Q.No.28. The reaction involved in the direct conversion of L-phenylalanine to L-tyrosine is
The direct conversion of L-phenylalanine to L-tyrosine occurs through hydroxylation catalyzed by phenylalanine hydroxylase (PAH), a key reaction in amino acid metabolism relevant for CSIR NET Life Sciences. This MCQ tests understanding of the precise biochemical pathway, where option (a) is correct.
Question Breakdown
Q.No. 3: The reaction involved in the direct conversion of L-phenylalanine to L-tyrosine is:
(a) Hydroxylation
(b) Decarboxylation
(c) Transamination
(d) Reduction
Option Analysis
Hydroxylation (Correct): PAH adds a hydroxyl group at the para position of phenylalanine’s benzene ring using O₂ and tetrahydrobiopterin (BH₄) as cofactors, directly yielding tyrosine. This irreversible step is liver-specific and essential for tyrosine synthesis, with deficiency causing phenylketonuria (PKU).
Decarboxylation (Incorrect): Removes the carboxyl group from amino acids, producing amines like phenethylamine from phenylalanine or tyramine from tyrosine. No direct path to tyrosine exists.
Transamination (Incorrect): Transfers the amino group to α-ketoglutarate, forming phenylpyruvate from phenylalanine or p-hydroxyphenylpyruvate from tyrosine. This occurs downstream in tyrosine catabolism, not for direct conversion.
Reduction (Incorrect): Involves gain of electrons or hydrogen, unrelated to phenylalanine-tyrosine conversion. No such enzymatic reduction pathway applies here.
Biochemical Mechanism
PAH (EC 1.14.16.1), an iron-dependent monooxygenase, forms a Fe(IV)=O intermediate after BH₄ oxidation. This hydroxylates phenylalanine, followed by a NIH shift and tautomerization to tyrosine. BH₄ regenerates via dihydrobiopterin reductase.
CSIR NET Relevance
This reaction highlights aromatic amino acid catabolism, glucogenic/ketogenic nature of phenylalanine/tyrosine, and disorders like PKU. Master for metabolism units in exams.
