Enzymatic Hydrolysis of Hemicellulose
Hemicellulose is the second most abundant polysaccharide in nature. It comprises 20-40% of lignocellulosic biomass depending on the type of biomass used. The major component of hemicellu — lose is xylan. Unlike cellulose, hemicellulose is a branched polymer consisting of shorter chains, typically 500-3,000 sugar units. Hemicellulose is a more complex polysaccharide than cellulose, as it is composed of several types of polymers. The most prevalent hemicellulose is xylan, composed of d-xylopyranosyl units linked by P-1,4-glycosidic bonds.
In hardwood, the xylan backbone is modified with various side chains, including 4-O-methyl-D-glucuronic acid linked to the xylose units via a-1,2-glycosidic bonds and acetic acid that esteri — fies the xylose units at the O-2 or O-3 positions. In non-acetylated softwood xylans, in addition to uronic acids, there are l-arabi — nofuranose residues attached to the main chain by a-1,2 and/ or a-1,3-glycosidic linkages. In grasses, phenolic substitutions of p-cumaric acid and ferulic acid esterify the arabinofuranoses. The abundance and linkage types of these substitutions vary between xylans from different sources. The в-mannan-based polymers are also major components of hemicelluloses. Their backbone is made of P-1,4-linked mannose residues alone, or mannose and glucose residues randomly distributed. Galactoglucomannans also contain a-1,6-linked galactose side chains, and the 0-2 and 0-3 of the mannose units can be substituted with acetate groups. Arabinan and arabinogalactans are generally also classified as hemicelluloses, although they originate from the "hairy" parts of galacturonan or from cell wall glycoproteins. The arabinan comprises a backbone of a-1,5-linked L-arabinofuranosyl units, which are further decorated with a-1,2- and a-1,3-linked L-arabinofuranosides. The backbone of arabinogalactans is composed of P-1,3-linked galactose residues, which are substituted with ^-1,6-linked galactose units and a-1,3- linked L-arabinofuranosyl or arabinans side chains. Some common molecular motifs found in hemicellulose are shown in Figure 4.7 under the structure of hemicellulose in Chapter 4. Unlike cellulose, the structure and composition of hemicellulose can vary depending on the plant source.