A Dimeric Chlorite Dismutase Exhibits O₂‑Generating Activity and Acts as a Chlorite Antioxidant in <i>Klebsiella pneumoniae</i> MGH 78578

Chlorite dismutases (Clds) convert chlorite to O₂ and Cl^–, stabilizing heme in the presence of strong oxidants and forming the OO bond with high efficiency. The enzyme from the pathogen <i>Klebsiella pneumoniae</i> (<i>Kp</i>Cld) represents a subfamily of Clds that share most of their active site structure with efficient O₂-producing Clds, even though they have a truncated monomeric structure, exist as a dimer rather than a pentamer, and come from Gram-negative bacteria without a known need to degrade chlorite. We hypothesized that <i>Kp</i>Cld, like others in its subfamily, should be able to make O₂ and may serve an <i>in vivo</i> antioxidant function. Here, it is demonstrated that it degrades chlorite with limited turnovers relative to the respiratory Clds, in part because of the loss of hypochlorous acid from the active site and destruction of the heme. The observation of hypochlorous acid, the expected leaving group accompanying transfer of an oxygen atom to the ferric heme, is consistent with the more open, solvent-exposed heme environment predicted by spectroscopic measurements and inferred from the crystal structures of related proteins. <i>Kp</i>Cld is more susceptible to oxidative degradation under turnover conditions than the well-characterized Clds associated with perchlorate respiration. However, wild-type <i>K. pneumoniae</i> has a significant growth advantage in the presence of chlorate relative to a Δ<i>cld</i> knockout strain, specifically under nitrate-respiring conditions. This suggests that a physiological function of <i>Kp</i>Cld may be detoxification of endogenously produced chlorite