| Abstract
| - Catalase-peroxidases (KatGs) are bifunctional heme proteins, belonging to the family of class Iperoxidases, that are able to catalyze both catalatic and peroxidatic reactions within a peroxidase-likestructure. We investigated the electronic structure of reaction intermediates of the catalytic cycle of KatGsby means of density functional theory (DFT) QM/MM calculations. The outcome was that the ionizationstate of the KatG-specific covalent adduct (Met264-Tyr238-Trp111) affects the radical character of compoundI (Cpd I). Specifically, in the optimized structures, substantial radical character is observed on the proximalTrp330 when Tyr238 is protonated, whereas when Tyr238 is deprotonated the radical localizes on theMet+-Tyr(O-)-Trp adduct. These findings are not affected by protein thermal fluctuations, although detailsof the spin density distribution are affected by the geometry of the active site. Calculations provide structuresin good agreement with the crystal structure of BpKatG Cpd I. They also provide an explanation for theexperimental findings of the mobile and catalatic-specific residue Arg426 being 100% in conformation R inthe X-ray structure of BpKatG treated with organic peroxides. The role of different Cpd I forms in the catalaseand peroxidase reaction pathways is discussed.
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