| Sumario: | Indoleamine 2,3-dioxygenase (IDO) and tryptophan 23-dioxygenase (TDO) are heme-containing dioxygenases and catalyze oxidative cleavage of the pyrrole ring of L-tryptophan. On the basis of three recent crystal structures of these heme-containing dioxygenases, two new mechanistic pathways were proposed by several groups. Both pathways start with electrophilic addition of the Fe(ll)-bound dioxygen concerted with proton transfer (oxygen ene-type reaction), followed by either formation of a dioxetane intermediate or Criegee-type rearrangement. However, density functional theory (DFT) calculations do not support the proposed concerted oxygen ene-type and Criegee-type rearrangement pathways. On the basis of DFT calculations, we propose a new mechanism for dioxygen activation in these heme systems. The mechanism involves (a) direct electrophilic addition of the Fe(ll)-bound oxygen to the C2 or C3 position of the indole in a closed-shell singlet state or (b) direct radical addition of the Fe(lll)-superoxide to the 02 position of the indole in a triplet (or open-shell singlet) state. Then, a radical-recombination or nearly barrierless charge-recombination step from the resultant diradical or zwitterionic intermediates, respectively, proceeds to afford metastable dioxetane intermediates, followed by ring-opening of the dioxetanes. Alternatively, homolytic 0-0 bond cleavage from the diradical intermediate followed by oxo attack and facile C2-C3 bond cleavage could compete with the dioxetane formation pathway. Effects of ionization of the imidazole and negatively charged oxyporphyrin complex on the key dioxygen activation process are also studied.
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