Which enzyme catalyzes the hydroxylation of phenylalanine to tyrosine?

• A. Phenylalanine Hydroxylase
• B. Tyrosine Aminotransferase
• C. Phenylalanine Decarboxylase
• D. Tyrosine Hydroxylase

Answer: (A)

This item tests understanding of the enzyme that adds a hydroxyl group to phenylalanine to form tyrosine, the first step in phenylalanine metabolism. Phenylalanine hydroxylase catalyzes this reaction, using molecular oxygen and the cofactor tetrahydrobiopterin to convert phenylalanine into tyrosine, with dihydrobiopterin as a byproduct. The hydroxylation specifically occurs on the aromatic ring to yield tyrosine, setting up downstream pathways for catecholamines, melanin, and thyroid hormones.

The other enzymes perform different actions: tyrosine hydroxylase modifies tyrosine to make L-DOPA and does not act on phenylalanine; tyrosine aminotransferase transfers an amino group from tyrosine rather than adding a hydroxyl group to phenylalanine; and phenylalanine decarboxylase removes CO2 from phenylalanine to form phenethylamine, also not a hydroxylation of phenylalanine. Defects in the correct enzyme lead to phenylketonuria, highlighting why this specific hydroxylation step is essential.