ABSTRACT

BioconversionThe plant cell wall is a complex macromolecular structure that surrounds and protects the cell, and is a distinguishing characteristic of plants essential to their survival (Caffall and Mohnen, 2009). As a consequence of limited mobility, plants are plastic in their ability to withstand a variety of harsh environmental conditions and to survive attack by pathogens and herbivores. The structure formed by the polysaccharides, proteins, aromatic, and aliphatic compounds of the cell wall enables plants to flourish in diverse environmental niches. The primary plant cell wall surrounding growing plant cells is composed of cellulose microfibrils, organized in insoluble crystalline regions (sometimes silica and waxy substances), embedded in a network of hemicellulose polymers (glucuronoxylan, arabinoxylan,

glucomannan), a gel-like matrix of pectins, lignins, and proteins (Lynd et al., 2002; Wei et al., 2009). The relative amounts of each component vary in different cell walls. Cellulose and hemicelluloses are the two major polysaccharides targeted as sugar sources for biofuel production. Cellulose (about 35-50%) is a linear polysaccharide in the plant cell walls, consisting of D-glucose molecules bound together by β-1,4-glycoside linkages (Fig. 21.1). Cellobiose is the smallest repetitive unit of cellulose and can be converted into glucose. The cellulose-hydrolyzing enzymes (i.e., cellulases) are divided into three major groups: endoglucanases, cellobiohydrolases (exoglucanases), and β-glucosidases (Kumar et al., 2008). The endoglucanases catalyze random cleavage of internal bonds of the cellulosic chain, while cellobiohydrolases attack the chain ends from reducing or nonreducing residues, releasing cellobiose. β-Glucosidases are only active on cellooligosaccharides and cellobiose, and release glucose monomers units from the cellobiose (Fig. 21.1).Hemicellulose (about 20-35%) is an amorphous and heterogeneous group of branched polysaccharides (copolymer of

any of the monomers glucose, galactose, mannose, xylose, arabinose, and glucuronic acid); hemicellulose surrounds the cellulose fibers and provides a linkage between cellulose and lignin. Hemicelluloses in hardwood contain mainly xylans, while in softwood glucomannans are most common (Kumar et al., 2008). There are various enzymes responsible for the degradation of hemicellulose (Fig. 21.2). In xylan degradation, for instance, endo-1,4-β-xylanase, β-xylosidase, α-glucuronidase, α-L-arabinofuranosidase, and acetylxylan esterase act on the different heteropolymers available in nature. In glucomannan degradation, β-1,4-mannanase and β-mannosidase cleave the polymer backbone. Like cellulose, hemicellulose is also an important source of fermentable sugars for biorefining applications. Xylanases are being produced and used as additives in feed for poultry and as additives to wheat flour for improving the quality of baked products at the industrial scale (Niehaus et al., 1999).Pectin (about 2-20%) is a heteropolysaccharide consisting of a linear chain of α-(1-4)-linked D-galacturonic acid that forms the pectin backbone, a homogalacturonan in the primary cell walls. Into this backbone, there are regions where galacturonic acid is replaced by α-(1-2)-linked L-rhamnose. Pectin has also found widespread commercial use, especially in the textile industry and in the food industry as a thickener, texturizer, emulsifier, stabilizer, Wller in

confections, dairy products, bakery products, and so on (Liu et al., 2006). Despite these applications, pectins are similar to cellulose and hemicelluloses, being common waste materials that can be converted to soluble sugars, ethanol, and biogas (Doran et al., 2000; Hutnan et al., 2000). Many kinds of degrading enzymes are involved in pectin degradation (Fig. 21.3). They may be acting either by hydrolysis or by transelimination, the latter performed by lyases. Pectin-degrading enzymes, i.e., polymethylgalacturonase, (endo-) polygalacturonase pectin depolymerase, pectinase, exopolygalacturonase, and exopolygalacturanosidase hydrolyze the polygalacturonic acid chain of the pectin polymer by the addition of a water molecule (Jayani et al., 2005). α-L-Rhamnosidases hydrolyze rhamnogalacturonan in the pectic backbone. α-L-arabinofuranosidases hydrolyze the L-arabinose side chains, and endo-arabinase act on arabinan side chains in pectin (Takao et al., 2002). These two enzymes operate synergistically in degrading branched arabinan to yield L-arabinose. Polysaccharide lyases (PLs) cleave the galacturonic acid polymer by β-elimination and comprise, e.g., polymethylgalacturonate lyase (pectin lyase), polygalacturonate lyase (pectate lyase), and exopolygalacturonate lyase (pectate disaccharidelyase).Lignin (about 10-20%) is a highly complex 3-D polymer of different phenylpropane units bound together by ether and carbon-carbon bonds (Fig. 21.4).