Cellulolytic Enzyme Production and Enzymatic Hydrolysis for Second-Generation Bioethanol Production - Biotechnology in China: Biofuels and Bioenergy - page 3

Table 1 Comparison of dosage requirement of amylase, glucoamylase, and cellulase during
hydrolysis
Enzyme Activity Enzyme loading

Amylase a 192 KNU/mL 0.2 KNU/g starch

Glucoamylase b 652 AGU/mL 3.26 AGU/g starch

Cellulase c

100 FPU/mL 20 FPU/g glucan

KNU kilo Novo unit, AGU amyloglucosidase unit, FPU filter paperase unit
a

Liquozyme SC DS was used [ 10 ]

b

Spirizyme was used [ 10 ]

c

Accellerase TM 1000 was used [ 11 ]

making its enzymatic hydrolysis heterogeneous, which significantly compromises
the reaction rate. Therefore, higher cellulase loadings are needed for cellulose
hydrolysis compared with hydrolysis of starch by amylase and glucoamylase
(Table 1 ). Merino and Cherry [ 12 ] suggested that the required enzyme concen-
tration for cellulose degradation is 40-fold to 100-fold higher than that of starch
hydrolysis, resulting in a high enzyme cost during cellulose degradation. There-
fore, the breakdown of lignocellulosic biomass to liberate sugars is much more
expensive [ 13 ], which accounts for a large fraction of bioethanol production costs
[ 14 ]. One strategy is reducing the cellulase production cost through improving
cellulase titers during the fermentation, and another is optimizing the cellulase
composition to enhance the synergetic effect of different enzymatic components
during cellulose hydrolysis so that the specific activity of cellulase is improved and
the enzyme dosage is minimized.
In this chapter, we review the updated progress in cellulase research and pro-
duction and enzymatic cellulose hydrolysis for the production of the second-
generation bioethanol.

2 Lignocellulose Biodegradation

2.1 Cellulolytic Enzymes

Most known cellulolytic enzymes are produced and excreted by filamentous fungi,
among which Trichoderma reesei has received special attention for its hyperenzy-
matic production capabilities [ 15 - 17 ]. It has been subsequently used as a model
organism for investigations of enzymatic hydrolysis mechanisms. Proteomic anal-
ysis of these enzymes with high-resolution two-dimensional gel electrophoresis
revealed many glycoside hydrolases with this species [ 15 , 17 ], which can be divided
into two main groups: cellulases and hemicellulases.
Cellulases are enzymes that hydrolyze cellulose, a linear polysaccharide mole-
cule composed of repeated b-(1?4) linked D -glucopyranosyl (Glc) units. Multiple
types of cellulases have been discovered, including at least two exo-b-glucanases,