**1. Introduction**

Currently, the attention of scientists is focused on the depletion of reserves of fossil energy sources. Rapid population growth provides high energy needs for fossil resources such as coal, natural gas, and crude oil [1]. The burning of fossil fuels poses a threat to the environment due to the emission of greenhouse gases, which, in turn, lead to global climate change. As for renewable energy sources, fuels and products derived from cellulosic waste do not harm the environment and can be considered as an alternative to fossil energy sources. Plant waste not only supports economic development but also creates an ecologically friendly environment for the production of energy and biochemicals [2–10].

Biomass cellulose is a plant polysaccharide and is an almost inexhaustible source of renewable raw materials that can be converted enzymatically into glucose. In turn, glucose is a raw material for microbiological processes of obtaining liquid and gaseous fuels (ethanol, butanol, etc.), organic and amino acids, feed protein, and many other useful products of microbiological synthesis [11]. Coniferous and deciduous wood and its waste are of particular interest [12].

An important stage of bioconversion of cellulose-containing biomass, which prevents its commercial use, is the enzymatic conversion of cellulose into glucose. Natural wood and other lignocellulose materials are resistant to enzymes due to the crystalline structure of cellulose and the presence of lignin and hemicelluloseprotecting cellulose fibers [12]. The reactivity of natural crystalline cellulose (for example, cotton) or lignocellulose (wood of various species, grass straw) during the enzymatic conversion is low, which is accompanied by extremely low yields of glucose and other sugars. Effective enzymatic hydrolysis of cellulose-containing biomass requires its pretreatment to increase reactivity by destroying the crystal structure of cellulose and completely or partially removing the lignin. As methods of pretreatment, gamma irradiation, mechanical grinding in a ball mill, treatment with mineral acids (sulfuric, phosphoric), cadoxene, and alkali delignification are used [12].

The influence of such factors on the efficiency of enzymatic conversion as changes in the degree of crystallinity of cellulose (determined by X-ray diffraction), the availability of cellulose surface to enzyme molecules (measured by such methods as protein adsorption, as well as by thermal desorption of nitrogen-hydrogen mixture), the size of cellulose particles (by dispersion analysis using optical microscopy), and the degree of polymerization of cellulose (determined by the viscosity of cellulose solutions in cadoxene) was investigated. Based on the results of these studies, the influence of these parameters on the efficiency of enzymatic cellulose conversion was quantified—it linearly depends on a decrease in the degree of crystallinity and an increase in the surface area available to the enzyme molecules, but it does not depend much on the geometric size of cellulose particles and its degree of polymerization [13].

Deep cleavage of crystalline cellulose to soluble sugars (glucose) is carried out by a complex of cellulolytic enzymes, including endoglucanases (EG), cellobiohydrolase (CBH), and β-glucosidase (BGL). The activity of the components of this complex and their interaction determine the action of the enzymes on cellulose-containing substrates [14].

Many research groups are looking for new effective producers of cellulases and hemicellulases. Research is being conducted to improve existing strains of microorganisms in order to increase the production of various cellulases and reduce their cost. In this chapter, we compare the hydrolytic potential of multi-enzyme cellulose complexes produced by different strains of fungi and evaluate the role of different enzymes in the hydrolysis of pretreated cellulose-containing substrates.
