Theoretical study of model compound I complexes of horseradish peroxidase and catalase

Theoretical studies of the electronic structure and spectra of models for the ferric resting state and Compound I intermediates of horseradish peroxidase (HRP-I) and catalase (CAT-I) have been performed using the INDO-RHF/CI method. The goals of these studies were twofold: i) to determine whether the axial ligand of HRP is best described as imidazole or imidazolate, and ii) to address the long-standing question of whether HRP-I and CAT-I are a1u and a2u tau cation radicals. Only the imidazolate HRP-I model led to a calculated electronic spectra consistent with the experimentally observed significant reduction in the intensity of the Soret band compared with the ferric resting state. These results provide compelling evidence for significant proton transfer to the conserved Asp residue by the proximal histidine. The origin of the observed reduction of the Soret band intensity in HRP-I and CAT-I spectra has been examined and found to be caused by the mixing of charge transfer transitions into the predominantly porphyrin tau-tau transitions. For both HRP-I and CAT-I, the a1u porphyrin tau cation state is the lowest energy, and it is further stabilized by both the anionic form of the ligand and the porphyrin ring substituents of protoporphyrin-IX. The calculated values of quadrupole-splitting observed in the Mossbauer resonance of HRP-I and CAT-I are similar for the a1u and a2u tau cation radicals. Electronic spectrum of the a1u tau cation radical of HRP-I are more similar to the observed spectra, whereas the spectra of both a1u tau and a2u tau cation radicals of CAT-I resemble the observed spectra. These results also indicate the limitations of using any one observable property to try to distinguish between these states. Taken together, comparison of calculated and observed properties indicate that there is no compelling reason to invoke the higher energy a2u tau cation radical as the favored state in HRP-I and CAT-I. Both ground-state properties and electronic spectra are consistent with the a1u tau cation radical.