The biological pathway involved in the syngas fermentation is Woode Ljungdahl
pathway, which is also termed as acetyl-CoA pathway. The Woode Ljungdahl
pathway controls the reaction of carbon monoxide and hydrogen to produce two
products as acetate and acetyl-CoA. Woode Ljungdahl pathway has two branches
which are methyl branch and carbonyl branch and forms intermediate acetyl CoA
depending upon the type of substrate.
In methyl branch, the formate dehydrogenase enzyme reduces carbon monoxide
to formate. Formate reacts with tetrahydrofolate using a formyl-THF synthetase
enzyme with an consumption of ATP, producing formyl-tetrahydrofolate. Methyltetrahydrofolate is reduced to formyl-tetrahydrofolate by the application of different
tetrahydrofolate-dependent enzymes such as methenyl tetrahydrofolate
cyclohydrolase, methylene-tetrahydrofolate dehydrogenase, and methylenetetrahydrofolate reductase. Then, the methyl group is transferred to a corrinoid
iron-sulphur protein. Lastly, the enzyme complex acetyl CoA synthatase catalyses
the formation of acetyl CoA.
Acetic acid formation:
4 CO þ 2 H 2 O !
yields CH 3 COOH þ 2 CO 2
2 CO 2 þ 4 H 2 !
yields CH 3 COOH þ 2 H 2 O
Ethanol formation:
6CO þ 3H 2 O ! C 2 H 5 OH þ 4CO 2
2CO 2 þ 6H 2 ! C 2 H 5 OH þ 3H 2 O
In the carbonyl branch of the Woode Ljungdahl pathway, carbon dioxide dehydrogenase enzyme reduces carbon dioxide to carboxyl group. Carboxyl group reacts
with the methyl group to produce acetyl-CoA, which undergo further series of
reactions to produce biofuels. In the fermentation phase, alcohol dehydrogenase
enzyme catalyses the reaction of acetaldehyde to alcohol which is ethanol. In the
complete process, first is the non-growth phase where acetyl-CoA converts to
ethanol, otherwise the acetyl-CoA converts to acetate (Bengelsdorf et al. 2013;
Munasinghe and Khanal 2010; Caruso et al. 2019).
Syngas fermentation is one of the promising biofuel technologies because of
several advantages such as no pretreatment is required for biomass substrates, can
tolerate high amounts of sulphur compounds, high reaction specificity, cost-effective
process and CO/H 2 ratio is flexible. However, there are some limitations also such as
low volumetric productivity and sensitivity to organisms.
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pathway, which is also termed as acetyl-CoA pathway. The Woode Ljungdahl
pathway controls the reaction of carbon monoxide and hydrogen to produce two
products as acetate and acetyl-CoA. Woode Ljungdahl pathway has two branches
which are methyl branch and carbonyl branch and forms intermediate acetyl CoA
depending upon the type of substrate.
In methyl branch, the formate dehydrogenase enzyme reduces carbon monoxide
to formate. Formate reacts with tetrahydrofolate using a formyl-THF synthetase
enzyme with an consumption of ATP, producing formyl-tetrahydrofolate. Methyltetrahydrofolate is reduced to formyl-tetrahydrofolate by the application of different
tetrahydrofolate-dependent enzymes such as methenyl tetrahydrofolate
cyclohydrolase, methylene-tetrahydrofolate dehydrogenase, and methylenetetrahydrofolate reductase. Then, the methyl group is transferred to a corrinoid
iron-sulphur protein. Lastly, the enzyme complex acetyl CoA synthatase catalyses
the formation of acetyl CoA.
Acetic acid formation:
4 CO þ 2 H 2 O !
yields CH 3 COOH þ 2 CO 2
2 CO 2 þ 4 H 2 !
yields CH 3 COOH þ 2 H 2 O
Ethanol formation:
6CO þ 3H 2 O ! C 2 H 5 OH þ 4CO 2
2CO 2 þ 6H 2 ! C 2 H 5 OH þ 3H 2 O
In the carbonyl branch of the Woode Ljungdahl pathway, carbon dioxide dehydrogenase enzyme reduces carbon dioxide to carboxyl group. Carboxyl group reacts
with the methyl group to produce acetyl-CoA, which undergo further series of
reactions to produce biofuels. In the fermentation phase, alcohol dehydrogenase
enzyme catalyses the reaction of acetaldehyde to alcohol which is ethanol. In the
complete process, first is the non-growth phase where acetyl-CoA converts to
ethanol, otherwise the acetyl-CoA converts to acetate (Bengelsdorf et al. 2013;
Munasinghe and Khanal 2010; Caruso et al. 2019).
Syngas fermentation is one of the promising biofuel technologies because of
several advantages such as no pretreatment is required for biomass substrates, can
tolerate high amounts of sulphur compounds, high reaction specificity, cost-effective
process and CO/H 2 ratio is flexible. However, there are some limitations also such as
low volumetric productivity and sensitivity to organisms.
66
N. Jaiswal et al.