Volume 10 Part 1 Article 24: Wood Degradation, Phenoloxidases and Chemotaxonomy of Higher Fungi

Volume 10 Part 1 Article 24
Year 1979
Title: Wood Degradation, Phenoloxidases and Chemotaxonomy of Higher Fungi
Author: H.P. Molitoris

Abstract:

Fungi, lacking a photosynthetic apparatus, have to meet their energy requirements by degrading organic materials, primarily those of plant origin like wood. Wood is quantitatively and economically one of our most important natural products. Up to 30% of its dry matter consists of lignin, the most abundant aromatic substance in nature. Lignin is extremely resistant against degradation, and the pathway of its biological breakdown is still largely unknown. This is one of the reasons why we still do not have an economic use for lignin. Therefore, millions of tons of lignin from industry are wasted annually, a fact that our economy cannot much longer afford. Much research is still needed is this field.

It is known that fungi and other microorganisms degrade wood. Since wood-decaying fungi, especially white-rot fungi, which degrade cellulose and lignin, produce large amounts of phenoloxidases (laccase and tyrosinase), for this and other reasons, their participation in wood degradation is postulated (for literature see Rosch, 1967; Kirk, 1971; Kirk and Connors, 1977).

Phenoloxidases oxidize phenols and other substrates with molecular oxygen under the production of water and of colored substances, which polymerize to form melanin-like pigments (Fig. 1) (for literature on distribution, analysis, biochemical and biological function of phenoloxidases see Molitoris, 1976; Kirk and Connors, 1977). Laccase (EC 1.10.3.2, p-diphenol : oxygen oxidoreductase) oxidizes diphenols primarily, with production of free radicals and quinones.

Tyrosinase (EC 1.10.3.1, o-diphenol: oxygen oxidoreductase), is also able to hydroxylate monophenols (cresolase activity) to diphenols, which then are further oxidized to quinones (catecholase activity).

Both phenoloxidases are glycoproteins, contain copper in their active site, and are widespread in animals and plants, especially in fungi. (Bavendamm, 1928; Davidson et al., 1938; Lyr, 1958; Nobles, 1965; Kaarik, 1965). Although having been investigated for a considerable amount of time, the biological function of phenoloxidases is still hypothetical. There is evidence (for literature see: Molitoris, 1976) for their participation in:

1. Wood and lignin degradation.
2. Protective action (e.g. against irradiation, microbial degradation).
3. Detoxification (e.g. of heartwood toxins in wood decay).
4. Morphogenesis (e.g. in fungal fruit body formation).
5. Certain host/parasite relationships.
6. Terminal oxidation.

The results are inconclusive, however, because experiments were conducted with only a few fungi and mostly in vivo. If in vitro experiments were done, then mostly using crude extracts instead of purified enzymes. In addition, the properties of purified laccases were insufficiently known. In order to study the biological function of phenoloxidases, we first had to purify and characterize the pure enzymes. As prerequisites, the synthesis and the conditions for optimal yield had to be investigated.

This paper deals mainly with laccases and reports our experiments on synthesis, characterization and distribution among the fungi of different taxonomie and ecological.groups. Furthermore, certain chemotaxonomic, morphogenetic and genetic aspects of laccases were studied. Finally, experiments on the participation of phenoloxidases in wood and lignin degradation are reported. Our own work and that of other authors on lignin degradation is discussed and summarized in a diagram.

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