## Cofactor Error in Phenylalanine Hydroxylation **Key Point:** Phenylalanine hydroxylase uses **tetrahydrobiopterin (BH₄)**, NOT tetrahydrofolate, as its essential cofactor. ### Correct Cofactor Mechanism Phenylalanine hydroxylase catalyzes: $$\text{L-Phenylalanine} + \text{BH}_4 + O_2 \rightarrow \text{L-Tyrosine} + \text{dihydropteridine}$$ - **Tetrahydrobiopterin (BH₄)** is a pteridine cofactor that donates electrons - Tetrahydrofolate is involved in one-carbon transfer reactions (methylation, nucleotide synthesis), not hydroxylation - Deficiency of BH₄ synthesis or regeneration causes **atypical phenylketonuria** (variants 2–6) ### Why Other Options Are Correct | Statement | Validity | Explanation | |-----------|----------|-------------| | Tyrosine → catecholamines, thyroid hormones, melanin | ✓ Correct | Tyrosine is the precursor for dopamine, norepinephrine, epinephrine, T₃/T₄, and melanin | | PKU from phenylalanine hydroxylase deficiency | ✓ Correct | Classic PKU (variant 1) is autosomal recessive; accumulation causes intellectual disability if untreated | | Tyrosine conditionally essential | ✓ Correct | When phenylalanine intake is severely restricted, tyrosine must be supplied exogenously | **High-Yield:** BH₄ deficiency causes a small percentage of neonatal screening-positive cases with normal phenylalanine hydroxylase activity — a classic NEET PG trap.
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