Tetrahydrobiopterin is the reduced form of dihydrobiopterin, 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

Tetrahydrobiopterin is also part of the nitric oxide synthase enzyme.

1. Phenylalanine, Tyrosine, and Tryptophan Biosynthesis

2. Tyrosine Metabolism

3. The Nitric Oxide Home Page