EPR DETECTION AND CHARACTERIZATION OF LIGNIN PEROXIDASE PORPHYRIN PI-CATION RADICAL

Lignin peroxidase (LiP) from Phanerochaete chrysosporium catalyzes the H2O2 dependent one- and two-electron oxidations of substrates. The ca talytic cycle involves the oxidation of ferric-LiP by H2O2 by two elec trons to compound I,which is an oxoferryl heme and a free radical. It has been speculated that the unpaired electron is in a pi delocalized porphyrin radical. However, no direct evidence for the presence of the free radical has been reported. We present electron paramagnetic reso nance (EPR) detection and characterization of compound I of LiP. The L iP compound I EPR signal is different than those reported previously f or compound I of horseradish peroxidase and chloroperoxidase. However, the EPR signal of compound I of LiP (axial g tensor extending from g( perpendicular to) = 3.42 to g(parallel to) similar to 2) is very simil ar to the EPR signals of compound I of ascorbate peroxidase and catala se from Micrococcus lysodeikticus, in which the radical has been ident ified as a porphyrin pi-cation radical, On the basis of the analysis o f our data and comparison with the earlier published results for compo unds I of other peroxidases, we interpret the LiP compound I signal by a model for exchange coupling between an S = 1 oxyferryl [Fe=O](2+) m oiety and a porphyrin pi-cation radical (S ' = 1/2) [Schulz, C. E., et al. (1979) FEES Lett. 103, 102-105]. The exchange coupling is charact erized by ferromagnetic rather than an antiferromagnetic interaction b etween the two species. The ferric-LiP EPR signal suggests that the ir on in the heme is in near perfect orthogonal symmetry and provides add itional evidence of the ferromagnetic interaction between the oxoferry l iron center and the porphyrin pi-cation radical.