3 Биологические мембраны. Обмен веществом (1160072), страница 18
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Energetics of the Na+K+ ATPase The concentration of Na + inside a vertebrate cell is about12 HIM, and the cell is bathed in blood plasma containing about 145 mM Na + . For a typical cell with atransmembrane potential of -0.07 V (inside nega-Chapter 10 Biological Membranes and Transporttive relative to outside), what is the free-energychange for transporting 1 mol of Na + out of the cellat 37 °C?7. Action of Ouabain on Kidney Tissue Ouabainspecifically inhibits the Na + K + ATPase activity ofanimal tissues but is not known to inhibit anyother enzyme. When ouabain is added in gradedconcentrations to thin slices of living kidney tissue,it inhibits oxygen consumption by 66%. Explainthe basis of this observation.
What does it tell usabout the use of respiratory energy by kidney tissue?8. Membrane Protein Topology The receptor forthe hormone epinephrine in animal cells is an integral membrane protein (Mr 64,000) that is believedto span the membrane seven times. Show that aprotein of this size is capable of spanning the membrane seven times. If you were given the aminoacid sequence of this protein, how would you goabout predicting which regions of the protein formthe membrane-spanning helices?9. Energetics of Symport Suppose that you determined experimentally that a cellular transportsystem for glucose, driven by symport of Na + ,could accumulated glucose to concentrations 25times greater than in the external medium, whilethe external [Na+] was only ten times greater thanthe intracellular [Na + ].
Is this a violation of thelaws of thermodynamics? If not, how do you explain this observation?10. Location of a Membrane Protein An unknownmembrane protein, X, can be extracted from disrupted erythrocyte membranes into a concentratedsalt solution. Isolated X can be cleaved into fragments by proteolytic enzymes. But treatment oferythrocytes, first with proteolytic enzymes, followed by disruption and extraction of membranecomponents, yields intact X. In contrast, treatmentof erythrocyte "ghosts" (which consist of only membranes, produced by disrupting the cells and wash-297ing out the hemoglobin) with proteolytic enzymes,followed by disruption and extraction, yields extensively fragmented X.
What do these experimentsindicate about the location of X in the plasmamembrane? On the basis of this information, do theproperties of X resemble those of glycophorin orthose of ankyrin?11. Membrane Self-Sealing Cell membranes areself-sealing—if they are punctured or disruptedmechanically, they quickly and automatically reseal. What properties of membranes are responsible for this important feature?12. Lipid Melting Temperatures Membrane lipidsin tissue samples obtained from different parts ofthe leg of a reindeer show different fatty acid compositions.
Membrane lipids from tissue near thehooves contain a larger proportion of unsaturatedfatty acids than lipids from tissue in the upper partof the leg. What is the significance of this observation?13. Flip-Flop Diffusion The inner face of thehuman erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. The outer face consists predominantly of phosphatidylcholine and sphingomyelin.Although the phospholipid components of themembrane can diffuse in the fluid bilayer, this sidedness is preserved all all times.
How?14. Membrane Permeability At pH 7, tryptophancrosses a lipid bilayer membrane about 1,000 timesmore slowly than does the closely related substance indole:Suggest an explanation for this observation.674Part III Bioenergetics and MetabolismKonrad BlochFeodor Lynen1911-1979John CornforthGeorge Popjakanimal cells, this cyclization results in the formation of lanosterol,which contains the four rings characteristic of the steroid nucleus.Lanosterol is finally converted into cholesterol in a series of about 20reactions, including the migration of some methyl groups and the removal of others. Elucidation of this extraordinary biosynthetic pathway, one of the most complex known, was accomplished by Konrad Bloch,Feodor Lynen, John Cornforth, and George Popjak in the late 1950s.Cholesterol is the sterol characteristic of animal cells, but plants,fungi, and protists make other, closely related sterols instead of cholesterol, using the same synthetic pathway as far as squalene-2,3-epoxide.
At this point the synthetic pathways diverge slightly, yieldingother sterols: stigmasterol in many plants and ergosterol in fungi, forexample (Fig. 20-35).Cholesterol Has Several FatesCholesterolacyl-CoA-cholestei'olacyl transferasei AC AT.Fatty acyl-CoACoA-SHMost of the cholesterol synthesis in vertebrates takes place in the liver.A small fraction of the cholesterol made there is incorporated into themembranes of hepatocytes, but most of it is exported in one of threeforms: biliary cholesterol, bile acids, or cholesteryl esters.
Bile acidsand their salts are relatively hydrophilic cholesterol derivatives thatare synthesized in the liver and aid in lipid digestion (p. 480). Cholesteryl esters are formed in the liver through the action of acyl-CoAcholesterol acyl transferase (ACAT). This enzyme catalyzes thetransfer of a fatty acid from coenzyme A to the hydroxyl group of cholesterol (Fig.
20-36), converting the cholesterol into a more hydrophobic form. Cholesteryl esters are transported in secreted lipoproteinparticles to other tissues that use cholesterol, or are stored in the liver.All growing animal tissues need cholesterol for membrane synthesis, and some organs (adrenal gland and gonads, for example) use cholesterol as a precursor for steroid hormone production (discussedlater).
Cholesterol is also a precursor of vitamin D (see Fig. 9-19).Cholesterol and Other Lipids AreCarried on Plasma LipoproteinsCholesteryl esterFigure 20—36 Synthesis of cholesteryl esters converts cholesterol into an even more hydrophobicform for storage and transport.Cholesterol and cholesteryl esters, like triacylglycerols and phospholipids, are essentially insoluble in water.
These lipids must, however,be moved from the tissue of origin (liver, where they are synthesized,or intestine, where they are absorbed) to the tissues in which they willbe stored or consumed. They are carried in the blood plasma from oneChapter 20 Lipid BiosynthesisPhospholipidmonolayer,ApoB-100Chylomicrons (x 60,000)Free (unesterified)cholesterolTriacylglycerolsCholesteryl esters(a)Figure 20-37 (a) Structure of a low-density lipoprotein (LDL).
Apolipoprotein B-100 (apoB-100) isone of the largest single polypeptide chains known,with 4,636 amino acid residues (Mr 513,000).(b) Four classes of lipoproteins visualized in theelectron microscope after negative staining. Fromtop to bottom: chylomicrons (50-200 nm in diameter); VLDL (28-70 nm); LDL (20-25 nm); and HDL(8-11 nm). For properties of lipoproteins, see Table20-2.tissue to another as plasma lipoproteins, macromolecular complexesof specific carrier proteins called apolipoproteins with various combinations of phospholipids, cholesterol, cholesteryl esters, and triacylglycerols.Apolipoproteins ("apo" designates the protein in its lipid-free form)combine with lipids to form several classes of lipoprotein particles,spherical complexes with hydrophobic lipids in the core and the hydrophilic side chains of protein amino acids at the surface (Fig.
20-37a).Differing combinations of lipids and proteins produce particles of different densities, ranging from very low-density lipoproteins (VLDL) tohigh-density lipoproteins (HDL), which may be separated by ultracentrifugation (Table 20-2, p. 676) and visualized by electron microscopy(Fig. 20-37b).Each class of lipoprotein has a specific function, determined by itspoint of synthesis, lipid composition, and apolipoprotein content.
Atleast nine different apolipoproteins are found in the lipoproteins ofhuman plasma (Table 20-3); they can be distinguished by their size,their reactions with specific antibodies, and their characteristic distribution in the lipoprotein classes. These protein components act as signals, targeting lipoproteins to specific tissues or activating enzymesthat act on the lipoproteins.VLDL (x 180,000)LDL (x 180,000)HDL (x 180,000)(b)676Part III Bioenergetics and MetabolismTable 20-2 Major classes of human plasma lipoproteins: some propertiesComposition (wt %)LipoproteinDensity (g/mL)ChylomicronsVLDLLDLHDL<1.0060.95-1.0061.006-1.0631.063-1.210ProteinPhospholipidsFree cholesterolCholesteryl estersTriacylglycerols21091817232085524231237158550104Source: Modified from Kritchevsky, D. (1986) Atherosclerosis and nutrition.
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