6 Wood Discoloration

4
Wood Cell Wall Degradation
4.1
Enzymes and Low Molecular Agents
In view of the historical development of the research on wood degradation
by fungi, this chapter starts with the enzymes involved in the decay of the
woody cell wall, although it is now commonly accepted that non-enzymatic,
low molecular weight metabolites are involved as precursors and/or co-agents
with enzymatic cell wall degradation.
Under the conditions within microbial cells, namely an aqueous environment
with pH values around 6 and temperatures of 1–50 ◦ C, most reactions
would run off only very slowly. Enzymes reduce the amount of the necessary
activation energy as biocatalysts and control the reaction by substrate and
effect specificity. More than 3,000 enzymes are described.
Comparable with the lock/key principle, enzymes possess an active center,
into which the substrate must fit, and which thus controls the conversion of
the correct substrate (substrate specificity). The protein portion of the enzyme
decides on the way of the reaction (effect specificity). Enzymes may consist
only of protein or contain additional cofactors (e.g., Mg 2+ ,Mn 2+ ) or coenzymes
(e.g., vitamin B1). Before the conversion of the substrate into a product, the
enzyme substrate complex is formed: enzyme E + substrate S → enzyme
substrate complex ES → enzyme E + product P.
Studies on fungal polysaccharide hydrolyzing enzymes have shown a structural
design composed of two functional domains, a catalytic core responsible
for the actual hydrolysis and a conserved cellulose-binding terminus, with an
intervening, glycosylated hinge region. A large number of genes encoding cellulases,
hemicellulases, glucanases, amylolytic enzymes, and those hydrolyzing
various oligosaccharides have been cloned from fungi. The best-studied organisms
are Trichoderma reesei, Phanerochaete chrysosporium, andAgaricus
bisporus in respect of cellulases and hemicellulases, and several Aspergillus
species in respect of amylolytic enzymes, pectinases and hemicellulases (reviews
by Penttilä and Saloheimo 1999; Kenealy and Jeffries 2003). For example,
papain cleavage of cellobiohydrolase (CHB) from P. chrysosporium separated
the catalytic domain from the hinge and binding domains. Restriction mapping
and sequence analysis of cosmid clones showed a cluster of three structural
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