3 Биологические мембраны. Обмен веществом (1160072), страница 17
Текст из файла (страница 17)
Membranes are composed of lipids and proteins in varying combinations that arespecific to each species, cell type, and organelle.The fluid mosaic model describes certain featurescommon to all biological membranes. The lipid bilayer is the basic structural unit. Fatty acyl chainsof phospholipids and the steroid nucleus of sterolsare oriented toward the interior of the bilayer;their hydrophobic interactions stabilize the bilayerbut allow the structure to be flexible.
Lipids andmost proteins are free to diffuse laterally withinthe membrane, and the hydrophobic moieties ofthe lipids undergo rapid thermal motion, makingthe interior of the bilayer fluid. Fluidity is affectedby temperature, fatty acid composition, and sterolcontent. Cells strive to maintain a constant fluiditywhen external circumstances change.Peripheral membrane proteins are loosely associated with the membrane through electrostaticinteractions and hydrogen bonds or by covalentlyattached lipid anchors. Integral membrane proteins associate with the lipid bilayer by hydrophobic interactions with their nonpolar amino acidside chains, which are oriented toward the outsideof the protein molecule. Some membrane proteinsspan the lipid bilayer several times, with hydrophobic sequences of about 20 amino acids, eachcapable of forming a transmembrane a helix.
Suchhydrophobic sequences can be detected and used topredict the structure and transmembrane disposition of these proteins. The lipids and proteins ofthe membrane are inserted into the bilayer withspecific sidedness; the membrane is structurallyand functionally asymmetric. Many membraneproteins contain covalently attached polysaccharides of various degrees of complexity. Plasmamembrane glycoproteins are always oriented withthe carbohydrate-bearing domain on the extracellular surface. Annexins and fusion proteins mediate the fusion of two membranes, which accompanies processes such as endocytosis and exocytosis.The lipid bilayer is impermeable to polar substances. Water is an important exception; it is ableto diffuse passively across the bilayer.
Other polarspecies cross biological membranes only by way ofspecific membrane proteins. Ion channels providehydrophilic pores through which select ions candiffuse, moving down their electrical or chemicalconcentration gradients.The movement of many ions and compoundsacross cellular membranes is catalyzed by specifictransport proteins (transporters), which, like enzymes, show saturation and substrate specificity.Transport via these systems may be passive (downthe electrochemical gradient, hence independent ofmetabolic energy) such as glucose transport intoerythrocytes, or active (against the gradient, anddependent on metabolic energy). The energy inputfor active transport may come from light, oxidationreactions, ATP hydrolysis, or cotransport of someother solute. Some transporters carry out symport,the simultaneous passage of two species in thesame direction; others mediate antiport, in whichtwo species move in opposite directions, but simultaneously.
An example of antiport is the chloridebicarbonate exchanger of erythrocytes. In animalcells, the differences in cytosolic and extracellularconcentrations of Na + and K+ are established andmaintained by active transport via the Na + K +ATPase, and the resulting Na + gradient is used asan energy source by a variety of symport and antiport systems.There are three general types of ion-pumpingATPases. P-type ATPases undergo reversible phosphorylation during their catalytic cycle, and areinhibited by the phosphate analog vanadate.V-type ATPases produce gradients of protonsacross the membranes of a variety of intracellularorganelles, including plant vacuoles.
F-type protonpumps (ATP synthases) are central to energyconserving mechanisms in mitochondria and chloroplasts. Ion-selective channels such as the acetylcholine receptor act in the passage of electrical signals in neurons, muscle cells, and other cellssensitive to a variety of stimuli.Further ReadingGeneralFinean, J.B., Coleman, R., & Mitchell, R.H.(1984) Membranes and Their Cellular Functions,3rd edn, Blackwell Scientific Publications, Oxford.An introductory text on membrane composition,structure, and function.Chapter 10 Biological Membranes and TransportHarold, F.M. (1986) The Vital Force: A Study ofBioenergetics, W.H.
Freeman and Company, NewYork.Chapters 9 and 10 of this excellent book concern theenergetics and mechanisms of transport, andChapter 4 is a discussion of the energy of ion gradients.Jain, M.K. (1988) Introduction to Biological Membranes, 2nd edn, John Wiley & Sons, Inc., NewYork.A textbook of membranology, longer and more advanced than Finean et al.Martonosi, A.N. (ed) (1985) The Enzymes of Biological Membranes, 2nd edn, Plenum Press, NewYork.This four-volume set has 61 individual reviews covering many of the topics in this chapter, includingthe electron microscopy of membranes, proteinlipid interactions in membranes, the energetics ofactive transport, and the Na+K+ ATPase.Stein, W.D. (1986) Transport and Diffusion AcrossCell Membranes, Academic Press, Inc., New York.This excellent textbook on biological transport covers all the transport systems described in this chapter, at a more advanced level.Tanford, C.
(1980) The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2ndedn, John Wiley & Sons, Inc., New York.A description of the forces that stabilize micelles,bilayers, and membranes, in rigorous physicalchemical terms.Molecular Constituents of MembranesBretscher, M.S. (1985) The molecules of the cellmembrane.
Sci. Am. 253 (October), 100-109.Supramolecular Architecture of MembranesFasman, G.D. & Gilbert, W.A. (1990) The prediction of transmembrane protein sequences andtheir conformation: an evaluation. Trends Biochem. Sci. 15, 89-92.Short but clear survey of several methods of predicting transmembrane helices from sequence (hydropathy plots).Mcllhinney, R.A.J. (1990) The fats of life: the importance and function of protein acylation. TrendsBiochem. Sci. 15, 387-391.A short but good summary of the kinds of proteinsthat contain covalently bound lipid, and the functional significance of lipid attachment.Rothman, J.E. & Lenard, J.
(1977) Membraneasymmetry. Science 195, 743-753.295Excellent summary of the experimental evidencethat both lipids and proteins of membranes areasymmetrically disposed in the bilayer.Singer, S.J. & Nicolson, G.L. (1972) The fluidmosaic model of the structure of cell membranes.Science 175, 720-731.The classic statement of the model.Unwin, N. & Henderson, R. (1984) The structureof proteins in biological membranes.
Sci. Am. 250(February), 78-94.Describes the technical problems and solutions indetermining the structure of integral membraneproteins, with emphasis on bacteriorhodopsin.Solute Transport across MembranesForgac, M. (1989) Structure and function of vacuolar class of ATP-driven proton pumps. Physiol. Rev.69, 765-796.Hille, B. (1988) Ionic channels: molecular pores ofexcitable membranes. Harvey Led. 82, 47-69.Jennings, M.L. (1989) Structure and function ofthe red blood cell anion transport protein. Annu.Rev.
Biophys. Biophys. Chem. 18, 397-430.Detailed, advanced treatment of this transporter.Kaback, H.R. (1989) Molecular biology of activetransport: from membrane to molecule to mechanism. Harvey Led. 83, 77-105.Description of the galactoside permease of E. coli.Lienhard, F.E., Slot, J.W., James, D.E., & Mueckler, M.M. (1992) How cells absorb glucose. Sci. Am.266 (January), 86-91.Introductory level description of the glucose transporter and its regulation by insulin.Lodish, H.F. (1988) Anion-exchange and glucosetransport proteins: structure, function and distribution.
Harvey Led. 82, 19-46.Numa, S. (1989) A molecular view of neurotransmitter receptors and ionic channels. Harvey Led.83, 121-165.Pedersen, P.L. & Carafoli, E. (1987) Ion motiveATPases. I. Ubiquity, properties, and significanceto cell function. Trends Biochem. Sci. 12, 146-150.II. Energy coupling and work output. Trends Biochem. Sci. 12, 186-189.White, J.M. (1990) Viral and cellular membranefusion proteins. Annu.
Rev. Physiol. 52, 675-697.Part II Structure and Catalysis296Problems40 rForce applied hereto compressmonolayer30(a)201. Determining the Cross-Sectional Area of a LipidMolecule When phospholipids are layered gentlyonto the surface of water, they orient at the airwater interface with their head groups in thewater and their hydrophobic tails in the air. Theexperimental apparatus pictured above (a) pushesthese lipids together by reducing the surface areaavailable to them. By measuring the force necessary to push the lipids together, it is possible todetermine when the molecules are packed tightlytogether in a continuous monolayer; when thatarea is approached, the pressure needed to furtherreduce the surface area increases sharply (b). Howwould you use such an experimental apparatus todetermine the average area occupied by a singlelipid molecule in a lipid monolayer?2.
Evidence for Lipid Bilayer In 1925, E. Gorterand F. Grendel used an apparatus like that described in Problem 1 to determine the surface areaof a lipid monolayer formed by lipids extractedfrom erythrocytes of several animal species. Theyused a microscope to measure the dimensions ofindividual cells, from which they calculated theaverage surface area of one erythrocyte. They obtained the data shown below. Were these investigators justified in concluding that "chromocytes[erythrocytes] are covered by a layer of fatty substances that is two molecules thick" (i.e., a lipidbilayer)?Numberof cells(per mm3)Total surfacearea of lipidmonolayerfrom cells (m2)Total surfacearea of oneerythrocyteAnimalVolume ofpackedcells (mL)DogSheepHuman401018,000,0009,900,0004,740,000622.950.479829.899.4(Aim2)Source: Data from Gorter, E. & Grendel, F.
(1925) On bimolecular layers of lipoids on thechromocytes of the blood. J. Exp. Med. 41, 439-443.100.2 0.4 0.6 0.8 1.0 1.2 1.4Area (nm2/molecule)(b)1.63. Length of a Fatty Acid Molecule The carboncarbon bond distance for single-bonded carbonssuch as those in a saturated fatty acyl chain isabout 0.15 nm. Estimate the length of a single molecule of palmitic acid in its fully extended form. Iftwo molecules of palmitic acid were laid end to end,how would their total length compare with thethickness of the lipid bilayer in a biological membrane?4. Temperature Dependence of Lateral DiffusionThe experiment described in Figure 10-12 wasdone at 37 °C.
If, instead, the whole experimentwere carried out at 10 °C, what effect would youpredict on the rate of cell-cell fusion, and the rateof membrane protein mixing? Why?5. Synthesis of Gastric Juice: Energetics Gastricjuice (pH 1.5) is produced by pumping HC1 fromblood plasma (pH 7.4) into the stomach. Calculatethe amount of free energy required to concentratethe H + in 1 L of gastric juice at 37 °C. Under cellular conditions, how many moles of ATP must behydrolyzed to provide this amount of free energy?(The free-energy change for ATP hydrolysis undercellular conditions is about - 5 8 kJ/mol, as we willexplain in Chapter 13.)6.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
