Q.30 Which one of the following metabolic intermediates is common to glycolysis,
nucleotide synthesis and glycogen synthesis?
(A) Citrate
(B) Oxaloacetate
(C) Glucose 6–phosphate
(D) Glycerol 3–phosphate

Glucose 6-phosphate serves as the common metabolic intermediate in glycolysis, nucleotide synthesis, and glycogen synthesis.

Option Analysis

Citrate forms in the citric acid cycle from acetyl-CoA and oxaloacetate, playing no direct role in glycolysis, nucleotide synthesis via the pentose phosphate pathway, or glycogen synthesis.

Oxaloacetate participates in the citric acid cycle, gluconeogenesis, and amino acid metabolism but does not connect to glycolysis intermediates, pentose phosphate pathway for nucleotides, or glycogen buildup.

Glucose 6-phosphate acts as the key branch point: it advances glycolysis after hexokinase phosphorylation, enters the pentose phosphate pathway for ribose-5-phosphate in nucleotide synthesis, and converts to glucose-1-phosphate for glycogen synthesis.

Glycerol 3-phosphate derives from glycolysis (dihydroxyacetone phosphate) for lipid synthesis and shuttle systems but lacks involvement in nucleotide or glycogen pathways.

Correct Answer

(C) Glucose 6-phosphate stands out as the shared intermediate across all three pathways.

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Glucose 6-phosphate emerges as the pivotal metabolic intermediate common to glycolysis, nucleotide synthesis, and glycogen synthesis, regulating energy, storage, and biosynthetic needs in cells. This molecule, formed by phosphorylating glucose via hexokinase or glucokinase, directs metabolism based on cellular demands like ATP production, DNA/RNA building blocks, or glucose reserves.

Role in Glycolysis

Glycolysis breaks down glucose for energy, starting with glucose 6-phosphate isomerizing to fructose 6-phosphate. This step commits glucose to ATP-generating reactions, yielding pyruvate under anaerobic conditions.

Connection to Nucleotide Synthesis

Glucose 6-phosphate fuels the pentose phosphate pathway’s oxidative phase, producing ribose-5-phosphate for nucleotide backbones and NADPH for redox balance. Cells prioritize this route during rapid division for DNA/RNA synthesis.

Link to Glycogen Synthesis

For storage, glucose 6-phosphate converts to glucose-1-phosphate via phosphoglucomutase, then UDP-glucose for glycogen polymerase addition. Insulin promotes this in liver and muscle post-feeding.

Why Other Options Fail

• Citrate: TCA cycle exclusive.

• Oxaloacetate: Gluconeogenesis/TCA focus.

• Glycerol 3-phosphate: Glycerolipid shuttle.

This integration suits CSIR NET Life Sciences prep, highlighting metabolic crossroads.