Dye-Decolorizing Peroxidases Maintain High Stability and Turnover on Kraft Lignin and Lignocellulose Substrates

Fungal enzyme systems for the degradation of plant cell wall lignin, consisting of, among others, laccases and lignin-active peroxidases, are well characterized. Additionally, fungi and bacteria contain dye-decolorizing peroxidases (DyP), which are also capable of oxidizing and modifying lignin constituents. Studying DyP activity on lignocellulose poses challenges due to the heterogeneity of the substrate and the lack of continuous kinetic methods. In this study, we report the kinetic parameters of bacterial DyP from Amycolatopsis 75iv2 and fungal DyP from Auricularia auricula-judae on insoluble plant materials and kraft lignin by monitoring the depletion of the cosubstrate of the peroxidases with a H₂O₂ sensor. In the reactions with spruce, both enzymes showed similar kinetics. On kraft lignin, the catalytic rate of bacterial DyP reached 30 ± 2 s^-1, whereas fungal DyP was nearly 3 times more active (81 ± 7 s^-1). Importantly, the real-time measurement of H₂O₂ allowed the assessment of continuous activity for both enzymes, revealing a previously unreported exceptionally high stability under turnover conditions. Bacterial DyP performed 24,000 turnovers of H₂O₂, whereas the fungal DyP achieved 94,000 H₂O₂ turnovers in 1 h with a remaining activity of 40 and 80%, respectively. Using mass spectrometry, the depletion of the cosubstrate H₂O₂ was shown to correlate with product formation, validating the amperometric method.