Cellulolytic Enzyme Production and Enzymatic Hydrolysis for Second-Generation Bioethanol Production - Biotechnology in China: Biofuels and Bioenergy - page 9

suggested that the irreversibly bound enzyme molecules undergo a conformational
change and are inactivated. This shows that the cellulose degradation mechanism is
still far from being fully understood, and many problems remain.
2.3 Factors Affecting Enzymatic Hydrolysis
The enzymatic hydrolysis of cellulose involves EG, CBH, and bG working in
synergy as discussed in Sect. 2.2 . The last step in this complicated process
involves the degradation of cellobiose to glucose by bG. However, T. reesei only
secretes low levels of bG, resulting in the accumulation of cellobiose during
hydrolysis, which in turn contributes to the inhibition of CBH and EG activity.
This inhibition effect caused by cellobiose accumulation is more than that caused
by glucose accumulation [ 44 ]. Supplementing the native T. reesei enzyme cocktail
with additional bG from other species of fungi with higher specific activities,
better affinity for substrates, and lower inhibition by glucose is therefore common
to mitigate cellobiose inhibition [ 44 , 45 ]. Zhang et al. [ 9 ] showed that the addition
of commercial Aspergillus niger bG preparation with 30 cellobiase units per gram
of glucan was enough to eliminate cellobiose inhibition completely. Chen et al.
[ 44 ] showed improvement in saccharification with the addition of purified bG
from Penicillium decumbens to the cellulase cocktail of T. reesei. Ma et al. [ 46 ]
introduced the bG I encoding sequence pbgl1 from P. decumbens into the genome
of T. reesei Rut C-30 and placed it under the control of the cbh1 promoter from
T. reesei. This recombinant T. reesei strain presented significantly increased bG
activity, filter paper activity, and saccharification of pretreated cornstalk compared
with the parent strain. Shen et al. [ 47 ] reported the heterologous expression of the
bG-encoding gene bgl1 from Saccharomycopsis fibuligera in the industrial yeast
species Saccharomyces cerevisiae. This recombinant yeast strain gained cellobi-
ose-hydrolyzing ability, and improved biomass hydrolysis efficiency during
cellulose degradation when used in combination with cellulase preparations.
Han and Chen [ 48 , 49 ] extracted bG from postharvest corn stover, and showed
that supplementation of commercial fungal cellulase with this purified bG is more
effective than supplementation with bG from A. niger.
Lignocellulosic materials are primarily composed of three components: cellulose,
hemicelluloses, and lignin. These are entangled together, forming a complicated
matrix in which cellulose is well protected. Most cellulolytic microbes encode an
array of hemicellulases along with cellulases to digest hemicelluloses, break down
the matrix, and thus increase accessibility of their cellulases to cellulose. Kumar and
Wyman [ 50 ] identified the relationship between xylanase/xylosidase activity
and cellulose hydrolysis activity. Zeng and Chen [ 51 ] reported that the addition of
ferulic acid esterase to steam-exploded rice straw accelerates its hydrolysis rate by
breaking some of the ester bonds between lignin and hemicellulose, and improving
accessibility to cellulose. Zhang et al. [ 9 ] reported that supplementation of pretreated