**1. Introduction**

Alternative renewable fuel as bioethanol in the form of biofuel derived from biomass can contribute sources to replace fossil fuel-based conventional energy sources [1].

Cellulases are important enzymes in cellulose degradation that occurs in nature, this degradation involves a system of extracellular multienzymes and have wide application [2, 3].

Cellulase enzymes play an important role in industrial processes, representing about 20% of the global enzyme market worldwide and presenting a wide range of application, from food, feed, textile, pulp and pulp industries. An application that has been growing in recent years is the conversion of biomass into fermentable sugars for the production of biofuels [4–6].

Cellulases act on cellulosic fiber, catalyzing the degradation of β-1,4-glycocydic bonds [7] and includes three different types that act synergistically, based on classification the mode of action and specificities of the substrate, these: endoglucanases (EC 3.2.1.4) that randomly hydrolyze β-1.4 bonds in the cellulose molecule; cellobiohydrolases or exoglucanases (EC 3.2.1.91) which release a cellobiose unit and act procedurally at the end of the chain; and β-glycosidases (EC 3.2.1.21) that hydrolysis cellobiose to glucose [2, 8].

The construction of a high-quality system for the production of these enzymes is important for its application in the process of saccharification of biomass involved in the biofuel production process [9]. Current efforts have focused on fungal cellulases to transform lignocellulosic biomass into fermentable sugars that can be converted into ethanol. This process will allow the production of renewable fuel from cellulosic biomass [10].

Advances in the production of recombinant enzymes focus on the search for industrially viable microorganisms capable of producing enzymes under various conditions, expressing them in a highly efficient manner, aiming at the synthesis of several copies of genes and a strong promoter. Several species of fungi are capable of synthesizing and secreting high amounts of cellulase; most studies with fungal species use linearized plasmid, since these are encompassed to chromosomal DNA, improving its stability and expression efficiency [11].

For genetic engineering, the main expression systems are: *E. coli*, a bacteria classified as belonging to the *Bacteria* Domain*, Proteobacteria philum; Gammaproteobacteria* class, *Enterobacteriales* order and *Enterobacteriaceae* family, which has a high rate of development, easy to manipulate, transform and capture of plasmids, and can grow with high cell density. *E. coli* is generally transformed with self-replicated plasmid that does not integrate with chromosomal DNA and continues to replicate independently of cell divisions [12, 13]; and *Pichia pastoris,* a yeast classified as belonging to the Fungi Kingdom*, Eucomycota* division*; Ascomycota* subdivision*;* class *Hemoascomycetes,* the order *Endomycetales,* family *Sacharomycetaceae* and subfamily *Sacharomycetoideae.* A remarkable physiological characteristic of this yeast is the fact that it is methyltrophic, that is capable of growing in culture medium containing methanol as the only source of carbon and the ability to secrete high amounts of extracellular proteins [14, 15].

Due to the advance in the techniques of recombinant expression, the production systems of recombinant enzymes are promising strategies for the efficient production of industrial cellulase that can increase productivity in several industrial applications, including biomass in the processing of biofuels and thus meet the increasing demands of this enzyme [16].

The cost of obtaining sugars from the biomass of sugarcane bagasse for fermentation is still high, mainly due to the low enzymatic yield of fungal production. Thus, it generates the need for cellulase supplementation to these enzymatic cocktails. To resay these restrictions, molecular biology combined with recombinant DNA technology is a viable tool in enzymatic production. In subsequent topics, the production of endoglucanases, exoglucanases and β-glucosidase of fungi cloned in *E. coli* and *Pichia pastoris* will be addressed.

#### **2. Lignocellulosic biomass**

Lignocellulosic biomass is characterized mainly by the presence of two carbohydrate polymers (cellulose and hemicellulose), as well as an aromatic polymer called lignin, in addition to other components found in smaller amounts, such as ash, pectin, proteins, non-structural carbohydrates (glucose, fructose and sucrose)

#### *Recombinant Fungal Cellulases for the Saccharification of Sugarcane Bagasse DOI: http://dx.doi.org/10.5772/intechopen.98363*

and lipids. Most of the biomass of lignocellulosic materials is composed of cellulose (40–50%), hemicellulose (20–30%) and lignin (10–25%) and the specific composition of lignocellulosic biomass varies depending on different factors, mainly plant species, age, growth stage and environmental factors, genetic variability, and cultivation conditions of plant material [2, 17, 18].

Lignocellulosic biomass has a complex internal structure and several of its main components also have complex structures. Cellulose and hemicellulose are polysaccharides composed of simple sugars while lignin is a complex network of aromatic alcohols. In general, hemicelluloses and lignin provide an amorphous matrix in which crystalline cellulose microfibrils are dispersed [2, 18].

Corn straw, sugarcane bagasse, rice straw and wheat bran are promising and abundant lignocellulosic raw products from plant residues in the United States, South America, Asia and Europe [19].

### **3. Heterologous systems**

A potential tool to develop better industrial production of cellulase are techniques of heterologous expression. This technology leads to enzyme yields at an economically viable level, since it allows the creation of microbial strains that express sets of adapted and synergistically active enzymes, within a single cell or combining different strains [20]. There are a variety of protein expression systems available, including bacterial and yeasts expression systems.

For the bacterial expression system, the most used is *Escherichia coli*, whose genetic characteristics are already well described. In addition, it has easy of manipulation, has an abundance of commercially available strains and vectors and has great ability to express recombinant genes with high yields [20–22].

As an alternative to the bacterial system, yeasts are often used, where *Pichia pastoris* yeast has become the most widely used host system for the expression of many heterologous proteins with relative ease of technique and at lower costs than those of most other eukaryotic systems [23–25].

Data from the last 15 years describing the recombinant fungal cellulases candidates for cellulose hydrolysis produced in the expression systems *E. coli*, *P. pastoris* or other different systems are presented below.
