Enzymology of laccases and tyrosinases

Laccases (EC 1.10.3.2) and tyrosinases (EC 1.14.18.1) are copper-containing redox enzymes. They oxidize typically various phenolic compounds and use molecular oxygen as an electron acceptor. They are common enzymes in nature and can be found in plants, mammals, and microbes. Laccases have been studied in numerous applications including pulp bleaching, fibre functionalization, baking, and textile dye decolorization (1). Laccases are currently used in large scale applications e.g. in denim bleaching. The enzyme is commercially available. Tyrosinases have been used in enzymatic synthesis or modification of high-value-added compounds like coumestrol, and L-DOPA. Reports on the use of tyrosinases for grafting of chitosan on proteinous fibres have also been published (2). Laccases contain four tightly coordinated copper atoms in their active site. They oxidize their substrates with a one-electron removal mechanism, which usually results in formation of free radicals. Radicals can start to react non-enzymatically e.g. to form polymers or they can be further oxidized enzymatically (3). The biological role of laccases is related to synthesis or degradation of lignin, pigment production, sporulation and pathogenesis. Most of the studies have been performed with fungal laccases, which usually have very broad substrate specificity and reasonably high redox potential. Tyrosinases have two copper atoms in their active site. Laccases and tyrosinases have been traditionally differentiated on the bases of their substrate specificity and sensitivity to inhibitors, although their substrate specificity do overlap. Tyrosinases are capable of oxidizing various mono- and diphenolic compounds, however, the reaction mechanism in tyrosinase-catalyzed reaction is different from the reaction mechanism of laccases (4). Tyrosinases catalyze ortho-hydroxylation of monophenols and subsequent oxidation of diphenols to quinones. Radicals are not formed in tyrosinase-catalyzed reactions. The physiological role of tyrosinases is related to pigment formation. Plant tyrosinases are involved in enzymatic browning reactions. References 1. Xu, F. (1999) Recent progress in laccase study: properties, enzymology, production, and applications. In The Encyclopedia of Bioprocessing Technology: Fermentation, Biocatalysis and Bioseparation (Flickinger, M.C. & Drew, S.W. eds), pp. 1545-1554. John Wiley & Sons, NY. 2. Sampaio, S., Taddei, P., Monti P., Buchert, J. and Freddi G. (2005) Enzymatic grafting of chitosan onto Bombyx mori silk fibroin: kinetic and IR vibrational studies. J. Biotechnology 116:21-33. 3. Thurston, C. (1994) The structure and function of fungal laccases. Microbiology 140:19-26. 4. Decker H., and Tuczek F. (2000) Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism, TIBS 25: 392-397.