Question

All of the following interconversions of monosaccharides (or derivatives) require a nucleotide linked sugar intermediate except A. galactose 1 -phosphate to glucose 1 -phosphate. B. glucose 6 -phosphate to mannose 6 -phosphate. C. glucose to glucuronic acid. D. glucuronic acid to xylose. E. glucosamine 6 -phosphate to \(N\) -acerylneuraminic acid (a sialic acid).

Short answer

A. Galactose 1-phosphate to glucose 1-phosphate B. Glucose 6-phosphate to mannose 6-phosphate C. Glucose to glucuronic acid D. Glucuronic acid to xylose E. Glucosamine 6-phosphate to N-acerylneuraminic acid (a sialic acid) Answer: B. Glucose 6-phosphate to mannose 6-phosphate

Step-by-step solution

Identifying nucleotide-linked sugar intermediates

Nucleotide-linked sugar intermediates are compounds where a sugar is linked to a nucleotide, which usually acts as a cofactor in various biosynthetic pathways. These intermediates play a crucial role in carbohydrate metabolism and the formation of glycosidic bonds.

Analyze each given interconversion

Now, let's analyze the interconversions given in the options to identify which one doesn't involve a nucleotide-linked sugar intermediate: A. Galactose 1-phosphate to glucose 1-phosphate: This interconversion takes place in the Leloir pathway, which does not involve a nucleotide-linked sugar intermediate. An enzyme called galactose-1-phosphate uridyltransferase (GALT) transfers a uridine monophosphate (UMP) moiety from Uridine diphosphate glucose (UDPG) to galactose 1-phosphate, forming glucose 1-phosphate. B. Glucose 6-phosphate to mannose 6-phosphate: In this interconversion, Phosphomannose isomerase (PMI) mediates the cis-trans isomerization of the hydroxyl group at C-2 position to convert glucose 6-phosphate into mannose 6-phosphate without involving nucleotide-linked sugar intermediates. C. Glucose to glucuronic acid: This interconversion involves the formation of UDP-glucuronic acid, which is a nucleotide-linked sugar intermediate. The enzyme UDP-glucose dehydrogenase catalyzes the oxidation of glucose linked to uridine diphosphate (UDP), which then forms UDP-glucuronic acid. D. Glucuronic acid to xylose: This interconversion also involves the formation of a nucleotide-linked sugar intermediate. UDP-glucuronic acid undergoes epimerization to form UDP-xylose, and then xylose is formed after hydrolysis. E. Glucosamine 6-phosphate to N-acerylneuraminic acid (a sialic acid): This interconversion involves several enzymatic steps and formation of CMP-N-acetylneuraminic acid (a nucleotide-linked sugar intermediate) during the sialic acid synthesis pathway.

Identify the correct answer

By analyzing the options, we can conclude that the interconversion of glucose 6-phosphate to mannose 6-phosphate (Option B) does not involve a nucleotide-linked sugar intermediate. Hence, the correct answer is Option B.