Several biological methods for lignocellulose recycling based on the enzymology of cellulose, hemicelluloses, and lignin degradation have been developed. To date, processes that use lignocellulolytic enzymes or microorganisms could lead to promising, environmentally friendly technologies. The relationship between cellulose and hemicellulose in the cell walls of higher plants is much more intimate than was previously thought. It is possible that molecules at the cellulose-hemicellulose boundaries, and those within the crystalline cellulose, require different enzymes for efficient hydrolysis.

Cellulase: Cellulases responsible for the hydrolysis of cellulose are composed of a complex mixture of enzymes with different specificities to hydrolyze the p-1,4-glycosidic linkages (Fig. 2A). Cellulases can be divided into three major enzyme activity classes (Goyal et al., 1991; Rabinovich et al., 2002). These are endoglucanases or endo-1-4-p-glucanase (EC 3.2.1.4), exoglucanase or cellobiohydrolase (EC 3.2.1.91), and p-glucosidase (EC 3.2.1.21). Endoglucanases, are thought to initiate attack randomly at multiple internal sites in the amorphous regions of the cellulose fiber, which opens-up sites for subsequent attack by the cellobiohydrolases. Cellobiohydrolases remove cellobiose from the ends of both sides of the glucan chain. Moreover, cellobiohydrolase can hydrolyze highly crystalline cellulose. p — glucosidase hydrolyzes cellobiose and in some cases short chain cellooligosaccharides to glucose.

Hemicellulase: Xylan is the main carbohydrate found in hemicelluloses. Its complete degradation requires the cooperative action of a variety of hydrolytic enzymes (Fig. 2B). Xylanases are frequently classified according to their action on distinct substrates: endo-1,4- p-xylanase (endoxylanase) (EC 3.2.1.8) generates xylooligosaccharides from the cleavage of xylan while 1,4-p-xylosidase (EC 3.2.1.37) produces xylose from xylobiose and short chain xylooligosaccharides. In addition, xylan degradation needs accessory enzymes, such as a-L- arabinofuranosidase (EC 3.2.1.55), a-4-O-methyl-D-glucuronidase (EC 3.2.1.39), acetyl xylan esterase (EC 3.1.1.72), ferulic acid esterase (EC 3.1.1.73), and p-coumaric acid esterase (EC 3.1.1.-), acting synergistically, to efficiently hydrolyze wood xylans. In the case of acetyl-4-O — methylglucuronoxylan, which is one of the most common hemicelluloses, four different enzymes are required for degradation: endo-1,4-p-xylanase, acetyl esterase (EC 3.1.1.6), a — glucuronidase, and p-xylosidase. The degradation of O-acetyl galactoglucomannan starts with the rupture of the polymer by endomannanase (EC 3.2.1.78). Acetylglucomannan esterase (EC 3.1.1.-) removes acetyl groups, and a-galactosidase (EC 3.2.1.22) eliminates galactose residues. Finally, p-mannosidase (EC 3.2.1.85) and p-glucosidase break down the endomannanase-generated oligomeric p-1,4 bonds (Thomson, 1993; Li et al., 2000; Perez et al., 2002).