# Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes
## 2 Lignocellulose: Structure and Functions  
Lignocellulose is a complex and heterogeneous material constituted by clusters of dried cell walls whose primary role is to confer mechanical resistance and protection against microbial invasions [1]; therefore, lignocellulosic has evolved to be highly resistant towards a broad range of different abiotic and biotic stresses. Lignocellulose is composed of lignin (10-30%), hemicellulose (20-30%), cellulose (35-45%), and, to a lower extent, by pectin, proteins, extractives, and ash [5] (Figure 1). Cellulose is organized in fibrillary structures that engage hemicellulose through hydrogen bonds; these structures, in turn, are occluded by lignin that confers hydrophobicity and robustness to the entire assembly [6]. The cellulosic microfibril is a homopolymer comprising (20-40) \(\beta\)-1,4-glucan chains that interact with each other through hydrogen bonds; its chemical repeat unit is cellobiose, a disaccharide of \(\beta\)-1,4-p-glucose. On the opposite, hemicellulose is a branched heteropolymer whose composition varies depending on the plant species. The most abundant hemicellulose derived from agricultural feedstock (e.g., straw, corn cob, bran) is represented by xylan, a \(\beta\)-1,4-polymer whose repeat unit is xylobiose, a disaccharide formed by \(\beta\)-1,4-d-xylose. Xylan can be modified by the acetylation of d-xylose units in the C2 or C3 (less frequent) position or by the addition of l-arabinose units (i.e., arabin xylan) [7], Lignin is formed by cross-linked aromatic polymers whose composing units (i.e., monolignols) are coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [8]. The relative abundance of each monolignol is not conserved in lignin but varies between the different plant species. In primary cell walls, the \"cellulose-hemicellulose-lignin\" assembly is surrounded by pectin, an acid polysaccharide mainly composed by \(\alpha\)-1,4-p-galacturonic acid units. On the contrary, in secondary cell walls, the presence of pectin is strongly reduced in favor of an increased amount of lignin (Figure 1).  
Pectin is characterized by four regions with different structures and compositions: homogalacturonan, xylogalacturonan, and type I-II rhamnogalacturonan. Importantly, non-substituted p-galacturonic acid residues can bind di-cationic ions (mainly Ca++) through the carboxylic groups of two residues from adjacent chains, increasing the viscosity of the matrix that, in turn, hinders the accessibility of CWLEs to the inner layers, such as those represented by the hemicellulose-cellulose assembly [9]. Despite the low abundance in lignocellulosic (less than 5%), pectin may affect the accessibility of CWLEs to cellulose, as shown by the increased saccharification of leaf and stem material obtained from transgenic wheat plants with altered pectin structure [9]. Moreover, pectin has a prominent role in alerting plants for the presence of pathogens [10] (see Sections 4.2 and 5.3).  
Besides being the most abundant carbon source on Earth [11], lignocellulosic exploitation in energy production is impeded by its complex structure and composition.  
This review will focus mainly on CWLEs with degrading activity towards cellulose, hemicellulose, and pectin; further notions on CWLEs with degrading activity towards lignin, such as lacases and polyphenol-oxidases, are described in other recent reviews [12; 13; 14].