HYDROGEN PEROXIDE GENERATION BY BACILLUS SUBTILIS OXALATE DECARBOXYLASE IN THE ABSENCE OF SUBSTRATE

Megan Booth and Alexander Angerhofer

Department of Chemistry, University of Florida, Gainesville FL 32611-7200, USA

Oxalate decarboxylase (OxDC) from Bacillus subtilis is a bicupin enzyme possessing a Mn ion in each of its two cupin folds. In 99.8% of all turnovers it acts as a decarboxylase by catalyzing the breakdown of oxalate into formate and CO₂. However, in 0.2% of turnover events the enzyme redirects its chemistry to oxidase activity and generates hydrogen peroxide as a product. The mechanism for this reaction is believed to involve oxygen binding at one of the two Mn centers and subsequent formation of a superoxide radical intermediate when one of these centers is oxidized from Mn²⁺ to Mn³⁺.  The superoxide generated upon oxidation has the potential to convert into hydrogen peroxide.  Current work utilizing the colorimetric ferrous-xylenol orange assay has shown that even without substrate, hydrogen peroxide is still generated at low pH in low concentrations (<1 µM). This suggests that a small percentage of the Mn in the virgin enzyme is already oxidized and produces superoxide before oxalate has even been added.