Q. 73 The precursor for the synthesis of aromatic amino acids is
(A) phosphoenolpyruvate
(B) pyruvate
(C) oxaloacetate
(D) 𝛼-ketoglutarate
The precursor for the synthesis of aromatic amino acids—phenylalanine, tyrosine, and tryptophan—is phosphoenolpyruvate (PEP), making option (A) correct.
Shikimate Pathway Overview
Aromatic amino acids form via the shikimate pathway in plants, bacteria, and fungi, absent in animals. PEP condenses with erythrose-4-phosphate to produce chorismate, the key branch point for phenylalanine, tyrosine, and tryptophan biosynthesis. This seven-step process links carbohydrate metabolism to essential amino acid production.
Why Phosphoenolpyruvate?
PEP, from glycolysis, provides the carbon skeleton foundation. It reacts with erythrose-4-phosphate (from pentose phosphate pathway) via DAHP synthase, initiating shikimate formation leading to chorismate. No other glycolysis intermediate directly serves this role for aromatic amino acids.
Option Analysis
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(A) Phosphoenolpyruvate: Correct, as it directly enters shikimate pathway for chorismate synthesis.
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(B) Pyruvate: Incorrect; pyruvate fuels alanine synthesis and TCA cycle entry, not aromatic amino acids.
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(C) Oxaloacetate: Incorrect; precursor for aspartate family amino acids like lysine, threonine, and asparagine via aspartate.
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(D) α-Ketoglutarate: Incorrect; precursor for glutamate family amino acids including glutamine, proline, and arginine.
| Option | Precursor Role | Amino Acid Family |
|---|---|---|
| (A) Phosphoenolpyruvate | Shikimate pathway entry | Aromatic (Phe, Tyr, Trp) |
| (B) Pyruvate | Alanine, TCA anaplerosis | Alanine |
| (C) Oxaloacetate | Aspartate transamination | Aspartate family |
| (D) α-Ketoglutarate | Glutamate transamination | Glutamate family |
