I. Structure and Properties of Lipids

Transcription

1 I. Structure and Properties of Lipids Lipids: A diverse group of compounds characterized by their low solubility in water and a high solubility in organic solvents such as chloroform and methanol. Nonpolar Amphipathic Polar or Hydrophilic Nonpolar or Hydrophobic (Hydrocarbon) Nonpolar Diverse Biological Functions: Principal storage of energy: Fats (solids) and Oils (liquids) Major Structural Element of Biological Membranes Partition of cells and subcellular organelles: Protective coatings and insulators: Oils and waxes Functional roles: Enzyme cofactors Intracellular and intercellular signalling molecules light absorbing molecules emulsifying agents

2 A. Storage Lipids Oils and fats are composed of fatty acid derivatives which serve as storage of energy in most organisms. Long chain hydrocarbon groups Fatty acid CO 2 + H 2 O ATP (energy) 1) Fatty Acids: consists of a long-chain hydrocarbon chain and carboxylic acid. Commonly occurring fatty acids have an even number of carbons R - COOH Example: 16 C fatty acid CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 C O OH Structural Properties: Length: Hydrocarbon chain: Typical length Carbons Even number of Carbons Carboxylic acid group (Carboxylate) 2

3 Degree of Saturation: Saturated (no double bonds) Unsaturated (one or more cis double bonds) Monounsaturated (one double bond) Polyunsaturated fatty acids (2 or more double bonds) Additional Structural Properties: Animals and plants fatty acids are linear in structure Micro-organisms: - Also branched fatty acids or fatty acids containing hydroxyl groups or 3 carbon cyclic rings Physical Properties: Dependent on carboxyl group, length and degree of unsaturation of fatty acids Solubility in Water: Carboxylate group accounts for slight solubility in water RCOOH RCOO - + H + Decreased solubility in water : Longer fatty acid chains Melting Point: Packing of lipids -- Length and Degree of unsaturation Decrease in melting point: shorter fatty acid chain larger the number of double bonds Saturated (12 24 carbons) Solid at room temperature Unsaturated Liquid at room temperature Examples: M.P. Length: 14:0 54 o C 16:0 63 o C 18:0 69 o C 3

4 Degree of Unsaturation: 18: o C 18: o C 18:2 9, o C 4

5 Nomenclature: Common names, Systematic names, Abbreviations Abbreviations (Symbols): # carbons:#double bonds ( positions of double bonds ) Some Biologically Important Fatty Acids Abbreviated Name Structure Systematic Name Common Name MP o C 12:0 CH 3 (CH 2 ) 10 COOH n-dodecanoic acid Lauric acid :0 CH 3 (CH 2 ) 12 COOH n-tetradecanoic acid Myristic acid :0 CH 3 (CH 2 ) 14 COOH n-hexadecanoic acid Palmitic acid :0 CH 3 (CH 2 ) 16 COOH n-octodecanoic acid Stearic acid :1( 9 ) cis-9-octodecenoic Oleic acid 13.4 acid 18:2( 9,12 ) cis,cis-9,12 Linoleic acid -5 Octadecadieoic acid 20:4( 5,8,11,14 ) Arachidonic acid Fatty Acid Composition of Some Dietary Lipids 5

6 2) Triglycerides (also called Triacylglycerols) Fatty acid esters of glycerol Structure: 3 Fatty acids linked by an ester bond to glycerol Triglycerides are highly insoluble in water Simple (same fatty acids) or complex (different fatty acids) Properties of Triglycerides: - Primarily: Storage of Energy and Insulation: Major energy source of animals Animals: Triglycerides are found as oily globules in adipocytes (fat cells) Typical fat content men 21%; women 26% Triglycerides - high source of energy compared to glycogen Triglycerides source of insulation for some animals Specialized functions: Control of density in Sperm Whale 6

7 Distribution of Saturated & Unsaturated fatty acids: Animal fats primarily composed of saturated fatty acids - solid Vegetable oils primarily composed of fatty acids - liquid Degradation: Reaction: Hydrolysis of the ester bond to produ ce glycerol and fatty acids Soaps: Chemical - Saponification of triglycerides (strong base and heat) Lipase: Enzymatic hydrolysis of triglycerides Triglyceride 3 Fatty acids + Glycerol 3) Waxes: Energy stores and water impermeable coats 7

8 Structure: Esters of long chain fatty acids (R) having long chain alcohols (R') R-COO-R' Beeswax R = Palmitic acid (C16) R = 1-Triacontanol (C30) R-COOH + R OH R-COO-R 8

9 B. Membrane Lipids Amphipathic lipids are a major constitutent of biological membranes. Their molecular properties are responsible for the formation of the lipid bilayer essential barrier for the selective passage of polar molecules and ions into and out of cells. General Characteristics: Amphipathetic lipids consist of a polar (hydrophilic) head group and a nonpolar (hydrophobic) tail: Most membrane lipids (Polar lipids) have two nonpolar tails important for the formation of lipid bilayers. Polar Head Group Hydrophobic Tails Major classes of lipids found in biological membranes: Polar lipids: Glycerophospholipids Sphingolipids Nonpolar lipids: Sterols (cholesterol) 9

10 1) Glycerolphospholipids (Phosphoglycerides): General structure: Glyerol backbone; two fatty acid esterified at positions 1 & 2 and a polar head group linked to position 3 of the glycerol backbone via phosphodiester bond. Common Glycerolphospholipids: Based on polar head groups (X) 10

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12 Phospholipases: Enzymes responsible for the specific degradation of phospholipids Phospholipase A1 O Phospholipase A2 CH 2 O C O R1 CH O C R2 O Phospholipase C CH 2 O P O - O Choline Phospholipase D Ether-linked Phospholipids: In some animal tissues and unicellular organisms fatty acids are linked to the glycerol backbone through ether bonds. Found in large quantities in heart tissue Released from white blood cells (basophils) to stimulate platelet aggregation and modulate other tissue responses. 12

13 2) Sphingolipids: General structure: Sphingosine backbone, a long fatty acid chain linked to the backbone via a amide bond and a polar head group Three major subclasses: 1. Sphingomyelins (polar head group - phosphocholine or phosphoethanolamine) generally called sphingophospholipids) present in large amounts in the plasma membrane of most cells. 2. Neutral glycolipids or glycosphingolipids generally with 1-6 neutral sugars cerebroside a head group X = 1 sugar residue globoside has a head group X = 2 sugar residues 3. Gangliosides complex glycosphingolipids - polar head groups contain sugar residues with one or more negatively charged sialic acid (N-acetyl-neuraminic acid ) residue. 13

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15 Sphinoglipids are structurally similar to glycerophospholipids. Phosphatidylcholine Sphingomyelin Sphingomyelin is found in high amounts in myelin sheath and the plasma membrane 15

16 Role of Sphingolipids in Recognition: 1. Neutral glycolipids contain antigenic determinants (sugar residues) for A,B,O Blood Typing Core Oligosaccharide chain = Glc Gal GalNAc Gal Fuc Where Glc Glucose Gal Galatose GalNAc N-acetyl-glactosamine Fuc Fucose Glc-Gal-GalNAc-Gal-Fuc O-Antigen (Type O- RBC) GalNAc Glc-Gal-GalNAc-Gal-Fuc A-Antigen (Type A- RBC) Gal Glc-Gal-GalNAc-Gal-Fuc B-Antigen (Type B- RBC) 16

17 2. Gangliosides are most complex glycosphingolipids and an oligosaccharide polar group with one or more sialic acid residues. Common gangliosides are G m1, G m2 and G m3 found on the surface of many animal cells G M1 serves as a receptor for Cholera Toxin a protein produced by the bacterium Vibrio cholerae 17

18 Sphingolipidoses (lipid storage diseases) - linked to defects in Sphingolipid degradation Examples: Niemann-Pick Disease: defect in sphinomylinase leads to accumulation of sphingomylin in the brain, spleen, liver, etc. resulting in mental retardation & early death Tay-Sachs Disease - deficiency in hexosaminidase A (1:300,000) accumulation of Ganglioside GM2 retardation in development, paralysis, blindmenss & early death 18

19 3. Sterol (Cholesterol) Structure: Four fused rings (3 containing 6 carbons; 1 containing 5 carbons) Hydrocarbon side chain extending from C 17 Polar hydroxyl group extending from C 3 Double bond C 5 to C 6 Isoprene: Precursor for the biosynthesis of Cholesterol and many other lipids including some vitamins and hormones. 19

20 C. Other Lipids: Derived from cholesterol Bile salts: Lipids facilitates the emulsification of fats during digestion. Steroid Hormones: 1. Cortisol, a glucocorticoid, regulates carbohydrate, protein and lipid metabolism and affects other processes including inflammatory reactions 2. Aldosterone, a mineralocorticoid, regulates salt and water excretion by the kidneys 3. Androgens and estrogens hormones involved in sexual development and function 20

21 Fat Soluble Vitamins Some Common Spices 21

22 Eicosanoids: Hormones that act on cells near site of synthesis. Eicosanoids are derived from 20-carbon polyunsaturated fatty acid arachidonic acid - stored as a fatty acid in phospholipids. They act at low concentrations and are involved in production of pain and fever, regulation of blood pressure, blood coagulation and reproduction. Nonsteroidal antiinflammatory drugs (NSAIDs) 22

23 D. Analysis of Lipids Lipids can be extracted from homogenized tissue or cells with organic solvents such as chloroform or benzene or a mixture of chloroform and methanol and further separated on the basis of their polarity by such techniques as: Adsorption Chromatography Thin Layer Chromatography Gas-liquid Chromatography Mass Spectroscopy 23