Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 204

Fig. 3 Complex mixture of enzymes for degrading hemicelluloses (modified from [ 18 , 26 ]). The
example depicted is cross-linked glucuronoarabinoxylan. The complex composition and structure
of hemicellulose require multiple enzymes to break down the polymer into sugar monomers,
primarily xylose, but other pentose and hexose sugars are also present in hemicelluloses.
A variety of debranching enzymes (red) act on diverse side chains hanging off the xylan
backbone (blue). These debranching enzymes include arabinofuranosidase, feruloyl esterase,
acetylxylan esterase, and a-glucuronidase (see Table 3 ). As the side chains are released, the xylan
backbone is exposed and made more accessible to cleavage by xylanase. b-Xylosidase cleaves
xylobiose into two xylose monomers; this enzyme can also release xylose from the end of the
xylan backbone or a xylo-oligosaccharide

Table 3 Glycoside hydrolase
(GH) and carbohydrate
esterase (CE) enzyme
families for degrading
hemicelluloses (adapted
from [ 18 ])

Enzyme Enzyme families

Endoxylanase GH5, 8, 10, 11, 43

b-Xylosidase GH3, 39, 43, 52, 54

a-L-Arabinofuranosidase GH3, 43, 51, 54, 62

a-Glucuronidase GH4, 67

a-Galactosidase GH4, 36

Acetylxylan esterase CE1, 2, 3, 4, 5, 6, 7

Feruloyl esterase CE1

hydrolyze hemicellulose to its constituent sugars; that is to say, some of these
enzymes are multifunctional, with catalytic domains belonging to different enzyme
families. Their great diversity and that of other enzymes involved in hemicellulose
degradation present a remarkably complicated enzymatic system whose more
thorough analysis may yield a greater understanding of hemicellulosic degradation.