DFT and EPR spectroscopic investigation into the effect of carboxylate binding in Bacillus subtilis Oxalate Decarboxylase

Justin Goodsell and Alex Angerhofer

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

Bacillus subtilis Oxalate Decarboxylase (OxDC) catalyzes the decomposition of oxalic acid to carbon dioxide and formate in the presence of oxygen while remaining net redox neutral.  The active site of the protein contains a manganese (II) ion in an octahedral environment with 3 histidines and 1 glutamate coming from the protein, leaving two open valencies which are likely occupied by water molecules in the resting state.  Although the metal center exists in a predominately manganese (II) oxidation state, in the active pH range a pre-catalytic manganese (III) can be detected via parallel mode EPR before addition of substrate.  This signal is highly affected by choice of buffering system. Recent spectroscopic evidence indicates that small carboxylates bind to the active site manganese atom with varied impacts to the EPR spectrum and are proposed to modulate the redox potential of the metal center, allowing catalysis to take place.  DFT calculations were performed in order to compare the predicted redox potentials based on the carboxylate introduced to the valence of the metal.