| Sumario: | The structures of Helicobacter pylori (HPC) and Penicillium vitale (PVC) catalases, each with two subunits in the crystal asymmetric unit, oxidized with peroxoacetic acid are reported at 1.8 and 1.7 Å resolution, respectively. Despite the similar oxidation conditions employed, the iron-oxygen coordination length 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 FeO single bond. The structure and electronic configuration of the oxoferryl heme and immediate protein environment is investigated further by QM/MM density functional theory calculations. Four different active site electronic configurations are considered, PoṙFeO, PoṙFeO⋯HisH, PoṙFeOH and PorFeOH (a protein radical is assumed in the latter configuration). The electronic structure of the primary oxidized species, PoṙFeO, differs qualitatively between HPC and PVC with an A-like porphyrin radical delocalized on the porphyrin in HPC and a mixed A-like ‘fluctuating’ radical partially delocalized over the essential distal histidine, the porphyrin, and, to a lesser extent, the proximal tyrosine residue. This difference is rationalized in terms of HPC containing heme b and PVC containing heme d. It is concluded that compound I of PVC contains an oxoferryl PoṙFeO species with partial protonation of the distal histidine and compound I of HPC contains a hydroxoferryl PorFeOH with the second oxidation equivalent delocalized as a protein radical. The findings support the idea that there is a relation between radical migration to the protein and protonation of the oxoferryl bond in catalase.
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