Oxidation of a non-phenolic lignin model compound by two manganese peroxidases: evidence for implication of carboxylate and radicals.

BACKGROUND

Manganese peroxidase is one of the Class II fungal peroxidases that are able to oxidize the low redox potential phenolic lignin compounds. For high redox potential non-phenolic lignin degradation, mediators such as GSH and unsaturated fatty acids are required in the reaction. However, it is not known whether carboxylic acids are a mediator for non-phenolic lignin degradation.

RESULTS

The white rot fungus is one of the most potent fungi in degradation of lignocellulose and xenobiotics. Two manganese peroxidases (MnP1 and MnP2) from CD2 were over-expressed in and successfully refolded from inclusion bodies. Both MnP1 and MnP2 oxidized the phenolic compounds efficiently. Surprisingly, they could degrade veratryl alcohol, a non-phenolic lignin compound, in a Mn-dependent fashion. Malonate or oxalate was found to be also essential in this degradation. The oxidation of non-phenolic lignin was further confirmed by analysis of the reaction products using LC-MS/MS. We proved that Mn and a certain carboxylate are indispensable in oxidation and that the radicals generated under this condition, specifically superoxide radical, are at least partially involved in lignin oxidative degradation. MnP1 and MnP2 can also efficiently decolorize dyes with different structures.

CONCLUSIONS

We provide evidence that a carboxylic acid may mediate oxidation of non-phenolic lignin through the action of radicals. MnPs, but not LiP, VP, or DyP, are predominant peroxidases secreted by some white rot fungi such as and the selective lignocellulose degrader . Our finding will help understand how these fungi can utilize MnPs and an excreted organic acid, which is usually a normal metabolite, to efficiently degrade the non-phenolic lignin. The unique properties of MnP1 and MnP2 make them good candidates for exploring molecular mechanisms underlying non-phenolic lignin compounds oxidation by MnPs and for applications in lignocellulose degradation and environmental remediation.