Biomedical Engineering Reference
In-Depth Information
reesei and represents about 60% of the total secreted cellulases. It has been
shown that this enzyme is processive and hydrolyses the cellulose from
the reducing end (Divne et al. 1998). Cel6A (CBHII) hydrolyses cellulose
from the non-reducing end and represents about 10-15% of the total
cellulases. The two major secreted endoglucanases are Cel7B and Cel5A,
each representing up to 10% of the total cellulases. Two β-glucosidases
(BGL1/Cel3A and BGL2/Cel1A) have been isolated from the supernatants
of T. reesei , but the production of these enzymes is very low compared with
other cellulolytic fungi like A. niger or Ta. emersonii (Rahman et al. 2009)
and also suffer from product inhibition (Chauve et al. 2010). Therefore, the
major focus in enhancing the cellulolytic performance was in improving
BGL activity either by heterologous expression of other BGL's such as
Cel3A from Ta. emersonii (Murray et al. 2004) or by enzyme engineering
of the T. reesei Cel3A (Ayrinhac et al. 2011). Despite being the best known
producer of cellulases, the genome sequencing of this fungus revealed
fewer cellulases and hemicellulases than other sequenced fungi (King et
al. 2011), which will further stimulate improving the performance of this
strain by heterologous expression of other (hemi)cellulolytic activities.
Thermostable Enzyme Cocktails for
Lignocellulose Degradation
Thermophiles are gaining more interest today as a source of industrially
relevant thermostable enzymes. The benefits of using thermostable
enzymes are primarily related to the fl exibility of the process conditions
which require stable enzymes in a wide range of temperatures and
pH, easier mixing in a high solids environment and a reduced risk of
contamination during the hydrolysis process. Many fungal strains such
as Thermoascus auranticus, Ta. emersonii and Sporotrichum thermophile can
produce thermostable enzymes with an optimum temperature above 60°C
and an acidic pH (Turner et al. 2007). As acid pretreatment, one of the most
applied feedstock pretreatment methods, is at high temperature, there is
a lot of potential for using these enzymes at industrial scale (Viikari et
al. 2007). Cultivation of thermophiles, although economically interesting
due to reduced risk of contamination and reduced viscosity, so far led to
low biomass yields and therefore low protein titers (Turner et al. 2007).
Therefore, the high cost of large scale fermentation of thermophilic fungi
remains a bottleneck in the production of these highly effi cient enzymes
for biomass degradation.
Most of the developments in the fi eld of thermostable cellulases are
currently aiming towards recombinant production in selected high protein
producers as T. reesei (Viikari et al. 2007), but this can lead to improper
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