Structure of the Active Site of Lignin Peroxidase Isozyme H2: Native Enzyme, Compound III, and Reduced Form

The wood-degrading fungus Phanerochaete chrysosporium secretes a number of extracellular enzymes called lignin peroxidases which are involved in the degradation of both lignin and a number of persistent environmental pollutants. Lignin peroxidase isozyme H2, a glycosylated protein of approximately 40 kDa, contains a single heme. X-ray absorption spectroscopy (XAS) has been used to probe the local environment of the iron in the active site of resting enzyme, reduced enzyme, and compound III. For the native and reduced forms, respectively, the average Fe-pyrrole nitrogen distances are 2.055 and 2.02 Å (±0.015 Å); the Fe-proximal nitrogen distance is 1.93 and 1.91 Å (±0.02 Å) while the Fe-distal ligand distance is 2.17 and 2.10 Å (±0.03 Å). Although the results are not as well-defined, the active-site structure of compound III is largely 2.02 ± 0.015 Å for the average Fe-pyrrole nitrogen distance, 1.90 ± 0.02 for the Fe-proximal nitrogen, and 1.74 ± 0.03 Å for the Fe-distal ligand distance. The heme iron-pyrrole nitrogen distnce is more expanded in ligninase H2 than in other peroxidases. The possible significance of this is discussed in relation to other heme proteins.