Structure and function of the aromatic amino acid hydroxylases.

INTRODUCTION The three aromatic amino acid hydroxylases, phenylalanine hydroxylase (PheOH; EC 1.14.16.1), tyrosine hydroxylase (TyrOH; EC 1.14.16.2) and tryptophan hydroxylase (TrpOH; EC 1.14.16.4), constitute a family of enzymes that share many physical, structural and catalytic properties. PheOH catalyses the hydroxylation of phenylalanine to tyrosine and is the ratelimiting step in the only pathway to catabolize phenylalanine (for reviews see [1,2]). TyrOH catalyses the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (L-Dopa), which is the ratelimiting step in the biosynthesis of catecholamines (for reviews