Classification of Lipids PDF

(triacylglycerols): These are and oils...

(triacylglycerols): These are and oils (a) Fats acids with glycerol glycerol. ipids (Greek: lipos-fat) are of great 8reat of fatty esters fat and oil importance to the body as the chief The difference between is a concentrated storage form of energy, besides Thus, oil is only physical. their role in cellular structure and various other is solid at roomn while fat a liquid biochemical functions.' As such, lipids are a temperature. heterogeneous group of Compounds and, Waxes: Esters of fatty acids (usually therefore, it is difficult to define them precisely. (b) other than long chain) with alcohols Lipids may be regarded as organic substances alcohols may be aliphatic relatively insoluble in water, soluble in organic glycerol. These alcohol is most solvents (alcohol, ether etc.), actually or or or alicyclic. Cetyl Waxes are commonly found in waxes. potentially related to fatty acids and utilized by used in the preparation of candles, the living cells. lubricants, cosmotics, ointments, polishes Unlike the polysaccharides, proteins and etc. nucleic acids, lipids are not polymers. Further, 2. Complex (or compound) lipids: These lipidsare mostly small molecules are esters of fatty acids with alcohols containing lássification of lipids additional groups such as phosphate, Lipids are broadly classified (modified from nitrogenous carbohydrate, protein etc. base, into simple, complex, derived and They are further divided as follows Bloor) lipids, which further subdivided are miscellaneous (a) Phospholipids They contain phosphoric into different groups acid and frequently a nitrogenous base. 1. Simple lipids: Esters of fatty acids with This is in addition to alcohol and fatty alcohols. These are mainly of two types acids.  i) Glycerophospholipids: These phospho- 5Lipids protect the internal organs, serve as lipids contain glycerol as the alcohol insulating materials and give shape and smooth e.g. lecithin, cephalin. appearance to the body. ( Sphingophospholipids:Sphingosine is the alcohol in this group of phospho- FATTY ACIDS lipids e.g., sphingomyelin. (b) Glycolipids: Theselipids contain_a fatty Fatty acids are carboxylic acids with acid, carbohydrate and nitrogenous base They are the simplest The alcohol is sphingosine, hence they hydrocarbon side chain. also called as form of lipids. are glycosphingolipids. Glycerol and phosphate are absent cerebrosides, gangliosides. eg.Occurrence Fatty acids mainly occur in the esterified form (c) Lipoproteins : Macromolecular complexes as major constituents of various lipids. They are of lipids with proteins. also present as free (unesterified) fatty acids. (d) Other complex lipids: Sulfolipids, amino- Fatty acids of animal orgin are much simpler in lipids and lipopolysaccharides are among structure in contrast to those of plant origin the other complex lipids. which often contain groups _uch as epoxy, keto, 3. Derived lipids: These are the derivatives hydroxy and cyclopentane rings. obtained on the hydrolysis of group 1 and group 2 lipids which possess the characteristics of Even and odd carbon fatty acids lipids. These include glycerol and other aktohols, in natural Most of the fatty acids that occur fatty acids, mono- and diacylglycerols lipid (fat) of even carbons (usually 14C- 20C). lipids are soluble vitamins, steroid hormones, hydro- This is due to the fact that biosynthesis of fatty carbons and ketone bodies. acids mainly occurs with the sequential addition 4. Miscellaneous lipids: These include a of 2 carbon units. Palmitic acid (16C) and stearic acid (18C) are the most common. Among large number of compounds possessing the characteristics of lipids e.g., carotenoids, the odd chain fatty acids, (propionic acid (3C) squalene, hydrocarbons such as pentacosane (in andvaleric acid (5C) are well known bees wax), terpenes etc. Saturated and unsaturated NEUTRAL LIPIDS: The lipids which are lipids. These to as neutral fatty acids uncharged arereferred are monodi-, and triacylglycerols, cholesterol Saturated fatty acids do not contain double and colesteryl esters. bonds, while unsaturated fatty acids contain one more double bonds. Both saturated and UPúnctions of lipids unsaturated fatty acids almost equally occur in functions the natural lipids. Fatty acids with one double Lipids perform several impotant bond are monounsaturated, and those with 2 or reserve of They the concentrated fuel are more double bonds are collectively known as the body (triacylglycerols). lyunsaturated fatty acids (PUFA). constituents of membrane Lipids are the regulate the membrane Nomenclature of fatty acids structuree and and cholesterol). permeability (phospholipids The naming of a fatty acid (systematic name) AThey serve source as a of Tat sodicis based on the hydrocarbon from which it is vitamins (A, D, E and K). derived. The saturated fatty acids end with a while the cellular metabolic Suttixanoic (e.g., octanoic acid) ALipids are important as unsaturated tatty acids end with a suffix -enoic regulators (steroid hormones and prostaglandins).  Reichert-Meissi (RM) number It is defined as There are two classes of phospholipids the number of ml 0.1 N KOH required to 1. Glycerophospholipids (or phosphoglyce completely neutralize the soluble volatile fatty rides) that contain glycerol as the alcohol. distilled from 5 g fat. RM number is useful acids in testing the purity of butter since it contains a 2. Sphingophospholipids (or sphingomyelins) that contain sphingosine as the alcohol good concentration of volatile fatty acids (butyric acid, caproic acid and caprylic acid). This is in Glycerophospholipids contrast to other fats and oils which have a negligible amount of volatile fatty acids. Butter Glycerophospholipids are the major lipids that occur in biological membranes. They consist has a RM number in the range 25-30, while it is less than 1 for most other edible oils. Thus any of glycerol 3-phosphate esteritied at its Ci and C2 with fatty acids) Usually, C Contains a adulteration of butter can be easily tested by Saturated fatty acid while C2 contains this sensitive RM number. unsaturated fatty acich Acid number : It is defined as the number of 1. Phosphatidic acid: This the simplest is mg of KOH required to completely neutralize phospholipid. It does not OCCur in good free tatty acids present in one gram fat or oil. concentration in the tissues. Basically, In normal circumstances, refined oils should phosphatidic acid is an intermediate in the be decomposition--due free from any free to chemical fatty acids. or Oils, bacterial on synthesis of triacylglycerols and phospholipids. The other( glycerophospholipids containing contamination-yield free fatty acids. Therefore, different nitrogenous bases or other groups may oils with increased acid number are unsafe for as the derivatives of phosphatidic be regarded human consumption. acid. These are 2.0ecithins(phosphatidylcholine: in the PHOSPHOLIPIDS the most abundant group of phospholipids (Greek: cell membranes. Chemically,lecithin compound lipids lecithos-egg yolk) is atphosphatidic acid with These are complex or Phosphatidylcholines in addition to fatty choline as the base) containing. phosphoric acid, choline. alcohol (Fig.3.3). represent the storage form of body's acids. nitrogenous base and BIOMEDICAL/CLINICAL cONCEPTs membranes, s o u r c e of fat soluble important to the body as constituents of ELipids are (steroid hormones and prostaglandins) vitamins and metabolic regulators (A, D, E and K) concentrated fuel stored in the adipose tissue are ETriacylglycerols (fats) primarily and around certain organs reserves of the body. Fats found in the subcutaneous tissue serve as thermal insulators. acids-linoleic and linolenic acid-are essential to humans, the EThe unsaturated fatty o r toad skin. deficiency of which causes phrynoderma employed in the treatment of leprosy. EBThe cyclic fatty acid, namely chaulmoogric acid,is (deterioration). etc. undergo rancidity ESFats and oils on exposure to air, moisture, bacteria antioxidants (vitamin E, hydroquinon This can be prevented by the addition of certain gallic acid). hydroxyanisole antioxidants-namely propyl gallate, butylated *n Jood preservation, used. and butylated hydroxytoluene-are commonly  CH2-0-C-R CH-O- -R1 R2-C-O-CH Ra-C-OCH CH3 CH2-O-P-0-CH-CHa-N-CH3 CH2-0-P-O Choline O (2) Lecithin (phosphatidylcholine) (1) Phosphatidic acid *********************************** **************************************** *** ** O CH2-O-C-R1 OH OH Ra--0-CH CH2-0-P-O H CH2-0-C-R OH HOOH Ra-C-O-CH 9 CHa-O-P-0-CHa-CHa-NHa OH myo-Inositol H O Ethanolamine (3) Cephalin (phosphatidylethanolamine) (4) Phosphatidylinositol - - - - - ----*--------------- ----. - - - - - - ----********************.**" ************************* CH2-0-CH=CH-R CHa-O-C-R Ra-C-O-CH R2-C-O-CH CH2-0-P-0-CH2-CH2-NH2 CHa-O-P-0-CH2-CH-COO Ethanolamine NH Serine (5) Phosphatidylserine (6) Plasmalogen (phosphatidalethanolamine) o----...**.**********.******* -*. *****-** ****************** O CH2-O-C-R, CH2-0-P -O-CH2 R2-C-O-CH O HC-OH O 0 HC-O--Rg CH2-0-P-O-CH2 R4-C-OH2 O Phosphatidylglycerol- (7) Cardiolipin (diphosphatidylglycerol) ************** **********************e*******************-**~***. -***-.-**--------- -~-****-.******** Ceramide Sphingosine OH CH-(CH2)12-CH=CH-CH-CH-NH-C-R CH2 O-P-O CH3 O-CH2-CHe-N CH Choline CH3 (8) Sphingomyelin Fig. 3.3 Structures of phospholipids.  (a)Dipalmitoyl lecithin is nipoulant Sphingomyelins phosphatidylcholine found in lungs. It is a Sphingosine is an amino alohol present in surtace active agent and prevents ihe adherence of inner surlace of the sphingomyelins (phingophospholipids). Ihey do not contain glycerol at all. Sphingosine is attached lungs due to surface tensionKRespiratory by an amicde linkage to a fatty acid to produce distress syndrome in infants is a disorder characterized by theabsence of lipalmitoyl ceramide)Iheacohol group of sphingosineis lecithirn bound to phosphoryleholine in sphingomyelin slructure (Fig.3.3).5phingomyelins are important (b) Lysolecithin is formed by removal of the constituents of myelin and are found in good fatty acid either at C, or C, of lecithin. (quantity in brain and nervous tissues. 3. Cephalins (phosphatidylethamolamine) Teramide, acts as a second messenger Ethanolamine is the nitrogenous base present in (signaling molecule) by regulating programmed cephalins. Thus, lecithin and cephalin differ with cell death (apoptosis), cell ycle and cell regard to the base. ditterentiation. (A ceramide containing d 4. Phosphatidylinositol: The steroisomer 30-carbon fatty acid is a major component of skin, apd it regulates skin's water permeability myo-inositol is attached to phosphatidic acid to give phosphatidylinositol(PI). This is an important Fhctions of phospholipids component of cell membranes. The action ofL Phospholipids constitute an important group certain hormones (e.8. oxytOCin, vasopressin) is of compound lipids that perform a wide variety mediated through PLS of functions 5. Phosphatidylserine: Theacid amino 1. In association with proteins, phospholipids serine is present in this group of glycerophos- form the structural components of membranes pholipids. Phosphatidylthreonine is also tound in and regulate membrane permeability. certain tissues. 2Phospholipids (lecithin, cephalin and 6. Plasmalogens: When a fatty acid is cardiolipin)in the mitochondria maintain the attached by an etherlinkage atC, of glycerol in conformation of electron transport chain the glycerophospholipids, the resultant components, and thus cellular respiration. Compound isplasmalogen. Phosphatidal- 3. Phospholipids participate in the absorption ethanolamine is the most important which is of fat from the intestine. similar in structure to phosphatidylethanolamine but for the ether linkage (in place of ester). An APhospholipids are esential for the synthesis of different lipoproteins, and thus unsaturated fatty acid occurs at C. Choline, inositol and serine may substitute ethanolamine participate in the transport of lipids. to other plaamalogens. Accumulation of fat in liver (fatty liver) can give be prevented by phospholipids, hence they are Cardiolipin: It is sd named as it was first regarded as lipotropic factors. isolated from heart muscle. Structurallya. Arachidonic acid, an unsaturated fatty acid cardiolipin consists of two) molecules_of liberated from phospholipids, serves as a phosphatidicacid held by an additional glyçerol precursor for the synthesis of eicosanoids (prosta- through phosphate groups. IS an important glandips, prostacyclins, thromboxanes etc.). componentof inner mitochondrial membraqe. in the reverse and essential for mitrochondrial function. Phospholipids participate in the cholesterol transport and thus help ECreased _cardiolipin levels may result in removal of cholesterol from the body. mitochondrial dysfunction) aging hypo act as surfactants (agents hyroidismardioskeletal myopathy (Barth XPhospholipids tension). For instance, syndrome) Cardiolipin is the only phospho Owering surface phosphatidylcholine is an important 8lyceride that possesses antigenic properties dipalmitoyl  Sphingosine OH O CHa-(CHa) 12-CH=CH-CH-CH-NH--CH(OH)-(CH2)21-CH3 Cerebronic acid CHOH fatty acid) HO H O-CH2 OR HH OH Fig. 3.4: Structure of galactosylceramide (R = H).. For sulfagalactosylceramide is a sulfatide (R = SO4). the ceramide). The lung surfactant. Respiratory distress syndrome in sequence attached to infants is associated with insufficient production ganglioside, GM2 that accumulates in Tay-Sachs of this surfactant. disease is represented next (outline structure). Ceramide ephalins, an important group of phospho- lipids participate in blood clotting Glucose 46Phosphatidylinositol is the SOurce of second messengersinositol triphosphate and Galactosse diacylglyceol, that are involved in the action of somehoromones N-Acetyl N-Acetyl neuraminic acid galactosamine GLYCOLIPIDS Glycolipids (glycosphingolipids) are important cLIPOPROTEINS constituents of cell membrane and nervous tissues (particularly the brain). Cerebrosides are Lipoproteins are molecular complexes of with proteins. They are the transport the simplest form of glycolipids. They(contain a lipids ceramide Gphingosine attached to a fatty acid) vehicles for lipids in the circulation. There are five types of lipoproteins, namely chylomicrons, and one or more sugars.c Galactocerebroside and glucocerebroside are very low density_lipoproteins (VLDL), low (galactosylceramide) the most important glycolipids. The structure density lipoproteins (LDL), high density of galactosylceramide is given in Fig.3.4. It lipoproteins (HDL) and free fatty acid- contains the fatty acid cerebronic acid. albumin complexes. Their structure, separation, derived metabolisr and diseases are discussed together Sulfagalactosylcerarmide is the sulfatide (Chapter 14). from galactosytceramide.) Gangliosides :These are predominantly found in ganglions and are the most complex form of STEROIDS - glycosphingolipidsyThey are derivatives of (the cerebrosides and contain one or more molecules Steroids are the compounds containing a of N-acetylneuraminic acid (NANA), the most éyclic steroid nucleus) (or ring) namely importantstatic acid. The structure of NANA is cyclopentanoperhydrophenanthrene (CPPP). It given in carbohydrate chemistry (Refer Fig.2.11) consists of a phenanthrene nucleus (rings A, B The most important gangliosides present in and C) to which a cyclopentane ring (D) is the brain are GM, GM2, GD, and GT, attached. (Grepresents ganglioside while M, D and T The structure and numbering of CPPP are indicate mono-, di- or fri- sialic acid residues, shown in Fig.3.5. The' steroid nucleus represents and the number denotes the carbohydrate saturated carbons, unless specifically shown as  B Steroid nucleus Cholesterol HO Ergosterol HO Fig. 3.5 : Structures of steroids (A, B, C-Perhydro phenanthrene: D-Cyclopentarne). double bonds. The methyl side chains (19 & 18) attached to carbons 10 & 13 are shown as single usually contain a bonds. At carbon 17, steroids side chain. There are several steroids in the biological include cholesterol, bile acids, system. These adrenocortical vitamin D, sex hormones, hormones, sitosterols, cardiac glycosides and contains one or more alkaloids. If the steroid hydroxyl groups it is commonly known as sterol (means solid alcohol). CHOLESTEROL Cholesterol, exclusively found in animals, is the most abundant animal sterol. It is widely distributed in all cells and is a major component of cell membranes and lipoproteins. Cholesterol (Greek: Cchole-bile) was first isolated from bile. Cholesterol literally means 'solid alcohol trom bile.