Pathway engineering of Escherichia coli for one-step fermentative production of L-theanine from sugars and ethylamine

Co-utilization of carbon sources in microorganisms for the bioproduction of chemicals

Carbon source is crucial for the cell growth and metabolism in microorganisms, and its utilization significantly affects the synthesis efficiency of target products in microbial cell factories. Compared with a single carbon source, co-utilizing carbon sources provide an alternative approach to optimize the utilization of different carbon sources for efficient biosynthesis of many chemicals with higher titer/yield/productivity. However, the efficiency of bioproduction is significantly limited by the sequential utilization of a preferred carbon source and secondary carbon sources, attributed to carbon catabolite repression (CCR). This review aimed to introduce the mechanisms of CCR and further focus on the summary of the strategies for co-utilization of carbon sources, including alleviation of CCR, engineering of the transport and metabolism of secondary carbon sources, compulsive co-utilization in single culture, co-utilization of carbon sources via co-culture, and evolutionary approaches. The findings of representative studies with a significant improvement in the bioproduction of chemicals via the co-utilization of carbon sources were discussed in this review. It suggested that by combining rational metabolic engineering and irrational evolutionary approaches, co-utilizing carbon sources can significantly contribute to the bioproduction of chemicals.

Efficient production of γ-aminobutyric acid using engineered Escherichia coli whole-cell catalyst

γ-Aminobutyric acid (GABA) has been widely used in the food, feed, pharmaceutical, and chemical industry fields. Previously, we developed a whole-cell catalyst capable of converting L-glutamate (L-Glu) into GABA by overexpressing the glutamate decarboxylase gene (gadz11) from Bacillus sp. Z11 in Escherichia coli BL21(DE3). However, to enhance cell permeability, a freeze-thaw treatment is required, and to enhance GADZ11 activity, pyridoxal 5'-phosphate (PLP) must be added to the reaction system. The aim of this study is to provide a more efficient approach for GABA production by engineering the recombinant E. coli above. First, the inducible expression conditions of the gadz11 in E. coli were optimized to 37 °C for 6 h. Next, an ideal engineered strain was produced via increasing cell permeability by overexpressing sulA and eliminating PLP dependence by constructing a self-sufficient system. Furthermore, an efficient whole-cell biocatalytic process was optimized. The optimal substrate concentration, cell density, and reaction temperature were 1.0 mol/L (the molecular ratio of L-Glu to L-monosodium glutamate (L-MSG) was 4:1), 15 and 37 °C, respectively. Finally, a whole-cell bioconversion procedure was performed in a 3-L bioreactor under optimal conditions. The strain could be reused for at least two cycles with GABA yield, productivity and conversion ratio of 206.2 g/L, 117.8 g/L/h and 100.0%, respectively. This is currently the highest GABA productivity from a mixture of L-Glu and L-MSG reported without the addition of cofactors or additional treatment of cells. This work demonstrates that the novel engineered E. coli strain has the potential for application in large-scale industrial GABA production.

Metabolic engineering of Escherichia coli for efficient production of L-5-hydroxytryptophan from glucose

Background

5-hydroxytryptophan (5-HTP), the direct biosynthetic precursor of the neurotransmitter 5-hydroxytryptamine, has been shown to have unique efficacy in the treatment of a variety of disorders, including depression, insomnia, and chronic headaches, and is one of the most commercially valuable amino acid derivatives. However, microbial fermentation for 5-HTP production continues to face many challenges, including low titer/yield and the presence of the intermediate L-tryptophan (L-Trp), owing to the complexity and low activity of heterologous expression in prokaryotes. Therefore, there is a need to construct an efficient microbial cell factory for 5-HTP production.

Results

We describe the systematic modular engineering of wild-type Escherichia coli for the efficient fermentation of 5-HTP from glucose. First, a xylose-induced T7 RNA polymerase-P_T7 promoter system was constructed to ensure the efficient expression of each key heterologous pathway in E. coli. Next, a new tryptophan hydroxylase mutant was used to construct an efficient tryptophan hydroxylation module, and the cofactor tetrahydrobiopterin synthesis and regeneration pathway was expressed in combination. The L-Trp synthesis module was constructed by modifying the key metabolic nodes of tryptophan biosynthesis, and the heterologous synthesis of 5-HTP was achieved. Finally, the NAD(P)H regeneration module was constructed by the moderate expression of the heterologous GDH_esi pathway, which successfully reduced the surplus of the intermediate L-Trp. The final engineered strain HTP11 was able to produce 8.58 g/L 5-HTP in a 5-L bioreactor with a yield of 0.095 g/g glucose and a maximum real-time productivity of 0.48 g/L/h, the highest values reported by microbial fermentation.

Conclusion

In this study, we demonstrate the successful design of a cell factory for high-level 5-HTP production, combined with simple processes that have potential for use in industrial applications in the future. Thus, this study provides a reference for the production of high-value amino acid derivatives using a systematic modular engineering strategy and a basis for an efficient engineered strain development of 5-HTP high-value derivatives.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01920-3.

Efficient cell factories for the production of N-methylated amino acids and for methanol-based amino acid production

The growing world needs commodity amino acids such as L-glutamate and L-lysine for use as food and feed, and specialty amino acids for dedicated applications. To meet the supply a paradigm shift regarding their production is required. On the one hand, the use of sustainable and cheap raw materials is necessary to sustain low production cost and decrease detrimental effects of sugar-based feedstock on soil health and food security caused by competing uses of crops in the feed and food industries. On the other hand, the biotechnological methods to produce functionalized amino acids need to be developed further, and titres enhanced to become competitive with chemical synthesis methods. In the current review, we present successful strain mutagenesis and rational metabolic engineering examples leading to the construction of recombinant bacterial strains for the production of amino acids such as L-glutamate, L-lysine, L-threonine and their derivatives from methanol as sole carbon source. In addition, the fermentative routes for bioproduction of N-methylated amino acids are highlighted, with focus on three strategies: partial transfer of methylamine catabolism, S-adenosyl-L-methionine dependent alkylation and reductive methylamination of 2-oxoacids.

Research progress of pathway and genome evolution in microbes

Microbes can produce valuable natural products widely applied in medicine, food and other important fields. Nevertheless, it is usually challenging to achieve ideal industrial yields due to low production rate and poor toxicity tolerance. Evolution is a constant mutation and adaptation process used to improve strain performance. Generally speaking, the synthesis of natural products in microbes is often intricate, involving multiple enzymes or multiple pathways. Individual evolution of a certain enzyme often fails to achieve the desired results, and may lead to new rate-limiting nodes that affect the growth of microbes. Therefore, it is inevitable to evolve the biosynthetic pathways or the whole genome. Here, we reviewed the pathway-level evolution including multi-enzyme evolution, regulatory elements engineering, and computer-aided engineering, as well as the genome-level evolution based on several tools, such as genome shuffling and CRISPR/Cas systems. Finally, we also discussed the major challenges faced by in vivo evolution strategies and proposed some potential solutions.

Metabolic Engineering of Pseudomonas putida for Fermentative Production of l-Theanine

N-alkylated amino acids are intermediates of natural biological pathways and can be found incorporated in peptides or have physiological roles in their free form. The N-ethylated amino acid l-theanine shows taste-enhancing properties and health benefits. It naturally occurs in green tea as major free amino acid. Isolation of l-theanine from Camilla sinensis shows low efficiency, and chemical synthesis results in a racemic mixture. Therefore, biochemical approaches for the production of l-theanine gain increasing interest. Here, we describe metabolic engineering of Pseudomonas putida KT2440 for the fermentative production of l-theanine from monoethylamine and carbon sources glucose, glycerol, or xylose using heterologous enzymes from Methylorubrum extorquens for l-theanine production and heterologous enzymes from Caulobacter crescentus for growth with xylose. l-Theanine (15.4 mM) accumulated in shake flasks with minimal medium containing monoethylamine and glucose, 15.2 mM with glycerol and 7 mM with xylose. Fed-batch bioreactor cultures yielded l-theanine titers of 10 g L^-1 with glucose plus xylose, 17.2 g L^-1 with glycerol, 4 g L^-1 with xylose, and 21 g L^-1 with xylose plus glycerol, respectively. To the best of our knowledge, this is the first l-theanine process using P. putida and the first compatible with the use of various alternative carbon sources.