The first and rate limiting step in serotonin (5-HT) biosynthesis is catalyzed by tryptophan hydroxylase. The enzyme requires iron(II), tetrahydrobiopterin, and dioxygen cofactors for the hydroxylation of L-tryptophan to 5-hydroxytryptophan. Tryptophan hydroxylase belongs to a small family of monooxygenases that utilize tetrahydrobiopterins. Other members are phenylalanine hydroxylase and tyrosine hydroxylase (Fitzpatrick, PF, 1999; Walther et al, 2003).
