Chapter 16: Carbohydrates

Transcription

1 Vocabulary Aldose: a sugar that contains an aldehyde group as part of its structure Amylopectin: a form of starch; a branched chain polymer of glucose Amylose: a form of starch; a linear polymer of glucose Anomer: one of the possible stereoisomers formed when a sugar assumes the cyclic form Anomeric Carbon: the chiral center created when a sugar cyclizes Cellulose: a polymer of glucose; an important structural material in plants Configuration: the three-dimensional arrangement of groups around a chiral carbon atom Deoxy Sugar: a sugar in which one of the hydroxyl groups has been reduced to a hydrogen Diastereomers: nonsuperimposable, non-mirror-image stereoisomers Disaccharide: two monosaccharides (monomeric sugars) linked by a glycosidic bond Enantiomers: mirror-image, nonsuperimposable stereoisomers Epimers: stereoisomers that differ only in configuration around one of several chiral carbon atoms Fischer projection: a two-dimensional representation of the stereochemistry of threedimensional molecules: Furanose: a cyclic sugar with a six-membered ring, named for its resemblance to the ring system in furan Furanoside: a glycoside involving a furanose Glucose: a monosaccharide; a ubiquitous metabolite Glyceraldehyde: the simplest carbohydrate that contains a chiral carbon, the starting point of a system of describing optical isomers Glycogen: a polymer of glucose; an important energy storage molecule in animals Glycoside: a compound in which one or more sugars is bonded to another molecule Haworth Projection Formulas: a perspective representation of the cyclic forms of sugars Hemiacetal: a compound that is formed by reaction of an aldehyde with an alcohol and is found in the cyclic structure of sugars Hemiketal: a compound that is formed by reaction of a ketone with an alcohol and is found in the cyclic structure of sugars Heteropolysaccharide: a polysaccharide that contains more than one kind of sugar monomer Homopolysaccharide: a polysaccharide that contains only one kind of sugar monomer Hyaluronic Acid: a polysaccharide found in the lubricating fluid of joints Ketose: a sugar that contains a ketone group as part of its structure Lignin: a polymer of coniferyl alcohol; a structural material found in woody plants Monosaccharide: a compound that contains a single carbonyl group and two or more hydroxyl groups Mucopolysaccharide: a polysaccharide that has a gelatinous consistency Oligosaccharide: a few sugars linked by glycosidic bonds Pectin: a polymer of galacturonic acid; it occurs in the cell walls of plants Peptidoglycan: a polysaccharide that contains peptide crosslinks; it is found in bacterial cell walls Pyranose: a cyclic form of a sugar containing a five-membered ring; it was named for its resemblance to pyran Pyranoside: a glycoside involving a pyranose Reducing Sugar: a sugar that has a free carbonyl group, one that can react with an oxidizing agent Polysaccharide: a polymer of sugars Starch: a polymer of glucose that plays an energy-storage role in plants

2 Chapter Summary What is unique about the structures of sugars? The simplest examples of carbohydrates are monosaccharides, compounds that each contain a single carbonyl group and two or more hydroxyl groups. Monosaccharides frequently encountered in biochemistry are sugars that contain from three to seven carbon atoms. Sugars contain one or more chiral centers; the configurations of the possible stereoisomers can be represented by Fischer projection formulas. What happens if a sugar forms a cyclic molecule? Sugars exist predominantly as cyclic molecules rather than in an open-chain form. Haworth projection formulas are more realistic representations of the cyclic forms of sugars than are Fischer projection formulas. Many stereoisomers are possible for five- and six-carbon sugars, but only a few of the possibilities are encountered frequently in nature. What are some oxidation reduction reactions of sugars? Monosaccharides can undergo various reactions. Oxidation reactions make up one important group. What are some important esterification reactions of sugars? Esterification of sugars to phosphoric acid plays an important role in metabolism. What are glycosides, and how do they form? The most important reaction of sugars by far is the formation of glycosidic linkages, which give rise to oligosaccharides and polysaccharides. What are some other important derivatives of sugars? Amino sugars are the basis of cell wall structures. What makes sucrose an important compound? Three important examples of oligosaccharides are the disaccharides sucrose, lactose, and maltose. Sucrose is common table sugar. It is a disaccharide formed when a glycosidic bond forms between glucose and fructose. Are any other disaccharides important to us? Lactose occurs in milk, and maltose is obtained via the hydrolysis of starch.

3 How do cellulose and starch differ from one another? In polysaccharides, the repeating unit of the polymer is frequently limited to one or two kinds of monomer. Cellulose and starch differ in the anomeric form of their glycosidic bonds: the a form in starch and the b form in cellulose. Is there more than one form of starch? Starch exists in two polymeric forms, the linear amylose and the branched amylopectin. How is glycogen related to starch? Starch, found in plants, and glycogen, which occurs in animals, differ from each other in the degree of branching in the polymer structure. What is chitin? Cellulose and chitin are polymers based on single kinds of monomer units glucose and N- acetylglucosamine, respectively. Both polymers play structural roles in organisms. What role do polysaccharides play in the structure of cell walls? In bacterial cell walls, polysaccharides are cross-linked to peptides. Plant cell walls consist primarily of glucose. Do polysaccharides play any specific roles in connective tissue? Glycosaminoglycans are a type of polysaccharide based on a repeating disaccharide in which one of the sugars is an amino sugar and at least one of them has a negative charge owing to the presence of a sulfate group or a carboxyl group. They play a role in joint lubrication and also in the blood clotting process. How are carbohydrates important in the immune response? In glycoproteins, carbohydrate residues are covalently linked to the polypeptide chain. Such glycoproteins can play a role in the recognition sites of antigens. A common example is the ABO blood group, in which the three major blood types are distinguished by sugar molecules attached to the protein.

4 Questions and Answers 1. Define the following terms: polysaccharide, furanose, pyranose, aldose, ketose, glycosidic bond, oligosaccharide, glycoprotein Polysaccharide: a polymer of simple sugars, which are compounds that contain a single carbonyl group and several hydroxyl groups Furanose: a cyclic sugar that contains a five-membered ring similar to that in furan Pyranose: a cyclic sugar that contains a six-membered ring similar to that in pyran Aldose: a sugar that contains an aldehyde group Ketose: a sugar that contains a ketone group Glycosidic Bond: the acetal linkage that joins two sugars Oligosaccharide: a compound formed by the linking of several simple sugars (monosaccharides) by glycosidic bonds Glycoprotein: formed by the covalent bonding of sugars to a protein 2. Name which, if any, of the following are epimers of D-glucose: D-mannose, D-galactose, D-ribose. D-Mannose and D-galactose are both epimers of D-glucose, with inversion of configuration around carbon atoms 2 and 4, respectively D-ribose has only five carbons, but the rest of the sugars named in this question have six 3. Name which, if any, of the following groups are not aldose ketose pairs: D-ribose and D-ribulose, D-glucose and D-fructose, D-glyceraldehyde and dihydroxyacetone. All groups are aldose ketose pairs. 4. What is the difference between an enantiomer and a diastereomer? Enantiomers are nonsuperimposable, mirror-image stereoisomers. Diastereomers are nonsuperimposable, nonmirror-image stereoisomers. 5. How many possible epimers of d-glucose exist?

5 Four epimers of d-glucose exist, with inversion of configuration at a single carbon. The possible carbons at which this is possible are those numbered two through five. 6. Why are furanoses and pyranoses the most common cyclic forms of sugars? Furanoses and pyranoses have five-membered and six-membered rings, respectively. It is well known from organic chemistry that rings of this size are the most stable and the most readily formed. 7. How many chiral centers are there in the open-chain form of glucose? In the cyclic form? There are four chiral centers in the open-chain form of glucose (carbons two through five). Cyclization introduces another chiral center at the carbon involved in hemiacetal formation, giving a total of five chiral centers in the cyclic form. 8. Following are Fischer projections for a group of five-carbon sugars, all of which are aldopentoses. Identify the pairs that are enantiomers and the pairs that are epimers. (The sugars shown here are not all of the possible five-carbon sugars.) Enantiomers: (a) and (f), (b) and (d). Epimers: (a) and (c), (a) and (d), (a) and (e), (b) and (f). 9. The sugar alcohol often used in sugarless gums and candies is L-sorbitol. Much of this alcohol is prepared by reduction of d-glucose. Compare these two structures and explain how this can be.

6 L-Sorbitol was named early in biochemical history as a derivative of L-sorbose. Reduction of d-glucose gives a hydroxy sugar that could easily be named D-glucitol, but it was originally named L-sorbitol and the name stuck. 10. Consider the structures of arabinose and ribose. Explain why nucleotide derivatives of arabinose, such as ara-c and ara-a, are effective metabolic poisons. Arabinose is an epimer of ribose. Nucleosides in which arabinose is substituted for ribose act as inhibitors in reactions of ribonucleosides. 11. Two sugars are epimers of each other. Is it possible to convert one to the other without breaking covalent bonds? Converting a sugar to an epimer requires inversion of configuration at a chiral center. This can be done only by breaking and re-forming covalent bonds. 12. How does the cyclization of sugars introduce a new chiral center? Two different orientations with respect to the sugar ring are possible for the hydroxyl group at the anomeric carbon. The two possibilities give rise to the new chiral center. 13. What is unusual about the structure of N-acetylmuramic acid (Figure 16.18) compared with the structures of other carbohydrates? This compound contains a lactic acid side chain.

7 14. What is the chemical difference between a sugar phosphate and a sugar involved in a glycosidic bond? In a sugar phosphate, an ester bond is formed between one of the sugar hydroxyls and phosphoric acid. A glycosidic bond is an acetal, which can be hydrolyzed to regenerate the two original sugar hydroxyls. 15. Define the term reducing sugar. A reducing sugar is one that has a free aldehyde group. The aldehyde is easily oxidized, thus reducing the oxidizing agent. 16. What are the structural differences between vitamin C and sugars? Do these structural differences play a role in the susceptibility of this vitamin to air oxidation? Vitamin C is a lactone (a cyclic ester) with a double bond between two of the ring carbons. The presence of the double bond makes it susceptible to air oxidation. 17. Name two differences between sucrose and lactose. Name two similarities. Similarities: sucrose and lactose are both disaccharides, and both contain glucose. Differences: sucrose contains fructose, whereas lactose contains galactose. Sucrose has an a,b(1 S 2) glycosidic linkage, whereas lactose has a b(1 S 4) glycosidic linkage. 18. Draw a Haworth projection for the disaccharide gentibiose, given the following information: it is a dimer of glucose, the glycosidic linkage is b(1 --> 6), and the anomeric carbon not involved in the glycosidic linkage is in the a configuration. 19. What is the metabolic basis for the observation that many adults cannot ingest large quantities of milk without developing gastric difficulties? In some cases, the enzyme that degrades lactose (milk sugar) to its components glucose and galactose is missing. In other cases, the enzyme isomerizes galactose to glucose for further metabolic breakdown.

8 20. Draw Haworth projection formulas for dimers of glucose with the following types of glycosidic linkages: A) A b(1 S 4) linkage (both molecules of glucose in the b form), B) An a,a(1 S 1) linkage C) A b(1 S 6) linkage (both molecules of glucose in the b form) 21. A friend asks you why some parents at her child s school want a choice of beverages served at lunch, rather than milk alone. What do you tell your friend? Milk contains lactose. Many people are sensitive to lactose and require an alternative beverage. 22. What are some of the main differences between the cell walls of plants and those of bacteria? The cell walls of plants consist mainly of cellulose, whereas those of bacteria consist mainly of polysaccharides with peptide crosslinks. 23. How does chitin differ from cellulose in structure and function? Chitin is a polymer of N-acetyl-b-d-glucosamine, whereas cellulose is a polymer of d- glucose. Both polymers play a structural role, but chitin occurs in the exoskeletons of invertebrates and cellulose primarily in plants. 24. How does glycogen differ from starch in structure and function? Glycogen and starch differ mainly in the degree of chain branching. Both polymers serve as vehicles for energy storage, glycogen in animals and starch in plants.

9 25. What is the main structural difference between cellulose and starch? Both cellulose and starch are polymers of glucose. In cellulose, the monomers are joined by a b-glycosidic linkage, whereas in starch they are joined by an a-glycosidic linkage. 26. What is the main structural difference between glycogen and starch? Glycogen exists as a highly branched polymer. Starch can have both a linear and a branched form, which is not as highly branched as that of glycogen. 27. How do the cell walls of bacteria differ from those of plants? Plant cell walls consist almost exclusively of carbohydrates, whereas bacterial cell walls contain peptides 28. Pectin, which occurs in plant cell walls, exists in nature as a polymer of d-galacturonic acid methylated at carbon 6 of the monomer. Draw a Haworth projection for a repeating disaccharide unit of pectin with one methylated and one unmethylated monomer unit in a(1 S 4) linkage. Repeating disaccharide of pectin 29. Advertisements for a food supplement to be taken by athletes claimed that the energy bars contained the two best precursors of glycogen. What were they? Glucose and fructose

10 30. Explain how the minor structural difference between a- and b-glucose is related to the differences in structure and function in the polymers formed from these two monomers. Differences in structure: cellulose consists of linear fibers, but starch has a coil form. Differences in function: cellulose has a structural role, but starch is used for energy storage 31. All naturally occurring polysaccharides have one terminal residue, which contains a free anomeric carbon. Why do these polysaccharides not give a positive chemical test for a reducing sugar? The concentration of reducing groups is too small to detect. 32. An amylose chain is 5000 glucose units long. At how many places must it be cleaved to reduce the average length to 2500 units? To 1000 units? To 200 units? What percentage of the glycosidic links are hydrolyzed in each case? (Even partial hydrolysis can drastically alter the physical properties of polysaccharides and thus affect their structural role in organisms.) To 2500, one place (0.02%). To 1000, four places (0.08%). To 200, 24 places (0.48%). 33. Suppose that a polymer of glucose with alternating a(1 S 4) and b(1 S 4) glycosidic linkages has just been discovered. Draw a Haworth projection for a repeating tetramer (two repeating dimers) of such a polysaccharide. Would you expect this polymer to have primarily a structural role or an energy-storage role in organisms? What sort of organisms, if any, could use this polysaccharide as a food source? This polymer would be expected to have a structural role. The presence of the b- glycosidic linkage makes it useful as food only to termites or to ruminants, such as cows and horses; these animals harbor bacteria capable of attacking the b-linkage in their digestive tracts

11 34. Glycogen is highly branched. What advantage, if any, does this provide an animal? Because of the branching, the glycogen molecule gives rise to a number of available glucose molecules at a time when it is being hydrolyzed to provide energy. A linear molecule could produce only one available glucose at a time 35. No animal can digest cellulose. Reconcile this statement with the fact that many animals are herbivores that depend heavily on cellulose as a food source. The digestive tract of these animals contains bacteria that have the enzyme to hydrolyze cellulose 36. How does the presence of a-bonds versus b-bonds influence the digestibility of glucose polymers by humans? Hint: There are two effects. Humans lack the enzyme to hydrolyze cellulose. In addition, the fibrous structure of cellulose makes it too insoluble to digest, even if humans had the necessary enzyme. 37. How do the sites of cleavage of starch differ from one another when the cleavage reaction is catalyzed by a-amylase and b-amylase? The enzyme b-amylase is an exoglycosidase, degrading polysaccharides from the ends. The enzyme a-amylase is an endoglycosidase, cleaving internal glycosidic bonds 38. What is the benefit of fiber in the diet? Fiber binds many toxic substances in the gut and decreases the transit time of ingested food in the digestive tract, so that harmful compounds such as carcinogens are removed from the body more quickly than would be the case with a low-fiber diet 39. How would you expect the active site of a cellulase to differ from the active site of an enzyme that degrades starch? A cellulase (an enzyme that degrades cellulose) needs an active site that can recognize glucose residues joined in a b-glycosidic linkage and hydrolyze that linkage. An enzyme that degrades starch has the same requirements with regard to glucose residues joined in an a-glycosidic linkage

12 40. Would you expect cross-linking to play a role in the structure of polysaccharides? If so, how would the cross-links be formed? Cross-linking can be expected to play a role in the structures of polysaccharides where mechanical strength is an issue. Examples include cellulose and chitin. These crosslinks can be readily formed by extensive hydrogen bonding. (See Figure ) 41. Compare the information in the sequence of monomers in a polysaccharide with that in the sequence of amino acid residues in a protein. The sequence of monomers in a polysaccharide is not genetically coded, and, in this sense, it does not contain information 42. Why is it advantageous that polysaccharides can have branched chains? How do they achieve this structural feature? It can be useful for polysaccharides to have a number of ends, characteristic of a branched polymer, rather than the two ends of a linear polymer. This would be the case when it is necessary to release residues from the ends as quickly as possible. Polysaccharides achieve this by having 1 S 4 and 1 S 6 glycosidic linkages to a residue at a branch point 43. Why is the polysaccharide chitin a suitable material for the exoskeleton of invertebrates such as lobsters? What other sort of material can play a similar role? Chitin is a suitable material for the exoskeleton of invertebrates because of its mechanical strength. Individual polymer strands are cross-linked by hydrogen bonding, accounting for the strength. Cellulose is another polysaccharide cross-linked in the same way, and it can play a similar role 44. Could bacterial cell walls consist largely of protein? Why or why not? Bacterial cell walls are not likely to consist largely of protein. Polysaccharides are easily formed and confer considerable mechanical strength. They are likely to play a large role. 45. Some athletes eat diets high in carbohydrates before an event. Suggest a biochemical basis for this practice.

13 Athletes try to increase their stores of glycogen before an event. The most direct way to increase the amount of this polymer of glucose is to eat carbohydrates. 46. You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation reduction titration involving iodine. You get a starch indicator from the stockroom. Why do you need it? Iodine is the reagent that will be added to the reaction mixture in the titration. When the end point is reached, the next drop of iodine will produce a characteristic blue color in the presence of the indicator. 47. Blood samples for research or medical tests sometimes have heparin added. Why is this done? Heparin is an anticoagulant. Its presence prevents blood clotting. 48. Based on what you know about glycosidic bonds, propose a scheme for formation of covalent bonds between the carbohydrate and protein portions of glycoproteins. Glycosidic bonds can be formed between the side-chain hydroxyls of serine or threonine residues and the sugar hydroxyls. In addition, there is the possibility of ester bonds forming between the side-chain carboxyl groups of aspartate or glutamate and the sugar hydroxyls. 49. What are glycoproteins? What are some of their biochemical roles? Glycoproteins are ones in which carbohydrates are covalently bonded to proteins. They play a role in eukaryotic cell membranes, frequently as recognition sites for external molecules. Antibodies (immunoglobulins) are glycoproteins. 50. Briefly indicate the role of glycoproteins as antigenic determinants for blood groups. The sugar portions of the blood-group glycoproteins are the source of the antigenic difference