ABSTRACT
Lignin is extremely recalcitrant to degradation. By linking to both cellulose and hemicelluloses, it creates a barrier to any solutions or enzymes and prevents the penetration of lignocellulolytic enzymes into the lignocellulosic structure.
However, some fungi and some bacteria are able to degrade lignin efficiently.
Lignocellulolytic enzyme-producing fungi are widespread, and include species from the ascomycetes, and basidiomycetes phyla such as white-rot and brown-rot fungi.
Fungi degrade lignin by secreting enzymes collectively termed ligninases or lignin-modifying enzymes which are classified into two categories, the heme peroxidases (lignin peroxidases, manganese peroxidases, versatile peroxidases, dye decolorizing peroxidases) and the laccases (phenol oxidases). The peroxidases are heme-containing enzymes with catalytic cycles that involve the activation by H2O2 and substrate reduction of compound I and compound II intermediates. Laccases are multi-copper oxidoreductases. In addition to the peroxidases and laccases, fungi produce other accessory oxidases such as aryl-alcohol oxidase and the glyoxal oxidase that generate the hydrogen peroxide required by the peroxidases. Fungi can also generate free hydroxyl radicals by cellobiose dehydrogenase catalyzed reactions, low-molecular-weight peptides/quinone redox cycling and glycopeptide-catalyzed Fenton reactions.
