Resonance Raman Spectroscopic Characterization of Compound III of Lignin Peroxidase

Resonance Raman (RR) spectra of several compounds III of lignin peroxidase (LiP) have been measured at 90 K with Soret and visible excitation wavelengths. The samples include LiPIIIa (or oxyLiP) prepared by oxygenation of the ferrous enzyme, LiPIIIb generated by reaction of the native ferric enzyme with superoxide, LiPIIIc prepared from native LiP plus H₂O₂ followed by removal of excess peroxide with catalase, and LiPIII* made by addition of excess H₂O₂ to the native enzyme. The RR spectra of these four products appear to be similar and, thus, indicate that the environments of these hexacoordinate, low-spin ferriheme species must also be very similar. Nonetheless, the Soret absorption band of LiPIII* is red-shifted by 5 nm from the 414-nm maximum common to LiPIIIa, -b, and -c [Wariishi, H., & Gold, M. H. (1990) J. Biol. Chem. 265, 2070–2077]. Analysis of the iron-porphyrin vibrational frequencies indicates that the electronic structures for the various compounds III are consistent with an Fe^IIIO₂^•− formulation. The spectral changes observed between the oxygenated complex and the ferrous heme of lignin peroxidase are similar to those between oxymyoglobin and deoxymyoglobin. The contraction in the core sizes in compound III relative to the native peroxidase is analyzed and compared with that of other heme systems. EPR spectra confirm that the high-spin ferric form of the native enzyme, with an apparent g = 5.83, is converted into the EPR-silent LiPIII* upon addition of excess H₂O₂. Its magnetic behavior may be explained by antiferromagnetic coupling between the low-spin Fe^III and the superoxide ligand. The Fe-O₂ stretching vibration of the oxygenated peroxidase is observed at 563 cm⁻¹ and shifts to 538 cm⁻¹ with ¹⁸O isotope. The Fe-histidine stretching vibration is observed at 245 cm⁻¹ in ferrous peroxidase and appears to shift to 276 cm⁻¹ in the oxygenated complex.