title: Reassessment of the reaction mechanism in the heme dioxygenases.
abstract: Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme enzymes that catalyze the O(2)-dependent oxidation of L-tryptophan to N-formyl-kynurenine. Previous proposals for the mechanism of this reaction have suggested that deprotonation of the indole NH group, either by an active-site base or by oxygen bound to the heme iron, as the initial step. In this work, we have examined the activity of 1-Me-L-Trp with three different heme dioxygenases and their site-directed variants. We find, in contrast to previous work, that 1-Me-L-Trp is a substrate for the heme dioxygenase enzymes. These observations suggest that deprotonation of the indole N(1) is not essential for catalysis, and an alternative reaction mechanism, based on the known chemistry of indoles, is presented.
authors.affiliation: Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK.
authors.forename: Nishma
authors.initials: N
authors.lastname: Chauhan
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Thackray
authors.forename: Sara A
authors.initials: SA
authors.lastname: Rafice
authors.forename: Graham
authors.initials: G
authors.lastname: Eaton
authors.forename: Michael
authors.initials: M
authors.lastname: Lee
authors.forename: Igor
authors.initials: I
authors.lastname: Efimov
authors.forename: Jaswir
authors.initials: J
authors.lastname: Basran
authors.forename: Paul R
authors.initials: PR
authors.lastname: Jenkins
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Mowat
authors.forename: Stephen K
authors.initials: SK
authors.lastname: Chapman
authors.forename: Emma Lloyd
authors.initials: EL
authors.lastname: Raven
mesh_terms: D002384:Catalysis; D002625:Chemistry, Organic / Q000379:methods*; D049308:Dioxygenases / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase / Q000737:chemistry; D007211:Indoles / Q000737:chemistry; D007700:Kinetics; D007737:Kynurenine / Q000737:chemistry; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D010100:Oxygen / Q000737:chemistry; D011522:Protons; D014364:Tryptophan / Q000737:chemistry; D014366:Tryptophan Oxygenase / Q000737:chemistry
chemical_list: D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D007211:Indoles; D011522:Protons; D007737:Kynurenine; D006418:Heme; D014364:Tryptophan; D049308:Dioxygenases; D014366:Tryptophan Oxygenase; D010100:Oxygen