Directional Degradation of Lignocellulose by Phlebia radiata

Abstract

Suggested applications of bio–ligninolytic systems in pulp and paper industry are usually based on selective removal or in situ modification of lignin. For example, it is possible using lignin–degrading white– rot fungi to decrease the amount of energy needed for mechanical pulping, i.e. biopulping, or to facilitate pulp bleaching (Kirk and Chang, 1990; Messner and Srebotnik, 1994; Ragauskas, 2002, Hatakka et al., 2002). Besides wood, annual plants, e.g. cereal straw and grass species contain substantial amounts of cellulose, which could be used as feed for ruminants, as a growth substrate for microorganisms and also as raw material for pulp and paper industry. However, due to the close association between cellulose and other plant cell wall polymers, i.e. lignin and hemicellulose (Eriksson et al., 1990; Daniel, 1994) cellulose is not easily available as a carbon or fiber source unless the structure of lignin is chemically or biologically modified or lignin is partially removed. Microbial delignification may be considered as a noteworthy alternative (Kirk and Farrell, 1987; Hatakka et al., 1989; Lewis and Yamamoto, 1990; Sarikaya and Ladish, 1997; Scott et al., 2002). In these cases a key question is how to break down the connection between lignin and carbohydrates, especially hemicellulose. The knowledge of the critical factors that are involved in the biodegradation of lignin carbohydrate complex (LCC) would help to understand and regulate selective delignification. The white–rot fungi that belong to Basidiomycetes are the most efficient of all known lignin degraders (Eriksson et al., 1990, Hatakka, 2001) but they also degrade cellulose and hemicellulose. Among these fungi Phlebia radiata is a very efficient degrader of lignin (Hatakka and Uusi–Rauva, 1983), it preferentially degrades lignin (Ander and Eriksson, 1977; Hatakka, 1983, 1994) but also was characterized as cellulase (Rogalski et al., 1993; Longa, 1997) and hemicellulase producer (Rogalski et al., 1993a; Tokarzewska– Zadora, 2001). Phlebia. radiata produces directly lignin–modifying enzymes, including lignin peroxidase (ligninase, LiP; EC 1.11.1.14), manganese peroxidase (MnP; EC 1.11.1.13), laccase (EC 1.10.3.2) and dioxygenase (EC 1.13.11), which have been characterized (Niku–Paavola et al., 1988; 1990; Hatakka et al., 1989; Karhunen et al., 1990; Lundell, 1993; Vares et al., 1995; Rogalski et al., 1996). The second group of enzymes involved in lignin degradation does not attack wood components directly. They include aryl alcohol oxidase and glyoxal oxidase providing H2O2 for the peroxidases, and also the feed–back type enzymes as glucose oxidase and cellobiose:quinone oxidoreductase (cellobiose dehydrogenase) that play an important role in joining the metabolic chains during the bioconversion of high– molecular mass wood constituents. All these enzymes Directional Degradation of Lignocellulose by Phlebia radiata