19. Ribulose-5-phosphate epimerase is involved in which one of the following processes?
(A) Glycolysis
(B) TCA cycle
(C) Glycosylation
(D) Pentose phosphate pathway

Ribulose-5-phosphate epimerase, also known as ribulose-phosphate 3-epimerase (RPE), catalyzes the reversible interconversion of ribulose-5-phosphate to xylulose-5-phosphate in the non-oxidative phase of the pentose phosphate pathway. The correct answer to the multiple-choice question is (D) Pentose phosphate pathway. This enzyme supports nucleotide synthesis and NADPH production essential for biosynthetic processes.​

Correct Answer

(D) Pentose phosphate pathway
RPE operates in the pentose phosphate pathway’s non-oxidative branch, converting D-ribulose-5-phosphate to D-xylulose-5-phosphate, which enables interconversions for ribose-5-phosphate production used in nucleic acids. This step links the pathway to glycolysis by producing glycolytic intermediates like fructose-6-phosphate and glyceraldehyde-3-phosphate.​

Glycolysis Explanation

Glycolysis breaks down glucose to pyruvate through 10 enzymatic steps, generating ATP and NADH without involving ribulose-5-phosphate or epimerization at the C3 position. Key enzymes include hexokinase, phosphofructokinase, and pyruvate kinase, focusing on hexose phosphorylation and cleavage. RPE plays no direct role here, as glycolysis handles C6 sugars exclusively.​

TCA Cycle Explanation

The TCA (Krebs) cycle oxidizes acetyl-CoA to CO2 in mitochondria, producing NADH, FADH2, and GTP via enzymes like citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. It processes C2 units from pyruvate, with no pentose phosphates or epimerase activity involved. RPE absence aligns with TCA’s focus on C4-C6 tricarboxylic acids.​

Glycosylation Explanation

Glycosylation attaches sugars to proteins/lipids in the ER/Golgi, using nucleotide sugars like UDP-glucose or dolichol-linked oligosaccharides via glycosyltransferases. While pentose phosphates contribute indirectly via ribose for nucleotide synthesis, RPE itself does not participate in glycan assembly. This process emphasizes O- or N-linked modifications unrelated to epimerase catalysis.​