6 Wood Discoloration

4.1 Enzymes and Low Molecular Agents 89
Table 4.1. Some characteristics of enzymes for wood degradation
altogether six classes of enzymes with subgroups:
1. oxidoreductases, 2. transferases, 3. hydrolases,
4. lyases, 5. isomerases, 6. ligases
lignin degradation by oxidoreductases:
lignin peroxidase EC 1.11.1.14
manganese-peroxidase EC 1.11.1.13
laccase EC 1.10.3.2
hemicellulose degradation by hydrolases:
endo-1,4-β-xylanase EC 3.2.1.8, xylan 1,4-β-xylosidase EC 3.2.1.37
mannan endo-1,4-β-mannosidase EC 3.2.1.78, β-mannosidase EC 3.2.1.25 etc.
cellulose degradation by hydrolases:
cellulase EC 3.2.1.4
β-glucosidase EC 3.2.1.21
cellulose 1,4-β-cellobiosidase EC 3.2.1.91 etc.
ectoenzymes:
extracellular degradation of macromolecules pectin, hemicellulose, cellulose, lignin
outside the hypha
[uptake of degradation products (carbohydrate oligomers, dimers, monomers,
lignin degradation products)]
intracellular enzymes:
metabolic transformation within the hypha to hyphal biomass, metabolites, energy
endoenzyme:
attack within the substrate, often randomly
exoenzyme:
attack at the non-reducing substrate end
enzyme activity:
in former times (but still used):
international unit: 1 U = 1 µmol/min, (1 U = 16.67nkat)
currently:
kat (katal, catalytic activity): 1 kat = 1 mol/s
The enzymes for the degradation of the cellulose and hemicelluloses within
thewoodycellwallbelongpredominantlytothehydrolases,whichcleaveglucosidic
bonds. Briefly (and thus not totally correctly), cellulose is hydrolyzed
by cellulase, cellulose 1,4-β-cellobiosidase and β-glucosidase. The hemicelluloses
xylan and mannan are degraded by endo-1,4-β-xylanase and mannan
endo-1,4-β-mannosidase, respectively, followed by xylan 1,4-β-xylosidase and
β-mannosidase and further enzymes for the side chains. Lignin is oxidatively
degraded by the oxidoreductases lignin peroxidase and manganese peroxidase.
Enzymatic wood degradation was summarized e.g., by Eriksson et al. 1990,
Shimada 1993, Jennings and Lysek 1999, Goodell et al. 2003.
Cellulose, hemicellulose, and lignin are as macromolecules too large to be
taken up into the hypha. Therefore, the molecules are first degraded by extra-
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