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Peroxidase-Type Reactions Suggest a Heterolytic/Nucleophilic O–O Joining Mechanism in the Heme-Dependent Chlorite Dismutase

Mayfield, Jeffrey A., Blanc, Béatrice, Rodgers, Kenton R., Lukat-Rodgers, Gudrun S., DuBois, Jennifer L.
Biochemistry 2013 v.52 no.40 pp. 6982-6994
Dechloromonas, biochemistry, cations, chemical bonding, heme, hydrogen peroxide, pH, peracetic acid
Heme-containing chlorite dismutases (Clds) catalyze a highly unusual O–O bond-forming reaction. The O–O cleaving reactions of hydrogen peroxide and peracetic acid (PAA) with the Cld from Dechloromonas aromatica (DaCld) were studied to better understand the Cl–O cleavage of the natural substrate and subsequent O–O bond formation. While reactions with H₂O₂ result in slow destruction of the heme, at acidic pH heterolytic cleavage of the O–O bond of PAA cleanly yields the ferryl porphyrin cation radical (compound I). At alkaline pH, the reaction proceeds more rapidly, and the first observed intermediate is a ferryl heme. Freeze-quench EPR confirmed that the latter has an uncoupled protein-based radical, indicating that compound I is the first intermediate formed at all pH values and that radical migration is faster at alkaline pH. These results suggest by analogy that two-electron Cl–O bond cleavage to yield a ferryl-porphyrin cation radical is the most likely initial step in O–O bond formation from chlorite.