# Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes
## 3 Degradation of Lignocellulose by CWLEs
### Cellulases and Lytic Polysaccharide Mono-Oxygenases  
Degradation of cellulose is carried out by glycoside hydrolases and oxidoreductase [19]. These enzymes synergistically cooperate to degrade amorphous and crystalline regions of cellulose, respectively (Figure 2a). Glycoside hydrolases include endo- and exoglucanases, cellobiohydrolases, and \(\beta\)-glucosidase with cellobiase activity. Endo-1,4-\(\beta\)-glucanases mainly cleave the hairy region of cellulose, releasing fragments with different lengths. Concomitantly, the exo-acting cellobiohydrolases depolymerize such fragments into cellobiose, which is converted into glucose by \(\beta\)-glucosidase [20]. Cellobiohydrolases are processive enzymes and are divided in two main subgroups depending on their capability of starting the degradation from the non-reducing or the reducing end of the cellulose chain. In general, cellobiohydrolases are characterized by a GH domain linked to a CBM that enhances the binding of the substrate and confers high specificity to the catalysis (Table 1). Importantly, endoglucanase and cellobiohydrolase are sensitive to product-inhibition mechanisms since they are both inhibited by cellobiose [21]. Indeed, lack of \(\beta\)-glucosidase activity in commercial enzymatic blends is one of the major bottlenecks at the industrial level since it negatively impacts the efficiency of cellulose degradation [22]. It is worth mentioning that not all the \(\beta\)-glucosidase are characterized by cellobiase activity (i.e., capable of degrading cellobiose into two d-glucose units; therefore, \(\beta\)-glucosidase are divided into three main subgroups depending on their substrate specificity, namely, aryl-\(\beta\)-glucosidase, pure cellobiases, and broad-specificity \(\beta\)-glucosidase. \begin{table}
\begin{tabular}{c c c} \hline
**SUBSTRATE** & **CWLE** & **AA** \\ \hline \multirow{6}{*}{CELLULOSE} & Lytic Polysaccharide Mono-Oxygenase & 9, 10, 15, 16. \\  & Cellobiose dehydrogenase & 3. \\  & endo-1,4-\(\beta\)-glucanase & **GH** \\  & & 5, 6, 7, 8, 9, 10, 12, 44, 45, 48, 51, 74, 124. \\  & Exoglucanase & 1, 5, 9. \\  & Cellobiohydrolase & 6, 7, 9, 48. \\  & \(\beta\)-glucosidase & 1, 3, 5, 9, 30, 116. \\ \hline HEMICELLULOSE & & \\ \hline \multirow{2}{*}{Xylan} & endo-1,4-\(\beta\)-xylanase & 5, 8, 11, 30, 43, 51, 98. \\  & \(\beta\)-xylosidase & 1, 2, 3, 30, 39, 43, 51, 52, 54, 116, 120. \\  & endo-1,4-\(\beta\)-mananase & 26, 45, 113, 134. \\  & \(\beta\)-manosidase & 1, 2, 5, 164. \\  & endo-1,4-\(\beta\)-galactosidase & 53. \\  & \(\beta\)-galactosidase & 1, 2, 35, 42, 59, 147, 165. \\  & Endoxyoglucanase & 5, 9, 12, 16, 44, 45. \\  & Exoxyoglucanase & 3, 74. \\ Arabinose branched & \(\iota\)-\(\alpha\)-Arabinofuranosidase & 2, 3, 43, 51, 54, 62. \\ \hline \multirow{6}{*}{PECTIN} & \multirow{6}{*}{Homogalacturonan} & endo-1,4-\(\alpha\)-polysgalacturonase & 28. \\  & Exopolygalacturonase & 28. \\ \cline{1-1}  & & **PL** \\ \cline{1-1}  & & 1, 2, 3, 9, 10. \\ \hline \end{tabular}
\end{table} 
Table 1: Distribution of conserved domains in major cell wall lytic enzymes (CWLE) families. Auxiliary activity (AA), polysaccharide lyase (PL), and glycoside-hydrolase (GH) domains are indicated. Numeration of conserved domains is in accordance with the CAZy Database (www.cazy.org). The crystalline region of cellulose is the substrate of cellulolytic oxidases (Figure 2a), such as lytic polysaccharide monooxygenase (LPMO). LPMO breaks cellulose chains by oxidative cleavages, thus improving the action of endoglucanases and cellobiohydrolases [23]. Depending on the type of LPMO, oxygen can be introduced at the C1 (producing gluconic acid) or at the C4 position (producing 4-ketoglucose) or at both C positions of the cleavage site as in the case of LPMO10A from _Thermobifida fusca_ and LPMO9A from _Thermoascus aurantiacus_[24, 25].