Laccase-catalyzed oxidation of Mn2+ in the presence of natural Mn3+ chelators as a novel source of extracellular H2O2 production and its impact on manganese peroxidase

A purified and electrophoretically homogeneous blue laccase from the litter-decaying basidiomycete Stropharia rugosoannulata with a molecular mass of approximately 66 kDa oxidized Mn2+ to Mn3+, as assessed in the presence of the Mn chelators oxalate, malonate, and pyrophosphate. At rate-saturating concentrations (100 mM) of these chelators and at pH 5.0, Mn3+ complexes were produced at 0.15, 0.05, and 0.10 mumol/min/mg of protein, respectively. Concomitantly, application of oxalate and malonate, but not pyrophosphate, led to H2O2 formation and tetranitromethane (TNM) reduction indicative for the presence of superoxide anion radical. Employing oxalate, H2O2 production, and TNM reduction significantly exceeded those found for malonate. Evidence is provided that, in the presence of oxalate or malonate, laccase reactions involve enzyme-catalyzed Mn2+ oxidation and abiotic decomposition of these organic chelators by the resulting Mn3+, which leads to formation of superoxide and its subsequent reduction to H2O2. A partially purified manganese peroxidase (MnP) from the same organism did not produce Mn 31 complexes in assays containing 1 mM Mn2+ and 100 mM oxalate or malonate, but omitting an additional H2O2 source. However, addition of laccase initiated MnP reactions. The results are in support of a physiological role of laccase-catalyzed Mn2+ oxidation in providing H2O2 for extracellular oxidation reactions and demonstrate a novel type of laccase-MnP cooperation relevant to biodegradation of lignin and xenobiotics.