Oxidative polymerization is based on the oxidative coupling of a monomer to the growing polymer and only continues if the growing chain is also oxidized to a radical, either by a peroxidase or by a peroxidase-generated monomer radical as mediator. Direct oxidation of lignin macromolecule by one class III peroxidase from plant was proved in this study. The elucidation of a unique catalytic mechanism of one key enzyme in lignin synthesis may drive application of engineered plant material for lignocellulose biorefinery processing as well as improve the applicability of the peroxidase/H2O2 catalyst to green polymer chemistry. Expressed as insoluble forms in E. coli, native cationic cell wall peroxidase (CWPO-C) from the poplar tree and mutant variants were successfully reactivated via refolding experiments and used to elucidate the previously presumed existence of an electron transfer (ET) pathway in the CWPO-C structure. Their catalytic properties were fully characterized through various analyses including steady state kinetic, direct oxidation of lignin macromolecules and their respective stabilities during the polymerization reactions. The analysis results proved that the 74th tyrosine residue on the CWPO-C surface plays an important role in catalyzing the macromolecules via supposed ET mechanism. Mutation of Tyr74 residue to tryptophan increased the radical resistance of peroxidase up to 10 times dramatically while maintaining its capability to oxidize lignin macromolecules. Furthermore, extension of poly(catechin) as well as lignin macromolecules with CWPO-C Y74W mutant clearly showed that this radical resistant peroxidase mutant can increase the molecular weight of various kinds of polyphenolics by using surface located active site. The anti-oxidation activity of the synthesized poly(catechin) was also confirmed by xanthine oxidase (XO) assay in this study.

Synthesis of high molecular weight lignin by cationic cell wall peroxidase mutants of poplar containing radical-robust surface active site

245th National Meeting (2013)

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