Q.29 Inter-conversion of UDP-glucose and UDP-galactose is catalyzed by
(A) an oxidase
(B) a kinase
(C) an epimerase
(D) a mutase
The inter-conversion of UDP-glucose and UDP-galactose is catalyzed by an epimerase, so the correct option is (C) an epimerase. UDP-galactose 4-epimerase specifically catalyzes the reversible conversion between UDP-galactose and UDP-glucose in galactose metabolism. This enzyme changes the configuration around carbon 4 of the hexose moiety, which is a classic epimerase reaction.
Correct Answer Explained: Epimerase (Option C)
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UDP-glucose and UDP-galactose differ only in the configuration at carbon 4 of the hexose part of the molecule, making them C-4 epimers of each other.
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Epimerases are a subclass of isomerases that catalyze the reversible interconversion of epimers by changing configuration around a single stereogenic center.
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In galactose metabolism, UDP-galactose 4-epimerase converts UDP-galactose to UDP-glucose (and vice versa), enabling efficient use of dietary galactose by channeling it into glycolysis via glucose derivatives.
Therefore, the correct answer is: (C) an epimerase.
Why Option A (Oxidase) Is Incorrect
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Oxidases are enzymes that catalyze oxidation–reduction reactions where molecular oxygen acts as the electron acceptor.
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The conversion of UDP-glucose to UDP-galactose does involve transient oxidation–reduction at C-4 within the enzyme’s active site, but the overall classification is not oxidase; the net reaction is a stereochemical inversion, not a net oxidation of the substrate.
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Thus, while redox chemistry is involved mechanistically, the enzyme’s functional and IUBMB classification is epimerase, not oxidase.
Why Option B (Kinase) Is Incorrect
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Kinases transfer phosphate groups from ATP to substrates (phosphorylation), commonly acting on sugars like glucose to form glucose-6-phosphate.
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In UDP-glucose ↔ UDP-galactose interconversion, there is no addition or removal of a phosphate group; the UDP moiety and overall phosphorylation state remain unchanged.
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Since no phosphorylation occurs, a kinase cannot be the correct enzyme class for this reaction.
Why Option D (Mutase) Is Incorrect
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Mutases catalyze intramolecular group transfers, typically shifting a functional group from one position to another on the same molecule (for example, phosphoglucomutase converting glucose-1-phosphate to glucose-6-phosphate).
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In the UDP-glucose/UDP-galactose reaction, the positions of groups on the carbon skeleton are not relocated; only the spatial configuration (stereochemistry) at a single carbon changes.
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Because the reaction is a configurational change at one chiral center rather than an intramolecular group transfer, the enzyme is not a mutase but an epimerase.
Summary for Exam Use
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Question: Inter-conversion of UDP-glucose and UDP-galactose is catalyzed by?
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Correct option: (C) an epimerase
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Key concept: UDP-glucose and UDP-galactose are C-4 epimers; the enzyme that interconverts epimers is called an epimerase, specifically UDP-galactose 4-epimerase in this pathway.
