Technical Reference of Biomass Polymers
and Associated Enzymes
Cellulose
Cellulose is completely composed of glucose. The glucose units are b-(1,4) linked with no side chains attached, allowing for crystalline packing of individual cellulose chains. This is different from starch, which is also composed entirely of glucose. The glucose units in starch are a-(1,4) linked with a-(1,6) linked side chains attached, allowing for amorphous packing of individual starch chains.
Cellulases
Three broad categories:
Endo-cellulases - enzymes that cleave at internal glycosidic linkages in cellulose and generate new reducing ends in the middle of the cellulose chain. These enzymes produce a range of cellodextrin products. Endo-cellulases may also act on these initial cellodextrin products to produce glucose, cellobiose, or cellotetraose.
Cellobiohydrolases - enzymes that typically cleave cellobiose or cellotetraose from either the reducing or non-reducing ends of the cellulose chain.
β-glucosidases - enzymes that produce glucose from cellobiose, cellotetraose, or other cellodextrins. These enzymes may also produce glucose from the ends of cellulose chains at a low rate.
Hemicellulose (Xylan)
The terms hemicellulose and xylan are often used interchangeably, though this is not completely accurate. Most hemicellulose found in biomass is based on a xylose backbone, and in this case the terms are interchangeable. Hemicelluloses are heterogeneous polymers of pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose), and sugar acids. Unlike cellulose, hemicelluloses are not chemically homogeneous. Hardwood hemicelluloses contain mostly xylans, whereas softwood hemicelluloses contain mostly glucomannans.
Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of (1,4)-linked β-D-xylopyranose units. Besides xylose, xylans may contain arabinose, glucuronic acid or its 4-O-methyl ether, and acetic, ferulic, and p-coumaric acids. Xylans from different sources differ in composition; the frequency and composition of branches are dependent on the source of xylan. The backbone consists of O-acetyl, α- L-arabinofuranosyl, α-(1,2)-linked glucuronic or 4-O- methylglucuronic acid substituents with about 80% of the backbone being highly substituted with monomeric side-chains of arabinose or glucuronic acid linked to O-2 and/or O-3 of xylose residues, and also by oligomeric side chains containing arabinose, xylose, and sometimes galactose residues. However, unsubstituted linear xylans have also been isolated. Xylans can be categorized as linear homoxylan, arabinoxylan, glucuronoxylan, and glucuronoarabinoxylan.
Hemicellulases (or xylanases)
Three broad categories:
Endo-hemicellulases (or endo-xylanases) - enzymes that cleave at internal glycosidic linkages in xylan and generate new reducing ends in the middle of the hemicellulose chain. These enzymes produce a range of products. Endo-hemicellulases may also act on these initial xylodextrin products to produce xylose, or other xylooliglsaccharides.
β-xylosidases - enzymes that produce xylose from xylodextrins and xylans. These enzymes may also produce arabinose from arabinoxylan at a low rate.
Other enzymes - These include a-arabinofuranosidases, a-glucuronidases, and ferulyl and acetyl esterases.
β-glucan
Beta-glucans are polymers of glucose. There are two types differentiated by the linkages found in them. Yeast and fungal β-glucans are linear polymers of β-(1,3)-linked glucose with β-(1,6)-linked glucose side chains attached. Cereal β-glucans are linear polymers of alternating β-(1,3) and β-(1,4) linked glucose residues with no side chains attached.
β-glucanases
Two broad categories:
Yeast-type - enzymes that cleave internal β-(1,3)-linkages in yeast or fungal β-glucans.
Cereal-type - enzymes that cleave internal β-(1,4)-linkages in cereal beta-glucans. Many cellulases and some xylanases also possess this activity.
