A Tryptophan Neutral Radical in the Oxidized State of Versatile Peroxidases from Pleurotus Eryngii: A combined Multifrequency EPR and DFT study

Versatile peroxidases are heme enzymes that combine catalytic properties of lignin peroxidases and manganese peroxidases, being able to oxidizeMn2 as well as phenolic and non-phenolic aromatic compounds in the absence of mediators. The catalytic process (initiated by hydrogen peroxide) is the same as in classical peroxidases, with the involvement of 2 oxidizing equivalents and the formation of the so-called Compound I. This latter state contains an oxoferryl center and an organic cation radical that can be located on either the porphyrin ring or a protein residue. In this study, a radical intermediate in the reaction of versatile peroxidase from the ligninolytic fungus Pleurotus eryngii with H2O2 has been characterized by multifrequency (9.4 and 94 GHz) EPR and assigned to a tryptophan residue. Comparison of experimental data and density functional theory theoretical results strongly suggests the assignment to a tryptophan neutral radical, excluding the assignment to a tryptophan cation radical or a histidine radical. Based on the experimentally determined side chain orientation and comparison with a high resolution crystal structure, the tryptophan neutral radical can be assigned to Trp164 as the site involved in long-range electron transfer for aromatic substrate oxidation.