Tryptophan is hydroxylated Authored: Jassal, B, 2008-10-01 13:18:42 EC Number: 1.14.16.4 Edited: Jassal, B, 2008-01-08 13:51:41 Pubmed10872454 Pubmed12379098 Pubmed12511643 Reactome Database ID Release 43209828 Reactome, http://www.reactome.org ReactomeREACT_15352 Reviewed: D'Eustachio, P, 2008-05-28 08:30:54 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).