Comparing Ligninolytic Capabilities of Bacterial and Fungal Dye-Decolorizing Peroxidases and Class-II Peroxidase-Catalases

We aim to clarify the ligninolytic capabilities of dye-decolorizing peroxidases (DyPs) from bacteria and fungi, compared to fungal lignin peroxidase (LiP) and versatile peroxidase (VP). With this purpose, DyPs from Amycolatopsis sp., Thermomonospora curvata, and Auricularia auricula-judae, VP from Pleurotus eryngii, and LiP from Phanerochaete chrysosporium were produced, and their kinetic constants and reduction potentials determined. Sharp differences were found in the oxidation of nonphenolic simple (veratryl alcohol, VA) and dimeric (veratrylglycerol-β- guaiacyl ether, VGE) lignin model compounds, with LiP showing the highest catalytic efficiencies (around 15 and 200 s−1·mM−1 for VGE and VA, respectively), while the efficiency of the A. auricula-judae DyP was 1–3 orders of magnitude lower, and no activity was detected with the bacterial DyPs. VP and LiP also showed the highest reduction potential (1.28–1.33 V) in the rate-limiting step of the catalytic cycle (i.e., compound-II reduction to resting enzyme), estimated by stopped-flow measurements at the equilibrium, while the T. curvata DyP showed the lowest value (1.23 V). We conclude that, when using realistic enzyme doses, only fungal LiP and VP, and in much lower extent fungal DyP, oxidize nonphenolic aromatics and, therefore, have the capability to act on the main moiety of the native lignin macromolecule.

Lignin oxidation mechanisms under oxygen delignification conditions. Part 2: Advanced methods for the detailed characterization of lignin oxidation mechanisms

Advanced analysis methods have been developed to follow the reactions of lignin during alkaline oxygen delignification conditions more comprehensively than before. This aim was attained by monitoring both the lignin macromolecule and the dissolved reaction products. Softwood (SW) and hardwood (HW) kraft spent liquor lignins were studied as substrates under various reaction conditions. The decrease in the contents of different types of free phenolic hydroxyl groups and the concurrent formation of carboxylic acids was followed by 31P NMR of the phosphitylated products. In addition, the formation of acidic degradation products with low molecular weight was determined by capillary zone electrophoresis (CE). This way, it was possible to distinguish the carboxylic acids bound to the lignin macromolecule from the cleaved reaction products, even if they partly co-precipitated during sample preparation. Peak deconvolution was applied to get information on syringyl type phenolic structures and on C(5) condensed guaiacyl structures in hardwood lignin. Pyrolysis-GC/MS was applied to provide additional information about the distribution of guaiacyl/syringyl/p-hydroxyphenyl (G/S/H) type lignin subunits, as well as changes in the phenylpropane side chain.