Biomodification of lignin compounds by Abortiporus biennis and Phanerochaete chrysosporium and investigation of related enzymes by transcriptomic analysis

Abstract

The objective of the present study was to understand biomodification mechanism of lignin compounds by two white rot basidiomycetes with different enzyme system, Abortiporus biennis and Phanerochaete chrysosporium. Based on this understanding, for lignin degradation by white rot basidiomycetes, changes of enzyme system and culture condition were applied. Finally, it was investigated what kinds of enzymes were involved in degradation of lignin compounds by transcriptomic analysis. In this study, biomodification products of monolignols and synthetic lignin composed of monolignols were examined for inferring accurate biomodification mechanism. As a result of analysis of biomodification of monolignols by white rot basidiomycetes, A. biennis and P. chrysosporium leads to not only degradation but also polymerization of monolignols. To degrade monolignols, the addition of ascorbic acid prevented a drastic increase of the molecular weight of monolignols, furthermore, various degraded products including acid compounds were formed from monolignols by A. biennis and P. chrysosporium. Based on these results, biomodification products of synthetic lignin were examined and changes of enzyme system of basidiomycetes for lignin degradation under the ligninolytic condition were evaluated. At first, A. biennis degraded synthetic lignin during initial incubation day (5-10 days), thereafter polymerized them. This phenomenon was occurred in case of monolignols. Therefore, the optimal conditions for lignin degradation by A. biennis were examined through the changes of enzyme system and culture condition. Ligninolytic enzyme system of A. biennis was based on MnP-laccase system, thus enzyme system of A. biennis was changed by addition of laccase mediator. As a result, when both enzyme system and surrounding conditions were simultaneously changed by addition of laccase mediator and reducing agents, whole cell of A. biennis assisted considerable degradation of synthetic lignin. To verify correlation between lignin degradation and complex enzyme system of whole cell of A. biennis, transcriptomic analysis was carried out. As a result, genes of laccase were highly expressed as expected. Upregulation of various extracellular enzymes such as MnP and aryl alcohol oxidase were observed in A. biennis exposed to DHP. These results suggested strongly involvement of various extracellular enzymes functioning in bond cleavage in lignin degradation. Catalytic enzyme system of P. chrysosporium is controlled by two peroxidases, LiP and MnP. Based on this catalytic system, P. chrysosporium also induced degradation and polymerization of synthetic lignin. To degrade synthetic lignin under the ligninolytic treatment, reducing agents, ascorbic acid and α-tocopherol during oxidative reaction were used for stabilizing unstable radicals generated. Contrary to the result of A. biennis, it was noteworthy that P. chrysosporium with two reducing agents produced aromatic compounds (syringic acid and 2,6-dimethoxybenzodiol) and succinic acid as well as degraded lignin polymer. Transcriptomic analysis of P. chrysosporium provided information about various enzymes related to lignin degradation and aromatic catabolic pathway. Consequently, extracellular catalytic system of P. chrysosporium attacked synthetic lignin, resulting in production of aromatic compounds derived from lignin molecules. Thereafter, aromatic compounds were metabolized in short-cut TCA cycle of P. chrysosporium and were finally converted to succinic acid. In conclusion, two different enzyme systems of A. biennis and P. chysosporium produced different degradation products. MnP-LAC complex system of A. biennis induced production of depolymerized lignin polymer. On the other hand, P. chysosporium with metabolic cycle related-enzymes preferentially catalyzed production of valuable lignin derived-chemical, succinic acid. Consequently, two basidiomycetes were suitable for lignin degradation with change of enzyme system and culture condition, so they can be used as novel biocatalysts for lignin application and biotechnological application of basidiomycetes.