8 1 MCB 450 Lecture 8 Fatty acids and lipids Membranes 8 2 Outline 1 Fatty acids structure nomenclature properties 2 Lipids components structure properties 3 Functions of lipids energy storage components of cell membranes 4 Membrane lipids phospholipids phosphoglycerides sphingolipids cholesterol 5 Lipid bilayers why they form bilayer fluidity lipid mobility and asymmetry 8 3 Five classes of lipids 1 Fatty acids Major building blocks of membranes Main sources of fuel Can be saturated or unsaturated fatty acids 2 Triglycerides Storage form of fatty acids source of glycerol 3 Phospholipids Main component of cell membranes 4 Glycolipids Important constituents of membranes 5 Sterols Polycyclic hydrocarbons Membrane components Variety of functions e g component of electron carriers hormones 8 4 Major membrane components fatty acids FA 1 Carboxylic acids with l o n g hydrocarbon chains 2 Chain can be fully saturated or contain one or more double bonds a few have 3 C rings OH groups or methyl group branches CARBOXYL HEADGROUP IONIZED AT PHYSIOLOGICAL pH 1 C atom closest to carboxyl C carbon 2 3 4 5 6 7 8 9 10 C atom most distant from carboxyl C carbon 11 12 13 14 15 16 8 5 Unsaturated fatty acids Double bond introduces rigid 30 bend 18 1 Oleic acid 18 2 Double bonds almost always have cis configuration Linoleic acid 8 6 Fatty acid nomenclature 1 Specifies Chain length double bonds separated by colon 16 C saturated palmitic acid 16 0 18 C oleic acid w 1 double bond 18 1 CH3 CH2 14COOH CH3 CH2 7CH CH CH2 7COOH Position of double bond specified by superscript s following 18 C oleic acid has its double bond between C 9 C 10 18 1 9 20 C FA w double bonds between C 9 C 10 C 12 C 13 20 2 9 12 Carboxyl C carbon 1 DO NOT include C O when asked for the number of double bonds 8 7 Fatty acid nomenclature 2 FA have systematic names and many have common names too Chain length double bonds separated by colon n normal unbranched 12 0 14 0 16 0 laurate myristate palmitate acid n dodecanoate n tetradecanoate n hexadecanoate 18 1 9 18 2 9 12 oleate linoleate cis 9 octadecenoate cis cis 9 12 octadecadienoate configuration of each double bond is indicated The most commonly occurring FA have even s of carbons in unbranched chains even s result from biosynthesis from C2 units Fatty acid nomenclature is important because it also tells us about the physical properties of fatty acids and lipids that contain them 8 8 Cis polyunsaturated fats are essential components of our diet FROM VEGETABLE OIL 18 3 9 12 15 Last C C bond is 3 bonds in from C hence 3 fatty acids 20 5 5 8 11 14 17 FROM FISH SHELLFISH 8 9 Physical properties of fatty acids 1 Physical properties of FA and compounds that contain them are largely determined by their hydrocarbon chain length and degree of saturation which influence 1 Solubility the longer the fatty acyl chain and the fewer double bonds the lower the solubility in H2O 2 Melting temperature 8 10 Fatty acids solubility and melting point trends Common name Systematic name Carbon skeleton Melting point C Solubility in water mg g solvent 30 C lauric acid dodecanoic ate 12 0 44 myristic tetradecanoate 14 0 54 palmitic hexadecanoate 16 0 63 stearic octadecanoate 18 0 70 palmitoleic cis 9 hexadecenoic 16 1 9 1 oleic cis 9 octadecenoic 18 1 9 13 linoleic cis cis 9 12 octadecadienoic 18 2 9 12 5 linolenic cis cis cis 9 12 15 octadecatrienoic 18 3 9 12 15 11 0 063 Sat Tm 0 024 0 008 0 003 MORE DOUBLE BONDS LOWER MELTING TEMP 8 11 Physical properties of fatty acids 2 Hydrocarbon chain length and degree of saturation influence Melting temperatures higher for saturated FA Free rotation about each C C bond allows each chain to adopt a fully extended form FA chains can pack together tightly stabilized by hydrophobic interactions with many atoms all along one molecule in van der Waals contact with atoms in neighboring chains FAs with double bonds can t pack together as tightly interactions between them are weaker It takes less thermal energy to disorder arrays consisting of or containing unsaturated fatty acids hence they have lower melting points 8 20 Classes of fatty acid containing lipids glycerol 8 13 Triacylglycerols Tristearin 3 x C18 0 3 FA ester linked to glycerol FA can vary 8 14 Saponification Ester linkage is susceptible to alkaline hydrolysis saponification http chemistry2 csudh edu rpendarvis ester fats html 8 15 Most natural plant and animal fats triacylglycerols 8 16 Triacylglycerols are the storage form of fatty acids Triacylglycerols accumulate in lipid droplets in adipocytes fat cells in adipose tissue 8 17 Triacylglycerols are a major energy reserve Glycogen glucose stores can maintain biological activity for 18 24 hrs Triacylglycerol stores allow survival for several weeks 8 18 Why are triacylglycerols Tg such a good fuel reserve 1 Fatty acids are richer in energy more reduced than carbohydrates Complete oxidation of Tg yields 38 kJ energy 1g compared to 17 kJ g from protein or carbohydrate 2 Tg can be stored more efficiently Tg are hydrophobic stored in anhydrous form Glycogen a polar carbohydrate binds 2 x its weight in water 1 g fat stores 6 x as much energy as 1 g hydrated glycogen 3 Tg provides enough stored energy to last for weeks Trivia The typical 70 kg man has 11 kg in fat storage The equivalent amount of energy in the form of glycogen would add an extra 55 kg 8 19 Carbon containing fuel molecules Octane C8H18 HYDROCARBON TAILS Saturated C8 0 fatty acid CARBOHYDRATE 8 20 Membrane lipids 1 Three common types phospholipids glycolipids cholesterol 2 Amphipathic one end of molecule is highly hydrophobic the other hydrophilic soluble in water insoluble in water 8 21 Classes of fatty acid containing lipids others L glycerol 3 phosphate 8 22 The simplest phosphoglyceride 8 23 Phosphoglycerides NEGATIVE CHARGE Major constituents of cellular membranes 8 24 Phosphoglycerides PE PC PS H PI Inositol 1 H 8 25 Sphingolipids 1 IN EUKARYA BOTH CHAINS RESISTANT TO ALKALINE HYDROLYSIS Fatty acyl chain amide linked to NH2 on C 2 8 26 Sphingolipids 2 Sphingomyelin abundant component of myelin membranous sheath that surrounds insulates axons HO l ll H C C N l H OH OH OH O COO OH OH 8 27 Cholesterol 4 FUSED RINGS PLANAR AND RELATIVELY RIGID 8 28 Cholesterol nucleus is made from 5 C isoprene units CH3 l CH2 C CH CH2 In cholesterol biosynthesis two 15 C units condense head to head C1 C1 8 29 Cholesterol derivatives 1 Five families of steroid hormones Androgens Estrogens Progestins
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