Implementation of a Reductive Route of One-Carbon Assimilation in Escherichia coli through Directed Evolution

Engineering a solar formic acid/pentose (SFAP) pathway in Escherichia coli for lactic acid production

Microbial CO2 fixation into lactic acid (LA) is an important approach for low-carbon biomanufacturing. Engineering microbes to utilize CO2 and sugar as co-substrates can create efficient pathways through input of moderate reducing power to drive CO2 fixation into product. However, to achieve complete conservation of organic carbon, how to engineer the CO2-fixing modules compatible with native central metabolism and merge the processes for improving bioproduction of LA is a big challenge. In this study, we designed and constructed a solar formic acid/pentose (SFAP) pathway in Escherichia coli, which enabled CO2 fixation merging into sugar catabolism to produce LA. In the SFAP pathway, adequate reducing equivalents from formate oxidation drive glucose metabolism shifting from glycolysis to the pentose phosphate pathway. The Rubisco-based CO2 fixation and sequential reduction of C3 intermediates are conducted to produce LA stoichiometrically. CO2 fixation theoretically can bring a 20% increase of LA production compared with sole glucose feedstock. This SFAP pathway in the integration of photoelectrochemical cell and an engineered Escherichia coli opens an efficient way for fixing CO2 into value-added bioproducts.

Engineering a Novel Acetyl-CoA Pathway for Efficient Biosynthesis of Acetyl-CoA-Derived Compounds

Acetyl-CoA is an essential central metabolite in living organisms and a key precursor for various value-added products as well. However, the intracellular availability of acetyl-CoA limits the efficient production of these target products due to complex and strict regulation. Here, we proposed a new acetyl-CoA pathway, relying on two enzymes, threonine aldolase and acetaldehyde dehydrogenase (acetylating), which can convert one l-threonine into one acetyl-CoA, one glycine, and generate one NADH, without carbon loss. Introducing the acetyl-CoA pathway could increase the intracellular concentration of acetyl-CoA by 8.6-fold compared with the wild-type strain. To develop a cost-competitive and genetically stable acetyl-CoA platform strain, the new acetyl-CoA pathway, driven by the constitutive strong promoter, was integrated into the chromosome of Escherichia coli. We demonstrated the practical application of this new acetyl-CoA pathway by high titer production of β-alanine, mevalonate, and N-acetylglucosamine. At the same time, this pathway achieved a high-yield production of glycine, a value-added commodity chemical for the synthesis of glyphosate and thiamphenicol. This work shows the potential of this new acetyl-CoA pathway for the industrial production of acetyl-CoA-derived compounds.

Electrical-biological hybrid system for carbon efficient isobutanol production

We have developed an electrical-biological hybrid system wherein an engineered microorganism consumes electrocatalytically produced formate from CO2 to supplement the bioproduction of isobutanol, a valuable fuel chemical. Biological CO2 sequestration is notoriously slow compared to electrochemical CO2 reduction, while electrochemical methods struggle to generate carbon-carbon bonds which readily form in biological systems. A hybrid system provides a promising method for combining the benefits of both biology and electrochemistry. Previously, Escherichia coli was engineered to assimilate formate and CO2 in central metabolism using the reductive glycine pathway. In this work, we have shown that chemical production in E. coli can benefit from single carbon substrates when equipped with the RGP. By installing the RGP and the isobutanol biosynthetic pathway into E. coli and by further genetic modifications, we have generated a strain of E. coli that can consume formate and produce isobutanol at a yield of >100% of theoretical maximum from glucose. Our results demonstrate that carbon produced from electrocatalytically reduced CO2 can bolster chemical production in E. coli. This study shows that E. coli can be engineered towards carbon efficient methods of chemical production.

Revisiting Nature's "Unifying Patterns": A Biological Appraisal

Effective bioinspiration requires dialogue between designers and biologists, and this dialogue must be rooted in a shared scientific understanding of living systems. To support learning from "nature's overarching design lessons" the Biomimicry Institute has produced ten "Unifying Patterns of Nature". These patterns have been developed to engage with those interested in finding biologically inspired solutions to human challenges. Yet, although well-intentioned and appealing, they are likely to dishearten biologists. The aim of this paper is to identify why and propose alternative principles based on evolutionary theory.

Genetic and biocatalytic basis of formate dependent growth of Escherichia coli strains evolved in continuous culture

The reductive glycine pathway was described as the most energetically favorable synthetic route of aerobic formate assimilation. Here we report the successful implementation of formatotrophy in Escherichia coli by means of a stepwise adaptive evolution strategy. Medium swap and turbidostat regimes of continuous culture were applied to force the channeling of carbon flux through the synthetic pathway to pyruvate establishing growth on formate and CO₂ as sole carbon sources. Labeling with ¹³C-formate proved the assimilation of the C1 substrate via the pathway metabolites. Genetic analysis of intermediate isolates revealed a mutational path followed throughout the adaptation process. Mutations were detected affecting the copy number (gene ftfL) or the coding sequence (genes folD and lpd) of genes which specify enzymes implicated in the three steps forming glycine from formate and CO₂, the central metabolite of the synthetic pathway. The mutation R196S present in methylene-tetrahydrofolate dehydrogenase/cyclohydrolase (FolD) abolishes the inhibition of cyclohydrolase activity by the substrate formyl-tetrahydrofolate. The mutation R273H in lipoamide dehydrogenase (Lpd) alters substrate affinities as well as kinetics at physiological substrate concentrations likely favoring a reactional shift towards lipoamide reduction. In addition, genetic reconstructions proved the necessity of all three mutations for formate assimilation by the adapted cells. The largely unpredictable nature of these changes demonstrates the usefulness of the evolutionary approach enabling the selection of adaptive mutations crucial for pathway engineering of biotechnological model organisms.

Type II methanotrophs: A promising microbial cell-factory platform for bioconversion of methane to chemicals

Methane, the predominant element in natural gas and biogas, represents a promising alternative to carbon feedstocks in the biotechnological industry due to its low cost and high abundance. The bioconversion of methane to value-added products can enhance the value of gas and mitigate greenhouse gas emissions. Methanotrophs, methane-utilizing bacteria, can make a significant contribution to the production of various valuable biofuels and chemicals from methane. Type II methanotrophs in comparison with Type I methanotrophs have distinct advantages, including high acetyl-CoA flux and the co-incorporation of two important greenhouse gases (methane and CO₂), making it a potential microbial cell-factory platform for methane-derived biomanufacturing. Herein, we review the most recent advances in Type II methanotrophs related to multi-omics studies and metabolic engineering. Representative examples and prospects of metabolic engineering strategies for the production of suitable products are also discussed.

Activation and competition of lipoylation of H protein and its hydrolysis in a reaction cascade catalyzed by the multifunctional enzyme lipoate-protein ligase A

Protein lipoylation is essential for the function of many key enzymes but barely studied kinetically. Here, the two-step reaction cascade of H protein lipoylation catalyzed by the multifunctional enzyme lipoate-protein ligase A (LplA) was quantitatively and differentially studied. We discovered new phenomena and unusual kinetics of the cascade: (a) the speed of the first reaction is faster than the second one by two orders of magnitude, leading to high accumulation of the intermediate lipoyl-AMP (Lip-AMP); (b) Lip-AMP is hydrolyzed, but only significantly at the presence of H protein and in competition with the lipoylation; (c) both the lipoylation of H protein and its hydrolysis is enhanced by the apo and lipoylated forms of H protein and a mutant without the lipoylation site. A conceptual mechanistic model is proposed to explain these experimental observations in which conformational change of LplA upon interaction with H protein and competitive nucleophilic attacks play key roles.

Expression of Formate-Tetrahydrofolate Ligase Did Not Improve Growth but Interferes With Nitrogen and Carbon Metabolism of Synechocystis sp. PCC 6803

The introduction of alternative CO2-fixing pathways in photoautotrophic organism may improve the efficiency of biological carbon fixation such as minimizing the carbon loss due to photorespiration. Here, we analyzed the effects of creating a formate entry point into the primary metabolism of the cyanobacterium Synechocystis sp. PCC 6803. The formate-tetrahydrofolate ligase (FTL) from Methylobacterium extorquens AM1 was expressed in Synechocystis to enable formate assimilation and reducing the loss of fixed carbon in the photorespiratory pathway. Transgenic strains accumulated serine and 3-phosphoglycerate, and consumed more 2-phosphoglycolate and glycine, which seemed to reflect an efficient utilization of formate. However, labeling experiments showed that the serine accumulation was not due to the expected incorporation of formate. Subsequent DNA-microarray analysis revealed profound changes in transcript abundance due to ftl expression. Transcriptome changes were observed in relation to serine and glycine metabolism, C1-metabolism and particularly nitrogen assimilation. The data implied that ftl expression interfered with the signaling the carbon/nitrogen ratio in Synechocystis. Our results indicate that the expression of new enzymes could have a severe impact on the cellular regulatory network, which potentially hinders the establishment of newly designed pathways.

Recent progress in metabolic engineering of microbial formate assimilation

Formate can be efficiently produced via electrochemical or photochemical catalytic conversion of CO₂, and it can be directly used as an organic carbon source by microorganisms. In theory, formate can be used as the sole carbon source for the microbial production of high-value-added chemicals. Consequently, the construction of efficient formate-assimilation pathways in microorganisms is essential for the utilization of cheap, renewable one-carbon compounds. This paper summarizes new methods of formate synthesis, as well as the natural formate utilization pathways of microorganisms with their advantages and disadvantages. Furthermore, it reviews recent progress in the design of utilization pathways for formate in microbial cells through metabolic engineering and synthetic biology. Besides, we also use the pathway-prediction algorithm comb-FBA to rationally design completely new one-carbon compounds utilization pathways. The pathway with the highest efficiency, named GAA, was corroborated by the in vitro experiments showing a carbon molar yield up to 88%. Finally, it discusses the main problems and challenges presently existing in the pathway design and strain improvement for microbial utilization of formate. KEY POINTS: • Natural and artificial design pathways of formate-assimilation was summarized. • Recent progresses in different hosts and approaches of using one-carbon compounds was reviewed. • Metabolic engineering and synthetic biology methods to improve formate utilization were discussed.

Quantitative analysis of glycine related metabolic pathways for one-carbon synthetic biology

Glycine is an essential one-carbon (C1) metabolite nested in a complex network of cellular metabolism. Glycine and its related metabolic pathways have important biochemical and biomedical implications and have thus been studied for a long time. However, quantitative and systems level knowledge about the interactions and regulations of the pathways are severely limited, especially for the purpose of reengineering the relevant pathways for C1-based biotechnological processes using synthetic biology and metabolic engineering approaches. In fact, quantitative analytic methods are missing for some of the key players of the glycine-related pathways, prominently the glycine cleavage system and folate cycle, particularly for intracellular processes under physiological conditions. Here, we pinpoint the existing gaps and highlight the need and challenges for future development.

A critical comparison of cellular and cell-free bioproduction systems

Conversion of biological feedstocks into value-added chemicals is mostly performed via microbial fermentation. An emerging alternative approach is the use of cell-free systems, consisting of purified enzymes and cofactors. Unfortunately, the in vivo and in vitro research communities rarely interact, which leads to oversimplifications and exaggerations that do not permit fair comparison of the two strategies and impede synergistic interactions. Here, we provide a comprehensive account for the advantages and drawbacks associated with each strategy, and further discuss recent research efforts that aim to breach the limits of cellular and cell-free production. We also explore emerging hybrid solutions that integrate the benefits of both worlds and could expand the boundaries of biosynthesis.

Quantitative study of H protein lipoylation of the glycine cleavage system and a strategy to increase its activity by co-expression of LplA

Glycine cleavage system (GCS) plays a key role in one-carbon (C1) metabolism related to the biosynthesis of a number of key intermediates with significance in both biomedicine and biotechnology. Despite extensive studies of the proteins (H, T, P and L) involved and the reaction mechanisms of this important enzyme complex little quantitative data are available. In this work, we have developed a simple HPLC method for direct analysis and quantification of the apo- and lipoylated forms (H_apo and H_lip) of the shuttle protein H, the latter (H_lip) is essential for the function of H protein and determines the activity of GCS. Effects of temperature, concentrations of lipoic acid and H_apo and the expression of H protein on its lipoylation were studied. It is found that H_lip is as low as only 20–30% of the total H protein with lipoic acid concentration in the range of 10–20 μM and at a favorable temperature of 30 °C. Furthermore, H_apo seems to inhibit the overall activity of GCS. We proposed a strategy of co-expressing LplA to improve the lipoylation of H protein and GCS activity. With this strategy the fraction of H_lip was increased, for example, from 30 to 90% at a lipoic acid concentration of 20 μM and GCS activity was increased by more than 2.5 fold. This work lays a quantitative foundation for better understanding and reengineering the GCS system.

Metabolic engineering advances and prospects for amino acid production

Amino acid fermentation is one of the major pillars of industrial biotechnology. The multi-billion USD amino acid market is rising steadily and is diversifying. Metabolic engineering is no longer focused solely on strain development for the bulk amino acids L-glutamate and L-lysine that are produced at the million-ton scale, but targets specialty amino acids. These demands are met by the development and application of new metabolic engineering tools including CRISPR and biosensor technologies as well as production processes by enabling a flexible feedstock concept, co-production and co-cultivation schemes. Metabolic engineering advances are exemplified for specialty proteinogenic amino acids, cyclic amino acids, omega-amino acids, and amino acids functionalized by hydroxylation, halogenation and N-methylation.