Laccase Role and Function in Lignin Biodegradation

Laccases classified as polyphenoloxidases are now widely accepted as p-diphenoloxidases (EC 1.10.3.2) (Tuor et al. 1995) and are widespread. The enzyme is found in many plant species (Mayer and Harel 1979) and is widely distributed in fungi (Mayer 1987) including wood-rotting fungi where it is often associated with lignin peroxidase, manganese dependent peroxidase, or both (Nerud and Misurcova 1996; Tuor et al. 1995). They are remarkably nonspecific as to reducing substrates and show much in common with another copper-containing oxidase, tyrosinase (monophenol mono-oxygenase; EC 1.14.18.1).

Laccases are blue copper oxidases and participate in electron transfer in biological systems by catalyzing the four-electron reduction of dioxygen to water with the simultaneous oxidation of organic substrates (Reinhammar and Malmstrom 1981; Thurston 1994). A minimum of four copper atoms, distributed in three spectroscopically distinct binding sites, appears necessary to facilitate efficient catalysis. The Type I (blue) copper center has a strong absorbance near 600nm and gives rise to the enzyme's blue color. The other two copper centers known as the Type II (normal) and the Type III involve a pair of magnetically coupled cupric ions (Farver and Pecht 1981; Li et al. 1992; Morpurgo et al. 1993). Studies into the mechanism of the reduction of O2 to H2O by laccases have shown that a bridge between the coupled binuclear center and the Type II center defines a trinuclear cluster as the active site (Solomon 1988).

Plant laccase mechanism and function has been investigated predominantly on Rhus vernicifera. It is thought that the enzyme has a protective function, possibly causing formation of a natural polyphenolic polymerisate in the case of tree damage (Reinhammar and Malmstrom 1981). A plant laccase has also been found to be involved in lignification (Bao et al. 1993) and the role of laccase in lignification has been reviewed extensively by O'Malley et al. (1993).

Fungal laccases oxidize phenols and phenolic substructures of lignin with subsequent polymerization or depolymerization (Higuchi 1989). These pathways are considered to proceed via phenoxy radicals of the phenolic units. The role of laccase in lignin biodegradation remains largely unresolved, since strains of the extensively studied white-rot fungus, P. chrysosporium, and several other Basidiomycetes are active in lignin degradation but do not produce laccase under the ligninolytic conditions employed in laboratory studies (Nerud and Misurcova 1996; Thurston 1994; Tuor et al. 1995).

In some fungi, laccase has a function seemingly unrelated to ligninolysis. For example, in Aspergillus nidulans, the enzyme appears to be essential for the synthesis of the spore pigments (Clutterbuck 1972).

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