| Abstract
| - The structures of Helicobacter pylori (HPC) and Penicillium vitale (PVC) catalases, each withtwo subunits in the crystal asymmetric unit, oxidized with peroxoacetic acid are reported at 1.8 and 1.7 Åresolution, respectively. Despite the similar oxidation conditions employed, the iron−oxygen coordinationlength is 1.72 Å for PVC, close to what is expected for a FeO double bond, and 1.80 and 1.85 Å for HPC,suggestive of a Fe−O single bond. The structure and electronic configuration of the oxoferryl heme andimmediate protein environment is investigated further by QM/MM density functional theory calculations.Four different active site electronic configurations are considered, Por•+−FeIVO, Por•+−FeIVO···HisH+,Por•+−FeIV−OH+ and Por−FeIV−OH (a protein radical is assumed in the latter configuration). The electronicstructure of the primary oxidized species, Por•+−FeIVO, differs qualitatively between HPC and PVC withan A2u-like porphyrin radical delocalized on the porphyrin in HPC and a mixed A1u-like “fluctuating” radicalpartially delocalized over the essential distal histidine, the porphyrin, and, to a lesser extent, the proximaltyrosine residue. This difference is rationalized in terms of HPC containing heme b and PVC containingheme d. It is concluded that compound I of PVC contains an oxoferryl Por•+−FeIVO species with partialprotonation of the distal histidine and compound I of HPC contains a hydroxoferryl Por−FeIV−OH with thesecond oxidation equivalent delocalized as a protein radical. The findings support the idea that there is arelation between radical migration to the protein and protonation of the oxoferryl bond in catalase.
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