3 Биологические мембраны. Обмен веществом (1160072), страница 10
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Some membrane proteins have more or less complex arrays ofcovalently bound carbohydrates, which may make up from 1 to 70% ofthe total mass of these glycoproteins. In the rhodopsin of the vertebrate eye, a single hexasaccharide makes up 4% of the mass; inglycophorin, a glycoprotein of the plasma membrane of erythrocytes,60% of the mass consists of complex polysaccharide units covalentlyattached to specific amino acid residues. Ser, Thr, and Asn residuesare often the points of attachment (see Fig. 11-23). In general, plasmaLipid moietyPoint of attachmentOOFigure 10—3 Covalently attached lipids of severaltypes anchor membrane proteins to the lipid bilayer.
The farnesyl side chain is an isoprenoid(p. 256).NH— CH*—C— NHAmino-terminal GlvMyristoylPalmitoylPalmitoylS-CH,-CH-NI HMethyl ester ofcarboxyl-terminal CysOOC-0—CH 2glycan -O-P-O-CH 2 -CH 2 -N-C1""""C H - O - P - O - inositolOC—O-CHIIIIII2IA-Carboxyl-terminalamino acidO"Phosphatidylinositol-glycanmembranes contain many glycoproteins, but intracellular membranessuch as those of mitochondria and chloroplasts rarely contain covalently bound carbohydrates.
The sugar moieties of surface glycoproteins influence the protein folding, transport to the cell surface, andreceptor functions of these glycoproteins.Certain membrane proteins are covalently attached to one or morelipids, which probably serve as hydrophobic anchors, holding the proteins to the membrane. The lipid moiety on some membrane proteins isa fatty acid, attached in amide or ester linkage; other proteins have along-chain isoprenoid covalently attached, and others are joinedthrough a complex polysaccharide (a glycan; see Chapter 11) to a molecule of phosphatidylinositol (Fig. 10-3).The Supramolecular Architecture of MembranesAll biological membranes share certain fundamental properties.
Theyare impermeable to most polar or charged solutes, but permeable tononpolar compounds; are 5 to 8 nm thick; appear trilaminar (threelayered) when viewed in cross section with the electron microscope (seeFig. 10—1). The combined evidence from electron microscopy, chemicalcomposition, and physical studies of permeability and of the motion of271272Phospholipidheads (polar)Outer faceFatty acyltails (nonpolar)Carbohydrate moietyof glycoproteinCholesterolLipid —bilayerPeripheral proteinInner faceFigure 10-4 The fluid mosaic model for membranestructure. The fatty acyl chains in the interior ofthe membrane form a fluid, hydrophobic region.Integral membrane proteins float in this sea oflipid, held by hydrophobic interactions with theirnonpolar amino acid side chains. Both proteins andlipids are free to move laterally in the plane of thebilayer, but movement of either from one face ofthe bilayer to the other is restricted.
The carbohydrate moieties attached to some proteins and lipidsof the plasma membrane are invariably exposed onthe extracellular face of the membrane.Outer face of bilayerTotal phospholipid50 f40 30 j -Phosphatidylcholine10or10 20 -40 50Peripheral proteinwith covalent lipid anchorindividual protein and lipid molecules within membranes supports thefluid mosaic model for the structure of biological membranes (Fig.10-4).
Amphipathic phospholipids and sterols form a lipid bilayer,with the nonpolar regions of lipids facing each other at the core of thebilayer and their polar head groups facing outward. In this lipid bilayer, globular proteins are embedded at irregular intervals, held byhydrophobic interactions between the membrane lipids and hydrophobic domains in the proteins. Some proteins protrude from one or theother face of the membrane; most span its entire width. The orientation of proteins in the bilayer is asymmetric, giving the membrane"sidedness"; the protein domains exposed on one side of the bilayer aredifferent from those exposed on the other side, reflecting functionalasymmetry.
The individual lipid and protein subunits in a membraneform a fluid mosaic; its pattern, unlike a mosaic of ceramic tile andmortar, is free to change constantly. The membrane mosaic is fluidbecause the interactions among lipids, and between lipids and proteins, are noncovalent, leaving individual lipid and protein moleculesfree to move laterally in the plane of the membrane.We will now look at some of these features of the fluid mosaicmodel in more detail, and consider the experimental evidence that supports it.Sphingomyelin2030 rIntegral proteinsPhosphatidylethanolamineIPhosphatidylserinePhosphatidylinositolInner face of bilayerFigure 10—5 The distribution of specific erythrocyte membrane lipids between the inner and outerface is asymmetric.A Lipid Bilayer Is the Basic Structural ElementWe saw in Chapter 9 that lipids, when suspended in water, spontaneously form bilayer structures that are stabilized by hydrophobic interactions (see Fig.
9-14). The thickness of biological membranes (5 to8 nm, measured by electron microscopy) is about that expected for alipid bilayer 3 nm thick with proteins protruding on each side. X-raydiffraction by membranes shows the distribution of electron densityexpected for a bilayer structure.
Liposomes (lipid vesicles) formed inthe laboratory show the same relative impermeability to polar solutesas is seen in biological membranes (although the latter are permeableto solutes for which they have specific transporters). In short, all evidence indicates that biological membranes are constructed of lipidbilayers.Membrane lipids are asymmetric in their distribution on the twofaces of the bilayer, although the asymmetry, unlike that of membraneproteins, is not absolute. In the plasma membrane, for example, certain lipids are typically found primarily in the outer face of the bilayer,and others in the inner (cytoplasmic) face (Fig. 10-5).Chapter 10 Biological Membranes and TransportMembrane Lipids Are in Constant Motion273Paracrystalline state (solid)Although the lipid bilayer structure itself is stable, the individualphospholipid and sterol molecules have great freedom of motion withinthe plane of the membrane (Fig.
10-6). They diffuse laterally so fastthat an individual lipid molecule can circumnavigate an erythrocyte ina few seconds. The interior of the bilayer is alsofluid;individual hydrocarbon chains of fatty acids are in constant motion produced by rotation about the carbon-carbon bonds of the long acyl side chains.The degree of fluidity depends on lipid composition and temperature. At low temperature, relatively little lipid motion occurs and thebilayer exists as a nearly crystalline (paracrystalline) array. Above atemperature that is characteristic for each membrane, lipids can undergo rapid motion. The temperature of the transition from paracrystalline solid to fluid depends upon the lipid composition of themembrane.
Saturated fatty acids pack well into a paracrystallinearray, but the kinks in unsaturated fatty acids (see Fig. 9-1) interferewith this packing, preventing the formation of a paracrystalline solidstate. The higher the proportion of saturated fatty acids, the higher isthe solid-to-fluid transition temperature of the membrane.The sterol content of a membrane also is an important determinant of this transition temperature. The rigid planar structure of thesteroid nucleus, inserted between fatty acyl side chains, has two effectsonfluidity:below the temperature of the solid-to-fluid transition, sterol insertion prevents the highly ordered packing of fatty acyl chains,and thus fluidizes the membrane. Above the thermal transition point,the rigid ring system of the sterol reduces the freedom of neighboringfatty acyl chains to move by rotation about carbon-carbon bonds, andthus reduces the fluidity in the core of the bilayer.
Sterols thereforetend to moderate the extremes of solidity and fluidity of the membranes that contain them.Both microorganisms and cultured animal cells regulate their lipidcomposition so as to achieve a constant fluidity under various growthconditions. For example, when cultured at low temperatures, bacteriasynthesize more unsaturated fatty acids and fewer saturated onesthan when cultured at higher temperatures (Table 10-3). As a result ofthis adjustment in lipid composition, membranes of bacteria culturedat high or low temperature have about the same degree of fluidity.Thermal motion ofacyl side chains(solid>fluid transition)FluidstateTransbilayerdiffusion("flip-flop")Table 10—3 Fatty acid composition of E. coli cellscultured at different temperaturesPercentage of total fatty acids*Fatty acid10 °C20 °C30 °C40 °CMyristic (14:0)Palmitic (16:0)Palmitoleic (16:1)Oleic(18:l)HydroxymyristicRatio of unsaturated:saturated!41826381342524341042923301084891282.92.01.60.38Source: Data from Marr, A.G.
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