Lignin degradation and/or modification by basidiomycetes is
the key step in lignocellulose decay. Therefore, the enzymes
and mechanisms involved in lignin attack are described
below. For a discussion of the subsequent steps in the degradation
of wood polysaccharides, we recommend other
reviews.
Laccases have been known for many years in plants,
fungi, and insects, where they play a variety of roles, including
synthesis of pigments, fruit-body morphogenesis, and
detoxification. Their production in fungal plate cultures
was considered to be a characteristic unique to white-rot
basidiomycetes, although some brown-rot fungi produce
laccase in liquid cultures. These phenoloxidases have a
low redox potential that allows direct oxidation only of phenolic
lignin units, which often comprise less than 10% of the total
polymer. The interest in laccases for biotechnological applications
increased with the discovery of their ability to oxidize
high redox potential substrates in the presence of synthetic
mediators, which allows the degradation of xenobiotic compounds
and chlorine-free bleaching of paper pulp.
Natural mediators involved in lignin biodegradation remain to
be identified, although some lignin-derived phenols could act
as efficient laccase mediators.