The first reaction in this pathway converts phenylalanine to tyrosine, coupled to the conversion of tetrahydrobiopterin to 4a-hydroxytetrahydrobiopterin, catalyzed by phenylalanine hydroxylase. Deficiencies in this enzyme are responsible for the commonest form of phenylketonuria (PKU) in humans. This reaction functions both as the first step in the pathway by which the body disposes of excess phenylalanine, and as a source of the amino acid tyrosine. The next two reactions are responsible for the regeneration of tetrahydrobiopterin from 4a-hydroxytetrahydrobiopterin (Blau et al. 2001).