Biodegradation of lignin

Lignin is an aromatic polymer forming up to 30% of woody plant tissues, providing rigidity and resistance to biological attack. Because it is insoluble, chemically complex, and lacking in hydrolysable linkages, lignin is a difficult substrate for enzymatic depolymerization. Certain fungi, mostly basidiomycetes, are the only organisms able to extensively biodegrade it; white-rot fungi can completely mineralize lignin, whereas brown-rot fungi merely modify lignin while removing the carbohydrates in wood. Several oxidative and reductive extracellular enzymes (lignin peroxidase, manganese peroxidase, laccase, and cellobiose:quinone oxidoreductase) have been isolated from ligninolytic fungi; the role of these enzymes in lignin biodegradation is being intensively studied. Enzymatic combustion, a process wherein enzymes generate reactive intermediates, but do not directly control the reactions leading to lignin breakdown, has been proposed as the mechanism of lignin biodegradation. The economic consequences of lignin biodegradation include wood decay and the biogeochemical cycling of woody biomass. Efforts are being made to harness the delignifying abilities of white-rot fungi to aid wood and straw pulping and pulp bleaching. These fungi can also be used to degrade a variety of pollutants in wastewaters and soils, to increase the digestibility of lignocellulosics, and possibly to bioconvert lignins to higher value products. Key words: delignification, white-rot fungi, biobleaching, lignin peroxidase, manganese peroxidase, laccase.