Synthetic biology optimizes carbon conservation and carbon fixation during microbial carbon metabolism

Since the Industrial Revolution, carbon dioxide (CO2) emissions have increased significantly, and global warming caused by excessive CO2 emissions has attracted great attention worldwide. With the development of biotechnology, the modification of natural carbon metabolic pathways using microorganisms as cell factories to achieve bettter carbon conservation, and the utilization of natural carbon fixation pathways and artificial carbon sequestration pathways to convert renewable carbon sources into alternative energy sources is the promising green energy solutions. In this review, with the ability of microbial systems optimizing carbon conservation and carbon fixation during their metabolic processes as the main criteria, we summarize the progress made in recent years in the design and synthesis of artificial carbon conservation pathways and artificial carbon fixation pathways, and conduct a comparative analysis to discuss the value of using microorganisms as cell factories for low-carbon sustainable production. The conversion of carbon sources to chemicals using acetyl-CoA as precursor is achieved mainly through the EMP pathway in the natural carbon metabolic pathway,and the carbon loss of metabolic pathway is caused by the decarboxylation process of pyruvate conversion to acetyl-CoA. In order to improve efficiency of carbon fixation, the researchers used synthetic biology to redesign and construct the carbon metabolic process of heterotrophic microorganisms, which can bypass the pyruvate decarboxylation process and achieve maximum carbon conservation during metabolism, thus improving the effective use of carbon sources. Herein, the non-oxidative artificial glycolytic pathway and its optimized construction of EP-Bifido pathway are discussed in detail. In addition, the introduction of effective CO2 fixation pathways by optimizing natural carbon fixation pathways or designing artificial carbon fixation cycle pathways is also an important direction to construct artificial carbon saving pathways. There are two main ways to introduce one-carbon compounds into the metabolic pathway, one is through the direct utilization of one-carbon compounds such as formate and methanol, and there have been many excellent reviews on this type of carbon fixation pathway. Herein, the carbon fixation pathway summarized in this paper is to realize the utilization of CO2 by introducing CO2 into the metabolic pathway. The artificial carbon fixation pathway has the advantages of short pathway and high efficiency of carbon fixation enzymes, although some carbon fixation pathways cannot be realized in vivo, its carbon fixation efficiency is often better than the natural carbon fixation pathway. In the process of microbial fermentation, the production and release of CO2 are often accompanied by the production of NAD(P) H and ATP, and the process of CO2 fixation during carbon fixation is accompanied by the consumption of NAD(P)H. In the process of chemical production by heterotrophic microorganisms, excess reducing power is theoretically required to achieve a net gain in CO2 fixation. The carbon in CO2 is in the highest valence state +4, it's the most completely oxidized form of carbon-based compounds. The conversion of CO2 to the reduced product (carbon with an average valence of -2) requires NAD(P)H to provide electrons for the process. The main forms of reducing power in the carbon fixation pathway are NAD(P)H and the reduced state Fd. By enhancing some pathways such as the pentosephosphate pathway or the introduction of exogenous energy supply systems, the deficiency of NAD(P)H, can be compensated, thus benefiting the production of target chemicals. With the development of synthetic biology, more and more CO2 mechanisms of fixation will be explored and developed to reconfigure microbial metabolism for efficient biomanufacturing and start a positive cycle of industrial decarbonization.