6 Wood Discoloration

4.4 Cellulose Degradation 97
metabolism of cellobiose instead of β-glucosidase.TheroleofCDHforcellulose
degradation was discussed (Hyde and Wood 1997; Kruså et al. 2005). It
was hypothesized that CDH act as link between cellulolytic and ligninolytic
pathways (Temp and Eggert 1999).
Insoluble, native cellulose is attacked comparatively slowly by a system of
cellulolytic enzymes. A limited introduction of substituents into the cellulose
molecule reduces the number of hydrogen bonds of cellulose chains in proportion
to the degree of substitution (DS) and the pattern of occurrence along
the cellulose chain. Depending on these features and the nature of the substituent,
water solubility of cellulose derivatives may be obtained at DS values
between 0.4 and 0.7, and cellulose loses its ordered structure and becomes
enzymatically accessible. Cellulose acetates up to a DS of 1.4 were deacetylated
by various enzyme preparations (Altaner et al. 2001).
For in vitro-degradation tests, carboxymethylcelluloses (CMC) are often
used as soluble cellulose substrate (e.g., Schmidt and Liese 1980). The molecular
structure of CMCs was characterized e.g., by Saake et al. (2000).
Pure crystalline cellulose substrates, like cotton or Avicel, are degraded by
white and soft-rot fungi. Most brown-rot fungi hardly show enzyme activity
against crystalline celluloses and attack only pre-treated cellulose derivatives
(Highley 1988; Enoki et al. 1988), because brown-rot fungi do not possess the
synergistic endo/exo glucanase system, but have only endoglucanases. Within
the woody cell wall, brown-rot fungi, however, depolymerize cellulose rapidly.
Thus, the presence of lignin, lignin breakdown products, hemicelluloses, or
simple sugars was postulated.
Due to the limitation of enzyme accessibility to the woody cell wall by its
pore sizes, the conceptions on cellulose degradation within wood by brownrot
fungi focused both on non-enzymatic procedures and enzymatic mechanisms
(e.g., Eriksson et al. 1990; Highley and Illman 1991; Micales 1992;
Ritschkoff et al. 1992; Goodell 2003). Bailey et al. (1968) postulated as preceeding
non-enzymatic agent a “precellulolytic phase”. Koenigs (1974) and
others showed that cellulose was oxidatively degraded by Fenton reagents
[Fe(II) + H2O2 → Fe(III) + OH − +OH 0 ]. Since ferrous iron is required in Fenton
reactions, which is, however, absent in oxygenated wood decay processes,
asearchforamechanismtoreduceironwasmade.H2O2 can also react with
copper ions and some chromium, vanadium and nickel species to generate
OH 0 (Halliwell 2003).
Numerous investigations stress the participation of oxalic acid (e.g., Green
et al. 1991a, 1993; Micales 1992), as the acid reduces Fe 3+ to Fe 2+ , which forms
from H2O2 the reactive hydoxylradical, which then depolymerizes the cellulose.
In several brown-rot fungi, like Coniophora puteana, Serpula lacrymans
and Oligoporus placenta, extracellular H2O2 was proven (Ritschkoff et al. 1990,
1992; Ritschkoff and Viikari 1991; Backa et al. 1992; Tanaka et al. 1992). Serpula
lacrymans dissolved by means of oxalic acid iron from stonewool, which
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