Nitriles are prized motifs in medicines and natural products, yet enzymatic synthesis routes are extremely scarce. The cyanobacterial enzyme AetD can install a −CN on 5,7-dibromo-l-tryptophan via an oxidative rearrangement with nitrogen migration and C−C/C−N cleavage. To elucidate the catalytic mechanism, we constructed the computational reactant model of AetD based on its crystal structure and performed extensive QM/MM calculations. Our results revealed that the dioxygen activation proceeds by spin crossover and direct O−O cleavage to form a diiron(IV,IV)-oxo species. This powerful oxidant affects hydrogen atom transfer (HAT)-triggered, proton-coupled electron transfer (PCET)-driven aziridine formation: Fe1(IV)O abstracts the Cβ−H and Fe2(IV)O abstracts the N−H via PCET, leading to ring closure. The aziridine intermediate further undergoes H abstraction and a diiron-mediated PCET to form an azirine, whose ring then opens by homolytic Cα−N scission accompanied by proton transfer from metal (Fe1) to the substrate. The following OH rebound to Cα and heterolytic Cα−Cβ cleavage produce 5,7-dibromo-indole-3-carbonitrile. The three H abstractions and Cα−Cβ scission are necessary for the N migration, and each of these steps corresponds to a barrier of 20−23 kcal/mol. Overall, the diiron site functions as (i) an HAT/PCET oxidant, (ii) an ET relay station during PCET and Cα−N homolysis, and (iii) a Lewis-acidic catalyst for OH rebound and proton relays. These findings provide a detailed mechanistic picture of AetD catalysis and highlight the cooperative redox between the two iron centers, offering mechanistic insights applicable to the design of biomimetic catalysts for nitrile synthesis.
Zhang et al. (Wed,) studied this question.