Chapter 7

Which of the following techniques is not commonly used to study oligosaccharide structures? A) X-ray crystallography B) Matrix-assisted laser desorption/ionization mass spectroscopy (MALDI-MS) C) Nuclear magnetic resonance (NMR) D) Complete chemical synthesis E) Oligosaccharide microarrays

A) X-ray crystallography

(a) Define "reducing sugar." (b) Sucrose is a disaccharide composed of glucose and fructose (Glc(alpha1-->2)Fru). Explain why sucrose is not a reducing sugar, even though both glucose and fructose are.

(a) A reducing sugar is one with a free carbonyl carbon that can be oxidized by Cu2+ or Fe3+. (b) The carbonyl carbon is C-1 of glucose and C-2 of fructose. When the carbonyl carbon is involved in a glycosidic linkage, it is no longer accessible to oxidizing agents. In sucrose (Glc(alpha-->2)Fru), both oxidizable carbons are involved in the glycosidic linkage.

(a) Draw the structure of any aldohexose in the pyranose ring form. (b) Draw the structure of the anomer of the aldohexose you drew above. (c) How many asymmetric carbons (chiral centers) does each of these structures have? (d) How many stereoisomers of the aldohexoses you drew are theoretically possible?

(a) Any of the hexoses drawn with a six-membered ring, as shown in Fig. 7-7 on p. 239, is correct. The hydroxyls at C-2, C-3, and C-4 can point either up or down. (b) For the anomer, the structure should be identical to the first, except that the hydroxyl group at C-1 should point up if it pointed down in your first structure, and vice versa. (c) The number of chiral centers is 5; all are carbons except C-6. (d) The number of possible stereoisomers for a compound with n chiral centers is 2n; in this case, 25, or 32 possible isomers.

Draw the structure of the repeating basic unit of: (a) amylose (b) cellulose.

(a) For the structure of amylose, see Fig. 7-14a, p. 245. The repeating unit is alpha-D-glucose linked to alpha-D-glucose; the glycosidic bond is therefore (alpha1 --> 4). (b) Cellulose has the same structure as amylose, except that the repeating units are beta-D-glucose and the glycosidic bond is (beta1--> 4). (See Fig. 7-15a, p. 246.)

Define each in 20 words or less: (a) anomeric carbon; (b) enantiomers; (c) furanose and pyranose; (d) glycoside; (e) epimers; (f) aldose and ketose.

(a) The anomeric carbon is the carbonyl carbon atom of a sugar, which is involved in ring formation. (b) Enantiomers are stereoisomers that are nonsuperimposable mirror images of each other. (c) Furanose is a sugar with a five-membered ring; pyranose is a sugar with a six-membered ring. (d) A glycoside is an acetal formed between a sugar anomeric carbon hemi-acetal and an alcohol, which may be part of a second sugar. (e) Epimers are stereoisomers differing in configuration at only one asymmetric carbon. (f) An aldose is a sugar with an aldehyde carbonyl group; a ketose is a sugar with a ketone carbonyl group.

Describe the structure of a proteoglycan aggregate such as is found in the extracellular matrix.

A proteoglycan aggregate is a supramolecular assembly of proteoglycan monomers. Each monomer consists of a core protein with multiple, covalently linked polysaccharide chains. Hundreds of these monomers can bind noncovalently to a single extended molecule of hyaluronic acid to form large structures.

Sketch the principal components of a typical proteoglycan, showing their relationships and connections to one another.

A typical proteoglycan consists of a core protein with covalently attached glycosaminoglycan polysaccharides, such as chondroitin sulfate and keratin sulfate. The polysaccharides generally attach to a serine residue in the protein via a trisaccharide (gal-gal-xyl). (See Fig. 7-24, p. 253.)

Which of the following statements about heparan sulfate is not true? A) Sulfation of heparan sulfate to form NS domains is important for its role as an anti-coagulant. B) Heparan sulfate can promote protein-protein interactions via the NS domains. C) The secondary structure of heparan sulfate is completely random. D) The NA domains of heparan sulfate contain no sulfation. E) The core repeating structure of heparan sulfate is made up of alternating GlcNAc and GlcA.

C) The secondary structure of heparan sulfate is completely random.

Match these molecules with their biological roles. a) glycogen (b) starch (c) trehalose (d) chitin (e) cellulose (f) peptidoglycan (g) hyaluronate (h) proteoglycan __ viscosity, lubrication of extracellular secretions __ carbohydrate storage in plants __ transport/storage in insects __ exoskeleton of insects __ structural component of bacterial cell wall __ structural component of plant cell walls __ extracellular matrix of animal tissues __ carbohydrate storage in animal liver

Ans: g; b; c; d; f; e; h; a

The number of structurally different polysaccharides that can be made with 20 different monosaccharides is far greater than the number of different polypeptides that can be made with 20 different amino acids, if both polymers contain an equal number (say 100) of total residues. Explain why.

Because virtually all peptides are linear (i.e., are formed with peptide bonds between the alpha-carboxyl and alpha-amino groups), the variability of peptides is limited by the number of different subunits. Polysaccharides can be linear or branched, can be alpha- or beta-linked, and can be joined 1--> 4, 1 --> 3, 1--> 6, etc. The number of different ways to arrange 20 different sugars in a branched oligosaccharide is therefore much larger than the number of different ways a peptide could be made with an equal number of residues.

Following complete hydrolysis of a sample of glycogen and a sample of cellulose, which of the following must be true? A) The glycogen sample is more soluble than the cellulose sample. B) The cellulose sample is more soluble than the glycogen sample. C) Both samples consist of a mixture of alpha-D-glucose and beta-D-glucose. D) The glycogen sample has a higher ratio of alpha-D-glucose than the cellulose sample. E) The cellulose sample contains only beta-D-glucose.

C) Both samples consist of a mixture of alpha-D-glucose and beta-D-glucose.

Describe the differences between a proteoglycan and a glycoprotein.

Both are made up of proteins and polysaccharides. In proteoglycans, the carbohydrate moiety dominates, constituting 95% or more of the mass of the complex. In glycoproteins, the protein constitutes a larger fraction, generally 50% or more of the total mass.

Which of the following statements concerning sialic acid residues on glycoproteins is true? A) Sialic residues on erythrocytes are recognized by lectins, leading to removal of the erythrocytes. B) Sialic residues on ceruloplasmin are recognized by lectins, leading to removal of ceruloplasmin. C) Sialic residues are removed by neuraminidases. D) The anti-viral drug oseltamavir accelerates the removal of sialic acid residues. E) Both A and B above

C) Sialic residues are removed by neuraminidases.

When forming the disaccharide maltose from two glucose monosaccharides: A) water is eliminated. B) a hemiacetal is converted to an acetal. C) the resulting dissacharide is no longer a reducing sugar. D) Both A and B E) A, B, and C above

D) Both A and B

Which of the following statements about hydrogen bonding in glycogen and cellulose is true? A) Glycogen forms more internal H-bonds than cellulose. B) Extensive internal hydrogen bonding makes cellulose more water soluble than glycogen. C) Extensive hydrogen bonding with water makes cellulose more soluble than glycogen. D) Glycogen primarily forms hydrogen bonds within a single chain. E) The hydrogen bonding in cellulose favors a helical conformation.

D) Glycogen primarily forms hydrogen bonds within a single chain.

Which of the following is a heteropolysaccharide? A) Cellulose B) Chitin C) Glycogen D) Hyaluronate E) Starch

D) Hyaluronate is a heteropolysaccharide. (made of alternating residues of D-glucuronic acid and N-acetylglucosamine)

Which of the following is a dominant feature of the outer membrane of the cell wall of gram negative bacteria? A) Amylose B) Cellulose C) Glycoproteins D) Lipopolysaccharides E) Lipoproteins

D) Lipopolysaccharides is a dominant feature of the outer membrane of the cell wall of gram negative bacteria.

Which of the following is not a reason that it is difficult to study oligosaccharide composition from biological systems? A) Oligosaccharides are often branched. B) Oligosaccharides often have a high negative charge density. C) Oligosaccharides have a variety of linkages (e.g., alpha1->6 or beta1->4). D) Oligosaccharides have too much conformational flexibility. E) There are no specific glycosidase enzymes that can be used to selectively digest oligosaccharides.

D) Oligosaccharides have too much conformational flexibility.

D-Glucose is called a reducing sugar because it undergoes an oxidation-reduction reaction at the anomeric carbon. One of the products of this reaction is: A) D-galactose. B) D-gluconate. C) D-glucuronate. D) D-ribose. E) muramic acid.

D-Glucose is called a reducing sugar because it undergoes an oxidation-reduction reaction at the anomeric carbon. One of the products of this reaction is: B) D-gluconate.

Which of the following is not a reducing sugar? A) Fructose B) Glucose C) Glyceraldehyde D) Ribose E) Sucrose

E) Sucrose is NOT a reducing sugar. (Sucrose does not have a reducing end, as it is made of the anomeric C of glucose and hydroxyl group of fructose via a alpha1-->beta4 linkage.)

From the abbreviated name of the compound Gal(beta1-->4)Glc, we know that: A) C-4 of glucose is joined to C-1 of galactose by a glycosidic bond. B) the compound is a D-enantiomer. C) the galactose residue is at the reducing end. D) the glucose is in its pyranose form. E) the glucose residue is the anomer.

From the abbreviated name of the compound Gal(beta1-->4)Glc, we know that: A) C-4 of glucose is joined to C-1 of galactose by a glycosidic bond.

What are some of the biochemical effects of the oligosaccharide portions of glycoproteins?

Hydrophilic carbohydrates can alter the polarity and solubility of the proteins. Steric and charge interactions may influence the conformation of regions of the polypeptide and protect it from proteolysis.

Hemoglobin glycation is a process where _______ is _______ attached to hemoglobin. A) glycerol; covalently B) glucose; enzymatically C) glucose; non-enzymatically D) N-acetyl-galactosamine; enzymatically E) galactose; non-enzymatically

Hemoglobin glycation is a process where _______ is _______ attached to hemoglobin. C) glucose; non-enzymatically

In glycoproteins, the carbohydrate moiety is always attached through the amino acid residues: A) asparagine, serine, or threonine. B) aspartate or glutamate. C) glutamine or arginine. D) glycine, alanine, or aspartate. E) tryptophan, aspartate, or cysteine.

In glycoproteins, the carbohydrate moiety is always attached through the amino acid residues: A) asparagine, serine, or threonine.

What are lectins? What are some biological processes which involve lectins?

Lectins are proteins that bind to specific oligosaccharides. They interact with specific cell-surface glycoproteins thus mediating cell-cell recognition and adhesion. Several microbial toxins and viral capsid proteins, which interact with cell surface receptors, are lectins.

Explain why all mono- and disaccharides are soluble in water.

Monosaccharides and disaccharides are soluble in water because these compounds have many hydroxyl groups, each of which can hydrogen bond with water. (See chapter 4.)

Describe the process by which "old" serum glycoproteins are removed from the mammalian circulatory system.

Newly synthesized serum glycoproteins bear oligosaccharide chains that end in sialic acid. With time, the sialic acid is removed. Glycoproteins that lack the terminal sialic acid are recognized by asialoglycoprotein receptors in the liver, internalized, and destroyed.

The biochemical property of lectins that is the basis for most of their biological effects is their ability to bind to: A) amphipathic molecules. B) hydrophobic molecules. C) specific lipids. D) specific oligosaccharides. E) specific peptides.

The biochemical property of lectins that is the basis for most of their biological effects is their ability to bind to: D) specific oligosaccharides.

Starch and glycogen are both polymers of: A) fructose. B) glucose1-phosphate. C) sucrose. D) alpha-D-glucose. E) beta-D-glucose.

Starch and glycogen are both polymers of: D) alpha-D-glucose.

The basic structure of a proteoglycan consists of a core protein and a: A) glycolipid. B) glycosaminoglycan. C) lectin. D) lipopolysaccharide. E) peptidoglycan.

The basic structure of a proteoglycan consists of a core protein and a: B) glycosaminoglycan.

Describe one biological advantage of storing glucose units in branched polymers (glycogen, amylopectin) rather than in linear polymers.

The enzymes that act on these polymers to mobilize glucose for metabolism act only on their nonreducing ends. With extensive branching, there are more such ends for enzymatic attack than would be present in the same quantity of glucose stored in a linear polymer. In effect, branched polymers increase the substrate concentration for these enzymes.

Explain how it is possible that a polysaccharide molecule, such as glycogen, may have only one reducing end, and yet have many nonreducing ends.

The molecule is branched, with each branch ending in a nonreducing end. (See Fig. 7-14c, p. 5.)

To possess optical activity, a compound must be: A) a carbohydrate. B) a hexose. C) asymmetric. D) colored. E) D-glucose.

To possess optical activity, a compound must be: C) asymmetric.

What is the biological advantage to an organism that stores its carbohydrate reserves as starch or glycogen rather than as an equivalent amount of free glucose?

The polymers are essentially insoluble and contribute little to the osmolarity of the cell, thereby avoiding the influx of water that would occur with the glucose in solution. They also make the uptake of glucose energetically more feasible than it would be with free glucose in the cell.

The reference compound for naming D and L isomers of sugars is: A) fructose. B) glucose. C) glyceraldehyde. D) ribose. E) sucrose.

The reference compound for naming D and L isomers of sugars is: C) glyceraldehyde.

Explain in molecular terms why humans cannot use cellulose as a nutrient, but goats and cattle can.

The ruminant animals have in their rumens microorganisms that produce the enzyme cellulase, which splits the (beta1 --> 4) linkages in cellulose, releasing glucose. Humans do not produce an enzyme with this activity; the human digestive enzyme alpha-amylase can split only (alpha1 --> 4) linkages (such as those in glycogen and starch).

The glycosaminoglycans are negatively charged at neutral pH. What components of these polymers confer the negative charge?

Uronic acids such as glucuronic acid, and sulfated hydroxyl groups, such as GalNAc4SO3- and GlcNAc6SO3-. (See Fig. 7-22, p. 250.)

When the linear form of glucose cyclizes, the product is a(n): A) anhydride. B) glycoside. C) hemiacetal. D) lactone. E) oligosaccharide.

When the linear form of glucose cyclizes, the product is a(n): C) hemiacetal.

When two carbohydrates are epimers: A) one is a pyranose, the other a furanose. B) one is an aldose, the other a ketose. C) they differ in length by one carbon. D) they differ only in the configuration around one carbon atom. E) they rotate plane-polarized light in the same direction.

When two carbohydrates are epimers: D) they differ only in the configuration around one carbon atom.

Which of following is an anomeric pair? A) D-glucose and D-fructose B) D-glucose and L-fructose C) D-glucose and L-glucose D) alpha-D-glucose and beta-D-glucose E) alpha-D-glucose and beta-L-glucose

Which of following is an anomeric pair? D) alpha-D-glucose and beta-D-glucose

Which of the following is an epimeric pair? A) D-glucose and D-glucosamine B) D-glucose and D-mannose C) D-glucose and L-glucose D) D-lactose and D-sucrose E) L-mannose and L-fructose

Which of the following is an epimeric pair? B) D-glucose and D-mannose

Which of the following monosaccharides is not a carboxylic acid? A) 6-phospho-gluconate B) gluconate C) glucose D) glucuronate E) muramic acid

Which of the following monosaccharides is not a carboxylic acid? C) glucose

Which of the following monosaccharides is not an aldose? A) erythrose B) fructose C) glucose D) glyceraldehyde E) ribose

Which of the following monosaccharides is not an aldose? B) fructose

Which of the following pairs is interconverted in the process of mutarotation? A) D-glucose and D-fructose B) D-glucose and D-galactose C) D-glucose and D-glucosamine D) D-glucose and L-glucose E) alpha-D-glucose and beta-D-glucose

Which of the following pairs is interconverted in the process of mutarotation? E) alpha-D-glucose and beta-D-glucose (Mutarotation is the porcess in which one ring form (say the alpha anomer) opens biefly into the linear form, then closes again to produce the beta anomer.)

Which of the following statements about starch and glycogen is false? A) Amylose is unbranched; amylopectin and glycogen contain many (alpha1-->6) branches. B) Both are homopolymers of glucose. C) Both serve primarily as structural elements in cell walls. D) Both starch and glycogen are stored intracellularly as insoluble granules. E) Glycogen is more extensively branched than starch.

Which of the following statements about starch and glycogen is false? C) Both serve primarily as structural elements in cell walls. (Starch and glycogen are storage molecules! Hence the granules and the toughness of chitin and cellulose!)

Why is it surprising that the side chains of tryptophan residues in proteins can interact with lectins? A) because the side chain of tryptophan is hydrophilic and lectins are hydrophobic. B) because the side chain of tryptophan is (-) charged and lectins are generally (+) charged or neutral. C) because the side chain of tryptophan can make hydrogen bonds and lectins cannot. D) because the side chain of tryptophan is hydrophobic and lectins are generally hydrophilic. E) none of the above.

Why is it surprising that the side chains of tryptophan residues in proteins can interact with lectins? D) because the side chain of tryptophan is hydrophobic and lectins are generally hydrophilic.