6
Alternatively, the use of isolated immobilized enzymes (Tamborini et al. 2018)
or cell-free extracts (Wu et al. 2015; Sun et al. 2014; Dudley et al. 2019) as catalysts
has partially circumvented the limitations shown by whole-cell catalysis for polymer synthesis. However, the difficulty of obtaining high-molecular-weight polymers by enzyme-catalyzed reactions is one of the main limitations to the applications
of biotechnological approaches in industrial polymer production.
From a policy-maker perspective, CO 2 -based (bio)chemically catalyzed polymer
synthesis is successful when and only when it can solidly show that CO 2 -based products can be superior to currently available products on the market ensuring lower
cost and lower fossil carbon content (Mac et al. 2017) as well as functional competitiveness. Therefore, further advancements are needed to improve the controllability
of chemical and physical properties of biologically derived intermediates to polymeric materials along with the productivity granted by engineered microorganisms.
This chapter illustrates how enzymatic synthesis complements traditional chemical synthesis of polymers to boost a sustainable innovation of CO 2 -based chemistry.
In summary, in this chapter, we described recent reports advancing the production
of different classes of polymeric building blocks by CO 2 -fixing microorganisms
pointing out the key metabolic engineering techniques enabling the processes.
Besides CO 2 - based monomers which could be straightforwardly functionalized in
polymer chemistry applications, the chapter described in vivo synthetic polymers of
potential commercial interest.
1.2 Advances in Microorganism Engineering for Improving
CO 2 Assimilation
If policy-makers seek to drive biotechnological innovation in polymer synthesis, it
is responsible to emphasize the carbon yield of products obtained by CO 2 -
assimilating microorganisms. In this perspective, controlling both the way of CO 2
entrance into and that of CO 2 exit from microorganisms is tremendously relevant in
the design of efficient bacterial cell factories. Indeed, it is worthwhile to note that
microorganisms themselves release CO 2 through several mechanisms, which include
photorespiration, decarboxylation in product synthesis pathways, redox imbalance,
and ATP generation, negatively affecting the carbon yield of the product of interest.
Therefore, to get net CO 2 gain by microorganisms in biotechnological applications,
metabolic engineering techniques have been devised both to enhance CO 2 fixation
and limit CO 2 release. Only some autotrophic microorganisms involving cyanobacteria, microalgae, archaea, and acetogens have been used in production processes
pointing at the concomitant CO 2 capture and production of high-value compounds.
In autotrophic microorganisms, CO 2 assimilation can be enhanced by increasing
the efficiency in CO 2 -fixation pathway, increasing the availability of reducing power
and ATP, and channeling the flux of carbon towards product formation. The efficiency of the pathway fixing CO 2 can be reinforced by improving the expression or
the catalytic properties of key enzymes. The existence of several alternative CO 2 -
A. A. Azim et al.
Alternatively, the use of isolated immobilized enzymes (Tamborini et al. 2018)
or cell-free extracts (Wu et al. 2015; Sun et al. 2014; Dudley et al. 2019) as catalysts
has partially circumvented the limitations shown by whole-cell catalysis for polymer synthesis. However, the difficulty of obtaining high-molecular-weight polymers by enzyme-catalyzed reactions is one of the main limitations to the applications
of biotechnological approaches in industrial polymer production.
From a policy-maker perspective, CO 2 -based (bio)chemically catalyzed polymer
synthesis is successful when and only when it can solidly show that CO 2 -based products can be superior to currently available products on the market ensuring lower
cost and lower fossil carbon content (Mac et al. 2017) as well as functional competitiveness. Therefore, further advancements are needed to improve the controllability
of chemical and physical properties of biologically derived intermediates to polymeric materials along with the productivity granted by engineered microorganisms.
This chapter illustrates how enzymatic synthesis complements traditional chemical synthesis of polymers to boost a sustainable innovation of CO 2 -based chemistry.
In summary, in this chapter, we described recent reports advancing the production
of different classes of polymeric building blocks by CO 2 -fixing microorganisms
pointing out the key metabolic engineering techniques enabling the processes.
Besides CO 2 - based monomers which could be straightforwardly functionalized in
polymer chemistry applications, the chapter described in vivo synthetic polymers of
potential commercial interest.
1.2 Advances in Microorganism Engineering for Improving
CO 2 Assimilation
If policy-makers seek to drive biotechnological innovation in polymer synthesis, it
is responsible to emphasize the carbon yield of products obtained by CO 2 -
assimilating microorganisms. In this perspective, controlling both the way of CO 2
entrance into and that of CO 2 exit from microorganisms is tremendously relevant in
the design of efficient bacterial cell factories. Indeed, it is worthwhile to note that
microorganisms themselves release CO 2 through several mechanisms, which include
photorespiration, decarboxylation in product synthesis pathways, redox imbalance,
and ATP generation, negatively affecting the carbon yield of the product of interest.
Therefore, to get net CO 2 gain by microorganisms in biotechnological applications,
metabolic engineering techniques have been devised both to enhance CO 2 fixation
and limit CO 2 release. Only some autotrophic microorganisms involving cyanobacteria, microalgae, archaea, and acetogens have been used in production processes
pointing at the concomitant CO 2 capture and production of high-value compounds.
In autotrophic microorganisms, CO 2 assimilation can be enhanced by increasing
the efficiency in CO 2 -fixation pathway, increasing the availability of reducing power
and ATP, and channeling the flux of carbon towards product formation. The efficiency of the pathway fixing CO 2 can be reinforced by improving the expression or
the catalytic properties of key enzymes. The existence of several alternative CO 2 -
A. A. Azim et al.