**3. Cellulose structure**

In this chapter, we focus on cellulose degradation by cellulases in order to enhance sugars release from biomass. Cellulose structure, allomorphs and its hydrolysis by cellulolytic or‐ ganisms such as fungi and bacteria, is also reviewed, as well as cellulases structure, CAZY classification, their synergistic activity and the recently cellulases identified by metagenomic

Another theme analyzed in this chapter is related to crystalline structure of cellulose, the main impediment to achieve full cellulose hydrolysis, and the role of proteins recently re‐ ported with cellulose disrupting activity that have improved saccharification processes. These proteins represent good candidates as an additive to enhance sugar production from

It has been estimated that the net CO2 fixation by land plants per year is approximately 56 X

Lignocellulose is a renewable organic material and is the major structural component of all cell plants. Lignocellulose plant biomass consists of three major components: cellulose (40–

**Production Tons Reference**

Minor components are proteins, lipids, pectin, soluble sugars and minerals (Table 2) [9]. It has a thickness of ~0.1 a 10 µm contrasting with <0.01 µm of cell membrane formed by pro‐

Examples of such biomass are angiosperms (hardwoods), gymnosperms (softwoods) and

graminaceous plants (grasses such as wheat, giant reed and *Miscanthus*).

(Table 1). Of this amount, 70% is estimated to represent plant cell walls (revised in [9]).

Assimilated CO2 56 X 109 [10] Plant biomass 170-200 X 109 [11] Cell walls 150-170 X109 [9] Lignocellulose 200 X109 [12] Cellulose 100 X 109 [13-14] Wheat straw 540 X 109 [15] Soybean straw 200 X 109 [16] Sugar cane bagasse 54 X 109 [17]

tons and that the worldwide biomass production by land plants is 170–200 X 109 tons

analysis, an excellent tool in this search of better cellulolytic activity.

120 Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization

**2. Structure and composition of the cell wall**

50 %), hemicellulose (20–40 %) and lignin (20–30 %) (Figure 1).

**Table 1.** Worldwide annual production of biomass

teins and phospholipids [18].

plant biomass.

109

Cellulose is the main component in the plant cell walls, and is made of parallel unbranched D-glucopyranose units linked by β-1,4-glycosidic bonds that form crystalline and highly or‐ ganized microfibrils through extensive inter and intramolecular hydrogen bonds and Van der Waals forces, amorphous cellulose correspond to regions where this bonds are broken and the ordered arrangement is lost (Figure 2).The cellulose chains aggregated into microfi‐ brils are reported to consist of 24 to 36 chains based on scattering data and information about the cellulose synthase [20-21].

Consecutive glucose molecules along chains in crystalline cellulose are rotated by 180º, meaning that the disaccharide (cellobiose) is the repeating unit [22].

Two different ending groups are found in each cellulose chain edge. At one end of each of the chains, a non-reducing group is present where a closed ring structure is found. A reduc‐ ing group with both an aliphatic structure and a carbonyl group is found at the other end of the chains. The cellulose chain is thus a polarized molecule and the new glucose residues are added at the non-reducing end allowing chain elongation (Figure 2) [23].

is important in determining the enzymatic digestibility of a cellulose sample. Crystallinity, is a measure of the weight fraction of the crystalline regions, is one of the most important measurable properties of cellulose that influences its enzymatic digestibility [19, 28-30].

A parameter termed the crystallinity index (CI) has been used to describe the relative amount of crystalline material in cellulose. Generally, in nature, crystallinity indexes range from 40% to 95%, the rest is amorphous cellulose [31]. The degree of polymerization, (DP) is the number of monomeric units in a polymer molecule, which in cellulose it ranges from 500

**polymerization**

Hydrolysis of Biomass Mediated by Cellulases for the Production of Sugars

**Ref.**

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

123

**Substrate Crystallinity index Degree of**

, Soluble; b, Insoluble.

Carboxymethyl cellulose (CMC)a NA 100-2000 [32]

Cellodextrins a NA 2-6 [32] Avicel b 0.5-0.6 300 [13] BC b 0.76-0.95 2000 [13] PASC b 0-0.04 100 [13] Cotton b 0.81-0.95 1000-3000 [13] Filter paper b 0-0.45 750 [13] Wood pulp b 0.5-0.7 500-1500 [13] Fluka Avicel PH-101 b 0.56-0.91 200-240\* [26] Fluka cellulose b 0.48-0-82 280\* [26] Sigma α-cellulose<sup>b</sup> 0.64 2140-2420\* [33]

The crystalline structure of cellulose has been studied since its discovery in the 19th century, its structure was first established by Carl von Nageli in 1858, and the result was later veri‐

In the past decades, many data on the polymorphism of cellulose were analysed, being the most reliable data published after 1984, when the results of NMR spectroscopic studies of

The repeating unit of the cellulose macromolecule includes six hydroxy groups and three oxygen atoms. Therefore, the presence of six hydrogen bond donors and nine hydrogen bond acceptors provides several possibilities for forming hydrogen bonds. Due to different

to 15,000 but varies depending the substrate (Table 2).

\*According to manufacturer's data. a

**5. Cellulose allomorphs**

cellulose were reported [36].

fied by X-ray crystallography [34-35].

**Table 2.** Some physical properties of cellulosic substrates

A wide variety of Gram-positive and Gram-negative bacterial species are reported to pro‐ duce cellulose, including *Clostridium thermocellum, Streptomyces* spp*., Ruminococcus* spp., *Pseudomonas* spp*., Cellulomonas* spp*., Bacillus* spp*., Serratia, Proteus, Staphylococcus* spp*.,* and *Bacillus subtilis* [24].

**Figure 2.** Crystalline and amorphous structure of cellulose. The crystalline structure is conserved by hydrogen bonds and Van der Waals forces, in amorphous structure exists twists and torsions that alter the ordered arrangement. Re‐ ducing and non-reducing are shown.
