Second-Generation Bioethanol: Advancement of Ethanologenic …
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yield by up to 38% in switchgrass plant (downregulation, overexpressing myb4).
These studies have opened the possibility of large cost reduction, especially through
genetically engineered crops.
3 Metabolism of Ethanologenic Microorganism
Hydrolysis of lignocellulosic biomass releases accessible sugars that comprise
mainly glucose and xylose, as well as small amounts of arabinose, galacturonic
acid, and rhamnose. Metabolism of ethanologens involves biochemical reaction that
converts these fermentable sugars into ethanol. While many microorganisms can
metabolize glucose and other hexose sugars, only a few microorganisms are able to
metabolize xylose and pentose sugars. However, these xylose-fermenting microorganisms such as Escherichia coli, Pachysolen tannophilus and Candida tropicalis
can only achieve low rate and yield of ethanol in comparison to glucose fermentation
(Jeffries 1981; Slininger et al. 1982). Moreover, pentose sugars are only utilized after
D-glucose has depleted in the fermentation, hence leads to an uneconomical long
fermentation time (Oreb et al. 2012; Bren et al. 2016). These challenges or undesirable traits of ethanologenic microbes truly require the understanding on pentose and
hexose metabolic pathways before the construction of industrial ethanologens can
be performed.
3.1 Glucose Catabolism
Glycolysis or also known as Embden–Meyerhof–Parnas pathway plays a major role
in initial catabolism of glucose into three carbon units, pyruvate. It occurs in two
stages namely ATP investment stage (stage 1) and ATP pay off stage (stage 2). In
the former, glucose is phosphorylated twice and is split to form two molecules of
glyceraldehyde-3-phosphate (G-3-P). Two ATP molecules are utilized in this stage,
which is regarded as an investment for further oxidation process. In the latter, G-3-P
is converted to pyruvate that yields four ATP and two NADH molecules. Due to the
consumption of two ATPs in stage 1, the net production of ATP per glucose molecule
is two.
In the presence of oxygen, the energy-rich pyruvate can be completely oxidized to
CO 2 and H 2 O that potentially yields 30 more ATP molecules. However, in the anaerobic condition, it can be converted to several types of reduced molecules depending
on the type of microorganisms. In ethanologenic microorganism such as yeast and
certain bacteria, pyruvate is decarboxylated to acetaldehyde and CO 2 . The acetaldehyde is then reduced by NADH to form ethanol, which is excreted later by the cell.
Technically, fermentation of glucose to ethanol does not face any grave challenges in
contract with xylose, since glucose is always the preferred carbon source that allows
faster growth than other sugars. (citation) Fig. 3 shows the overview of ethanol
metabolism.
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yield by up to 38% in switchgrass plant (downregulation, overexpressing myb4).
These studies have opened the possibility of large cost reduction, especially through
genetically engineered crops.
3 Metabolism of Ethanologenic Microorganism
Hydrolysis of lignocellulosic biomass releases accessible sugars that comprise
mainly glucose and xylose, as well as small amounts of arabinose, galacturonic
acid, and rhamnose. Metabolism of ethanologens involves biochemical reaction that
converts these fermentable sugars into ethanol. While many microorganisms can
metabolize glucose and other hexose sugars, only a few microorganisms are able to
metabolize xylose and pentose sugars. However, these xylose-fermenting microorganisms such as Escherichia coli, Pachysolen tannophilus and Candida tropicalis
can only achieve low rate and yield of ethanol in comparison to glucose fermentation
(Jeffries 1981; Slininger et al. 1982). Moreover, pentose sugars are only utilized after
D-glucose has depleted in the fermentation, hence leads to an uneconomical long
fermentation time (Oreb et al. 2012; Bren et al. 2016). These challenges or undesirable traits of ethanologenic microbes truly require the understanding on pentose and
hexose metabolic pathways before the construction of industrial ethanologens can
be performed.
3.1 Glucose Catabolism
Glycolysis or also known as Embden–Meyerhof–Parnas pathway plays a major role
in initial catabolism of glucose into three carbon units, pyruvate. It occurs in two
stages namely ATP investment stage (stage 1) and ATP pay off stage (stage 2). In
the former, glucose is phosphorylated twice and is split to form two molecules of
glyceraldehyde-3-phosphate (G-3-P). Two ATP molecules are utilized in this stage,
which is regarded as an investment for further oxidation process. In the latter, G-3-P
is converted to pyruvate that yields four ATP and two NADH molecules. Due to the
consumption of two ATPs in stage 1, the net production of ATP per glucose molecule
is two.
In the presence of oxygen, the energy-rich pyruvate can be completely oxidized to
CO 2 and H 2 O that potentially yields 30 more ATP molecules. However, in the anaerobic condition, it can be converted to several types of reduced molecules depending
on the type of microorganisms. In ethanologenic microorganism such as yeast and
certain bacteria, pyruvate is decarboxylated to acetaldehyde and CO 2 . The acetaldehyde is then reduced by NADH to form ethanol, which is excreted later by the cell.
Technically, fermentation of glucose to ethanol does not face any grave challenges in
contract with xylose, since glucose is always the preferred carbon source that allows
faster growth than other sugars. (citation) Fig. 3 shows the overview of ethanol
metabolism.