Biomedical Engineering Reference
In-Depth Information
the lignin; they exert catalysis through the small-molecule intermediate products,
reduce the cross-linking degree, and greatly improve the solubility of lignin. In fact,
the mechanism of biopulping has not been fully understood. (3) Also researched is
the biopulping process and the development of related bioreactors.
8.2.1
Mechanism of Biopulping
8.2.1.1
Enzymatic Degradation of Lignocellulose by White-Rot Fungi
White-rot fungi can secrete a variety of extracellular enzymes. Most of them can
secrete cellulase, hemicellulase, ligninase, pectinase, protease, and other enzymes.
In 1983, Tien [
9
] and Glenn et al. [
10
] found LiP. In 1984, Kuwahara et al. [
11
]
found MnP. In the 1990s, research on the catalytic characteristics of enzymes and
molecular biology were popular. Lac was used to remove pulp lignin in 1993, and
MnP was used for pulp [
12
] in 1996. Plant tissues are degraded by extracellular
enzyme used by the white-rot fungi. Ligninases are the most important enzymes
used in biopulping; they include LiP, MnP, Lac, aromatic-ring-splitting enzymes,
and the lactoperoxidase system [
13
].
The substrates that are catalyzed by Lac are not limited to nonphenolic lignin;
according to statistics, the number of substrates has reached 250 [
14
]. The types
of substrates according to their structures include phenols and their derivatives,
aromatic amines and their derivatives, carboxylic acids and their derivatives,
metal organic compounds, other nonphenolic substrates, and so on. Phenols
and their derivatives account for about half of the total substrates, including
o
-dihydroxybenzene and
p
-dihydroxy benzene and their derivatives. Lac cannot
oxidize phenol but catalyzes
-naphthol and some phenol derivatives. Lac is a
copper-containing protein, and it can catalyze polyphenols through four successive
one-electron transfer reactions to form quinones and free radicals using O
2
as the
recipient. In vitro, lignin monomers form polymers under the condition of the
Lac/O
2
. In the bacteria, it can degrade lignin with the help of other ligninolytic
enzymes.
'
8.2.1.2
Growth Process of White-Rot Fungi in Wood Chips
White-rot fungi can grow in the cells of all conifer and broadleaf trees. During
the growth process of white-rot fungi in wood chips, the mycelia enter into the
cell cavity of wood chips; they first grow in the ray parenchyma cells and then
penetrate into other cells through the pits or directly through the cell wall. The cell
wall immediately begins to be degraded after the decrease of the metabolic nutrients
(Figs.
8.1
,
8.2
,
8.3
, and
8.4
)[
15
].
White-rot fungi attack the cell wall components in the uneven distribution areas
and further pass through the primary wall and middle lamella. When white-rot fungi
