High redox potential peroxidases

Abstract

High redox potential peroxidases secreted by some basidiomycete fungi are unique enzymes that play a central role in the degradation of lignin (Kirk and Cullen, 1998). Plant biomass (bryophytes excluded) is mainly made up of two polysaccharides, cellulose and hemicelluloses, and the complex aromatic polymer of lignin (Fengel and Wegener, 1984). The main role of lignin in the plant cell wall is the protection of polysaccharides against the action of hydrolytic enzymes. It also contributes to stem rigidity and water transport, two important characteristics that facilitated land colonization by vascular plants. Such characteristics derive from the recalcitrance and structural complexity of lignin which includes up to three types of subunit derived from three different p-hydroxycinnamyl alcohols (Higuchi, 1997; Boerjan et al., 2003). These phenylpropanoid units are linked together by a variety of ether and C-C bonds forming a three dimensional network that confers lignin its mechanical resistance as well as an extremely high resistance to degradation. Biodegradation of the lignin polymer has been described as an "enzymatic combustion", where the aromatic units are oxidized by the hydrogen peroxide secreted by ligninolytic basidiomycetes in a reaction catalysed by high redox potential peroxidases, enzymes which are unique to this group of organisms (Kirk and Farrell, 1987). Due to the light colour of the wood decayed by ligninolytic basidiomycetes these organisms are also known as white-rot fungi (Martínez et al., 2005). By contrast, the so-called brown-rot basidiomycetes do not produce ligninolytic enzymes but instead transform wood into a lignin-enriched brown material. In addition to peroxidases, other fungal enzymes such as laccases and H2O2-producing oxidases are also involved in lignin biodegradation. Ligninolytic enzymes are of considerable importance for the natural degradation of lignocellulosic materials in terrestrial ecosystems, enabling the recycling of the organic carbon fixed by photosynthesis, and also for most industrial processes that utilise lignocellulosic biomass, including paper pulp manufacturing and bioethanol production both of which require the removal of lignin to exploit cellulose. © 2007 Springer.