No heme-chlorite adduct in the catalytic cycle of chlorite dismutase: Insight from QM/MM and dynamics calculations.

Chlorite dismutase (Cld) is one of the rare enzymes capable of forming oxygen‑oxygen bonds. Its catalytic cycle of Cld has been proposed to involve coordination of chlorite to the heme active site (Fe-O-Cl-O), followed by cleavage of one of the ClO bonds Fe = O + ClO, subsequent reorientation of the OCl to the ferryl to yield a Fe-O-O-Cl complex, and then eventually release of chloride and molecular oxygen. However, direct experimental confirmation/observation of a chlorite-heme adduct in Cld, or of the proposed reaction intermediates at the heme, is still wanting, except for an unusual pair of radical signals assigned to neighboring aromatic aminoacids. We report here the first QM/MM (quantum mechanical / molecular mechanics) dynamics computational analysis of the proposed Cld catalytic cycle. The results are in line with some previous assessments (including computations of more limited/simplified versions of the enzyme active site) but not with others, in suggesting that no real heme iron-chlorite coordination complex can be observed in Cld, and that the only potential reaction intermediate would be a ferryl species coupled to a radical delocalized on the surrounding functional groups.