ch7 - Carbohydrates and Glycobiology STEP~BY-STEP GUlBE...

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Unformatted text preview: Carbohydrates and Glycobiology STEP~BY-STEP GUlBE Major Concepts Monosaccharides are the monomeric subunits from which di-, oliga, and polysaccharides are fanned. They are classified according to the position of the carbonyl group (aldose or ketose), the total number of carbon atoms in the chain (triose to heptose), the isomeric form {L or D, epimers, enamel-s), Whether or not they occur in cyclic form, and Whether or not they have another chemical substituent. Polysaccharides vary in monomeric composition, type of glycosidic bond connecting the monosaccha— ride units, chain length and degree of branching, and biological function. Starch and glycogen are homopolysaccharides of glucose that function as fuel stores in plants and ani- mals, respectively. Both starch and glycogen are chains of glucose monomers connected by linear link- ages, {oil—>4) glycosidic bonds, but they have differ- ent degrees of branching through (ail—96) glycosidic bonds. Branching allows faster degradation of a poly- saccharide to individual monosaccharide units, which can then be used to generate biologically useful en- ergy. Cellulose is also a homopolymer of glucose, but its (Bled) linkages result in rigid extended polymers that are used by cells for support rather than as energy storage molecules. More complex heteropolysaccha- rides, composed of more than one type of monosac~ . charide, can be covalently cross—linked ‘oy peptides or proteins to form. peptidoglycans or proteoglycans. These carbohydrate-protein aggregates function to provide structural support (bacterial cell wall), con- nectivity (extracellular matrix), and lubrication (synw ovial fluid). lycoconjugates include proteoglycans, glycopra _ 7&8, and glycolipids. These hybrid molecules have arbohydrate portions that alter or augment the 3‘?- ._«,,. g3 functional properties of their protein and lipid com— ponents. Proteoglycans are important components of the ere traceliular matrix of multicellular animals. They are composed primarily (by mass) of glycosaminogly— can chains that are connected to a protein and are im- portant for structure and resilience. Glycoproteins contain smaller, more diverse oligosaccharides that are linked to amino acid residues of a protein via 0- or N-glycosidic bonds; these information—rich carbohy- drate portions commonly act as recognition sites for enzymes or receptors, or for targeting a protein to a specific cellular location. Membrane glycolipids such as the gangliosides, and bacterial lipopolysaccha- rides, are lipids that are covalently bound to oligosac- charide moieties. The diversity of oligosaccharide structure found in glyooconjugates makes these molecules informa- tional molecules, actually surpassing nucleic acids in the density of information carried. Lectins, oligosaccharide-binding proteins, are central in cell-cell recognition and signaling processes. Lectin~ carbohydrate reccgrution and binding are highly spe- cific, and, like other biochemical contacts, are medi» ated by multiple weak, noncovaient mteractions. Chemical analysis of carbohydrates makes use of some of the same techniques used for protein analysis. Full determination of the structure of complex het~ eropolysaccharides involves many steps not only be- cause there are a variety of monosaccharide subunits, but also because there are a variety of linkages possi— ble. Mass spectrometry and NMR spectroscopy are two important procedures used to characterize carbo- hydrates. 87 88 Chapter 7 Carbohydrates and Giycobioiogy What to Review . and the various conformations of biological mole- cules (Figs. 1—18 through 1—23). Answering the 19030an questions and fewewg the rel‘ ' Many sugars can be oxidized and are termed reduc» evant concepts, which you have already studied, should mg sugars. Be sure you anderstand the Overall com make $113 Chapter more understandabie' cept of oxidation—reduction reactions. 0 Review the structure of aldehydes, ketones, and hy- - Review the various analytical techniques used in droxyl groups (Fig. 1—45); ail of these groups are the study of amino acids and proteins (Figs. i—S, found in carbohydrates. 8—16 through 3—22). Which ones can be applied to - Simple sugars exist in a variety of isomeric forms. the StUdY 0f CErbOhYdrateS? Review What is meant by enantiomers (Fig. 1—49) Topics for Discussion Answering each of the following questions, especialiy in the context of a study group dis— cussion, should help you understand the important points of this chapter. 7.1 Monosaccharides and Disaccharides 1. Why are monosaccharides appropriately named “Garbo hydrates“? The Two Families of Monosaccharides Are Aldcses and Ketascs 2. Why are monosaccharides soluble in water but not in nonpolar soivents? 3. What are the naming conventions for monosaccharides? Mancsaccharides Have Asymmetric Centers 4. The word “ciaira ” is derived from the Greek word meaning “hand.” How does this relate to the structure of carbohydrates? 5. How many D isomers wouid an aidopentose have? How many totai isomers? 6. What is a possible explanation for the observation that most of the hexoses found in liv- ing organisms are D isomers? (Hint: see Chapter 6.) ’7. Is galactose an epiraer of mannose‘? The Common Monasaccharides Have Cyclic Structures 8. in an aqueous solution of D-glucose, Why will there always be a very small amount of the linear form of the monosaccharide? 9. Does the discussion on conformations of cyciic forms of monosaccharides suggest an explanation for Why the more commoa anomer of D-fructofaranose is the B anomer‘? (Hint: see Fig. 7—8.) 7.2 Polysacchatides Step-By-Step Guide 89 10. How many D isomers would a cycl‘izec aidopentose have? How many total isomers? Organisms Contain a Variety of Hexose Derivatives 11. What are some of the biologically more important monosacchatide denvatives? 12. Why is phosphorylation of sugars beneficial to a cell? Manosaccharides Are Reducing Agents 13. What is reduced and what is oxidized in the reaction between a monosaccharide and a ferric ion? ' Disaccharides Contain a Glycosidic Band 14. Be able to draw and give the complete name of the disaccharides commonly known as maltose, Iactose, and sucrose. Which of these is/are reducing sugars? 15. What are the structural and functional differences between homopolysaccharides and heteropolysaccharides? 16. Why are homopolysaccharides not useful as intonnational molecuies‘? Some Homapoiysaccharides Are Stored Forms of Fuels 17. Suppose you had three polysaccharides, amylcse, amylopectin, and glycogen, each with the same number of monosaccharide subunits. Which would be degraded the fastest, assuming that the enzymes acting to degrade the polysaccharides all worked at the same rate? Same Humapolysaccharides Serve Structural Roles 18. What are the similarities and differences between cellulose and chitin? Steric Factors and Hydrogen Bonding influence Homapclysaccharide Folding 19. What are the most important noncovalent bonds or interactions in cellulose? Bacterial and Algal Cell Walls Contain Structural Heteropalysaccharides 20. How does iysozyme act as a first defense against bacterial infection? Bow does penicillin combat bacterial infections? 90 Chapter 7 Carbohydrates and Glycobiology 2}. What chemical property of agarose contributes to its ability to trap large amounts of water? Glycosaminoglycans Are Hatempalysaccharldes of the Extracellular Matrix 22. What chemical property of glycosaminoglycans contributes to their role as lubricants? 23. HOW does heparin work to inhibit btood coagulation? 7.3 Glycocoajugates: Proteoglycaos, Glycoproteins, anti Glycolipids 24. One of the distinctions between proteogiycans and glycoproteins is entirety relative. The second half of the name usually indicates the predominant species. Proteogtyccms are primarily glycans (or polysaccharides), Whereas glycoprotea‘ns usualiy contain more protein (by weight). Proteaglycans Ara Glycosamincglycan-Ccntainng Macromolecules of the Cell Surface and Extracellular Matrix 25. HOW does the R-group of Ser make it a good amino acid for connecting to a carbohy- drate moiety? 26. If a proteoglycan aggregate is compared to an evergreen tree, with hyalaronate as the “trunk,” What compound would be the “needles” on this tree? What role do the core pro- teins play? 27. What is the relationship among glycosaminoglycans, proteoglycans, fibrous proteins, in- tegrins, and the extraceliular matrix? Glycoprcleins Have Covalently Attached Oligcsaccharides 28. How are the functions of glycoproteins different from those of glycosaminoglycans? 29. What types of iinkages connect oligosaccharides to proteins? 30. What kinds of biologically important information are encoded by the oligosaccharide portions of glycoproteins? 31. How does the addition of carbohydrate moieties alter the chemistry of giycoproteins and giycolipids‘? Step-By—Step Guide 91 Glycolipids and Lipopolysaocharides Are Membrane Components 32. How do glycolipids and lipopolysaccharides differ? 7.4 Carbohydrates as Informational Molecules: The Sugar Code 33. How can so much distinguishing uiforrnation be packed into an oligosaccharicie of corn- paratively few monosaccharide units? Lectins Are Proteins That Road the Sugar Code and Mediate Many Biological Processes 34. Why is it important that there are signals for removal and destruction of “old” celis and hormones? 35. Why do people of biood type 0 tend to have gastric ulcers more often than do people of blood type A or B? [satin-Carbohydrate Interactions Are Highly Specific and Often Polyvalent 36. A quick review of the 131:; of the histidine Rugroup will help in understanding the man- nose 6~phosphate receptor-mannose 6-phosphate interaction. 7.5 Working with Carbohydrates 3'7. What makes analysis of carbohydrates so much more difficult than analysis of proteins? 38. How can the specificity of lectins and glycosidases be used to identify and analyze carbohydrates? Discussion Questions for Study Groups ' How can such a simple difference——the difference between or and ,6 glycosidic bonds—~produce differences in ' function and three—dimensional structure as drastic as those seen between amylose and cellulose? S-domains of altered heparin sulfate bind to extracellular proteins and signaling molecules in several ways. Dis- cuss the different noncovaient interactions possible for binding on the basis of the chemical groups present in these molecules. 92 Chapter 7 Carbohydrates and Glycobiology SELF-TEST Do You Know the Terms? ’ IIIIIIII AGRflSS DOWN 6. A homopolysaccharide of glucose; it is lughly branched 1. A poiysaceharide is a polymer of repeating and found exciusiveiy in animal cells. 'monosaccharides. ’7. Formed by cyciization of a kerose sugar. 2. A sugar with a carbonyl group at (32 (or any position 8. A homopolysaccharide of g‘iucose units connected by other than Cd). {cl-o4) glycosidic bonds; found exclusively in plants. 3. Carbohyérate moieties are attached to glycoproteins 9. Simple sugars. through either N— or - bonds. 11. Heseropolysacchazides such as hyalnronate. 4. Six-membered ring form of sugars. 13. Glycogen and cellulose, with thousands of simpie sugar 5. Five-membered ring form of sugars. subunits, are examples. 7. A polysaccharide containing more than one type of 17. Oxidation of the carbonyi carbon oE sugars (except glu- sugar is a poiysaccharide; an example is shon— cose) results in the formation of acids. droitin sulfate. 18. A compound with an asymmetrical atom allowing forma— 9. Process that interconverts isomers of pyranoses. tion of mirror—image isomers has one or more 10. Lectins are proteins that bind to specific . centers. 12. Glycoconjugates containing protein and oiigosaccharide 21. B-D-glucuronate is an example of 2. acid. portions: for example, glycophorin A. 22. End of a poiysaccharide chain that is not invoiveci in a 14. A homopolysaccharide of glucose units connected by glycosidic bond and has a free anomeric carbon. {fi 144} glycosiclic bonds; it is found exciusively in 24:. Lactose and sucrose are examples. plants. 25. Polysaccharides cross-lurked by peptides; Eound in bac— 15. An isomer that differs at only one of two or more chiral terial cell walls. centers. Self-Test 93 16. Gangliosides, for exampie. 20. A sugar with the carbonyl group at C-1. 17. The a and (3 forms of a pyranose, for example. 23. Animal tissues have an extracelluiar composed 19. In the formation of pyranoses, linkage between the alde- of glycoconjugates and fibrous proteins. hyde on 0—1 and the alcohol on 0-5. Do You Know the Facts? 1. Which of the foliowing is not a characteristic of carbohydrates in cells? A. They serve as energy stores in plants and animals. B. They are major structural components of plant tissues. 0. They act as binding sites for proteins. D. They are organic catalysts. E. They play a role in cell—ceil recognition. 2. Which of the following contributes to the structural rigidity of celluiose? A. Adjacent glucose polymers are stabilized by hydrogen bonding. B. Glucose residues are joined by (oil—)4) linkages. C. Cellulose is a highly branched molecuie. D. The conformation of the glucose polymer is a coiled structure. E. Adjacent polymers are covalently iinired by short peptides. 3. Which of the following is an epirner of glucose? A. ri‘alose B. Idose C. Gulose D. Altrose E. Allose 4. Why are sugars usually found as phosphorylated derivatives in cells? A. Phosphoryiated sugars are important in reguiating cellular pH. B. Unphosphoryiated sugars can be transported across cell membranes. C. Unphosphorylated sugars are rapidly degraded by celluiar enzymes. 1). Phosphorylated sugars encode genetic information. . E. None of the above is a correct explanation. . Which of the following disacchanldes could be extended to form a celiulose polymer? - A. Sucrose B. 'i‘rehaloso C. Maltose I). Lactose E. None of the above . Which of the following is a heteropolysaccharide? What is its function? A. Giycogen B. Hyaloronate _ C. Starch . 13. Cellulose E. Chitin 94 Chapter 7 Carbohydrates and Glycobiology 7. (a) Draw the pyranose form of Ban-glucose. (b) How many isomers of glucopyranose are possible? (c) Describe this monosaooharide using at least four chemise} terms. 8. (a) Draw the furanose form of a»D—fruct0furanose. (b) How many isomers of fructofuranose are possible? ((2) Describe this monosaccharide using at least four chemicai terms. 9. In addition to simple sugars, What other compounds have you encountered in the text that exist as enantiomers? 10. Ceiluiose is a homopolysaccharide of glucose units. Why can’t this molecule be used by most organisms to generate energy? SelfnTest 95 11. Draw straight-chain {Fischer projection) and cyclic (Haworth perspective) representations of the following mol» ecules. Identify your Fischer projections as showing the L or the :3 form, and identify your Hawoxth perspectives as the or or {3 form. (a) Glyceraldehyde (b) Dihydroxyacetone (c) Ribose (ti) Galactose 12. Draw the Haworth perspective formulas for disaccharides containing two glucose molecules: (a) in an {cal—>4) linkage. (b) in a (Sheri) linkage. (c) in an Cori—>6) linkage. Applying What You Know .1. Why are many membrane proteins glycoproteins? . Describe the relationship between glycosaminoglycans and a proteoglycans aggregate. How is collagen involved in this arrangement? ' What structural differences between glycogen and cellulose explain their functional differences? "'Amylopectin is a homopolysaccharide containing BOO—6,000 glucose residues. Because a single glucose residue __ 3 approximately 0.001 Jurn long, the largest amylopectin molecules would be 6 am long, if all the residues were [arranged in a single chain. Plant cells store amylopectin in granules whose diameters are 3—100 pm. How do plant spells maximize the storage capacity of these (relatively) small structures? What other factorCs) probably have in- fluenced the way cells store glucose molecules? 96 Chapter 7 Carbohydrates and Glycobiology 5. The enzyme hyaluronidase is found as a component of some snake and insect toxins. How does it increase a toxin’s effectiveness? 1 observations: (1) In many glycoproteins, the oligosacchaiide portion at~ 6. Consider the following experimenta taches to the protein portion at sequences that form 6 bends. (2) The three—dnnensional protein architecture of glycoproteins is not affected by removal of the associated oligosaccharide (e). What do these observations tell you about the spatial placement of the oligosaccharides in glycoproteins? 2 as to the surface of the stomach by interactions between bacterial membrane lectins and specific oligosaccharides of membrane glycoproteins of gastric epithelial calls. One of the binding sites recognized byH. pylori} is the oligosaccharide called Lab. Chemically synthesized analogs of the Leb l oligosaccharide can be administered orally to inhibit bacterial attachment. Refer to Figure B—ea; draw a iigand _ to the H. pylori lectin-Leb oligosaccharide interaction, with and Without the presence of 7. The bacterium Helicooaoter pylori adher binding curve to illustra Lab analog. What type of interaction is this similar to? (Hint: see Chapter 6.) 8. The starch present in barley consists of polymers of glucose that are linked by two types of bonds: (ales) and (calm—)6) linkages. The enzymes found in malt (the dried, germinated barley that is rich in hydrolytic enzymes) are used to hydrolyze some of these bonds in starch to produce free glucose, which can then be fermented to ethanol and 002. rl‘here are other methods for making fermented beverages; for example, “spit beer" is made by a person chewing corn briefly and then spitting it into a container. The action of salivary amylase is similar to that of the barley enzymes. Neither the barley nor salivary enzymes can degrade starch completely; the pieces of starch left unhydrolyzed are highly branched and are referred to as limit dextrins. la “light” beer production, however, a mold enzyme is used to split all the limit dextrins to individual glucose molecules. What bond(s) must this accessory mold enzyme be able to cleave? Answers 97 ANSWERS Be You Know the Terms? Across Down 6. glycogen 1. home 7. herniketal 2. ketcse 8. starch 3. O-glycosidic 9. monosaccharides 4. pyranose 11. glycosanunoglycans 5. furanose 13. polysaccharides 7. hetero 17. aldom'c 9. mutarotation 18. chiral 10. oligosaccharides 21. uremic 12. glycoproteins 22. reducing 14. ceiiuiose , 24. disaccharides 15. epimer 25. peptidoglycans 16. glycolipids 17. anomers 19. herrflacetal 20. aldose 23. matrix 00 You Know the Facts? 10. The (Iii—>4) glycosidic bonds of cellulose can be hydrolyzed only by cellulase, which is produced by 1' D a few microorganisms, such as some bacteria and 2. A Mchonympha. 3- E 11. The different Fischer proéection formulas are 4. B shown in Figure 9—3. The Haworth perspective for» 5_ E mules are shown below. 6. B; Hyaluronate plays an important structural and (a) NOt mind as a (b) N“ found as a lubricating role in the extracellular matrix of anhnal ring St? 110mm ring 513111011111“ ‘3 tissues. ((3) KOCHE (d) HOCH2 O . (a) cagog H 0 H OH HO H a H H OH H OH H H H 3 0H 0H 30 l H OH OH H OH H OH fi~D~ribofuranose wn‘galacmpyranose Bflnpgiucose £2. (a) {eel—)4) linkage ‘ CH20H CHQOH (b) 32 lsomers (chiral carbons, n = 5; 2“ = 32) 0 0 (c) hexose, aldose, pyranose, reducing sugar H H H H H OH (3.) HOCH2 0 H0 on H O OH H H 021203 H H0 H OH H OH H OH (b) (5164) linkage OH H (SI-1203 CH20H _ _ O 0 or D fructose H H E H OH 1)) 16 isomers (chiral carbons, n = 4; 2” m 16) 03 E O OH H 0) hexose, ketose,furanose, reducing sugar H0 H H WHO acids other than glycine also have a chiral 3 OH H OH enters—the or carbon. 98 Chapter 7 Carbohydrates and Glycobiology (c) (ab—>6) linkage crayon o H H H HO on n O\ a on CHE O H H I?“ no OH H on H on Applying What You Know 1. Many plasma membrane proteins are glycoproteins, with the oligosaccharide portion invariably located on the external surface of the membrane. Oligosac— charide moieties on glycoproteins are not monoto— nous, but rich in structure} information. They act as biological markers, for example as “life-clocks“ for individual proteins or cells, signaling whether they should be allowed to circulate or should be destroyed. Glycosaminoglycans are heteropolysacchzu‘ides, a family of linear polymers composed of repeating disaccharide units. One of the two monosaccharides in the repeating disaccharide always has an amino group of some sort attached, hence glycosamtno— glycans. in a typical proteoglycan aggregate, a very tong strand of a certain type of glycosarninoglycan (hyaluronate), which can be thought of as the centre? trunk of a tree, has numerous “branches” {aggrecan core proteins) attached. To these core protein branches are attached many smaller glycosamino- glycan moiecules of other types (such as keratan suifate). In a typicai proteoglycan aggregate in hu- man cartilage, each hyaluronate trunk has about 100 core protein branches, and each core protein branch has many smaller glycosaminoglycan branches. Inter- woven with these enormous aggregates are fibrous proteins, such as coliagen, which give strength and resilience to the matrix. Both polysaccharides are homopolysaccharides, meaning they contain only a single type of monomeric unit; in this case, glucose. Cellulose is a linear, unbranched homopolysaccharide, whereas glycogen is branched. Both glycogen and cellulose have variable numbers of subunits, but both are very iarge molecules. The critical difference between the two is that glucose residues in ceiliflose have a ,8 configuration, whereas glucose residues in glycogen are linked in the 0: configuration. Glycogen is a polymer of (oi—+4)— linked subunits of glucose, with (DEE-#96j-Dflkeé branches every 8-}.2 residues. The most favorable threewdimensional conformation for glycogen is a tightly coiled helical structure stabilized by hydro— gen bonds. Cellulose is a polysaccharide of (til—>4) giycosidic bonds. Each glucose unit is turned 180° relative to its neighbors, yielding a straight, ex- tended chain. Multiple hydrogen bonds form be- tween neighboring chains on all sides. These structural differences correspond to the particular functions of the polysaccharides. Glycogen’s role is energy storage. When glucose is needed, its branched structure allows for rapid degradation (from many branch ends) by the cell. Cellulose has a structural role; it gives rigidity and strength to plant cell walis. Extensive hydrogen bonding between linear molecules of celiulose forrns sheets, and further hydrogen bonding be- tween stacked sheets makes for very tough, rigid masses of material. . Giucose storage moiecules are branched, with branches occurring every 24—30 residues in any lopectin and every 8—12 residues in glycogen. The most stable conformation for the adjacent and reta- tively rigid residues is a curved chain. The exten— sively branched molecules, therefore, pack more residues into a tighter space. Other factors probably influenced the evolu— tion of very large, branched molecules for storage. The enzymes that act on these polymers to mobilize glucose for metabolism act only on their nonreduc- ing ends. With extensive branching, more such ends are available for enzymatic attack than in the same quantity of glucose stored as a linear polymer. In ef- fect, branched polymers increase the substrate con» centration for these enzymes. Another significant consideration is the effect of the same number of individual glucose molecules on cell osmolarity. The increase in intracellular osmoiarity due to the presence of hundreds of thousands of glucose mol- ecules probably would be sufficient to rupture the cell. . Hyalurorudase degrades hyaluronate, an important structural glycosarninoglycan in animal tissues. lt hydrolyzes the (Biwezl) linkage between the repeat- ing disaccharide units. rl‘his aliows invasion of the tissue by the other components of the toxins such as those that act to disrupt cell membranes. . These two pieces of evidence, along with the highly hydrophiiic character of oligosaccharides, indicate that the oligosaccharide portions of glycoproteins extend from the surface of the proteins, rather than being involved in the internal structure. The presence of the analog of the Le” oligosacchaw ride alters binding of the Leb oligosaccharlde to the H. pylori iectin in a manner similar to a competitive :3" 3. l l inhibitor of an enzyme. The analog competes with the Le“ oligosaceharide for the binding site of the lectin, much as a competitive inhibitor competes with substrate for the active site of an enzyme. Barley and salivary enzymes can hydroiyze only the (ml—>4) linkages; not only can they not hydroiyze the (alwfi) linkages at the branch points, usually the (eel—>4) linkages just adjacent to the Coal—>6) linkages are not cieaved. These short polymers re- main in the final beer product, Where they account for ~22% of the total starch (and therefore calories) in regular beer. Answers 99 The mold enzyme used in Eight beer produc- tion can hydrolyze both the (ad-94) and (oil—>6) finkages. Because it is added before the fermenta— tion process, it hydrolyzes the starch completeiy, making all the glucose available to the yeast for fer— mentation. (This toads to higher alcohol content of the beer, wruch is then diluted, making light beer taste less than robust.) ’ ...
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This note was uploaded on 08/02/2011 for the course BCHM 461 taught by Professor Gerratana during the Spring '11 term at University of Maryland Baltimore.

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ch7 - Carbohydrates and Glycobiology STEP~BY-STEP GUlBE...

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