3 Биологические мембраны. Обмен веществом (1160072), страница 8
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Glycerophospholipids differ in the structureof the head group; common glycerophospholipidsare phosphatidylethanolamine and phosphatidylcholine. The polar heads of the glycerophospholipids carry electric charges at pH near 7. Thesphingolipids, also membrane components, containsphingosine, a long-chain aliphatic amino alcohol,but no glycerol. Sphingomyelin possesses, in addition to phosphoric acid and choline, two long hydrocarbon chains, one contributed by a fatty acidand the other by sphingosine.
Two other classes ofsphingolipids are neutral glycolipids and gangliosides, which contain various sugar components.Cholesterol, a sterol, is a precursor of many steroids and is also an important component ofplasma membranes of animal cells. All polar lipidsare amphipathic; they have polar or charged headsand nonpolar hydrocarbon tails.
They spontaneously form micelles, bilayers, and liposomes, stabilized by hydrophobic interactions.Some types of lipids, although present in relatively small quantities, play critical roles as cofactors or signals. Steroid hormones are derived fromsterols.
Phosphatidylinositol is hydrolyzed to yieldtwo intracellular messengers, diacylglycerol andinositol trisphosphate. Prostaglandins, thromboxanes, and leukotrienes are extremely potent hormonelike molecules derived from arachidonic acid.Vitamins A, D, E, and K are fat-soluble compoundsmade up of isoprene units. All play essential rolesin the metabolism or physiology of animals. Vitamin A furnishes the visual pigment of the vertebrate eye.
Vitamin D is parent to a hormone thatregulates calcium and phosphate metabolism. Vitamin E probably functions in the protection ofmembrane lipids from oxidative damage, and vitamin K is essential in the blood-clotting process.Ubiquinones and plastoquinones, also isoprenoidderivatives, function as electron carriers in animals and plants, respectively. Dolichols activateand anchor sugars on cellular membranes for usein the synthesis of certain complex carbohydratesand glycoproteins.In the determination of lipid composition, lipidsare extracted from tissues with organic solventsand separated by thin-layer or gas-liquid chromatography.
Individual lipids are identified by theirchromatographic behavior, their susceptibility tohydrolysis by specific enzymes, or by mass spectraldetermination of their molecular masses.Further ReadingGeneralGurr, M.I. & Harwood, J.L. (1990) Lipid Biochemistry. An Introduction, 4th edn, Chapman & Hall,London.A good general resource on lipid structure and metabolism, at the intermediate level.Harwood, J.L. & Russell, N.J. (1984) Lipids inPlants and Microbes, George Allen & Unwin, Ltd.,London.Short, clear descriptions of lipid types, their distribution, metabolism, and function in plants andmicrobes; intermediate level.266Part II Structure and CatalysisMead, J.F., Alfin-Slater, R.B., Howton, D.R., &Popjak, G.
(1986) Lipids: Chemistry, Biochemistryand Nutrition, Plenum Press, New York.An intermediate level textbook on chemical, metabolic, and nutritional aspects of lipids.Vance, D.E. & Vance, J.E. (eds) (1991) Biochemistry of Lipids, Lipoproteins and Membranes, NewComprehensive Biochemistry, Vol. 20, ElsevierScience Publishing Co., Inc., New York.An excellent collection of reviews on various aspectsof lip id structure, biosynthesis, and function.
Particularly germane are the chapters by K. Bloch(Cholesterol: evolution of structure and function);P.R. Cullis & M.J. Hope (Physical properties andfunctional roles of lipids in membranes); C.C.Sweeley (Sphingolipids); and W.L. Smith, P. Borgeat, & F.A. Fitzpatrick (The eicosanoids: cyclooxygenase, lipoxygenase, and epoxygenase pathways).Structural Lipids in MembranesHakamori, S.
(1986) Glycosphingolipids. Sci. Am.254 (May), 44-53.Ostro, M.J. (1987) Liposomes. Sci. Am. 256 (January), 102-111.Sastry, P.S. (1985) Lipids of nervous tissue: composition and metabolism. Prog. Lipid Res. 24, 6 9 176.A good discussion of the growing number of processes known to be controlled by metabolites ofinositol-containing membrane lipids.Machlin, L.J. & Bendich, A. (1987) Free radicaltissue damage: protective role of antioxidant nutrients.
FASEB J. 1, 441-445.Brief discussion of tocopherols as antioxidants andtheir role in preventing damage by oxygen free radicals.Shimizu, T. & Wolfe, L.S. (1990) Arachidonic acidcascade and signal transduction. J. Neurochem.55, 1-15.Role of arachidonic acid (20 : 4) as precursor to theeicosanoids.Snyder, F., Lee, T.-C, & Blank, M.L.
(1989)Platelet-activating factor and related ether lipidmediators: biological activities, metabolism, andregulation. Ann. N. Y. Acad. Sci. 568, 35-43.Vermeer, C. (1990) y-Carboxyglutamate-containing proteins and the vitamin K-dependent carboxylase. Biochem. J. 266, 625-636.Biochemical basis for the requirement of vitamin Kin blood clotting, and the importance of carboxylation in the synthesis of the blood-clotting proteinthrombin.Spector, A.A. & Yorek, M.A.
(1985) Membranelipid composition and cellular function. J. LipidRes. 26, 1015-1035.Viitala, J. & Jarnefelt, J. (1985) The red cell surface revisited. Trends Biochem. Sci. 10, 392-395.Includes discussion of the human A, B, and O bloodtype determinants.Lipids with Specific Biological ActivitiesResolution and Analysis of LipidsChojnacki, T. & Dallner, G. (1988) The biologicalrole of dolichol. Biochem. J. 251, 1-9.Kates, M. (1986) Techniques of Lipidology: Isolation, Analysis and Identification of Lipids, 2ndedn, Laboratory Techniques in Biochemistry andMolecular Biology, Vol. 3, Part 2 (Burdon, R.H.
&van Knippenberg, PH., eds), Elsevier Science Publishing Co., Inc., New York.Fisher, S.K., Heacock, A.M., & Agranoff, B.W.(1992) Inositol lipids and signal transduction in thenervous system: an update. J. Neurochem. 58,18-38.Problems1. Melting Points of Fatty Acids The meltingpoints of a series of 18-carbon fatty acids are stearic acid, 69.6 °C; oleic acid, 13.4 °C; linoleic acid,- 5 °C; and linolenic acid, -11 °C. What structuralaspect of these 18-carbon fatty acids can be correlated with the melting point? Provide a molecularexplanation for the trend in melting points.2.
Spoilage of Cooking Fats Some fats used incooking, such as olive oil, spoil rapidly upon exposure to air at room temperature, whereas others,such as solid shortening, remain unchanged. Why?3. Preparation of Bearnaise Sauce During thepreparation of bearnaise sauce, egg yolks are incorporated into melted butter to stabilize the sauceChapter 9 Lipidsand avoid separation. The stabilizing agent in theegg yolks is lecithin (phosphatidylcholine).
Suggestwhy this works.4. Hydrolysis of Lipids Name the products of mildhydrolysis of the following lipids with diluteNaOH:(a) l-stearoyl-2,3-dipalmitoylglycerol(b) l-palmitoyl-2-oleoylphosphatidylcholine5. Number of Detergent Molecules per MicelleWhen a small amount of sodium dodecyl sulfate(Na'CH 3 (CH 2 )iiOSO 3 ) is dissolved in water, thedetergent ions go into solution as monomeric species. As more detergent is added, a point is reached(the critical micelle concentration) at which themonomers associate to form micelles. The criticalmicelle concentration of SDS is 8.2 mM.
An examination of the micelles shows that they have an average particle weight (the sum of the molecularweights of the constituent monomers) of 18,000.Calculate the number of detergent molecules in theaverage micelle.6. Hydrophobic and Hydrophilic Components ofMembrane Lipids A common structural feature ofmembrane lipid molecules is their amphipathicnature. For example, in phosphatidylcholine, thetwo fatty acid chains are hydrophobic and thephosphocholine head group is hydrophilic. Foreach of the following membrane lipids, name thecomponents that serve as the hydrophobic andhydrophilic units:(a) phosphatidylethanolamine(b) sphingomyelin(c) galactosylcerebroside(d) ganglioside(e) cholesterol7. Properties of Lipids and Lipid Bilayers Lipidbilayers formed between two aqueous phases havethis important property: they form two-dimensional sheets, the edges of which close upon eachother, and undergo self-sealing to form liposomes.(a) What properties of lipids are responsible forthis property of bilayers? Explain.(b) What are the biological consequences of thisproperty with regard to the structure of biologicalmembranes?8.
Chromatographic Separation of Lipids A mixture of the following lipids is applied to a silica gelcolumn, and the column is then washed with progressively more polar solvents. The mixture consists of: phosphatidylserine, cholesteryl palmitate(a sterol ester), phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, palmitic acid, n-267tetradecanol, triacylglycerol, and cholesterol. Inwhat order do you expect the lipids to elute fromthe column?9.
Storage of Fat-Soluble Vitamins In contrast towater-soluble vitamins, which must be a part ofour daily diet, fat-soluble vitamins can be stored inthe body in amounts sufficient for many months.Suggest an explanation for this difference based onsolubilities.10. Alkali Lability of Triacylglycerols A commonprocedure for cleaning the grease trap in a sink isto add a product that contains sodium hydroxide.Explain why this works.11. Dependence of Melting Point on Fatty Acid Unsaturation Draw all of the possible triacylglycerols that you could construct from glycerol, palmiticacid, and oleic acid. Rank them in order of increasing melting point.12.
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