Heme dioxygenases catalytic enzymes

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

Actually, it is noteworthy that there are examples of heme-containing dioxygenases that bind 02 with formation of a heme—Fe(II)—02 complex. This is the case of indolamine-2,3-dioxygenase (EC 1.13.11.11) and tryptophan-2,3-dioxygenase, which catalyze the insertion of 02 into L-tryptophan to yield A-for-mylkynurenine [21], The catalytic cycle involves the ternary complex L-trypto-phan-iron(II) enzyme-02 as an active intermediate. In this ternary complex, which yields A-formylkynurenine and the Fe(II) enzyme, 02 and/or the substrate is activated [21], a situation clearly different from that found for PGHS. 

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