Biology Reference
In-Depth Information
ABSTRACT
A number of microorganisms, bacteria and filamentous fungi, are able to degrade
lignocellulosic components to various extents. However, only a few ones can degrade
lignins, among which wood-rotting fungi. White-rot fungi, the most frequent ones,
mineralize cell wall components (cellulose, hemicelluloses and lignins) and extensively
degrade lignins, which results in a bleached aspect of decayed wood. Fungal lignin
degradation involves secreted heme-peroxidases and laccases that use oxidants as
electron acceptors (H
2
O
2
and O
2
). The three main heme-peroxidases are lignin perox-
idases, manganese peroxidases and versatile peroxidases. Lignin and versatile
peroxidases are able to oxidize non-phenolic lignin units. By contrast, manganese
peroxidase needs diffusible Mn-chelated ions and mainly degrades lignin units with
free phenolic groups. Other peroxidases have been recently found, such as dye perox-
idases with potential applications in bioremediation and other industrial processes.
Peroxidases need the cooperation of other oxidases such as glyoxal or aryl-alcohol
oxidases to produce hydrogen peroxide. The recent availability of complete fungal
genomes opens innovative opportunities to annotate lignocellulolytic gene families.
Phylogenetic approaches are unique tools to comparatively analyse these data and
predict or infer gene function. The increasing number of putative lignin-degrading
enzymes emerging from genome analyses requires the development of high-throughput
expression systems to meet the demands of industries. Several expression systems have
been developed to produce more efficient recombinant 'ligninases' and auxiliary
enzymes. Fungi and yeasts are useful hosts for the production of recombinant enzymes,
but only a few ones are devoted to ligninase production. In this review, we give a survey
of the main fungal and enzymatic actors of lignin degradation before discussing the
phylogenetic inference strategy used to predict lignocellulolytic enzymes and reviewing
the recombinant production of these enzymes in fungal hosts.
I. INTRODUCTION: THE LIGNIN-DEGRADING
FUNGI AND THEIR FEATURES
A number of microorganisms, bacteria and filamentous fungi, are able to
degrade lignocellulosic components to various extents. The bacterial degra-
dation of lignified cell walls is both more limited and much slower than that
of lignocellulolytic fungi. Some species of the genera Cellulomonas, Bacillus
and Pseudomonas, and some Actinomycetes can degrade, to some degree, the
lignocellulosic components of wood (
Eriksson et al., 1990
). However, very
few of them are able to degrade lignins and it seems that wood-colonizing
bacteria act in synergy with lignolytic fungi (
Blanchette and Shaw, 1978
).
Of the more than one million species of fungi known today, only few have
the capability to degrade wood. These wood-rotting fungi belong to the
phylae Basidiomycota and Ascomycota. They are generally grouped into
soft-rot, brown-rot and white-rot fungi according to the characteristics of
the wood being degraded and to the aspect of the resulting lignocellulosic
residues.
