1. Sialic acid, also called N-acetyl-a-D-neuraminic acid, is often found in N-li
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Question
1. Sialic acid, also called N-acetyl-a-D-neuraminic acid, is often found in N-linked oligosaccharides involved in cell-cell interactions. Cancer cells synthesize much more sialic acid than normal cells. Inhibitors of sialic acid (N-acetyl-D-neuraminic acid) may be effective anti-cancer drugs.
Answer the following questions about the structure of sialic acid.
Circle the anomeric carbon on both structures of the compound.
Will sialic acid mutorotate between a and b forms? Explain.
Is sialic acid a “deoxy acid”? If so, which carbon has the deoxy group?
From the open chain form of sialic acid, you can see that can become a ketone. Label the ketone (NOT amide!) carbon on both forms of the molecule.
How many chiral carbons are in the closed ring form (left)? Underline them on the structure. Are there the same number of chiral carbons in the chain form? Explain.
OVER
Case Study: You have a 20-year-old patient who was normal at birth but soon displayed delayed development of intellect and motor skills. He has high levels of sialic acid in his urine and his blood.
a. You culture his fibroblasts. What are fibroblasts and why does it make sense to culture them?
b. The cultured cells have a high intracellular level of sialic acid and surprisingly high amounts of sialic acid in the lysosomes. What happens in lysosomes?
c. You do an experiment in which you add a radioactive sialic acid derivative to a sample of his lysosomes and to those of a normal person. Compared with a normal person, your patient’s degraded sialic acid leaves the lysosome very slowly. What does this tell you?
d. You measure the intracellular activities of N-acetylneuraminate lyase, sialidase, CMP-N-acylneuraminate phosphodiesterase, and acylneuraminate citidylyl transferase and find that they are normal. Are these all or mostly degrading or synthesizing enzymes?
e. Is the defect in your patient more likely to be in the sialic acid synthesis pathway or in the pathway for degrading it? Justify your choice.
COOH CH3 CH2OH CH2 HN O COOH CHOH OH OHH HC OH N-Acetyl--D-neuraminic acid CH2OH N-Acetyl--D-neuraminic acid (open-chain form)Explanation / Answer
Since this question has a lot of subparts, I will answer only first four according to the answering guidelines.
Q1. carbon number 2 on both the structures is the anomeric carbon.
Q2. Sialic acid does mutarotate between a and b forms in aqueous solution. However, it is a thermodynamically unfavorable reaction as sialic acid is known to mutarotate by four kinetically distinct patways. Therefore, the extent of water catalyzed mutarotation is very small.
Q3. Carbon 3 and 5 are dideoxy.
Q4. carbon 2 is a ketone group in the open chain which forms the closed ring.
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