maximal productivity of lactic acid (3.14 g L
−1 h
−1 ) was
attained; any decrease in concentration of xylose leads to
lower lactic acid yields (Abdel-Rahman et al. 2016). Bacillus
coagulans C106, a thermophilic strain, has also been
reported with the ability to ferment xylose to lactic acid with
a theoretical yield of 100% using PPP (Ye et al. 2013).
11.2 Bioethanol Production
Bioethanol also known as ethyl alcohol is one of the most
studied biofuels. It is considered as a petrol substitute and
less toxic fuel for transport vehicles (Rodrussamee et al.
2018) as its use can reduce Carbon dioxide (CO 2 ) emission
(Selim et al. 2018). Bioethanol is a high octane number
biofuel produced by fermentable sugars and is generally
classified into first, second, and third generations. The
first-generation bioethanol production utilizes food feedstock,
primarily starchy materials. Whereas bioethanol of second
generation is manufactured from LC biomass, such biomass
is generally easily available and is more cost-effective. Since
second-generation bioethanol is based on non-food feedstock, it does not compete with food supplies. Production of
third-generation bioethanol is rooted on algal cultivation
using waste streams from industries as substrate (Robak and
Balcerek 2018). In comparison to other raw materials such as
molasses and starchy substrates, LC biomass is considered as
most economical (Singla et al. 2012).
The ability of microorganisms to utilize glucose, xylose,
galactose, arabinose, and mannose is prerequisite to ethanol
production from biomass (Hahn-Hägerdal et al. 2006). Other
ideal attributes include the ability to display broad substrate
specificity as well as “ethanol tolerance”, substrate, temperature, and inhibitory compounds in hydrolysates (Pasha
et al. 2007). Generally, inhabitant xylose-fermenting yeasts,
for example, Pachysolen tannophilus (Jeffries et al. 1985),
P. stipitis (Toivola et al. 1984), Scheffersomyces (Candida) shehatae (Chandel et al. 2007), and Kluyveromyces
marxianus (Nitiyon et al. 2016; Rodrussamee et al.
2011) can ferment both glucose and xylose.
Among several microorganisms, yeasts have shown to
exhibit privileged yields of ethanol from xylose. So far, the
most comprehensively studied xylose-fermenting yeasts
include Pichia stipitis, Pachysolen tannophilus, and Candida shehatae. Among which C. shehatae and P. stipitis are
the most desirable inhabitant producers of ethanol with near
theoretical yield of 0.51 g ethanol g
−1 xylose.
According to a recent report, S. cerevisiae and P. stipitis
are two most potent model organisms in fermentation technology because of their higher productivity and ethanol
yield. They have considerably higher tolerance to ethanol
and certain inhibitors present in LC hydrolysates. Besides,
they can utilize various carbohydrate rich substrates through
fermentation processes (Selim et al. 2018; Vilela L de et al.
2015). Though these fermentation processes are influenced
by various toxic substances and inhibitory compounds that
restrict the enzymes activity resulting in failure to obtain
desired products (Wikandari et al. 2019). The cofactor
imbalance due to co-expression of XR and XDH is discussed
in detail in previous sections. It is considered that this
cofactor imbalance is the main reason for less efficient
xylose fermentation for bioethanol production.
Two key approaches including cloning of XR and XDH
and/or XI have been generally used to overcome this issue.
Moreover, additional genetic modifications like (i) overexpression of enzymes required for conversion of xylulose into
glycolysis intermediates, (ii) mutagenesis of aldose reductase (transforms xylose to xylitol), and (iii) overexpression
of heterologous xylose transporters have also been implemented. Besides all these genetic approaches to improve
xylose fermentation, ethanol yield by recombinant S. cerevisiae is still insignificant and simultaneous fermentation of
xylose and glucose is yet another bottleneck.
In an effort to control the repressive effect of glucose over
the utilization of xylose, Vilela et al. (2015) described
heterologous expression of a XI gene (xylA)
from Burkholderia cenocepacia into S. cerevisiae strain
that exhibited anaerobic fermentation of xylose, without
xylitol accumulation. Moreover, an evolutionary engineering strategy was manipulated through sequential batch fermentation on xylose to the xylA-expressing strain for
improving xylose fermentation. The resulting yeast strain
was capable of co-fermenting xylose and glucose, with
enhanced ethanol production. In another study, comparison
of the commercial strains with XI (from Clostridium
phytofermentans) or XR/XDH pathway showed that
recombinant strains of S. cerevisiae with both XI and
XR/XDH pathways are the most efficient ethanol producers
(Cunha et al. 2019). To further promote higher ethanol
production from xylose, heterologous genes, Sut1 and Sut2
for sugar transporters were expressed, the customized version of S. cerevisiae could only transport xylose (Du et al.
2010). Figure 2 shows pathways to produce ethanol from
xylose using either bacteria or yeast.
11.3 Xylitol Production
Xylitol is used as a sugar substitute with low caloric content
for diabetic patients and has anti-carcinogenic properties
(Takata et al. 2014; Mishra et al. 2013; Lee et al. 2013).
A number of researches have been conducted on xylitol
production from xylose using plant biomass as a source of
raw material (Kwak et al. 2019).
Majority of xylose-assimilating yeasts synthesize xylitol
by expressing xyl1 and xyl2 genes that encode XR and XDH,
258
R. Rashid et al.
−1 h
−1 ) was
attained; any decrease in concentration of xylose leads to
lower lactic acid yields (Abdel-Rahman et al. 2016). Bacillus
coagulans C106, a thermophilic strain, has also been
reported with the ability to ferment xylose to lactic acid with
a theoretical yield of 100% using PPP (Ye et al. 2013).
11.2 Bioethanol Production
Bioethanol also known as ethyl alcohol is one of the most
studied biofuels. It is considered as a petrol substitute and
less toxic fuel for transport vehicles (Rodrussamee et al.
2018) as its use can reduce Carbon dioxide (CO 2 ) emission
(Selim et al. 2018). Bioethanol is a high octane number
biofuel produced by fermentable sugars and is generally
classified into first, second, and third generations. The
first-generation bioethanol production utilizes food feedstock,
primarily starchy materials. Whereas bioethanol of second
generation is manufactured from LC biomass, such biomass
is generally easily available and is more cost-effective. Since
second-generation bioethanol is based on non-food feedstock, it does not compete with food supplies. Production of
third-generation bioethanol is rooted on algal cultivation
using waste streams from industries as substrate (Robak and
Balcerek 2018). In comparison to other raw materials such as
molasses and starchy substrates, LC biomass is considered as
most economical (Singla et al. 2012).
The ability of microorganisms to utilize glucose, xylose,
galactose, arabinose, and mannose is prerequisite to ethanol
production from biomass (Hahn-Hägerdal et al. 2006). Other
ideal attributes include the ability to display broad substrate
specificity as well as “ethanol tolerance”, substrate, temperature, and inhibitory compounds in hydrolysates (Pasha
et al. 2007). Generally, inhabitant xylose-fermenting yeasts,
for example, Pachysolen tannophilus (Jeffries et al. 1985),
P. stipitis (Toivola et al. 1984), Scheffersomyces (Candida) shehatae (Chandel et al. 2007), and Kluyveromyces
marxianus (Nitiyon et al. 2016; Rodrussamee et al.
2011) can ferment both glucose and xylose.
Among several microorganisms, yeasts have shown to
exhibit privileged yields of ethanol from xylose. So far, the
most comprehensively studied xylose-fermenting yeasts
include Pichia stipitis, Pachysolen tannophilus, and Candida shehatae. Among which C. shehatae and P. stipitis are
the most desirable inhabitant producers of ethanol with near
theoretical yield of 0.51 g ethanol g
−1 xylose.
According to a recent report, S. cerevisiae and P. stipitis
are two most potent model organisms in fermentation technology because of their higher productivity and ethanol
yield. They have considerably higher tolerance to ethanol
and certain inhibitors present in LC hydrolysates. Besides,
they can utilize various carbohydrate rich substrates through
fermentation processes (Selim et al. 2018; Vilela L de et al.
2015). Though these fermentation processes are influenced
by various toxic substances and inhibitory compounds that
restrict the enzymes activity resulting in failure to obtain
desired products (Wikandari et al. 2019). The cofactor
imbalance due to co-expression of XR and XDH is discussed
in detail in previous sections. It is considered that this
cofactor imbalance is the main reason for less efficient
xylose fermentation for bioethanol production.
Two key approaches including cloning of XR and XDH
and/or XI have been generally used to overcome this issue.
Moreover, additional genetic modifications like (i) overexpression of enzymes required for conversion of xylulose into
glycolysis intermediates, (ii) mutagenesis of aldose reductase (transforms xylose to xylitol), and (iii) overexpression
of heterologous xylose transporters have also been implemented. Besides all these genetic approaches to improve
xylose fermentation, ethanol yield by recombinant S. cerevisiae is still insignificant and simultaneous fermentation of
xylose and glucose is yet another bottleneck.
In an effort to control the repressive effect of glucose over
the utilization of xylose, Vilela et al. (2015) described
heterologous expression of a XI gene (xylA)
from Burkholderia cenocepacia into S. cerevisiae strain
that exhibited anaerobic fermentation of xylose, without
xylitol accumulation. Moreover, an evolutionary engineering strategy was manipulated through sequential batch fermentation on xylose to the xylA-expressing strain for
improving xylose fermentation. The resulting yeast strain
was capable of co-fermenting xylose and glucose, with
enhanced ethanol production. In another study, comparison
of the commercial strains with XI (from Clostridium
phytofermentans) or XR/XDH pathway showed that
recombinant strains of S. cerevisiae with both XI and
XR/XDH pathways are the most efficient ethanol producers
(Cunha et al. 2019). To further promote higher ethanol
production from xylose, heterologous genes, Sut1 and Sut2
for sugar transporters were expressed, the customized version of S. cerevisiae could only transport xylose (Du et al.
2010). Figure 2 shows pathways to produce ethanol from
xylose using either bacteria or yeast.
11.3 Xylitol Production
Xylitol is used as a sugar substitute with low caloric content
for diabetic patients and has anti-carcinogenic properties
(Takata et al. 2014; Mishra et al. 2013; Lee et al. 2013).
A number of researches have been conducted on xylitol
production from xylose using plant biomass as a source of
raw material (Kwak et al. 2019).
Majority of xylose-assimilating yeasts synthesize xylitol
by expressing xyl1 and xyl2 genes that encode XR and XDH,
258
R. Rashid et al.