Tryptophan radicals as reaction intermediates in versatile peroxidases: Multifrequency EPR, ENDOR and density functional theory studies

Versatile peroxidases are a new class of ligninolytic enzymes secreted by fungi from the group of white-rot basidiomycetes. Versatile peroxidase (VP) is a structural hybrid between lignin and manganese peroxidase. This hybrid combines the catalytic properties of the two above peroxidases being able to catalyze Mn(II)-dependent and Mn(II)-independent reactions. A long-range electron transfer mechanism has been inferred for the oxidation of big substrate molecules starting from an exposed tryptophan to the heme cofactor. A neutral tryptophan radical has been identified in VP fromBjerkandera adusta andPleurotus eryngii, after H₂O₂ activation and assigned to a specific tryptophan residue using multifrequency electron paramagnetic resonance and electron-nuclear double resonance spectroscopy. Comparative density functional theory calculations were performed for tryptophan neutral and cation radical species, considering also the effect of the polar environment surrounding the radical. The functional role of the radicals is discussed in the context of mechanistic models for VP.