Dihydrobiopterin is the oxidized form of tetrahydrobiopterin, an electron carrier in tyrosine biosynthesis (Figure 21.18). Tyrosine is only aromatic amino acid made in animals. The reaction is catalyzed by phenylalanine hydroxylase.

Phenylalanine + Dihydrobiopterin + O2 <=> Tyrosine + Tetrahydrobiopterin + H2O

Deficiency of phenylalanine hydroxylase is responsible for phenylketonuria (PKU), an autosomal recessive disease that results in production of too much phenylalanine, due to the block in synthesis of tyrosine.

1. Phenylalanine, Tyrosine, and Tryptophan Biosynthesis

2. Tyrosine Metabolism