Chapter 2 Enzymes of saprotrophic basidiomycetes

Decomposer fungi utilize dead organic matter that is mainly composed of cell wall polysaccharides and other biopolymers. These include cell wall polymers of plant origin (cellulose, hemicelluloses, lignin, pectin), cell wall polysaccharides of fungi (chitin) and nutrient reserve polysaccharide (starch) as well as proteins. Utilization of these polymers necessitates production of extracellular enzymes; the polysaccharide-based biopolymers are usually degraded by hydrolytic enzymes causing endo and/or exocleavage. Lyases and specific oxidases are also produced. Wood-rotting cellulolytic fungi have evolved complex systems of nonenzymatic cellulose cleavage based on the production of reactive oxygen species, but the detailed functioning and relative importance of this decomposition mechanism is still unclear. Lignin decomposition is catalyzed by a set of oxidases and peroxidases with auxiliary enzymes providing hydroxyl radicals, but it also include the provision of enzyme cosubstrates such as organic acids or aryl alcohols. The composition of ligninolytic systems is thus very complex and species-specific. Compared to decomposition of wood, far less is known about basidiomycete species decomposing litter. Some litter-decomposing fungi are apparently physiologically related to ligninolytic wood-rotters but the compositions and regulation of their ligninolytic systems is not so well characterized, and little is known of their enzymology in the natural soil environment. However, it seems clear that litter decomposers are able to degrade lignin as well as cellulose and hemicelluloses and probably also chitin and starch. Their ligninolytic system also plays an important role in the transformation of humic substances including humus formation and mineralization. The main gaps in our current knowledge are in the ecology of enzyme production under natural conditions and in estimating the role of decomposer basidiomycetes in complex biological processes in soils.