3 Биологические мембраны. Обмен веществом (1160072)
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Лекция 3.Биологические мембраны. ОбменвеществомC H A P T E RLipidsBiological lipids are a chemically diverse group of compounds, the common and defining feature of which is their insolubility in water. Thebiological functions of the lipids are equally diverse. Fats and oils arethe principal stored forms of energy in many organisms, and phospholipids and sterols make up about half the mass of biological membranes. Other lipids, although present in relatively small quantities,play crucial roles as enzyme cofactors, electron carriers, light-absorbing pigments, hydrophobic anchors, emulsifying agents, hormones,and intracellular messengers. This chapter introduces representativelipids of each type, with emphasis on their chemical structure andphysical properties.Storage Lipids0.4 /xmLipids play an important role in cell structureand function.
In this electron micrograph of thecytoplasm of the photosynthetic alga Euglena, thelipid-containing membranes of a chloroplast (upperright) and several mitochondria (surrounding thechloroplast and lower left) are visible. Two lipiddroplets, stores of chemical energy, can be seen inthe chloroplast. The gray oval structure at thelower right is a lipid-filled inclusion in thecytoplasm.240The fats and oils used almost universally as stored forms of energy inliving organisms are highly reduced compounds, derivatives of fattyacids. The fatty acids are hydrocarbon derivatives, at about the samelow oxidation state (that is, as highly reduced) as the hydrocarbons infossil fuels. The complete oxidation of fatty acids (to CO2 and H2O) incells, like the explosive oxidation of fossil fuels in internal combustionengines, is highly exergonic.We will introduce here the structure and nomenclature of the fattyacids most commonly found in living organisms.
Two types of fattyacid-containing compounds, triacylglycerols and waxes, are describedto illustrate the diversity of structure and physical properties in thisfamily of compounds.Fatty Acids Are Hydrocarbon DerivativesFatty acids are carboxylic acids with hydrocarbon chains of 4 to 36carbons. In some fatty acids, this chain is fully saturated (contains nodouble bonds) and unbranched; others contain one or more doublebonds (Table 9-1).
A few contain three-carbon rings or hydroxylgroups. A simplified nomenclature for these compounds specifies thechain length and number of double bonds, separated by a colon; the16-carbon saturated palmitic acid is abbreviated 16:0, and the 18carbon oleic acid, with one double bond, is 18:1. The positions of anydouble bonds are specified by superscript numbers following A (delta);a 20-carbon fatty acid with one double bond between C-9 and C-10 (C-lChapter 9 Lipids241Table 9-1 Some naturally occurring fatty acidsSystematicname*Common name(derivation)CH3(CH2)10COOHrc-DodecanoicacidLaurie acid(Latin laurus,laurel plant)14:0CH3(CH2)12COOHrc-Tetradecanoicacid16:0CH3(CH2)14COOH18:0CarbonskeletonStructure*12:0Meltingpoint (°C)Solubility at 30 °C(mg/g solvent)WaterBenzene44.20.0632,600Myristic acid(Latin Myristica,nutmeg genus)53.90.024874/I-HexadecanoicacidPalmitic acid(Greek palma,palm tree)63.10.0083348CH3(CH2)16COOHra-OctadecanoicacidStearic acid(Greek stear,hard fat)69.60.003412420:0CH3(CH2)18COOHrc-EicosanoicacidArachidic acid(Latin Arachis,legume genus)76.524:0CH3(CH2)22COOHrc-TetracosanoicacidLignoceric acid(Latin lignum,wood + cera, wax)86.016:1(A9)CH3(CH2)5CH==C1Palmitoleicacid-0.518: HA9)CH,(CHo)7CH==C3Oleic acid(Greek oleum,oil)13.418:2( A912)CH 3 (CH 2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOHa-Linoleic acid(Greek linon, flax)18:3( A9'12'15)CH 3 CH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 7 COOHLinolenic acid-11CH 3 (CH 2 ) 4 CH=CHCH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 3 COOHArachidonic acid-49.520:4( A581114)* All acids are shown in their un-ionized form.
At pH 7, all free fatty acids have an ionizedcarboxylate. Note that numbering of carbon atoms begins at the carboxyl group carbon.fThe prefix n- indicates the "normal" unbranched structure. For instance, "dodecanoic" simplyindicates 12 carbon atoms, which could be arranged in a variety of branched forms. Thus"rc-dodecanoic" specifies the linear, unbranched form.being the carboxyl carbon), and another between C-12 and C-13, is designated 20:2(A9'12), for example.
The most commonly occurring fattyacids have even numbers of carbon atoms in an unbranched chain of 12to 24 carbons (Table 9-1). As we shall see in Chapter 20, the evennumber of carbons results from the mode of synthesis of these compounds, which involves condensation of acetate (two-carbon) units.The position of double bonds is also regular; in most monounsaturated fatty acids the double bond is between C-9 and C-10 (A9), and theother double bonds of polyunsaturated fatty acids are generally A12and A15 (Table 9-1). The double bonds of polyunsaturated fatty acidsare almost never conjugated (alternating single and double bonds, asin —CH=CH—CH=CH—), but are separated by a methylene group(—CH=CH—CH2—CH=CH—).
The double bonds of almost all naturally occurring unsaturated fatty acids are in the cis configuration.-5Part II Structure and Catalysis242CarboxylgroupVVCH 2CH 2CH 2HydrocarbonchainCH 2 HVICH2 Hi^n 2 —\^ri 2CHCH22—CH2XCH3(a)(b)ff#MK)Saturatedfatty acidsMixture of saturated andunsaturated fatty acids(c)Figure 9-1 The packing of fatty acids depends ontheir degree of saturation, (a) Steanc acid i stearateat pH 7) is shown in its usual extended conformation, (b) The cis double bond (shaded) in oleic acid(oleate) does not permit rotation and introduces arigid bend in the hydrocarbon tail.
All the otherbonds are free to rotate, (c) Fully saturated fattyacids in the extended form pack into nearly crystalline arrays, stabilized by many hydrophobic interactions. The presence of one or more cis double bondsinterferes with this tight packing, and results inless stable aggregates.The physical properties of the fatty acids, and of compounds thatcontain them, are largely determined by the length and degree of unsaturation of the hydrocarbon chain.
The nonpolar hydrocarbon chainaccounts for the poor solubility of fatty acids in water. Laurie acid(12:0, MY 200), for example, has a solubility of 0.063 mg/g of w a t e r much less than that of glucose (Mr 180), which is 1,100 mg/g of water.The longer the fatty acyl chain and the fewer the double bonds, thelower the solubility in water (Table 9-1). The carboxylic acid group ispolar (and ionized at neutral pH), and accounts for the slight solubilityof short-chain fatty acids in water.The melting points of fatty acids and of compounds that containthem are also strongly influenced by the length and degree of unsaturation of the hydrocarbon chain (Table 9-1).
At room temperature(25 °C), the saturated fatty acids from 12:0 to 24:0 have a waxy consistency, whereas unsaturated fatty acids of these lengths are oily liquids. In the fully saturated compounds, free rotation around each ofthe carbon-carbon bonds gives the hydrocarbon chain great flexibility;the most stable conformation is this fully extended form (Fig. 9—la), inwhich the steric hindrance of neighboring atoms is minimized. Thesemolecules can pack together tightly in nearly crystalline arrays, withatoms all along their lengths in van der Waals contact with the atomsof neighboring molecules (Fig.
9-lc). A cis double bond forces a kink inthe hydrocarbon chain (Fig. 9-lb). Fatty acids with one or several suchkinks cannot pack together as tightly as fully saturated fatty acids(Fig. 9-lc), and their interactions with each other are thereforeweaker. Because it takes less thermal energy to disorder these poorlyordered arrays of unsaturated fatty acids, they have lower meltingpoints than saturated fatty acids of the same chain length (Table 9-1).In vertebrate animals, free fatty acids (having a free carboxylategroup) circulate in the blood bound to a protein carrier, serum albumin. However, fatty acids are present mostly as carboxylic acid deriva-0HOCH243Chapter 9 Lipids2C-0OHFigure 9—2 Glycerol and triacylglycerols.
The triacylglycerol shown here has identical fatty acids(palmitate, 18:0) in positions 1 and 3. When thereare two different fatty acids in positions 1 and 3 ofthe glycerol, C-2 (in red) of glycerol (shaded) becomes a chiral center (see Fig. 3—9). Biologicaltriacylglycerols have the L configuration.Triacylglycerol (general structure)tives such as esters or amides. Lacking the charged carboxylate group,these fatty acid derivatives are generally even less soluble in waterthan are the free carboxylic acids.8 /im(a)Triacylglycerols Are Fatty Acid Esters of GlycerolThe simplest lipids constructed from fatty acids are the triacylglycerols, also referred to as triglycerides, fats, or neutral fats. Triacylglycerols are composed of three fatty acids each in ester linkage with a singleglycerol (Fig.
9-2). Those containing the same kind of fatty acid in allthree positions are called simple triacylglycerols, and are named afterthe fatty acid they contain. Simple triacylglycerols of 16:0, 18:0, and18:1, for example, are tristearin, tripalmitin, and triolein, respectively.Mixed triacylglycerols contain two or more different fatty acids; toname these compounds unambiguously, the name and position of eachfatty acid must be specified.Because the polar hydroxyls of glycerol and the polar carboxylatesof the fatty acids are bound in ester linkages, triacylglycerols are nonpolar, hydrophobic molecules, essentially insoluble in water.
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