Polyphenol Oxidase Activity on Guaiacyl and Syringyl Lignin Units

Abstract 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 18 O 2 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.