Biology Reference
In-Depth Information
V. CONCLUSION: RECENT TRENDS IN FUNGAL
LIGNOCELLULOSICS DEGRADATION STUDIES
In the past decade, several approaches have thus been developed to screen the
natural fungal biodiversity leading to the discovery of new enzymatic targets.
Exploration of fungal secretomes is currently a useful method to find new
enzymes and to understand cooperation between the different enzymes
involved in lignin degradation pathways. It is clear now that this degradation
proceeds according to a multi-enzymatic process involving the main oxida-
tive enzymes in a first step (i.e. peroxidases and laccases) and numerous
auxiliary enzymes to help and achieve the degradation. Proteomics plays a
key role in this exploration, as well as transcriptomics for the dynamic study
of enzyme production. However, a full exploitation of the resulting data
requires an in-depth knowledge of the host genome.
Advances in (meta)genomics are expected to reveal many more fungal
ligninases through the mining of (meta)genomes using expert databases
(see FOLy database, PeroxyBase;
http://peroxibase.toulouse.inra.fr
)
. In this
context, the improvements of current expression systems by using high-
throughput approaches for gene cloning and expression are thus required
to fill the gap between putative proteins deduced from sequence data and
information on their catalytic and structural properties. The diversity of
enzymes involved in lignin degradation is increasing, even though it is
necessary to control their expression as active proteins and at significant
levels, using proteomic and transcriptomic tools. Such an increased diversity
will probably ask for reconsidering and changing the classification of these
enzymes. The best example is the discovery of many secreted heme-peroxidases
that do not fit with the current classes. Moreover, the EC classification seems
not suitable for these new enzymes and needs also to be modified (
Hofrichter
et al., 2010
).
The low specificity of redox enzymes involved in the degradation mechan-
isms of lignin makes it difficult to understand the whole processes and
therefore the role of certain enzymes such as laccase and CDH in lignin
degradation. Many works focused on model compounds used to characterize
the cleavage of specific bonds in lignins, but more comprehensive studies are
to be developed. Most of the difficulties are due to the inability to isolate the
native form of lignin encountered by fungi in nature and to recreate in vitro
the actual environment. In addition, time scale is quite different for the
fungus and the scientists. If enzymes alone are very efficient in vitro on
model substrates, the growth of fungi in natural environment is relatively
slow and is largely influenced by environmental factors such as humidity and
temperature. Cooperation between enzymes may play a major role in lignin
