Molecular biology of cellulolytic fungi

The synthesis, modification and hydrolysis of carbohydrates by glycosidase enzymes are some of the fundamental activities in nature. Enzymes responsible for these processes are produced across different organisms, genera and species including the kingdom fungi. Together with bacteria, fungi are responsible for the recycling of nature’s recalcitrant polymers such as lignocellulose which is mainly stored in the plant cell walls. The three main components of a plant cell wall are cellulose, hemicellulose and lignin in a percent ratio of about 40:30:30 (Sjöström 1981). White rot fungi are capable of degrading all three polymeric substances, including the polyphenolic lignin, whereas brown rot and soft rot fungi prefer the carbohydrate polymers of cellulose, formed of 13-1,4-linked D-glucopyranose units with no side branches and hemicellulose, of which the backbone structure consists of ß-1,4-linked D-xylopyranosyl units (xylan) or 13-1,4-linked D-mannose and D-glucose units (mannan) with sugar side chains that may be acetylated and/or methylated (reviewed in Tenkanen 1995). Earlier studies of lignocellulose hydrolysis have mainly concentrated on the biochemistry and molecular biology of cellulose degradation. More recently, the enzymology of lignin degradation (reviewed in Leonowicz et al. 1999) and especially molecular studies on the hydrolysis of hemicellulose have advanced considerably (e.g. de Vries et al. 2002). Xylan degradation has been studied in detail with genes and enzymes from Aspergillus (reviewed in de Vries et al. 2002) and lignin degradation with Phanerochaete chrysosporium (reviewed in Cameron et al. 2000). At present, some 20 enzymes involved in the degradation of lignocellulose have been described. In this chapter, we will concentrate on molecular aspects relating to cellulose hydrolysis.