Cellulolytic Enzyme Production and Enzymatic Hydrolysis for Second-Generation Bioethanol Production - Biotechnology in China: Biofuels and Bioenergy - page 4

or cellobiohydrolases (CBHs; EC 3.2.1.91) (CBH I and CBH II), four endo-b-
glucanases (EGs; EC 3.2.1.4) (EG I, EG II, EG III, EG V), and one b-glucosidase
(bG; EC 3.2.1.21) [ 18 ].
Hemicellulases are enzymes that degrade hemicelluloses, a group of polysac-
charides and one of major plant cell wall components. Unlike cellulose, which is
composed entirely of glucosyl moieties linked by only b-1, 4-glycosidic bonds,
various types of sugar moieties linked by different bonds, intramolecular archi-
tecture, and intermolecular interactions can be found within hemicelluloses.
Hemicelluloses also differ from the major plant cell wall structural component
cellulose in their much smaller polysaccharide chains. Hemicelluloses can serve to
cross-link cellulose microfibrils by interconnecting them as well as linking cel-
lulose molecules to other cell wall components [ 19 ]. Common hemicelluloses
include b-glucan (different from cellulose), xylan, xyloglucan, arabinoxylan,
mannan, galactomannan, arabinan, galactan, and polygalacturonan. Corresponding
to these hemicelluloses, various types of hemicellulases exist, each specific to a
kind of hemicellulose [ 19 - 23 ]. These enzymes can be clustered into two groups:
hemicellulases that attack the polysaccharide backbone and those that attack the
side chains.
Xylan, whose structure differs from plant to plant, is the second most abundant
polysaccharide in herbs and hardwoods, demanding the collaboration of a group of
enzymes during their degradation. Multiple enzymes including endo-b-xylanase
(EC 3.2.1.8), b-xylosidase (EC 3.2.1.37), a-glucuronidase (EC 3.2.1.139),
a- L -arabinofuranosidase (EC 3.2.1.55), and acetylxylan esterase (EC 3.1.1.6) act
synergistically in this process (Fig. 1 ). Endo-1,4-xylanases, cleave internal b-1,
4-xylosidic bonds on the xylan polysaccharide backbone. Unlike EGs, whose
cleavage sites are random, endo-1,4-xylanases recognize specific bonds for
cleavage on the basis of polysaccharides properties such as chain length and
branching levels [ 20 ]. Endoxylanases were initially classified into two groups by
their ability to hydrolyze the 1, 3-a- L -arabinofuranosyl branching points of ara-
binoxylans: hydrolyzing and nonhydrolyzing endoxylanases, which have different
pI values and molecular weights [ 21 ]. However, these patterns were shown to
account for only 70% of all endoxylanases, and a classification system of
all glycoside hydrolases (glycoside hydrolase families) is better recognized
now [ 22 ]. Products from xylan degradation by endoxylanases are a mixture of
b- D -xylopyranosyl oligomers of various lengths, which serve as substrates for
b-xylosidases that subsequently hydrolyze them to xylose from the nonreducing
end of these oligomers [ 20 ]. Multiple other enzymes are also involved in xylan
degradation, primarily due to the complex nature of these polysaccharides:
a- L -arabinofuranosidase cleaves the a-glycosidic bonds between arabinose and
xylose moieties in xylan; a-glucuronidases cleave the a(1?2) bonds linking the
(methyl) GlcU units in xylan [ 19 ]; acetylxylan esterase removes the O-acetyl
groups at the 2- and 3-positions of b- D -xylopyranosyl residues; ferulic acid
esterase (EC 3.1.1.73) cleaves the ester bond between the arabinose and ferulic
acid side chains; and p-coumaric acid esterase (EC 3.1.1.73) cleaves the ester bond
between arabinose and p-coumaric acid [ 20 ].