Polyphenol Oxidase Activity on Guaiacyl and Syringyl Lignin Units

Area of Science:

• Biotechnology and Biochemistry
• Lignin Valorization
• Enzymatic Catalysis

Background:

• Lignin processing yields heterogeneous guaiacyl (G) and syringyl (S) compounds, limiting their use.
• Polyphenol oxidases (PPOs) are enzymes involved in lignin degradation.
• Developing methods to reduce lignin heterogeneity is crucial for its industrial application.

Purpose of the Study:

• To investigate the enzymatic activity of six short PPOs from lignocellulose-degrading ascomycetes on G- and S-type phenolic compounds.
• To explore the potential of PPOs in reducing the heterogeneity and methoxylation degree of lignin-derived compounds.
• To elucidate novel enzymatic reactions catalyzed by PPOs on S-compounds.

Main Methods:

• Screening of six PPOs for activity against G- and S-type phenolic compounds.
• Enzymatic assays using guaiacol, vanillic acid, ferulic acid, syringol, syringic acid, and sinapic acid.
• Isotopic labeling studies with 18O2 to determine reaction mechanisms.
• Investigation of reaction products, including methoxy-ortho-diphenols and methanol.
• Assessment of the effect of reducing agents on PPO-catalyzed reactions.

Main Results:

• All six PPOs catalyzed ortho-hydroxylation of G-type compounds to methoxy-ortho-diphenols.
• A subset of PPOs exhibited activity on S-type compounds, yielding identical methoxy-ortho-diphenols.
• PPOs catalyzed a novel ortho-demethoxylation of S-compounds, producing methanol.
• 18O2 assays confirmed ortho-hydroxylation and ortho-demethoxylation mechanisms for S-compounds.
• Reducing agents steered PPO reactions towards methoxy-ortho-diphenol formation, preventing polymerization.

Conclusions:

• PPOs effectively reduce heterogeneity in G- and S-type lignin-derived compounds.
• A novel PPO-catalyzed ortho-demethoxylation of S-compounds was discovered.
• PPO application offers new pathways for lignin valorization and the production of plant-based chemicals and materials.