Chapter 10

Word origin. Account for the origin of the term carbohydrate.

Diversity. How many different oligosaccharides can be made by linking one glucose, one mannose, and one galactose residue? Assume that each sugar is in its pyranose form. Compare this number with the number of tripeptides that can be made from three different amino acids.

They go together like a horse and carriage. Match each term with its description. (a) Enantiomers _____ (b) Cellulose ____ (c) Lectins ______ (d) Glycosyltransferases ____ (e) Epimers _____ (f) Starch ____ (g) Carbohydrates _____ (h) Proteoglycan _____ (i) Mucoprotein _____ (j) Glycogen _____ 1\. Has the molecular formula of \(\left(\mathrm{CH}_{2} \mathrm{O}\right) n\) 2\. Monosaccharides that differ at a single asymmetric carbon atom 3\. The storage form of glucose in animals 4\. The storage form of glucose in plants 5\. Glycoprotein containing glycosaminoglycans 6\. The most abundant organic molecule in the biosphere 7\. N-Acetylgalactosamine is a key component of this glycoprotein 8\. Carbohydrate-binding proteins 9\. Enzymes that synthesize oligosaccharides 10\. Stereoisomers that are mirror images of each other

Couples. Indicate whether each of the following pairs of sugars consists of anomers, epimers, or an aldose-ketose pair. \(\sqrt{1}\) (a) D-glyceraldehyde and dihydroxyacetone (b) D-glucose and D-mannose (c) D-glucose and D-fructose (d) \(\alpha\) -D-glucose and \(\beta\) -D-glucose (e) D-ribose and D-ribulose (f) D-galactose and D-glucose

A lost property. Glucose and fructose are reducing sugars. Sucrose, or table sugar, is a disaccharide consisting of both fructose and glucose. Is sucrose a reducing sugar? Explain.

Meat and potatoes. Compare the structures of glycogen and starch.

Sweet proteins. List the key classes of glycoprotein, their defining characteristics, and their biological functions.

Life extender. What is the function of the carbohydrate moiety that is attached to EPO?

Carbohydrates-not just for breakfast anymore. Differentiate between a glycoprotein and a lectin.

Cushioning. What is the role of the glycosaminoglycan in the cushioning provided by cartilage?

Locks and keys. What does the fact that all organisms contain lectins suggest about the role of carbohydrates?

Undelivered mail. Not returned to sender. I-cell disease results when proteins normally destined to the lysosomes lack the appropriate carbohydrate- addressing molecule (p. 182 ). Suggest another possible means by which I-cell disease might arise.

Carbohydrates and proteomics. Suppose that a protein contains six potential \(N\) -linked glycosylation sites. How many possible proteins can be generated, depending on which of these sites is actually glycosylated? Do not include the effects of diversity within the carbohydrate added.

Many possibilities. Why are polysaccharides considered information-rich molecules?

Stereospecificity. Sucrose, a major product of photosynthesis in green leaves, is synthesized by a battery of enzymes. The substrates for sucrose synthesis, D-glucose and \(\mathrm{D}\) -fructose, are a mixture of \(\alpha\) and \(\beta\) anomers as well as acyclic compounds in solution. Nonetheless, sucrose consists of \(\alpha\) -D-glucose linked by its \(\mathrm{C}-1\) atom to the \(\mathrm{C}-2\) atom of \(\beta-\mathrm{D}-\) fructose. How can the specificity of sucrose be explained in light of the potential substrates?

Mutarotation. The specific rotations of the \(\alpha\) and \(\beta\) anomers of D-glucose are +112 degrees and \(+18.7 \mathrm{de}-\) grees, respectively. Specific rotation, \([\alpha]_{\mathrm{D}},\) is defined as the observed rotation of light of wavelength \(589 \mathrm{nm}\) (the D line of a sodium lamp) passing through \(10 \mathrm{cm}\) of a \(1 \mathrm{g} \mathrm{ml}^{-1}\) solution of a sample. When a crystalline sample of \(\alpha\) -D-glucopyranose is dissolved in water, the specific rotation decreases from 112 degrees to an equilibrium value of 52.7 degrees. On the basis of this result, what are the proportions of the \(\alpha\) and \(\beta\) anomers at equilibrium? Assume that the concentration of the open-chain form is negligible.

Periodate cleavage. Compounds containing hydroxyl groups on adjacent carbon atoms undergo carbon-carbon bond cleavage when treated with periodate ion \(\left(\mathrm{IO}_{4}^{-}\right) .\) How can this reaction be used to distinguish between pyranosides and furanosides if cis-glycans are cleaved more rapidly than trans-glycols?

Mapping the molecule. Each of the hydroxyl groups of glucose can be methylated with reagents such as dimethylsulfate under basic conditions. Explain how exhaustive methylation followed by the complete digestion of a known amount of glycogen would enable you to determine the number of branch points and reducing ends.