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Carboxybydrolases
of Trichoderma reesei: structure, mode of action,
regulation and application
Abstract:
Cellulose is the most abundant source of carbon in the
world. This plant polysaccharide is decomposed by several
microorganisms. One of the best producers of cellulases which has
found industrial applications are fungi from the type Trichoderma.
Apart from cellulases, other enzymes which occur in the culture
filtrates (i.e. xylanases, proteases, chitinases, and b-1,3-glucanases) are also involved in the hydrolysis of plants
biomass and play other significant roles. Three different types of
enzymes of T. reesei are involved in the degradation
of cellulose. At least two cellobiohydrolases (CBH), several
endo-1,4-glucanases (EG) and b-glucosidases
have been identified and characterized. CBHI is the dominant
enzyme of cellulolytic complex forming up to 60% of the secreted
protein by T. reesei. The proportion of CBHII amouts
to around 20% and endoglucanase, most of which endoglucanase I (EGI)
comprise 10%. b-glucosidases
account for only 1% of the total secreted proteins. Molecule of
CBHI (which is 18 nm long) consist of three domains: cellulose
binding domain, hinge domain and catalytic core domain. Genetic
improvements of T. reesei QM6a and its deratives for
cellulase production has been investigated in several laboratories
and hyperproductive mutants were obtained by treatment with UV
light, gamma irradiation or diethyl sulphate. The application of
genetic engineering to Trichoderma has made it possible to
modulate cellulase production in such away that new T. reesei
strains producing novel cellulase profiles are now available.
Irrespective of genetic engineering advancement, it seems that the
knowledge of regulatory mechanism of cellulases is required for
increased production of these enzymes. b-1,4-xylan,
next to cellulose is the most abundant hemicellulose fraction. Due
to structural heterogeneity, xylan degrading enzyme system
includes several hydrolytic enzymes. One of the best known of
these enzymes are endo-b-l,4-xylanases
which attact the main chain of xylan and b-xylosidase
which hydrolyses xylooligosaccharides to D-xylose. Recently, trend
of research seems to develop towards production cellulase-free b-1,4-xylanases.
Such preparations are used in kraft pulp for bleaching processes
as an alternative to chlorine-based methods.
1. Introduction.
2. Characteristic of cellulolytic enzymes complex of Trichoderma
reesei. 2.1. Cellobiohydrolases. 2.2. Endoglucanases.
2.3. b-glucosidases.
3. Molecular structure of cellulases. 4. Cellulases genes
from T. reesei. 5. Genetical modification of T. reesei.
5.1. Mutagenisation. 5.2. Genetic engineering. 6. Regulation
of production and secretion of cellulases. 6.1. Regulation of
cellulases formation on molecular level. 6.2. Induction and
inducers of production of cellulases. 6.3. Carbon catabolite
repression of biosynthesis of cellulases. 6.4. Secretory
pathways of cellulases of T. reesei. 7. Application
of cellulases of T. reesei. 8. Additional enzymes
of culture filtrates of T. reesei. 8.1. Xylanases.
8.2. Application of xylanolytic enzymes of T. reesei.
8.3. Proteases. 8.4. b-glucanases.
8.5. Chitynases. 8.6. Application of lytic enzymes of T. reesei.
9. Final remarks
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