Lodish H. - Molecular Cell Biology (5ed, Freeman, 2003) (794361), страница 64
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Despite theirdiverse shapes and functions, these biomembranes and allother biomembranes have a common bilayer structure.Because all cellular membranes enclose an entire cell oran internal compartment, they have an internal face (the surface oriented toward the interior of the compartment) and anexternal face (the surface presented to the environment).More commonly, the surfaces of a cellular membrane aredesignated as the cytosolic face and the exoplasmic face.
Thisnomenclature is useful in highlighting the topological equivalence of the faces in different membranes, as diagrammed inFigure 5-4. For example, the exoplasmic face of the plasmamembrane is directed away from the cytosol, toward the extracellular space or external environment, and defines theouter limit of the cell. For organelles and vesicles surroundedby a single membrane, however, the face directed away fromthe cytosol—the exoplasmic face—is on the inside in contact with an internal aqueous space equivalent to the extracellular space. This equivalence is most easily understood forvesicles that arise by invagination of the plasma membrane;this process results in the external face of the plasma membrane becoming the internal face of the vesicle membrane.Three organelles—the nucleus, mitochondrion, and chloroplast—are surrounded by two membranes; the exoplasmicsurface of each membrane faces the space between the twomembranes.Three Classes of Lipids Are Foundin BiomembranesA typical biomembrane is assembled from phosphoglycerides, sphingolipids, and steroids.
All three classes of lipidsare amphipathic molecules having a polar (hydrophilic) headgroup and hydrophobic tail. The hydrophobic effect and vander Waals interactions, discussed in Chapter 2, cause the tailgroups to self-associate into a bilayer with the polar headgroups oriented toward water (see Figure 5-2). Although thecommon membrane lipids have this amphipathic character incommon, they differ in their chemical structures, abundance,and functions in the membrane.Phosphoglycerides, the most abundant class of lipids inmost membranes, are derivatives of glycerol 3-phosphate(Figure 5-5a).
A typical phosphoglyceride molecule consistsof a hydrophobic tail composed of two fatty acyl chains esterified to the two hydroxyl groups in glycerol phosphateand a polar head group attached to the phosphate group.The two fatty acyl chains may differ in the number of carbons that they contain (commonly 16 or 18) and their degreeof saturation (0, 1, or 2 double bonds). A phosphogyceride is5.1 • Biomembranes: Lipid Composition and Structural Organizationin plasmalogens or the subtle differences in their threedimensional structure compared with that of other phosphoglycerides may have as-yet unrecognized physiologicsignificance.A second class of membrane lipid is the sphingolipids.All of these compounds are derived from sphingosine, anamino alcohol with a long hydrocarbon chain, and contain along-chain fatty acid attached to the sphingosine aminogroup.
In sphingomyelin, the most abundant sphingolipid,phosphocholine is attached to the terminal hydroxyl groupof sphingosine (Figure 5-5b). Thus sphingomyelin is a phospholipid, and its overall structure is quite similar to that ofphosphatidylcholine. Other sphingolipids are amphipathicglycolipids whose polar head groups are sugars. Glucosylcerebroside, the simplest glycosphingolipid, contains a singleglucose unit attached to sphingosine.
In the complex glycosphingolipids called gangliosides, one or two branchedsugar chains containing sialic acid groups are attached toclassified according to the nature of its head group. In phosphatidylcholines, the most abundant phospholipids in theplasma membrane, the head group consists of choline, a positively charged alcohol, esterified to the negatively chargedphosphate. In other phosphoglycerides, an OH-containingmolecule such as ethanolamine, serine, and the sugar derivative inositol is linked to the phosphate group. The negatively charged phosphate group and the positively chargedgroups or the hydroxyl groups on the head group interactstrongly with water.The plasmalogens are a group of phosphoglycerides thatcontain one fatty acyl chain, attached to glycerol by an esterlinkage, and one long hydrocarbon chain, attached to glycerol by an ether linkage (COOOC).
These molecules constitute about 20 percent of the total phosphoglyceridecontent in humans. Their abundance varies among tissuesand species but is especially high in human brain and hearttissue. The additional chemical stability of the ether linkageH132OOOHN+OHydrophobic tailPEHOCH3PN+O−OHOH6OH4521NHPSO−OHHOOO3OCH3PN+O−PCHN+OOHCH3CH3O(b) Sphingolipids FIGURE 5-5 Three classes ofHead group(a) PhosphoglyceridesOOOHOHPICH3CH3SMOOHOOOHHOOH(c) CholesterolOH151GlcCermembrane lipids. (a) Mostphosphoglycerides are derivatives ofglycerol 3-phosphate (red) containing twoesterified fatty acyl chains, constitutingthe hydrophobic “tail” and a polar “headgroup” esterified to the phosphate.
Thefatty acids can vary in length and besaturated (no double bonds) or unsaturated(one, two, or three double bonds). Inphosphatidylcholine (PC), the head groupis choline. Also shown are the moleculesattached to the phosphate group in threeother common phosphoglycerides:phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol(PI).
(b) Sphingolipids are derivatives ofsphingosine (red), an amino alcohol witha long hydrocarbon chain. Various fattyacyl chains are connected to sphingosineby an amide bond. The sphingomyelins(SM), which contain a phosphocholinehead group, are phospholipids. Othersphingolipids are glycolipids in whicha single sugar residue or branchedoligosaccharide is attached to thesphingosine backbone. For instance, thesimple glycolipid glucosylcerebroside(GlcCer) has a glucose head group.(c) Like other membrane lipids, the steroidcholesterol is amphipathic. Its singlehydroxyl group is equivalent to the polarhead group in other lipids; the conjugatedring and short hydrocarbon chain form thehydrophobic tail. [See H.
Sprong et al., 2001,Nature Rev. Mol. Cell Biol. 2:504.]152CHAPTER 5 • Biomembranes and Cell Architecturesphingosine. Glycolipids constitute 2–10 percent of the totallipid in plasma membranes; they are most abundant in nervous tissue.Cholesterol and its derivatives constitute the third important class of membrane lipids, the steroids. The basicstructure of steroids is a four-ring hydrocarbon. Cholesterol,the major steroidal constituent of animal tissues, has a hydroxyl substituent on one ring (Figure 5-5c).
Although cholesterol is almost entirely hydrocarbon in composition, it isamphipathic because its hydroxyl group can interact withwater. Cholesterol is especially abundant in the plasma membranes of mammalian cells but is absent from most prokaryotic cells. As much as 30–50 percent of the lipids in plantplasma membranes consist of certain steroids unique toplants.At neutral pH, some phosphoglycerides (e.g., phosphatidylcholine and phosphatidylethanolamine) carry no netelectric charge, whereas others (e.g., phosphatidylinositoland phosphatidylserine) carry a single net negative charge.Nonetheless, the polar head groups in all phospholipids canpack together into the characteristic bilayer structure.
Sphingomyelins are similar in shape to phosphoglycerides and canform mixed bilayers with them. Cholesterol and othersteroids are too hydrophobic to form a bilayer structure unless they are mixed with phospholipids.Most Lipids and Many Proteins Are LaterallyMobile in BiomembranesIn the two-dimensional plane of a bilayer, thermal motion permits lipid molecules to rotate freely around their long axes andto diffuse laterally within each leaflet.
Because such movements are lateral or rotational, the fatty acyl chains remain inthe hydrophobic interior of the bilayer. In both natural and ar-(a)Fluorescent reagentMembrane proteinCellBleached areaLabelBleach withlaserFluorescencerecovery123Fluorescence intensity (arb. units)(b)Fluorescence before bleaching300050%immobile200050%mobile1000Bleach50100Time (s)150▲ EXPERIMENTAL FIGURE 5-6 Fluorescence recoveryafter photobleaching (FRAP) experiments can quantify thelateral movement of proteins and lipids within the plasmamembrane.
(a) Experimental protocol. Step 1 : Cells are firstlabeled with a fluorescent reagent that binds uniformly to aspecific membrane lipid or protein. Step 2 : A laser light is thenfocused on a small area of the surface, irreversibly bleaching thebound reagent and thus reducing the fluorescence in theilluminated area. Step 3 : In time, the fluorescence of thebleached patch increases as unbleached fluorescent surfacemolecules diffuse into it and bleached ones diffuse outward. Theextent of recovery of fluorescence in the bleached patch isproportional to the fraction of labeled molecules that are mobilein the membrane.
(b) Results of FRAP experiment with humanhepatoma cells treated with a fluorescent antibody specific forthe asialoglycoprotein receptor protein. The finding that 50percent of the fluorescence returned to the bleached areaindicates that 50 percent of the receptor molecules in theilluminated membrane patch were mobile and 50 percent wereimmobile. Because the rate of fluorescence recovery isproportional to the rate at which labeled molecules move into thebleached region, the diffusion coefficient of a protein or lipid inthe membrane can be calculated from such data.
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