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Oxygen Activation during Oxidation of Methoxyhydroquinones by Laccase from Pleurotus eryngii

American Society for Microbiology
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Oxygen activation during oxidation of the lignin-derived hydroquinones 2-methoxy-1,4-benzohydroquinone (MBQH2) and 2,6-dimethoxy-1,4-benzohydroquinone (DBQH2) by laccase from Pleurotus eryngii was examined. Laccase oxidized DBQH2 more efficiently than it oxidized MBQH2; both the affinity and maximal velocity of oxidation were higher for DBQH2 than for MBQH2. Autoxidation of the semiquinones produced by laccase led to the activation of oxygen, producing superoxide anion radicals (Q·− + O2 ↔ Q + O2·−). As this reaction is reversible, its existence was first noted in studies of the effect of systems consuming and producing O2·− on quinone formation rates. Then, the production of H2O2 in laccase reactions, as a consequence of O2·− dismutation, confirmed that semiquinones autoxidized. The highest H2O2 levels were obtained with DBQH2, indicating that DBQ·− autoxidized to a greater extent than did MBQ·−. Besides undergoing autoxidation, semiquinones were found to be transformed into quinones via dismutation and laccase oxidation. Two ways of favoring semiquinone autoxidation over dismutation and laccase oxidation were increasing the rate of O2·− consumption with superoxide dismutase (SOD) and recycling of quinones with diaphorase (a reductase catalyzing the divalent reduction of quinones). These two strategies made the laccase reaction conditions more natural, since O2·−, besides undergoing dismutation, reacts with Mn2+, Fe3+, and aromatic radicals. In addition, quinones are continuously reduced by the mycelium of white-rot fungi. The presence of SOD in laccase reactions increased the extent of autoxidation of 100 μM concentrations of MBQ·− and DBQ·− from 4.5 to 30.6% and from 19.6 to 40.0%, respectively. With diaphorase, the extent of MBQ·− autoxidation rose to 13.8% and that of DBQ·− increased to 39.9%.


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