Biomedical Engineering Reference
In-Depth Information
cellulase in the lignocellulosic materials. Cellobiose is the main intermediate of
cellulose degradation, but it can induce strains to produce cellulase only at lower
concentrations (
10 mM), but inhibits at a concentration higher than 20 mM.
This critical concentration value for the shift of induction and inhibition effect
greatly differs from species to species. Gao et al. found that
<
-glucosidase isozymes
had three different subcellular localizations: intracellular, extracellular, and plasma
membrane bound. They summed up the “subregulatory network” model to under-
stand multisubcellular localization of
“
-glucosidase enzyme isozyme involved in
the formation of inducer in the process of conversion. Multisubcellulars localize
“
“
-GLase isozyme to constitute a subregulatory network that can further adjust
cells' tolerable dosage of cellobiose and critical threshold values for generating
direction, rate, and degree of strong inducer through further energy metabolization
and transglycosylation. The difference degree of species of the
-GLase isozyme
decides the difference of the inducing and inhibition effect of the same concentration
of cellobiose on cellulase synthesis [
34
,
35
].
“
(1) Cellulase-inducing synthesis
In the presence of cellulose, many of the filamentous fungi can synthesize cellulase
that is ten times higher than that of the glucose culture. Culture medium has a
significant effect on enzyme synthesis. So, it is speculated that the degradation
products of cellulose can be the inducer of cellulase synthesis, such as cellobiose,
lactose, sophorose, sorbitol, gentiobiosyl, methyl-
-glucosidase, and cellobiose
sugar lactone. They can improve the enzyme synthesis of many cellulose microor-
ganisms [
36
,
37
]. In
Escherichia coli
, lactose induces
“
-galactosidase synthesis
regularly. But, the inducing effect of fungi as an inducer compatible with substrate
is not so regular.
Generally, it is believed that oligosaccharides and derivatives released from the
polymeric substrate are truly inducers of cellulase expression. Sophorose is regarded
as a natural soluble inducer for
T. reesei
cellulase, which is synthesized by the
transglycosylation effect of
“
-glucosidase on the cellobiose [
38
]. However, the
synthesis of cellulase can be induced by sophorose powerfully only for specific
fungi, such as
T. reesei
,
Aspergillus terreus,
and
Penicillium purpurogenum
,but
has no inducing effect on other cellulase-producing fungi, such as
Penicillium
janthinellum
,
Penicillium chrysosporium
,
Aspergillus nidulans,
and
A. niger.
The adjusting effect of cellobiose has a dual nature. Previous studies have shown
that cellobiose induces the synthesis of cellulase for most fungi. But, when the
concentrations of cellobiose are different, the cellulase-inducing effect will be in
contradiction. Cellobiose has an inducing effect at low concentrations on cellulase
synthesis but an inhibiting effect when the concentration is higher than a certain
value. Cellobiose can be hydrolyzed by
“
-glucosidase to generate glucose and
inhibit the synthesis of cellulase and can induce the synthesis of cellulase by the
transglycosylation effect of
“
-glucosidase to generate sophorose. The balance of the
final result depends on cellobiose regulation of
“
-glucosidase enzyme hydrolysis
and transglycosylation [
39
]. In the natural process of cellulose hydrolysis, the
“
