6 Wood Discoloration

96 4 Wood Cell Wall Degradation
Early workers investigating brown-rot fungi assumed that only cellulolytic
enzymes were responsible for cellulose degradation. Cellulolytic activity was
initially described using terminology of C1-Cx (Reese et al. 1950): native (crystalline)
cellulose is prepared by C1 cellulase for the degradation by Cx cellulase,
as C1 cellulase loosens the crystalline areas by cleaving the hydrogen bridges
for the following attack by Cx cellulase.
The C1-Cx model was later refined to refer to the action of general classes
of exoglucanases and endoglucanases, respectively. As methods were further
refined more specific functionalities were defined and newly isolated enzymes
were found in brown-rot fungi. Brown-rot fungi produce several endo-1,4β-glucanases
and β-glucosidases, but typically lack exo-1,4-β-glucanase activity.
However, cellobiohydrolase and cellobiose dehydrogenase [cellobiose:
(acceptor) 1-oxidoreductase, EC 1.1.99.18] have been isolated from Coniophora
puteana. Brown-rot fungal wood degradation was recently reviewed by Goodell
(2003). White-rot and soft-rot fungi produce the full cellulolytic enzyme
system of endo- and-exoglucanases, and β-glucosidase.
The enzymes produced are thought to act in concert with each other as
well as with non-enzymatic systems. Attack occurs at the amorphous cellulose
regions (Cx action) by cellulase (“endoclucanase”, systematic name:
1,4-β-D-glucan 4-glucanohydrolase, EC 3.2.1.4), which endohydrolyzes 1,4-β-
D-glucosidic linkages in cellulose and other β-D-glucans. A combined action
takes place by cellulose 1,4-β-cellobiosidase (1,4-β-D-glucan cellobiohydrolase,
EC 3.2.1.91), which hydrolyzes 1,4-β-D-glucosidic linkages in cellulose
and cellotetraose, releasing cellobiose from the non-reducing ends (exoenzyme),
and by glucan 1,4-β-glucosidase (1,4-β-D-glucan glucohydrolase, EC
3.2.1.74), which acts on 1,4-β-D-glucans and related oligosaccharides and exohydrolyzes
successive glucose units from the ends. The final hydrolysis of
oligosaccharides is mediated by β-glucosidase (“cellobiase”,β-D-glucoside glucohydrolase,
EC 3.2.1.21), which acts on terminal, non-reducing β-D-glucose
residues with release of β-D-glucose. Cellobiose may be also attacked by cellobiose
dehydrogenase [cellobiose:(acceptor) 1-oxidoreductase, EC 1.1.99.18]
oxidizing cellobiose to cellobionolactone under reduction of O2 to H2O2,and
Fe 3+ to Fe 2+ (Kruså et al. 2005).
In the mold Trichoderma viride (T. reesei), three endoglucanases, two exoglucanases,
and several β-glucosidases were found (Eriksson et al. 1990). In
Sporotrichum pulverulentum Novobr. (anamorph of Phanerochaete chrysosporium),
five endoglucanases, one exoglucanase and two β-gucosidases, which
together with oxidizing enzymes (laccase and cellobiose: chinon oxidoreductase)
caused a combined degradation of cellulose and lignin. Uemura et al.
(1992) isolated six exoglucanases. In P. chrysosporium, cellulases have been
classified into eight different families among the glycoside hydrolases (Samejima
and Igarashi 2004). In addition, the importance of the cellobiose dehydrogenase
(CDH) was shown, as this enzyme could participate in the extracellular
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