Xylanases belong to the carbohydrate active enzyme (CAZymes) group and are placed under glycoside hydrolases class, which is further divided into clans and families (Juturu and Wu, 2012).
Agricultural Residues as Animal Feed
Mateus G. Godoy, … Denise M.G. Freire, in Current Developments in Biotechnology and Bioengineering, 2018
Xylanases (endo-β-1,4-xylanase, EC 126.96.36.199) are mainly responsible for the hydrolysis of β-1,4 bonds in plant xylan (Fig. 12.1), the main component of hemicellulose . Hemicellulose constitutes about 30% of plant cell walls , and because xylan is an important component of hemicelluloses, xylanases play a crucial role in the bioconversion of agro-industrial waste to biomass that can be used in animal feed .
Industrial-scale xylanase production can be accomplished using SSF by fungi, mainly Aspergillus and Trichoderma. Fungi are potentially useful for xylanase production because they secrete enzymes into the medium, and their enzyme levels are, generally, much higher than those of yeasts and bacteria [59,60].
Efforts to improve the digestibility of nutrients for monogastric and ruminant animals can affect the profitability of the animal-feed industry . Supplementing the diets of dairy cows and beef cattle with exogenous enzymes can significantly improve feed utilization and animal performance [61–63].
Enzyme products are different enzymes that are mixed with unknown amounts of each enzyme. Several studies have reported that the mixture of supplemental fibrolytic enzymes with high xylanase activity produced a positive response in dairy cows in confinement [64,65].
Other studies have evaluated the use of xylanases to improve nutrient digestibility in other ruminants and in pigs .
Differential Expression of the Microbial β-1,4-Xylanase, and β-1,4-Endoglucanase Genes
Arvind Kumar, Ram Naraian, in New and Future Developments in Microbial Biotechnology and Bioengineering, 2019
6.3.2 Regulation of β-1,4-Xylanase and β-1,4-Endoglucanase Gene Expression
Xylanases and endoglucanases are dynamic enzymes whose expression is precisely controlled by activation and repression mechanisms. The availability of a carbon source determines the operation of either activation or repression mechanism. The expression is induced only in the presence of respective substrates however, repressed when easily utilizable sugars are present. The most probable molecular inducers of xylanase and endoglucanase system are xylose, arabinose, lactose, cellobiose, sophorose, carboxymethyl cellulose, and intermediary hydrolytic products of polysaccharides (Amore et al., 2013; Herold et al., 2013; Xing et al., 2013) (Tables 6.7 and 6.8). The diversity of polysaccharide hydrolyzates and their concentrations regulate the expression and secretion of specific types of polysaccharide hydrolyzing enzymes and their isoforms. A specific set of isozymes were expressed in a concentration-dependent manner. For instance, low concentration of xylose induced the expression of Xyn1/XynA, Xyn2, and Xyn3/XynB endoxylanases while high concentration sustained only the expression of Xyn2 and Xyn3/XynB with repression of Xyn1/XynA (Xing et al., 2013). Although, the minimum concentrations of xylose or arabinose are not known that is sufficient for the induction of XlnR- and AraR-regulated xylanase genes. However, it has been demonstrated that 1 mM xylose is sufficient for the maximum induction of xylanolytic genes in A. niger (de Vries et al., 1999). Overall, the regulation of xylanase and endoglucanse genes seems similar in bacteria and filamentous fungi (most studied models), for example, T. reesei, N. crassa, and Aspergillus spp. However, the regulatory mechanisms are quite complex and present some differences even within filamentous fungi. This complexity is specifically due to the cross talk between the expression of xylanolytic and cellulolytic genes (Amore et al., 2013). In the following section we discuss very briefly the transcription factors regulating the expression of xylanase and cellulase encoding genes.