A Dimeric Chlorite Dismutase Exhibits O2-Generating Activity and Acts as a Chlorite Antioxidant in Klebsiella pneumoniae MGH 78578

摘要

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 Klebsiella pneumoniae (KpCld) 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 KpCld, like others in its subfamily, should be able to make O2 and may serve an in vivo 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 crystalstructures of related proteins. KpCld is more susceptibleto oxidative degradation under turnover conditions than the well-characterizedClds associated with perchlorate respiration. However, wild-type K. pneumoniae has a significant growth advantage in thepresence of chlorate relative to a Δcld knockoutstrain, specifically under nitrate-respiring conditions. This suggeststhat a physiological function of KpCld may be detoxificationof endogenously produced chlorite.