Problem 22
Locks and keys. What does the fact that all organisms contain lectins suggest about the role of carbohydrates?
Problem 23
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.
Problem 24
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.
Problem 25
Many possibilities. Why are polysaccharides considered information-rich molecules?
Problem 26
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?
Problem 28
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.
Problem 29
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?
Problem 30
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.
