thiolase (Fig. 4.). Acetoacetyl-CoA is then reduced to 3-hydroxybutyryl-CoA by a
NADPH-dependent acetoacetyl-CoA reductase, followed by the polymerization of
PHB via a poly-3-hydroxybutyrate synthase. The successful heterologous expression of the PHB genes in a plasmid-based approach was recently shown in C. autothenanogenum [111] as well as in C. coskatii [112]. In a recombinant
C. ljungdahlii strain, the same approach was without success [112]. So far, the titers
of PHB are rather low. Under autotrophic conditions with syngas, the production of
PHB was only 1.2% per cell dry weight in the recombinant C. coskatii strain,
whereas in C. autothenanogenum a production of 5.6% PHB could be achieved.
These low titers are caused by the limitation of reducing equivalents as well as ATP
[111]. Future optimizations should therefore address strategies to improve the
energy status of the cell. This could be achieved either by implementation of
additional energy conservation modules or by implementation of ATP-yielding
pathways.
5 Conclusion
Acetogens are on the advent of becoming promising candidates as catalysts toward
a sustainable, bio-based economy. Their outstanding metabolic product diversity
along with their wide range of substrate flexibility makes them well suited for the
biotechnological production of various compounds from several feedstocks
including industrial waste gas, methanol, formate and CO. With the well-studied
biochemistry of the WLP and its bioenergetics for some model acetogens and a
well-developed genetic toolbox for several acetogenic bacteria, the foundation for
the industrial use of acetogens as biocatalysts has been laid over the past decades.
A steadily increasing number of genome sequences available and additional
knowledge of bioenergetics will further help to develop optimized strains in the
future. Even though the metabolic engineering of acetogens is still in the beginning
and the industrial use as platform organisms has been limited to the production of
ethanol so far, their potential is tremendous.
References
1. Drake HL, Gößner AS, Daniel SL (2008) Old acetogens, new light. Ann NY Acad Sci
1125:100–128
2. Müller V (2003) Energy conservation in acetogenic bacteria. Appl Environ Microbiol
69:6345–6353
3. Ragsdale SW (2008) Enzymology of the Wood-Ljungdahl pathway of acetogenesis. Ann N
Y Acad Sci 1125:129–136
4. Schuchmann K, Müller V (2016) Energetics and application of heterotrophy in acetogenic
bacteria. Appl Environ Microbiol 82:4056–4069
5. Drake HL (1994) Acetogenesis, acetogenic bacteria, and the acetyl-CoA “Wood/Ljungdahl”
pathway: Past and current perspectives. In: Drake HL (ed) Acetogenesis. Springer, US, New
York, pp 3–60
124
D. Litty and V. Müller
NADPH-dependent acetoacetyl-CoA reductase, followed by the polymerization of
PHB via a poly-3-hydroxybutyrate synthase. The successful heterologous expression of the PHB genes in a plasmid-based approach was recently shown in C. autothenanogenum [111] as well as in C. coskatii [112]. In a recombinant
C. ljungdahlii strain, the same approach was without success [112]. So far, the titers
of PHB are rather low. Under autotrophic conditions with syngas, the production of
PHB was only 1.2% per cell dry weight in the recombinant C. coskatii strain,
whereas in C. autothenanogenum a production of 5.6% PHB could be achieved.
These low titers are caused by the limitation of reducing equivalents as well as ATP
[111]. Future optimizations should therefore address strategies to improve the
energy status of the cell. This could be achieved either by implementation of
additional energy conservation modules or by implementation of ATP-yielding
pathways.
5 Conclusion
Acetogens are on the advent of becoming promising candidates as catalysts toward
a sustainable, bio-based economy. Their outstanding metabolic product diversity
along with their wide range of substrate flexibility makes them well suited for the
biotechnological production of various compounds from several feedstocks
including industrial waste gas, methanol, formate and CO. With the well-studied
biochemistry of the WLP and its bioenergetics for some model acetogens and a
well-developed genetic toolbox for several acetogenic bacteria, the foundation for
the industrial use of acetogens as biocatalysts has been laid over the past decades.
A steadily increasing number of genome sequences available and additional
knowledge of bioenergetics will further help to develop optimized strains in the
future. Even though the metabolic engineering of acetogens is still in the beginning
and the industrial use as platform organisms has been limited to the production of
ethanol so far, their potential is tremendous.
References
1. Drake HL, Gößner AS, Daniel SL (2008) Old acetogens, new light. Ann NY Acad Sci
1125:100–128
2. Müller V (2003) Energy conservation in acetogenic bacteria. Appl Environ Microbiol
69:6345–6353
3. Ragsdale SW (2008) Enzymology of the Wood-Ljungdahl pathway of acetogenesis. Ann N
Y Acad Sci 1125:129–136
4. Schuchmann K, Müller V (2016) Energetics and application of heterotrophy in acetogenic
bacteria. Appl Environ Microbiol 82:4056–4069
5. Drake HL (1994) Acetogenesis, acetogenic bacteria, and the acetyl-CoA “Wood/Ljungdahl”
pathway: Past and current perspectives. In: Drake HL (ed) Acetogenesis. Springer, US, New
York, pp 3–60
124
D. Litty and V. Müller