Phenylalanine hydroxylase is an enzyme, which is necessary for the catalysis of phenylalanine to tyrosine. This catalysis is realized by hydroxylation of the aromatic side chain of phenylalanine. The enzyme needs tetrahydrobiopterin, BH4 a pteridine cofactor, and a non-heme iron for catalysis. In the reaction, molecular oxygen is cleaved. One oxygen atom is incorporated in BH4. The other oxygen atom is incorporated in phenylalanine. The reaction is shown in the following schema.
This reaction is the bottleneck in the metabolic pathway, which degrades phenylalanine. Mutations in the gene of PAH can cause the metabolomic disorder * phenylketonuria.