Cellulolytic Enzyme Production and Enzymatic Hydrolysis for Second-Generation Bioethanol Production - Biotechnology in China: Biofuels and Bioenergy - page 10

lignocellulose samples with xylanase and pectinase significantly increases the
conversion of cellulose and hemicellulose, with pectinase being more effective.
Hemicellulose degradation, particularly xylan, has also been shown to involve
multiple enzymes working in synergy. It was suggested that the acetyl groups in
acetylated b- D -xylopyranosyl residues hinder the binding of endoxylanases to the
xylan backbone [ 20 ], and acetylxylan esterases remove this hindrance and increase
the accessibility of endoxylanases to the xylan molecule. On the other hand,
b-xylosidases are believed to degrade xylooligosaccharides, the product of xylan
degradation by endoxylanases and also the inhibitor of the latter enzyme. The
presence of b-xylosidases is thus essential for the complete hydrolysis of xylan, as
well as the full activity of endoxylanases [ 20 , 52 ]. The synergistic effect of enzymes
involved in xylan degradation is a complicated process that demands further
investigation because of the heterogeneity and the complicated structure of the
substrate. Indeed, previous research identified the complexity of the synergism
during the degradation of xylan [ 53 ].
The enzymatic hydrolysis of lignocellulose is a complicated process that is
affected by many factors. Liu et al. [ 6 ] suggested that different pretreatment
methods result in different lignocellulosic biomass compositions and structures
after pretreatment and that these changes significantly influence the ethanol yield
from lignocellulosic biomass. Furthermore, the absorption and desorption of cel-
lulase were shown to be related to the lignin content of the biomass, which has a
significant effect on enzymatic hydrolysis [ 6 ]. Yu et al. [ 54 ] treated corn straw
biologically with white rot fungi prior to alkaline/oxidative pretreatment and
showed that this biological treatment decreases the lignin content significantly and
improves cellulase desorption. Many other pretreatment methods have also been
used by Chinese scientists, including microwave and alkaline pretreatment, dilute
acid pretreatment, organosolv pretreatment, steam explosion, biodelignification,
and hot compressed water pretreatment [ 4 ]. These pretreatment approaches gen-
erally aim to break the physical barriers of lignocellulose and remove lignin, which
competitively binds to CBH I and inhibits the binding of CBH I to cellulose
[ 4 , 55 ]. In lignocellulosic biomass, hemicelluloses are entangled with cellulose,
which protects the latter from cellulase degradation. In some pretreatment
approaches, hemicelluloses are also removed, thereby exposing cellulose to cel-
lulases and affecting the types of enzymes required for later enzymatic hydrolysis.
One such example is steam explosion, in which hemicelluloses are removed from
lignocellulose, thereby affecting the types of enzymes required and decreasing the
necessity for hemicellulases in subsequent enzymatic hydrolysis steps [ 56 ].
Aside from investigations on different pretreatment methods, multiple reports
have focused on other factors that affect cellulases and lignocellulose biodegra-
dation. Further investigation by Chinese scientists identified a series of nonenzyme
factors that improve the enzymatic hydrolysis efficiency of lignocellulosic
biomass. This suggests that reducing biomass recalcitrance using microbes is a
complicated process that involves multiple factors that are not fully understood.
Wang et al. [ 57 ] found a low molecular weight peptide called short fiber
generating factor in the supernatant of Trichoderma pseudokoningii S38 culture.