Electrochemistry and kinetics of fungal laccase mediators

The screening of potential redox mediators for laccase was performed using homogeneous Trametes hirsuta laccase. Heterogeneous (electrochemical) and homogeneous (oxidation by laccase) reactions of the different types of the enhancers (mediators) of the enzyme were investigated. It was discovered that derivatives of phenyl-methyl-pyrazolones and benzoic acid, as well as N-hydroxynaphthalimide were efficient substrates for the laccase. The characterization of several representatives from each class was carried out using electrochemical and enzyme kinetics methods. The kinetic parameters for the oxidation of phenyl-methyl-pyrazolones and 3-(6-hylroxy)-aminobenzoic acid were comparable to those for 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) oxidation by the laccase, whereas the rate of enzymatic oxidation of N-hydroxynaphthalimide was sufficiently lower. Electrochemical experiments demonstrated that only oxidation of phenyl-methyl-pyrazolones and N-hydroxynaphthalimide yielded several high-potential intermediates capable of oxidizing veratryl alcohol, which was used as a lignin model substrate, whereas derivatives of benzoic acid showed low-potential intermediate, which was not able to oxidized lignin model compound. Phenyl-methyl-pyrazolones was about 50% as effective in degrading veratryl alcohol compared to ABTS as judged from HPLC kinetic studies, whereas N-hydroxynaphthalimide showed the same efficiency as ABTS. Phenyl-methyl-pyrazolones and hydroxynaphthalimides may be of commercial interest for oxidoreductase-catalyzed biodegradation of different xenobiotics. (c) 2005 Elsevier B.V. All rights reserved.