Polyphenol Oxidase Activity on Guaiacyl and Syringyl Lignin Units

The natural heterogeneity of guaiacyl (G) and syringyl (S) compounds resulting from lignin processing hampers their direct use as plant-based chemicals and materials. Herein, we explore six short polyphenol oxidases (PPOs) from lignocellulose-degrading ascomycetes for their capacity to react with G-type and S-type phenolic compounds. All six PPOs catalyze the ortho-hydroxylation of G-type compounds (guaiacol, vanillic acid, and ferulic acid), forming the corresponding methoxy-ortho-diphenols. Remarkably, a subset of these PPOs is also active towards S-compounds (syringol, syringic acid, and sinapic acid) resulting in identical methoxy-ortho-diphenols. Assays with 18O2 demonstrate that these PPOs in particular catalyze ortho-hydroxylation and ortho-demethoxylation of S-compounds and generate methanol as a co-product. Oxidative (ortho−) demethoxylation of S-compounds is a novel reaction for PPOs, which we propose occurs by a distinct reaction mechanism as compared to aryl-O-demethylases. We further show that addition of a reducing agent can steer the PPO reaction to form methoxy-ortho-diphenols from both G- and S-type substrates rather than reactive quinones that lead to unfavorable polymerization. Application of PPOs opens for new routes to reduce the heterogeneity and methoxylation degree of mixtures of G and S lignin-derived compounds.

Bibliographic Details

Main Authors:	de O.G. Silva, Caio, Sun, Peicheng, Barrett, Kristian, Sanders, Mark G., van Berkel, Willem J.H., Kabel, Mirjam A., Meyer, Anne S., Agger, Jane W.
Format:	Article/Letter to editor biblioteca
Language:	English
Subjects:	Life Science,
Online Access:	https://research.wur.nl/en/publications/polyphenol-oxidase-activity-on-guaiacyl-and-syringyl-lignin-units
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