Laccase-Catalyzed Oxidation of Mn 2+ in the Presence of Natural Mn 3+ Chelators as a Novel Source of Extracellular H 2 O 2 Production and Its Impact on Manganese Peroxidase

Abstract

A purified and electrophoretically homogeneous blue laccase from the litter-decaying basidiomycete Stropharia rugosoannulata with a molecular mass of approximately 66 kDa oxidized Mn 2+ to Mn 3+ , 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, Mn 3+ complexes were produced at 0.15, 0.05, and 0.10 μmol/min/mg of protein, respectively. Concomitantly, application of oxalate and malonate, but not pyrophosphate, led to H 2 O 2 formation and tetranitromethane (TNM) reduction indicative for the presence of superoxide anion radical. Employing oxalate, H 2 O 2 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 Mn 2+ oxidation and abiotic decomposition of these organic chelators by the resulting Mn 3+ , which leads to formation of superoxide and its subsequent reduction to H 2 O 2 . A partially purified manganese peroxidase (MnP) from the same organism did not produce Mn 3+ complexes in assays containing 1 mM Mn 2+ and 100 mM oxalate or malonate, but omitting an additional H 2 O 2 source. However, addition of laccase initiated MnP reactions. The results are in support of a physiological role of laccase-catalyzed Mn 2+ oxidation in providing H 2 O 2 for extracellular oxidation reactions and demonstrate a novel type of laccase-MnP cooperation relevant to biodegradation of lignin and xenobiotics.