**6. Cellulose degradation**

Cellulose, a polysaccharide consisted of linear β-1,4-linked D-glucopyranose chains, re‐ quires three classes of enzymes for its degradation: β-1,4-endoglucanases (EGL), exogluca‐ nases/cellobiohydrolases (CBH), and β-glucosidase (BGL). The endoglucanases cleave cellulose chains internally mainly from the amorphous region, releasing units to be degrad‐ ed by CBHs and/or BGLs. The cellobiohydrolases cleave celobiose units (the cellulose-de‐ rived disaccahride) from the end of the polysaccharide chains [6]. Finally, β-glucosidases hydrolise cellobiose to glucose, the monomeric readily metabolisable carbon source for fun‐ gi [35]. These three classes of enzymes need to act synergistically and sequentially in order to degrade completely the cellulose matrix. After endo- and exo-cleaving (performed by EGLs and CBHs, respectively), the BGLs degrade the remaining oligosaccharides to glucose. A schematic view of cellulose degradation is depicted in the Figure 3.

**Figure 3.** Schematic view of cellulose degradation. Endoglucanases hydrolise cellulose bonds internally, while cello‐ biohydrolases cleave celobiose units from the ends of the polysaccharide chains. The released cellobiose units (disac‐ charide) are finally hydrolyzed by β-glucosidases, releasing glucose, the main carbon source readily metabolisable by

Microbial Degradation of Lignocellulosic Biomass

http://dx.doi.org/10.5772/54325

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Hemicellulose is a complex polysaccharide matrix composed of different residues branched in three kinds of backbones, named xylan, xyloglucan and mannan. The complexity of hemi‐ cellulose requires a concerted action of endo-enzymes cleaving internally the main chain, exo-enzymes releasing monomeric sugars, and accessory enzymes cleaving the side chains of the polymers or associated oligosaccharides, leading to the release of various mono- and

Xylan, a polymer composed by ß-1,4-linked D-xylose units, is degraded through the action of ß-1,4-endoxylanase, which cleaves the xylan backbone into smaller oligosaccharides, and ß-1,4-xylosidase, which cleaves the oligosaccharides into xylose. Fungal ß-1,4-endoxylanase are classified as GH10 or GH11 [40], differing from each other in substrate specificty [41]. Endoxylanases belonging to family GH10 usually have broader substrate specificity than en‐ doxylanases from family GH11 [33]. GH10 endoxylanases are known to degrade xylan back‐ bones with a high degree of substitutions and smaller xylo-oligosaccharides in addition to degrade linear chains of 1,4-linked D-xylose residues. Thus, GH10 endoxylanases are neces‐ sary to degrade completely substituted xylans [42]. ß-Xylosidases are highly specific for

fungi.

**7. Hemicellulose degradation**

disaccharides depending on hemicellulose type.

The most efficient cellulose-degrading fungi is *Trichoderma reesei*. The highly efficient degra‐ dation of cellulose by *T. reesei* is mainly due to the highly effectiveness of cellulases acting synergistically in this specie, although *T. reesei* does not have the biggest number of cellulas‐ es in the fungi kingdom [36]. The *T. reesei* has five characterized EGLs, two highly expressed CBHs and two characterized BGLs, the latter being expressed at low levels [37, 38] reviewed in [33]. In addition to being expressed at very low levels in *T. reesei*, the BGLs are strongly subjected to product inhibition [39]. These features reduce the utilization of *T. reesei* for in vitro saccharification of cellulose substrates and, in industrial applications, cellulase mix‐ tures from *T. reesei* are often supplemented with BGLs from Aspergilli, which are highly ex‐ pressed and tolerant to glucose inhibition [33].

**Figure 3.** Schematic view of cellulose degradation. Endoglucanases hydrolise cellulose bonds internally, while cello‐ biohydrolases cleave celobiose units from the ends of the polysaccharide chains. The released cellobiose units (disac‐ charide) are finally hydrolyzed by β-glucosidases, releasing glucose, the main carbon source readily metabolisable by fungi.
