Microbial Degradation of Lignin-Carbohydrate Complexes
White-rot fungi are the most efficient lignin degraders in nature and they play a key role in carbon recycling on Earth. In this chapter, the free-radical chemistry of white-rot fungi is reviewed in order to understand their potential role in the breakdown of lignin-carbohydrate complexes (LCCs). White-rot fungi secrete three classes of extracellular ligninolytic enzymes: one phenol oxidase; (1) laccase (Lac) and two heme-containing peroxidases; (2) lignin peroxidase (LiP) and (3) manganese peroxidase (MnP). LiP is capable of oxidizing phenolic and nonphenolic lignin substructures directly, while the majority of Lac and MnP enzymes cannot directly oxidize nonphenolic lignin substructures, which make up 70–90% of the lignin in woody plants (Kirk and Farrell 1987; Erikksson et al. 1990; Higuchi 1990). Fungal lignin degradation is an extracellular free-radical event that proceeds in concert with activation of molecular oxygen, redox cycling of free-radical mediators and transition metals, peroxidation of low molecular mass fungal metabolites, electron transfer reactions of carbohydrates, and a catalytic cycle involving a wide variety of oxidases, reductases, peroxidases and dehydrogenases, such as LiP, MnP, Lac, cellobiose dehydrogenase, cellobiose:quinone oxidoreductase, quinone reductase, arylalcohol oxidase, ferrireductase, glucose oxidase, galactose oxidase, pyranose oxidase, oxalate oxidase and glyoxal oxidase.
KeywordsStreptomyces Hydrolase Bagasse Penicillium Lyase
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