Reassessment of the reaction mechanism in the heme dioxygenases.
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.
Chauhan
Nishma
N
Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK.
Thackray
Sarah J
SJ
Rafice
Sara A
SA
Eaton
Graham
G
Lee
Michael
M
Efimov
Igor
I
Basran
Jaswir
J
Jenkins
Paul R
PR
Mowat
Christopher G
CG
Chapman
Stephen K
SK
Raven
Emma Lloyd
EL
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
D050503:Indoleamine-Pyrrole 2,3,-Dioxygenase; D007211:Indoles; D011522:Protons; D007737:Kynurenine; D006418:Heme; D014364:Tryptophan; D049308:Dioxygenases; D014366:Tryptophan Oxygenase; D010100:Oxygen