Abstract
With the goal of achieving carbon sequestration, emission reduction and cleaner production, biological methods have been employed to convert carbon dioxide (CO2) into fuels and chemicals. However, natural autotrophic organisms are not suitable cell factories due to their poor carbon fixation efficiency and poor growth rate. Heterotrophic microorganisms are promising candidates, since they have been proven to be efficient biofuel and chemical production chassis. This review first briefly summarizes six naturally occurring CO2 fixation pathways, and then focuses on recent advances in artificially designing efficient CO2 fixation pathways. Moreover, this review discusses the transformation of heterotrophic microorganisms into hemiautotrophic microorganisms and delves further into fully autotrophic microorganisms (artificial autotrophy) by use of synthetic biological tools and strategies. Rapid developments in artificial autotrophy have laid a solid foundation for the development of efficient carbon fixation cell factories. Finally, this review highlights future directions toward large-scale applications. Artificial autotrophic microbial cell factories need further improvements in terms of CO2 fixation pathways, reducing power supply, compartmentalization and host selection.
Highlights
The carbon element is the most important component in all types of living organic matter, accounting for approximately 50% of the dry weight of the organics
A series of CO2 fixation pathways were predicted in silico by evaluating the stoichiometric and thermodynamic feasibility of theoretical pathways, from which to recruit a route characterized by high energy-efficiency
A cocktail containing all of these enzymes was developed to investigate the feasibility of the proposed pathways in vitro, and further, all of these enzymes were introduced into heterotrophic model microorganisms to create artificial autotrophy. 13C-labeling method is a powerful tool to pinpoint carbon flow
Summary
Despite their naturally existing diversity, the application of CO2 fixation pathways for biomanufacturing valuable compounds directly from CO2 has been limited so far. The engineered strain maintained a slight growth using only formate and CO2 as substrates, confirming its feasibility for supplying energy and reducing power via NAD-dependent FDH oxidative activity Based on these pioneer studies, an outstanding work made an attempted to endow E. coli grown on formate or methanol and CO2 without any other organics via the rGly route. With the goal of shorting the process of one-carbon utilization and accelerating growth starting from formate, an artificial synthetic route was computationally designed, the formolase (FLS) pathway In this linear route, two natural enzymes with considerable activity toward substrate analogs were identified. The SACA process is characterized by carbon-conserved and ATP-independent processes, achieving high-efficiency of the pathway has been challenging, likely due to the toxicity of formaldehyde to cells and kinetic bottlenecks of enzymes To address this issue, renovating the host and implementing further protein engineering may represent promise solutions.
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