Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 19
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Thelinkages are named accordingly:thus lactose contains a (1 n 4)bond, and sucrose contains an(1 n 2) bond.HHOHGalactoseHCH2OHOHOH HH1 OHHOHOHGlucoseHOHCH2OHO2HH2OHHOCH2OHOHOHLactoseHHOHOHGlucoseHFructoseCH2OHOHOH HHOHH1 CH2OHO2OHSucroseHOCH2OHOHOHHH2.2 • Chemical Building Blocks of CellsCows and termites can break down cellulose because theyharbor cellulose-degrading bacteria in their gut.Many complex polysaccharides contain modified sugarsthat are covalently linked to various small groups, particularly amino, sulfate, and acetyl groups.
Such modificationsare abundant in glycosaminoglycans, major polysaccharidecomponents of the extracellular matrix that we describe inChapter 6.“essential” polyunsaturated fatty acids, linoleic acid (C18:2)and linolenic acid (C18:3), cannot be synthesized by mammalsand must be supplied in their diet. Mammals can synthesizeother common fatty acids. Two stereoisomeric configurations,cis and trans, are possible around each carbon-carbon doublebond:H2CCH2CTABLE 2-3HH2CCHFatty Acids Are Precursorsfor Many Cellular LipidsCCCH2HHCisBefore considering phospholipids and their role in the structure of biomembranes, we briefly review the properties offatty acids. Like glucose, fatty acids are an important energysource for many cells and are stored in the form of triacylglycerols within adipose tissue (Chapter 8).
Fatty acids alsoare precursors for phospholipids and many other lipids witha variety of functions (Chapter 18).Fatty acids consist of a hydrocarbon chain attached to acarboxyl group (OCOOH). They differ in length, althoughthe predominant fatty acids in cells have an even number ofcarbon atoms, usually 14, 16, 18, or 20. The major fattyacids in phospholipids are listed in Table 2-3. Fatty acidsoften are designated by the abbreviation Cx:y, where x is thenumber of carbons in the chain and y is the number of double bonds. Fatty acids containing 12 or more carbon atomsare nearly insoluble in aqueous solutions because of theirlong hydrophobic hydrocarbon chains.Fatty acids with no carbon-carbon double bonds are saidto be saturated; those with at least one double bond are unsaturated. Unsaturated fatty acids with more than one carboncarbon double bond are referred to as polyunsaturated.
TwoTransA cis double bond introduces a rigid kink in the otherwiseflexible straight chain of a fatty acid (Figure 2-18). In general, the fatty acids in biological systems contain only cisdouble bonds.Fatty acids can be covalently attached to another moleculeby a type of dehydration reaction called esterification, in whichthe OH from the carboxyl group of the fatty acid and a H froma hydroxyl group on the other molecule are lost. In the combined molecule formed by this reaction, the portion derivedfrom the fatty acid is called an acyl group, or fatty acyl group.This is illustrated by triacylglycerols, which contain three acylgroups esterfied to glycerol:OH3C(CH2)n COCH2OCHOCH2OH3C(CH2)n COH3C(CH2)n CTriacylglycerolFatty Acids That Predominate in PhospholipidsCommon Name ofAcid (Ionized Formin Parentheses)Abbreviation43Chemical FormulaSATURATED FATTY ACIDSMyristic (myristate)C14:0CH3(CH2)12COOHPalmitic (palmitate)C16:0CH3(CH2)14COOHStearic (stearate)C18:0CH3(CH2)16COOHOleic (oleate)C18:1CH3(CH2)7CHUCH(CH2)7COOHLinoleic (linoleate)C18:2CH3(CH2)4CHUCHCH2CHUCH(CH2)7COOHArachidonic (arachidonate)C20:4CH3(CH2)4(CHUCHCH2)3CHUCH(CH2)3COOHUNSATURATED FATTY ACIDS44H3CCHAPTER 2 • Chemical FoundationsHHHHHHHHHHHHHHCCCCCCCCCCCCCCHHHHHHHHHHHHHHOCOPalmitate(ionized form of palmitic acid)H3CHHHHHHHHHHHHHHHHCCCCCCCCCCCCCCCCHHHHHHHHHHHHHHOCOOleate(ionized form of oleic acid)▲ FIGURE 2-18 The effect of a double bond on the shapeof fatty acids.
Shown are space-filling models and chemicalstructures of the ionized form of palmitic acid, a saturated fattyacid with 16 C atoms, and oleic acid, an unsaturated one withIf the acyl groups are long enough, these molecules areinsoluble in water even though they contain three polar esterbonds. Fatty acyl groups also form the hydrophobic portionof phospholipids, which we discuss next.Phospholipids Associate Noncovalently to Formthe Basic Bilayer Structure of BiomembranesBiomembranes are large flexible sheets that serve as theboundaries of cells and their intracellular organelles andform the outer surfaces of some viruses.
Membranes literally define what is a cell (the outer membrane and the contents within the membrane) and what is not (the extracellularspace outside the membrane). Unlike the proteins, nucleicacids, and polysaccharides, membranes are assembled by thenoncovalent association of their component building blocks.18 C atoms. In saturated fatty acids, the hydrocarbon chain isoften linear; the cis double bond in oleate creates a rigid kink inthe hydrocarbon chain.
[After L. Stryer, 1994, Biochemistry, 4th ed.,W. H. Freeman and Company, p. 265.]The primary building blocks of all biomembranes are phospholipids, whose physical properties are responsible for theformation of the sheetlike structure of membranes.Phospholipids consist of two long-chain, nonpolar fattyacyl groups linked (usually by an ester bond) to small, highlypolar groups, including a phosphate. In phosphoglycerides,the major class of phospholipids, fatty acyl side chains are esterified to two of the three hydroxyl groups in glycerol. Thethird hydroxyl group is esterified to phosphate.
The simplestphospholipid, phosphatidic acid, contains only these components. In most phospholipids found in membranes, the phosphate group is esterified to a hydroxyl group on anotherhydrophilic compound. In phosphatidylcholine, for example,choline is attached to the phosphate (Figure 2-19). The negative charge on the phosphate as well as the charged or polargroups esterified to it can interact strongly with water.
TheFatty acid chainsOHydrophobic tailHydrophilic headCCH2O OCCHOH2CGlycerolPHOSPHATIDYLCHOLINE▲ FIGURE 2-19 Phosphatidylcholine, a typical phosphoglyceride. All phosphoglycerides are amphipathic, having ahydrophobic tail (yellow) and a hydrophilic head (blue) in whichglycerol is linked via a phosphate group to an alcohol. Either ofPhosphateOPOCH3H2COO−N+CH2CH3CH3Cholineor both the fatty acyl side chains in a phosphoglyceride may besaturated or unsaturated. In phosphatidic acid (red), the simplestphospholipid, the phosphate is not linked to an alcohol.2.2 • Chemical Building Blocks of Cellsphosphate and its associated esterified group, the “head”group of a phospholipid, is hydrophilic, whereas the fattyacyl chains, the “tails,” are hydrophobic.The amphipathic nature of phospholipids, which governstheir interactions, is critical to the structure of biomembranes.
When a suspension of phospholipids is mechanicallydispersed in aqueous solution, the phospholipids aggregateinto one of three forms: spherical micelles and liposomes andsheetlike, two-molecule-thick phospholipid bilayers (Figure2-20). The type of structure formed by a pure phospholipidor a mixture of phospholipids depends on several factors, including the length of the fatty acyl chains, their degree ofsaturation, and temperature. In all three structures, the hydrophobic effect causes the fatty acyl chains to aggregate andexclude water molecules from the “core.” Micelles are rarelyformed from natural phosphoglycerides, whose fatty acylchains generally are too bulky to fit into the interior of amicelle.
If one of the two fatty acyl chains is removed byhydrolysis, forming a lysophospholipid, the predominanttype of aggregate that forms is the micelle. Common detergents and soaps form micelles in aqueous solution that behave as tiny ball bearings, thus giving soap solutions theirslippery feel and lubricating properties.Under suitable conditions, phospholipids of the composition present in cells spontaneously form symmetric phospholipid bilayers. Each phospholipid layer in this lamellar45structure is called a leaflet.
The fatty acyl chains in eachleaflet minimize contact with water by aligning themselvestightly together in the center of the bilayer, forming ahydrophobic core that is about 3 nm thick (see Figure 2-20).The close packing of these nonpolar tails is stabilized by thehydrophobic effect and van der Waals interactions betweenthem. Ionic and hydrogen bonds stabilize the interaction ofthe phospholipid polar head groups with one another andwith water.A phospholipid bilayer can be of almost unlimited size—from micrometers (m) to millimeters (mm) in length orwidth—and can contain tens of millions of phospholipidmolecules. Because of their hydrophobic core, bilayers arevirtually impermeable to salts, sugars, and most other smallhydrophilic molecules.
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