**12. Cellulose biodegradation**

Although more than a dozen fungal species considered as cellulose degraders have been re‐ ported (including *T. viride, T. reesei, F. solani, A. niger, A. terreus, P. chrysosporium, B. adusta* and *P. sanguineus)* [3, 74]; and even with cellulases identified in nematodes (*Bursaphelenchus xylophilus,* a nematode infecting pine wood), yeast (*Aureobasidium pullulans)* and marine bac‐ teria (*Saccharophagus degradans)*, the search of new cellulases genes continues. This have led to the construction of metagenomic libraries and bioprospecting analysis from several envi‐ ronments: buffalo rumen, higher termite guts, bovine ruminal protozoan, decomposing pop‐ lar wood chips and hardwood forest leading to the identification of new genes and organisms with cellulolytic activities [107, 121-130].

Out of the currently existing 125 families, 15 correspond to cellulases (GHF 1,3, 5, 6, 7, 8, 9, 12, 44, 45, 48, 51, 74, 116, and 124), and 64 families group the cellulose binding domains (see http://www.cazy.org/). In [134] an excellent review of and classification system for many

Hydrolysis of Biomass Mediated by Cellulases for the Production of Sugars

http://dx.doi.org/10.5772/53719

135

The widely accepted mechanism for enzymatic cellulose hydrolysis involves synergistic ac‐ tions by endoglucanases (EGL, EC 3.2.1.4], exoglucanases or cellobiohydrolases (CBH, EC 3.2.1.74; 1,4-β-D-glucan-glucanhydrolase and EC 3.2.1.91; 1,4-β-D-glucan cellobiohydrolase),

Endoglucanases hydrolyse accessible intramolecular β-1,4-glucosidic bonds of cellulose chains randomly to produce new chain ends; exoglucanases processively cleave cellulose chains at the reducing and non-reducing ends to release soluble cellobiose or glucose; and βglucosidases hydrolyse cellobiose to glucose in order to eliminate cellobiose inhibition (13).

The activity of cellulase enzyme systems is much higher than the sum of the activity of its individual subunits; a phenomenon known as synergism, so they have to be considered not just simply a conglomerate of enzymes with components from all three cellulase types, but

These enzymes cleave internal linkages in amorphous cellulose filaments, generating oligo‐ saccharides with different sizes and creating new chain ends that can in turn be attacked by exoglucanases (135). The cellulolytic process is initiated by endoglucanases that randomly cleave internal linkages at amorphous regions of the cellulose fibre and creating new reduc‐ ing and non reducing ends that are susceptible to the action of cellobiohydrolases [136].

Endoglucanases are monomeric enzymes with a molecular weight that ranges from 22 to 45 kDa, although some fungi such as *Sclerotium rolfsii* and *Gloeophyllum sepiarium* have endo‐ glucanases twice this size [137]. In general, endoglucanases are not glycosylated; however, they sometimes may have relatively low amounts of carbohydrate (from 1 to 12%) [2]. Un‐ like other endoglucanases reported with optimum pH 4 to 5; the only known endoglucanase with a neutral pH optimum is that from the basidiomycete *Volvariella volvacea,* expressed in recombinant yeast. Basically, their optimum temperature ranges from 50 to 70 °C [138-139].

Exhaustively hydrolysing cellulose also requires the action of β-glucosidases (BGL) (EC 3.2.1.21), which hydrolyse cellobiose, releasing two molecules of glucose and thereby pro‐ vide a carbon source that is easy to metabolize. Fungi causing white and brown rot, mycor‐

According to [13], primary hydrolysis occurs on the surface of solid substrates and releases soluble sugars with a degree of polymerization (DP) up to 6 into the liquid phase upon hy‐ drolysis by endoglucanases and exoglucanases. This depolymerisation step performed by

rhizal fungi and plant pathogens produce these enzymes [2, 135].

These three hydrolysis processes occur simultaneously as shown in Figure 7.

CBD families is provided.

**14. Endoglucanases**

and β-glucosidases (BGL, EC 3.2.1.21).

as a mixture that efficiently hydrolyse cellulose fibres.

To have a better impression of the latest developments regarding fungal carbohydrate-active enzymes, the following sections will discuss the enzymes needed for cellulose degradation.

(*a*) Cel7A binding to cellulose, (*b*) recognition of a reducing end of a cellulose chain, (*c*) initial threading of the cellu‐ lose chain into the catalytic tunnel, (*d* ) threading and formation of a catalytically active complex, (*e*) hydrolysis in a processive cycle and ( *f )* product expulsion and threading of another cellobiose (shown in yellow in *e* and *f*). Image reproduced with publisher´s permission [131].

**Figure 6.** Activity on substrate of cellulase (exoglucanase, Cel7A) of *T. reesei.* The enzyme has a small carbohydratebinding domain (CBD) of 36-amino acid, a long flexible linker with O-glycan (dark blue), and a large catalytic domain (CD) with N-linked glycan (pink) that can thread a single chain of cellulose into the catalytic tunnel of 50 Å.
