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 horseradi sh peroxidase (HRP-I) and catalase (CAT-I) have been performed using t he INDO-RHF/Cl method. The goals of these studies were twofold: i) to determine whether the axial ligand of HRP is best described as imidazo le or imidazolate, and ii) to address the long-standing question of wh ether HRP-I and CAT-I are a(1u) and a(2u) pi cation radicals. Only the imidazolate HRP-I model led to a calculated electronic spectra consis tent with the experimentally observed significant reduction in the int ensity of the Soret band compared with the ferric resting state. These results provide compelling evidence for significant proton transfer t o the conserved Asp residue by the proximal histidine. The origin of t he observed reduction of the Soret band intensity in HRP-I and CAT-I s pectra has been examined and found to be caused by the mixing of charg e transfer transitions into the predominantly porphyrin pi-pi transiti ons. For both HRP-I and CAT-I, the a(1u) porphyrin pi cation state is the lowest energy, and it is further stabilized by both the anionic fo rm of the ligand and the porphyrin ring substituents of protoporphyrin -lX. The calculated values of quadrupole-splitting observed in the Mos sbauer resonance of HRP-I and CAT-I are similar for the a(1u) and a(2u ) pi cation radicals. Electronic spectrum of the a(1u) pi cation radic al of HRP-I are more similar to the observed spectra, whereas the spec tra of both a(1u) pi and a(2u) pi cation radicals of CAT-I resemble th e observed spectra. These results also indicate the limitations of usi ng any one observable property to try to distinguish between these sta tes. Taken together, comparison of calculated and observed properties indicate that there is no compelling reason to invoke the higher energ y a(2u) pi cation radical as the favored state in HRP-I and CAT-I. Bot h ground-state properties and electronic spectra are consistent with t he a(1u) pi cation radical.