Reassessment Of The Reaction Mechanism In The Heme Dioxygenases

摘要

Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxy-genase (TDO) are heme enzymes that catalyze oxidation of L-tryptophan to N-formyl-kynurenine (Scheme 1), in a mechanism that involves binding of dioxygen to reduced iron. The mechanistic details of this oxidation are not yet known, but early studies3 suggested base-catalyzed deprotonation of the indole NH group (Scheme 2A). This was based largely on the observation that 1-methyl-L-tryptophan (1-Me-L-Trp) is an inhibitor of dioxygenase activity and was plausible since base-catalyzed abstraction is not possible with the methylated compound. There are problems with this mechanism, however. To begin with, it is inconsistent with the chemistry of indoles, which typically react by electrophilic addition across the C_3 position and subsequent formation of a cation at N_1. In addition, the structure of human IDO (hIDO) reveals that there is no active-site base close enough for proton abstraction at N_1. The only polar active-site residue is Ser167, but this is not essential for activity. Although X. campestris TDO (XcTDO) does contain an active-site histidine (His55, equivalent to Ser167 in hIDO) which hydrogen bonds to the indole N_1, it is not essential for activity. Together, this led to the hypothesis that the bound dioxygen might act as the active-site base (Scheme 2B), with no involvement from active-site residues.