Molecular biology and structure-function of lignin-degrading heme peroxidases

Abstract

Three peroxidases involved in lignin degradation are produced by white-rot fungi. Lignin peroxidase (LiP) is characterized by oxidation of high redox-potential aromatic compounds (including veratryl alcohol) whereas manganese peroxidase (MnP) requires Mn2+ to complete the catalytic cycle and forms Mn3+ chelates acting as diffusing oxidizers. Pleurotus and Bjerkandera versatile peroxidase (VP) is able to oxidize Mn2+ as well as non-phenolic aromatic compounds, phenols and dyes. Phanerochaete chrysosporium has two gene families including ten LiP-type and three MnP-type genes coding different isoenzymes expressed during secondary metabolism. Two VP genes have been recently cloned from Pleurotus eryngii. Phanerochaete chrysosporium MnP and P. eryngii VP are induced by H2O2, being Mn2+ involved in regulation of their transcript levels. At least eighteen more ligninolytic peroxidase genes have been cloned from other white-rot fungi. Protein sequence comparison reveals that typical MnP from P. chrysosporium and two other fungi (showing a longer C-terminal tail) are separated from other ligninolytic peroxidases, which form two main groups including P. chrysosporium LiP and Pleurotus peroxidases respectively.LiP and MnP crystal structures and VP theoretical molecular models are available. The high redox potential of ligninolytic peroxidases seems related to the distance between heme iron and proximal histidine, and the ability of MnP to oxidize Mn2+ is due to a Mn-binding site formed by three acidic residues near the internal heme propionate. Pleurotus eryngii VP show higher sequence and structural affinities with P. chrysosporium LiP than MnP, but includes a Mn-binding site accounting for its ability to oxidize Mn2+. The functionality of this site was demonstrated by site-directed mutagenesis of MnP and VP. All fungal peroxidases, which exhibit similar topology (11–12 helices) and folding, also include binding sites for two structural Ca2+. Veratryl alcohol was first modeled near LiP heme, but evidence for oxidation at the protein surface via a long-range electron transfer pathway has accumulated. Chemical and site-directed mutagenesis modification confirmed that an exposed tryptophan is involved in veratryl alcohol oxidation however, multiple sites could be responsible for oxidation of different aromatic substrates and dyes by these peroxidases.