Oxygen activation in extradiol catecholate dioxygenases - a density functional study

Recent trapping and spectroscopic characterization of an O-2 adduct for the non-heme enzyme homoprotocatechuate 2,3-dioxygenase (HPCD) demonstrates it to be a Fe-III-superoxo species. This proposal is in direct opposition to the consensus mechanism (J. P. Emerson, E. G. Kovaleva, E. R. Farquhar, J. D. Lipscomb and L. Que, Jr., Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 7347-7352) in which the metal facilitates the transfer of electrons from the substrate to O-2 to form the reactive species in the mechanism without changing oxidation state. In this study we performed a detailed analysis of the electronic structure of the O-2 adduct for the mutant and native enzymes and the nature of oxygen activation in the reaction mechanism of HPCD using a combination of computational chemistry and theoretical Mossbauer spectroscopy. Our results are in agreement with the available experimental data and demonstrate that even for the native enzyme changes in the metal oxidation state are an important factor in oxygen activation.