6 Wood Discoloration

4.4 Cellulose Degradation 95
4.4
Cellulose Degradation
In the biosphere about 2.7 × 10 11 tofcarbonareboundinlivingorganisms.
According to Schwarz (2003) about 4 × 10 10 t cellulose are produced per year.
Cellulose occurs in all land plants, is always fibrillarly constructed and consists
of β-1,4-linked glucose anhydride units (glucopyranose). The substrate for cellulose
biosynthesis is UDP-glucose which is polymerized by cellulose synthase
(UDP-glucose: 1,4-β-D-glucan 4-β-D-glucosyltransferase, EC 2.4.1.12) to β-1,4
glucan chains. Depending on the wood species, the degree of polymerization
(DP) of native cellulose ranges from 10,000 to 15,000 glucose anhydride units.
In “native cellulose”, hydrogen bridges exist between the OH groups of neighboring
glucose units and neighboring cellulose molecules. Tidy (crystalline cellulose)
regions and areas of lower order (amorphous, paracrystalline cellulose)
alternate (Fengel and Wegener 1989; Fig. 4.3). In Boehmeria nivea, cellulose
crystals of about 300 glucose residues are interrupted vertically to the longitudinal
axis by an amorphous region of 4–5 glucose residues (Schwarz 2004). There
are different models for the arrangement of the cellulose molecules in the fibrils.
Bacterial cellulose degradation including the cellulosome was treated by
Schwarz (2003). There is still some uncertainty as how cellulose is degraded
by fungi. Differences occur between the various groups of fungi, brown, white,
and soft-rot fungi.
Fig.4.3. Diagram of enzymatic cellulose degradation
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