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Lipids: Properties, Functions, and Classification

This article provides an introduction to lipids, including their definition, functions, and types. It discusses fatty acids, neutral lipids, and phospholipids, as well as the classification of fatty acids based on chain length and saturation. It also explains the properties of fats and oils, and describes the reactions of fatty acids, including salt formation, ester formation, and lipid peroxidation.

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Lipids: Properties, Functions, and Classification

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  1. LIPIDS By Henry Wormser, Ph.D. PSC 3110 – Fall semester 2008

  2. Introduction • Definition: water insoluble compounds • Most lipids are fatty acids or ester of fatty acid • They are soluble in non-polar solvents such as petroleum ether, benzene, chloroform • Functions • Energy storage • Structure of cell membranes • Thermal blanket and cushion • Precursors of hormones (steroids and prostaglandins) • Types: • Fatty acids • Neutral lipids • Phospholipids and other lipids

  3. Fatty acids • Carboxylic acid derivatives of long chain hydrocarbons • Nomenclature (somewhat confusing) • Stearate – stearic acid – C18:0 – n-octadecanoic acid • General structure:

  4. Fatty acids • Common fatty acids n = 4 butyric acid (butanoic acid) n = 6 caproic acid (hexanoic acid) n = 8 caprylic acid (octanoic acid) n = 10 capric acid (decanoic acid)

  5. Fatty acids • common FA’s: n = 12: lauric acid (n-dodecanoic acid; C12:0) n = 14: myristic acid (n-tetradecanoic acid; C14:0) n = 16: palmitic acid (n-hexadecanoic acid; C16:0) n = 18; stearic acid (n-octadecanoic acid; C18:0) n = 20; arachidic (eicosanoic acid; C20:0) n= 22; behenic acid n = 24; lignoceric acid n = 26; cerotic acid

  6. Less common fatty acids iso – isobutyric acid anteiso odd carbon fatty acid – propionic acid hydroxy fatty acids – ricinoleic acid, dihydroxystearic acid, cerebronic acid cyclic fatty acids – hydnocarpic, chaulmoogric acid

  7. PHYTANIC ACID A plant derived fatty acid with 16 carbons and branches at C 3, C7, C11 and C15. Present in dairy products and ruminant fats. A peroxisome responsible for the metabolism of phytanic acid is defective in some individuals. This leads to a disease called Refsum’s disease Refsum’s disease is characterized by peripheral polyneuropathy, cerebellar ataxia and retinitis pigmentosa

  8. Less common fatty acids These are alkyne fatty acids

  9. Fatty acids • Fatty acids can be classified either as: • saturated or unsaturated • according to chain length: • short chain FA: 2-4 carbon atoms • medium chain FA: 6 –10 carbon atoms • long chain FA: 12 – 26 carbon atoms • essential fatty acids vs those that can be biosynthesized in the body: • linoleic and linolenic are two examples of essential fatty acid

  10. Unsaturated fatty acids • Monoenoic acid (monounsaturated) Double bond is always cis in natural fatty acids. This lowers the melting point due to “kink” in the chain

  11. Unsaturated fatty acids • Dienoic acid: linoleic acid

  12. Unsaturated fatty acids • Various conventions are in use for indicating the number and position of the double bond(s)

  13. Unsaturated fatty acids • Polyenoic acid (polyunsaturated)

  14. Unsaturated fatty acids • Monoenoic acids (one double bond): • 16:1, 9 w7: palmitoleic acid (cis-9-hexadecenoic acid • 18:1, 9 w9: oleic acid (cis-9-octadecenoic acid) • 18:1, 9 w9: elaidic acid (trans-9-octadecenoic acid) • 22:1, 13 w9: erucic acid (cis-13-docosenoic acid) • 24:1, 15 w9: nervonic acid (cis-15-tetracosenoic acid)

  15. Unsaturated fatty acids • Trienoic acids (3 double bonds) • 18:3;6,9,12 w6 : g-linolenic acid (all cis-6,9,12-octadecatrienoic acid) • 18:3; 9,12,15 w3 : a-linolenic acid (all-cis-9,12,15-octadecatrienoic acid) • Tetraenoic acids (4 double bonds) • 20:4; 5,8,11,14 w6: arachidonic acid (all-cis-5,8,11,14-eicosatetraenoic acid)

  16. Unsaturated fatty acids • Pentaenoic acid (5 double bonds) • 20:5; 5,8,11,14,17 w3: timnodonic acid or EPA (all-cis-5,8,11,14,17-eicosapentaenoic acid)* • Hexaenoic acid (6 double bonds) • 22:6; 4,7,10,13,16,19 w3: cervonic acid or DHA (all-cis-4,7,10,13,16,19-docosahexaenoic acid)* Both FAs are found in cold water fish oils

  17. Typical fish oil supplements

  18. Properties of fats and oils • fats are solids or semi solids • oils are liquids • melting points and boiling points are not usually sharp (most fats/oils are mixtures) • when shaken with water, oils tend to emulsify • pure fats and oils are colorless and odorless (color and odor is always a result of contaminants) – i.e. butter (bacteria give flavor, carotene gives color)

  19. Examples of oils • Olive oil – from Oleo europa (olive tree) • Corn oil – from Zea mays • Peanut oil – from Arachis hypogaea • Cottonseed oil – from Gossypium • Sesame oil – from Sesamum indicum • Linseed oil – from Linum usitatissimum • Sunflower seed oil – from Helianthus annuus • Rapeseed oil – from Brassica rapa • Coconut oil – from Cocos nucifera

  20. Non-drying, semi-drying and drying oils • based on the ease of autoxidation and polymerization of oils (important in paints and varnishes) • the more unsaturation in the oil, the more likely the “drying” process • Non-drying oils: • Castor, olive, peanut, rapeseed oils • Semi-drying oils • Corn, sesame, cottonseed oils • Drying oils • Soybean, sunflower, hemp, linseed, tung, oiticica oils

  21. Fatty acid reactions • salt formation • ester formation • lipid peroxidation

  22. Soaps • Process of formation is known as saponification • Types of soaps: • Sodium soap – ordinary hard soap • Potassium soap – soft soap (shaving soaps are potassium soaps of coconut and palm oils) • Castile soap – sodium soap of olive oil • Green soap – mixture of sodium and potassium linseed oil • Transparent soap – contains sucrose • Floating soap – contains air • Calcium and magnesium soaps are very poorly water soluble (hard water contains calcium and magnesium salts –these insolubilize soaps)

  23. Lipid peroxidation • a non-enzymatic reaction catalyzed by oxygen • may occur in tissues or in foods (spoilage) • the hydroperoxide formed is very reactive and leads to the formation of free radicals which oxidize protein and/or DNA (causes aging and cancer) • principle is also used in drying oils (linseed, tung, walnut) to form hard films

  24. Hydrogenated fats • hydrogenation leads to either saturated fats and or trans fatty acids • the purpose of hydrogenation is to make the oil/fat more stable to oxygen and temperature variation (increase shelf life) • example of hydrogenated fats: Crisco, margarine

  25. Neutral lipids • Glycerides (fats and oils) ;glycerides • Glycerol • Ester of glycerol - mono glycerides, diglycerides and triglycerides • Waxes – simple esters of long chain alcohols

  26. GLYCERIDES Function: storage of energy in compact form and cushioning

  27. Stereospecific numbering • carbon 2 of triglycerides is frequently asymmetric since C-1 and C-3 may be substituted with different acyl groups • by convention we normally draw the hydroxyl group at C-2 to the left and use the designation of sn2 for that particular substituent • C-1 and C-3 of the glycerol molecule become sn1 and sn3 respectively

  28. Analytical methods to evaluate lipids • saponification number • iodine value (Hanus method) • free fatty acids • acetyl number • Reichert-Meissl number • HPLC/GC (for more precise analysis)

  29. Saponification number • gives some clue as to the average size of fatty acids in a given sample of fat • defined as the number of milligrams of KOH needed to neutralize the fatty acids in 1 Gm of fat • butter (large proportion of short chain FAs) sap. no. 220 – 230 • oleomargarine (long chain FAs) sap. No is 195 or less

  30. Iodine number • measures the degree of unsaturation in a given amount of fat or oil • the iodine number is the number of grams of iodine absorbed by 100 grams of fat • Cottonseed oil: 103 –111 • Olive oil: 79 – 88 • Linseed oil: 175 –202 • frequently used to determine adulteration of commercial lots of oils

  31. Acetyl number some fatty acids have hydroxyl groups The acetyl number gives the proportion of these hydroxyl-containing fatty acids in a given sample of fat or oil

  32. Acetyl number • the acetyl number is the number of milligrams of KOH needed to neutralize the acetic acid of 1 Gm of acetylated fat • examples: • castor oil – 146 –150 • cod liver oil – 1.1 • cottonseed oil – 21 – 25 • olive oil – 10.5 • peanut oil – 3.5

  33. Reichert – Meissl number • measures the amount of volatile fatty acids (low MW and water soluble Fas) • the R-M number is the number of milliliters of 0.1N alkali required to neutralize the soluble fatty acids distilled from 5 Gm of fat • butter fat has a high R-M number

  34. WAXES • simple esters of fatty acids (usually saturated with long chain monohydric alcohols) Beeswax – also includes some free alcohol and fatty acids Spermaceti – contains cetyl palmitate (from whale oil) –useful for Pharmaceuticals (creams/ointments; tableting and granulation) Carnauba wax – from a palm tree from brazil – a hard wax used on cars and boats

  35. Spermaceti source Carnauba wax source Bee’s wax

  36. Waxes Examples of long chain monohydric alcohols found in waxes

  37. Phospholipids • the major components of cell membranes • phosphoglycerides Phospholipids are generally composed of FAs, a nitrogenous base, phosphoric acid and either glycerol, inositol or sphingosine

  38. Phosphatidyl inositol Commonly utilized in cellular signaling

  39. Sphingolipids Based on sphingosine instead of glycerol

  40. Sphingomyelin (a ceramide) It is a ubiquitous component of animal cell membranes, where it is by far the most abundant sphingolipid. It can comprise as much as 50% of the lipids in certain tissues, though it is usually lower in concentration than phosphatidylcholine

  41. Ether glycerophospholipids • Possess an ether linkage instead of an acyl group at the C-1 position of glycerol • PAF ( platelet activating factor) • A potent mediator in inflammation, allergic response and in shock (also responsible for asthma-like symptom • The ether linkage is stable in either acid or base • Plasmalogens: cis a,b-unsaturated ethers • The alpha/beta unsaturated ether can be hydrolyzed more easily

  42. Ether glycerophospholipids

  43. glycolipids There are different types of glycolipids: cerebrosides, gangliosides, lactosylceramides

  44. GLYCOLIPIDS • Cerebrosides • One sugar molecule • Galactocerebroside – in neuronal membranes • Glucocerebrosides – elsewhere in the body • Sulfatides or sulfogalactocerebrosides • A sulfuric acid ester of galactocerebroside • Globosides: ceramide oligosaccharides • Lactosylceramide • 2 sugars ( eg. lactose) • Gangliosides • Have a more complex oligosaccharide attached • Biological functions: cell-cell recognition; receptors for hormones

  45. Gangliosides • complex glycosphingolipids that consist of a ceramide backbone with 3 or more sugars esterified,one of these being a sialic acid such as N-acetylneuraminic acid • common gangliosides: GM1, GM2, GM3, GD1a, GD1b, GT1a, GT1b, Gq1b

  46. Ganglioside nomenclature • letter G refers to the name ganglioside • the subscripts M, D, T and Q indicate mono-, di-, tri, and quatra(tetra)-sialic-containing gangliosides • the numerical subscripts 1, 2, and 3 designate the carbohydrate sequence attached to ceramide

  47. Ganglioside nomenclature • Numerical subscripts: • 1. Gal-GalNAc-Gal-Glc-ceramide • 2. GalNAc-Gal-Glc-ceramide • 3. Gal-Glc-ceramide

  48. A ganglioside (GM1)

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