Abstract
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.
Highlights
Lignin is a complex polymer formed in the cell wall of vascular plants by radical coupling of three main and several additional aromatic precursors [1]
Bacterial AspDyP2 and TcuDyP, and fungal AauDyP, PerVPL and PchLiPA were produced in Escherichia coli and their kinetic constants on simple and dimeric lignin models, and the Eo 0 values of redox couples in their catalytic cycles were estimated under standard and comparable conditions
Dye-decolorizing peroxidases (DyPs) are an old enzyme family shared by archaea, bacteria and fungi
Summary
Lignin is a complex polymer formed in the cell wall of vascular plants by radical coupling of three main and several additional aromatic precursors [1]. With a total production of 100 million tons per year and an estimated annual growth around 2.2% [3], technical lignins can be used as renewable feedstocks to obtain addedvalue chemical products [4] In this way, the production of aromatic compounds from lignin exploitation could pump the use of biofuels as an economically sustainable alternative to fossil fuels, and broad the products portfolio of the cellulose industry in agreement with the bio-refinery concept [5]. The production of aromatic compounds from lignin exploitation could pump the use of biofuels as an economically sustainable alternative to fossil fuels, and broad the products portfolio of the cellulose industry in agreement with the bio-refinery concept [5] With this purpose, it is necessary to break down the recalcitrant lignin polymer, which has naturally evolved to confer plants resistance towards physical and microbial agents [6]. For these reasons, studying the specialized organisms being able to cause lignin decay in nature is one of the clues in obtaining efficient biocatalysts for lignin transformation in plant biorefineries [7]
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