Lipids of animal origin include animal fat, egg yolk, butter and cheese.
• Vegetable and cooking oils come from plants.
• Lipids are a group of compounds, featuring fats, oils, steroids, waxes, and related compounds.
• Physical properties distinguish lipids more than chemical properties.

Occurrence of Lipids

• Fat depots include subcutaneous tissues, mesenteric tissues, fatty tissues around the kidney, yellow bone marrow, and blood (lipoproteins).
• Food sources include milk, egg, meat, liver, fish oils, nuts, seeds and oils.

CLASSIFICATION OF LIPIDS

• Simple lipids are esters of fatty acids with glycerol or other higher alcohols.
• Compound or complex lipids are fatty acids esterified with alcohol, containing other groups such as phospholipids with phosphoric acid and non-phosphorylated lipids.
• Derived lipids are compounds from lipids or precursors like fatty acids and steroids.
• Lipids complexed to other compounds form another class.

Simple Lipids

• Neutral fats (Triacylglycerol, TG) are triesters of fatty acids with glycerol.
• Waxes are esters of fatty acids with higher monohydroxy aliphatic alcohols.
• True waxes are esters of higher fatty acids with cetyl alcohol (C16H33OH) or other higher straight chain alcohols.
• Cholesterol esters are esters of fatty acid with cholesterol.
• Vit A and Vit D esters are palmitic or stearic acids esters of Vit A (Retinol) or Vit D respectively.

Neutral Fat

• Neutral fats are also called as triacylglycerols (TAG) or triglycerides (TG).
• Neutral fats are esters of trihydric alcohol, glycerol with fatty acids.
• Triacylglycerols constitute a significant number of the fatty acids in plants and animals.
• Neutral fats are considered the most abundant lipids in nature.
• Neutral fats constitute about 98% of the lipids of Adipose tissue, 30% of plasma or liver lipids, and less than 10% of erythrocyte lipids.
• Glycerol with various fatty acids are esters.
• Esterification of glycerol with one molecule of fatty acid gives monoglyceride, with 2 molecules gives diglyceride.

Types of Triglycerides

• Simple triglycerides are when the three fatty acids connected to glycerol are of the same type, e.g., tripalmitin.
• Mixed triglycerides are those of different types, e.g., stearodiolein and palmito-oleo-stearin.
• Natural fats are mixtures of mixed triglycerides with a small amount of simple triglycerides.

Physical Properties of TAG

• Freshly prepared fats and oils are colorless, odorless and tasteless.
• Any color or taste is due to the association with foreign substances, e.g., the yellow color of body fat or milk fat results from carotene pigments.
• Fats typically have a specific gravity less than 1, causing them to float on water.
• They are insoluble in water but soluble in organic solvents like ether and benzene.
• Glycerides of lower FA melt at lower temperatures than those of the higher fatty acids
• TAGs undergo emulsification

Chemical Properties

• Triglycerides undergo HYDROLYSIS into fatty acids and glycerol by heated steam, acid, alkali, or enzyme (e.g., lipase of pancreas).
• During enzymatic and acid hydrolysis, glycerol and free fatty acids are produced.
• SAPONIFICATION is the hydrolysis of a fat by an alkali.
• Alkaline hydrolysis produces glycerol and salts of fatty acids (soaps).
• Soaps emulsify oily material, aiding in washing fatty materials.
• Neutral fats with unsaturated fatty acids can add halogens via HALOGENATION.
• Halogenation determines the degree of unsaturation of the fat or oil to determine its biological value
• HYDROGENATION OR HARDENING OF OILS is a type of addition reaction accepting hydrogen at the double bonds of unsaturated fatty acids.
• Hydrogenation occurs under high hydrogen pressure and is catalyzed by finely divided nickel or copper and heat.
• Hardening of oils leads to margarine manufacturing, e.g., change of oleic acid of fats into stearic acid.
• Saturation of all double bonds should be avoided, because it will lead to margarine that is very hard, of very low biological value and difficult to digest.

Advantages for Hydrogenated Oil

• Hydrogenated oil is more pleasant as a cooking fat.
• Hydrogenated oil is a utilizable as normal animal fats and oils.
• Hydrogenated oil is less liable to cause gastric or intestinal irritation and is easily stored and transported.

Disadvantages for Hydrogenated Oil

• Hydrogenated oil can lack fat-soluble vitamins (A, D, E and K) and essential fatty acids.
• Hydrogenated oil can be filled with saturated fatty acids

RANCIDITY

• Fats and oils have a tendency to become rancid.
• Rancidity is the appearance of an unpleasant smell and taste for fats and oils.
• Hydrolytic rancidity is due to partial hydrolysis of the triacyl glycerol molecules due to traces of hydrolytic enzymes present in naturally occurring fats and oils.
• Oxidative rancidity results from partial oxidation of unsaturated fatty acids, leading to the formation of peroxides of small molecular weight fatty acids by peroxides and free radicals.
• Oxidative rancidity will affect the integrity of biomembranes in vivo, leading to cell death.

Iodine Number

• The iodine number is the number of grams of iodine absorbed or taken up by 100 grams of fat or oil.
• Iodine Number is an index of the degree of unsaturation and demonstrates proportionality to the content of unsaturated fatty acids.
• Higher the iodine number, higher is the degree of unsaturation, e.g. iodine number of butter is 28, and that of sunflower oil is 130.
• The iodine number measures for the degree of unsaturation of the fat, as a natural property for it.
• Unsaturated fatty acids absorb iodine at their double bonds, hence iodine number increases and with it the biological value of the fat.
• Iodine Number is used for identification of the type of fat, detection of adulteration and determining fats biological value

Saponification Number

• The saponification number is the number of milligrams of KOH required to completely saponify (hydrolyze) one gram of fat.
• Saponification number is a measure of the average molecular size of the fatty acids present.
• The value is higher for fats containing short chain fatty acids
• The number of -COOH groups determines the amount of alkali needed to saponify a given quantity of fat.
• Fats with short chain fatty acids have more -COOH groups per gram than long chain fatty acids, meaning fats with short chains will have higher saponification number.

WAXES

• Waxes are solid simple lipids containing a monohydric alcohol (with a higher molecular weight than glycerol) esterified to long-chain fatty acids.
• Waxes are insoluble in water and fat solvents with a negative acrolein test result.
• Waxes are indigestible by lipases since Fats are not easily hydrolyzed.
• Waxes are of no nutritional value.
• Waxes are resistant to rancidity

Types of Waxes

• Waxes are distributed in nature as the secretion of certain insects as beeswax, protective coatings of the skins and furs of animals and leaves and fruits of plants.
• Waxes are classified into true waxes and wax-like compounds.
• True waxes include beeswax secreted by honeybees to form combs.
• Beeswax is a mixture of waxes with the chief constituent being mericyl palmitate.

Wax-Like Compounds

• Cholesterol esters, when prepared from wool-associated skin glands and secreted by sebaceous glands of the skin, are Lanolin (or wool fat).
• Lanolin comprises a very complex mixture containing free and esterified cholesterol (e.g., cholesterol-palmitate) with other sterols.

Complex Lipids

• Complex lipids are esters of fatty acids containing groups other than and in addition to an alcohol and fatty acids.
• Phospholipids are substituted fats containing phosphoric acid residue, a nitrogenous base and other substituents (e.g., phosphatidyl choline (Lecithin), Phosphatidyl ethanolamine(Cephalin), phosphatidyl inositols (Lipositols), phosphatidyl serine, plasmalogens, sphingomyelins).
• Glycolipids are lipids containing a carbohydrate moiety.
• Glycolipids contain a nitrogenous base in addition to fatty acids.
• Glycolipids contain a special alcohol called sphingosine or sphingol.
• Glycolipids lack phosphoric acid or glycerol.
• Glycolipds have two types of cerebrosides and gangliosides
• Sulpholipids are lipids characterised by possessing sulphate groups.
• Aminolipids are Proteolipids.
• Lipoproteins have lipids as a prosthetic group.

Phospholipids

• Phospholipids typically have phosphoric acid and a nitrogenous base.
• Phosphatidyl choline (Lecithin) is a phospholipid
• Phosphatidyl ethanolamine (Cephalin) is found in biomembranes with amphipathic properties.
• As a second messenger, phosphatidyl inositols (Lipositols) plays a vital role in the mediation of hormone action on biomembranes.
• Found in bio-membranes in brain and muscle, phosphatidyl serine and plasmalogens are phospholipids.
• Sphingomyelins are phospholipids found in the nervous system and combined with fatty acids to yield a component of glycosphingolipids called ceramide.

Glycerophospholipids

• Glycerophospholipids are phospholipids which contain glycerol alcohol, like lecithin and cephalin.
• Sphingosine alcohol defines Sphingophospholipids like sphingomyelin
• The alcohol present in sphingolipids is monohydric
• Sphingosine is synthesized in the body from serine and palmitic acid.

Glycolipids

• These lipids contain a fatty acid, carbohydrate and nitrogenous base.
• The alcohol is sphingosine, hence they are also glycosphingolipids.
• Cerebrosides contain two or more hexoses hexosamines that are attached to a ceramide molecule.
• Ceramide + Galactose + Glucose → Lactosyl ceramide
• Lactosyl ceramide is a component of erythrocyte membrane.
• Gangliosides are formed when ceramide oligo-saccharides have at least one molecule of NANA (N-acetyl neuraminic acid) (sialic acid) attached to them.
• Ceramide-Glucose-galactose-NANA is designated as GM3 (ganglioside M3).
• Gangliosides contribute to stability of paranodal junctions and ion channel clusters in myelinated nerve fibers.
• Autoantibodies to GM1 disrupt lipid rafts, paranodal or nodal structures, and ion channel clusters in peripheral motor nerves.
• Lipoproteins have lipids as prosthetic group to proteins.

Sulpholipids

• An abundant sulfolipid is sulfoquinovosyl diacylglycerol, which is composed of a glycoside of sulfoquinovose and diacylglycerol.
• Mammalian brain and kidney are the richest source of sulpholipids.

Derived Lipids

• Derivatives lipids are obtained by hydrolysis of sample and compound lipids.
• Fatty acids may be saturated, unsaturated or cyclic
• Monoglycerides (Monoacylglycerol) and Diglycerides (Diacylglycerol) are other derived lipids.
• Alcohols are derived lipids.
• Straight chain alcohols are water insoluble alcohols of higher molecular weight obtained on hydrolysis of waxes.
• Cholesterol and other steroids including Vit D are derived lipids.
• Alcohols containing the β-ionone ring include Vit A and certain carotenoids are derived lipids.
• Glycerol is a derived lipid.

Fatty Acids

• A fatty acid (FA) is an organic acid that occurs in a natural triglyceride and is a monocarboxylic acid.
• The chain length ranges from C4 to about 24 carbon atoms in fats obtained from hydrolysis.
• Oils are liquid while fats are solid at room temperature.
• Fats are called neutral fats.
• Straight chain FAs may feature saturated forms or unsaturated double bonds.
• Aliphatic, branched and cyclic chain

Nomenclature of Fatty Acids

• Systemic names for fatty acids denote long, unbranched hydrocarbon chains with a carboxyl (COOH) group at one end based on the hydrocarbon with the same number of carbon atoms.
• "Oic" is substituted for the final "e" in the name of the hydrocarbon
• Saturated acids end in “anoic" (e.g. octanoic acid).
• Unsaturated acids with double bonds end in enoic, e.g. octadecenoic acid (oleic acid).
• Carbon atoms are numbered from the – COOH carbon (carbon No. 1).
• The carbon adjacent to -COOH gr. is α-carbon.
• Carbon atom 3 is β-carbon.
• The end -CH3 carbon is the ω-carbon ('Omega' carbon).
• The carbon atoms are numbered from the carboxyl atom with 1.

Classification of Fatty Acids

• Classification by chain length includes short chain (2-8C), medium chain (10-12C) and long chain (> 12 C).
• Classification according to the degree of saturation creates saturated and unsaturated fatty acids.
• According to their biological value, lipids are classified as either essential or non essential.
• According to structure, lipids are classified as Aliphatic, branched and cyclic chain

According to Chain Length

• Short chain fatty acids contain 8 carbon atoms or less (from 2-8) e.g Acetic acid (2 C): CH3-COOH, Butyric (4 C): CH3-CH2-CH2-COOH
• Medium-chain fatty acids contain from 10-12 carbon atoms.
• Long chain fatty acids contain more than 12 carbon atoms e.g Palmitic (16 C): CH3-(CH2)14-COOH, Stearic (18 C): CH3-(CH2)16-COOH, Lignoceric (24 C): CH3-(CH2)22-COOH

According to the Degree of Saturation

• Saturated FAs have the general formula CnH2n+1 COOH and contain no double bonds.
• Saturated FA examples are Acetic acid CH3COOH, Propionic acid C2H5COOH, Butyric acid C3H7COOH, Caproic acid C5H11COOH, Palmitic acid C15 H31COOH, and Stearic acid C17 H35COOH
• Saturated fatty acids having 10 carbon or less number of carbon atoms are lower fatty acids acetic acid, butyric acid.
• Saturated fatty acids having more than 10 carbon atoms are higher fatty acids, e.g. palmitic acid, stearic acid.
• Milk contains significant amount of lower fatty acids.

Unsaturated FA

• Unsaturated fatty acids are classified further according to degree of unsaturation that contain one or more double bonds.
• Monoethenoid fatty acids contain one double bond.
• Mono unsaturated fatty acids have have the general formula of Cn H2n-1 COOH.
• Oleic acid C17H33 COOH (formula 18 : 1; 9) is found in nearly all fats The three polyunsaturated fatty acids of biologicalimportance are polyethenoid fats:
• Linoleic acid series (18 : 2; 9, 12) contains two double bonds between C9 and C10 and between C12 and C13, with the general formula CnH2n-3 COOH.
• Linoleic acid dietary sources include peanut oil, corn oil, cottonseed oil, soyabean oil and egg yolk.
• Linolenic acid series (18 : 3; 9, 12, 15) contains three double bonds between 9 and 10, 12 and 13, and 15 and 16, with the general formula CnH2n-5 COOH.
• Linolenic acid dietary sources include linseed oil, rapeseed oil, and soybean oil, fish viscera and liver oil (cod liver oil).
• Arachidonic acid series (20 : 4; 5, 8, 11, 14) contains four double bonds with the general formula CnH2n-7 COOH.
• Arachidonic acid dietary sources includes peanut oil and animal fats with liver fats.
• Sebaceous glands (sebum) and foods contain branched chain FAs.
• Phytanic acid in butter is an example of a branched chain FA.

Positional Symbolism of Double Bonds

• The position of a double bond is represented by the symbol Δ followed by a superscript number.
• E.g, Δ 9 indicates a double bond between carbons 9 and 10 of the fatty acid:

Substituted and Cyclic Fatty Acids

• In hydroxy fatty acid and methyl fatty acid, Substituted fatty acids feature hydrogen replacements of - OH group or - CH3 group respectively.
• Both saturated and unsaturated hydroxyl fatty acids, particularly with long chains, are found in nature, e.g. cerebronic acid of brain glycolipids, ricinoleic acid in castor oil.
• Cyclic fatty acids have cyclic groups present in some seeds, e.g. chaulmoogric acid (obtained from chaulmoograseeds) and hydnocarpic acid.

Structure of Fats

• Fats are composed of fatty acid + glycerol to yield an ester.

Fatty Acid Numbering

• A C18 fatty acid with one double bond is octadecenoic acid; with two double bonds, octadecadienoic acid; and with three double bonds, octadecatrienoic acid.
• Carbon atoms are numbered from the carboxyl carbon (carbon No. 1).
• The carbon atoms adjacent to the carboxyl carbon are α,β and carbons.
• The terminal methyl carbon is the ω or n-carbon.

Cis- and Trans- Double Bonds

• Cis -hydrogen atoms are on the same side of the double bond.
• Trans-hydrogen atoms are oriented in opposite directions.
• The cis double bond causes a bend in the molecule, whereas a trans double bond gives a straight molecule as in a saturated FA.

Essential Fatty Acids

• Essential fatty acids (EFA) cannot be synthesized in the body and must be provided in the diet.
• Linoleic acid, linolenic acid and arachidonic acid are essential fatty acids.
• Growth retardation and deficiency manifestation symptoms can result from a lack of EFA in the diet.
• Linoleic acid is most important as, arachidonic acid can be synthesised from linoleic acid by a three stage reaction

Non-Essential Fatty Acids

• Nonessential fatty acids synthesize through biochemical reactions.
• Conversion of essential amino acids such as alanine, asparagine, aspartic acid, cystine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine produces non-essential fatty acids.
• Common non-essential acids examples are stearic, oleic, palmitic, myristic, and lauric acids

Properties of Fatty Acids

• Hydrogenation
• Halogenation
• Melting Point
• The solubility in water decreases, while melting and boiling points increase with increases in chain length.
• Saturated and unsaturated fatty acids form salts with alkali by forming forming salts with alkali.
• Sodium and potassium salts of long chain fatty acids are called soaps.
• Calcium and magnesium soaps are insoluble.
• Calcium soaps are used in grease.
• Alkyl sulfate (R-CH2-O-SO2-ONa) and alkyl sulfonate (R-CH2-SO2-O-Na) are not precipitated by hard water and are used as detergents.

Hydrogenation and Halogenation of Fatty Acids

• Unsaturated fatty acids can be converted to saturated fatty acids by hydrogenation of the double bond.
• Hydrogenation has been shown to lead to solidification and saturation.
• When treated with halogens under mild conditions, the unsaturated fatty acids can take up two halogen atoms, at each double bond to form the halogenated derivative of the fatty acid.
• The number of halogen atoms taken up depends on the number of double bonds and helps index the degree of unsaturation.

Fatty Acid Esterification

• Ester Formation begins with Glycerol + fatty acid yielding Mono acyl glycerol.
• This is followed by Monoglyceride + fatty acid to form Di acyl glycerol.
• Finally Diglyceride + fatty acid completes the cycle to yield Triglyceride or tri acyl glycerol.
• All fatty acids undergo oxidation in the body to give energy.

Biomedical Importance of Essential Fatty Acids

• Essential Fatty Acids are structural elements in tissues.
• The brain, retina and other neural tissues are rich in long-chain polyunsaturated fatty acids.
• Essential Fatty Acids regulate structural elements of gonads.
• Vitellogenesis, pre-spawning and spawning occur synchronically with the high lipid content in gonads, and in serum.
• The fatty acid pattern in sperm is important for the function and stability of sperm membranes.
• Changes of the fatty acid pattern in sperm will bring the sterility in animals like boar.

Cholesterol

• Cholesterol is the most important sterol in animal tissues.
• Cholesterol is present in animals as free alcohol, or esterified with fatty acids such as -linoleic, oleic, and palmitic acids
• Steroid hormones, bile salts and vitamin D are Cholesterol byproducts
• Tissues contain varied cholesterol levels that serve structural and metabolic roles such as adrenal cortex (10%), and brain (2%)
• Cholesterol is synthesized in the body from acetyl-CoA (1gm/day, cholesterol absent in plants) and taken in the diet (0.3 gm/day as in butter, milk, egg yolk, brain, meat and animal fat).

Cholesterol Structure

• Composed of 27 carbon atoms, and 4 rings
• Contains one hydroxyl group at the third position.
• Has a double bond between carbon atoms 5 and 6.
• Exhibits an eight carbon sidechain, beta-oriented, attached to terminal carbon 17.

Properties of Cholesterol

• Cholesterol is a yellowish crystalline solid.
• Cholesterol crystals under the microscope have a notched appearance.
• It is insoluble in water but soluble in organic solvents e.g. chloroform, benzene and ether.
• Total blood cholesterol is approximately 200 mg/dL, of which 2/3 is esterified (chiefly to unsaturated fatty acids) and 1/3 is free cholesterol

Functions of Cholesterol

• Component of Cell membranes

• Regulates Nerve conduction

• Found in Bile salts

• Precursor to Steroid hormones

• Precursor to Vitamin D3

Steroids of Biomedical Importance

• Steroids of biomedical importance are derivatives of cholesterol formed of a steroid ring or nucleus.

Bile Acids

• Bile acids are produced through cholesterol oxidation in the liver.
• Cholic and chenodeoxycholic acids conjugate with glycine or taurine with end results of glycocholic, glycochenodeoxycholic, taurocholic and taurochenodeoxycholic acids.
• Bile acids react with sodium or potassium to create sodium or potassium bile salts.
• Human bile exhibits the four salt variations of glycocholic acid, deoxycholic acid, cholic acid and lithocholic acid.

Bile Acid Functions

• Bile acids assist emulsification of lipids during digestion.
• Bile acids assist digestion of other foodstuffs
• Activation of pancreatic lipase
• Promote digestion and absorption of fat-soluble vitamins.
• Solubilize cholesterol prevent gall stone formation

Miscellaneous Lipids

• Aliphatic hydrocarbons contain isooctadecane, found in liver fat and certain hydrocarbons found in bees wax and plant waxes.
• Carotenoids, squalene, and Vitamins E and K are Miscellaneous Lipids.
• Squalene is a hydrocarbon found in shark and mammalian liver and in human serum.