Phosphoenolpyruvate-supply module in Escherichia coli improves N-acetyl-d-neuraminic acid biocatalysis

Escherichia coli BL21(DE3) optimized deletion mutant as the host for whole-cell biotransformation of N‑acetyl‑D‑neuraminic acid

N‑Acetyl‑D‑neuraminic acid (Neu5Ac) is the crucial compound for the chemical synthesis of antiflu medicine Zanamivir. Chemoenzymatic synthesis of Neu5Ac involves N-acetyl-D-glucosamine 2-epimerase (AGE)-catalyzed epimerization of N-acetyl-D-glucosamine (GlcNAc) to N-acetyl-D-mannosamine (ManNAc), and aldolase-catalyzed condensation between ManNAc and pyruvate. Host optimization plays an important role in the whole-cell biotransformation of value-added compounds. In this study, via single-plasmid biotransformation system, we showed that the AGE gene BT0453, cloned from human gut microorganism Bacteroides thetaiotaomicron VPI-5482, showed the highest biotransformation yield among the AGE genes tested; and there is no clear Neu5Ac yield difference between the BT0453 coupled with one aldolase coding nanA gene and two nanA genes. Next, Escherichia coli chromosomal genes involved in substrate degradation, product exportation and pH change were deleted via recombineering and CRISPR/Cas9. With the final E. coli BL21(DE3) ΔnanA Δnag ΔpoxB as host, a significant 16.5% yield improvement was obtained. Furthermore, precursor (pyruvate) feeding resulted in 3.2% yield improvement, reaching 66.8% molar biotransformation. The result highlights the importance of host optimization, and set the stage for further metabolic engineering of whole-cell biotransformation of Neu5Ac.

An overview and future prospects of sialic acids

Sialic acids (Sias) are negatively charged functional monosaccharides present in a wide variety of natural sources (plants, animals and microorganisms). Sias play an important role in many life processes, which are widely applied in the medical and food industries as intestinal antibacterials, antivirals, anti-oxidative agents, food ingredients, and detoxification agents. Most Sias are composed of N-acetylneuraminic acid (Neu5Ac, >99%), and Sia is its most commonly used name. In this article, we review Sias in terms of their structures, applications, determination methods, metabolism, and production strategies. In particular, we summarise and compare different production strategies, including extraction from natural sources, chemical synthesis, polymer decomposition, enzymatic synthesis, whole-cell catalysis, and de novo biosynthesis via microorganism fermentation. We also discuss research on their physiological functions and applications, barriers to efficient production, and strategies for overcoming these challenges. We focus on efficient de novo biosynthesis strategies for Neu5Ac via microbial fermentation using novel synthetic biology tools and methods that may be applied in future. This work provides a comprehensive overview of recent advances on Sias, and addresses future challenges regarding their functions, applications, and production.

Biosynthesis of Putrescine from L-arginine Using Engineered Escherichia coli Whole Cells

Putrescine, a biogenic amine, is a highly valued compound in medicine, industry, and agriculture. In this study, we report a whole-cell biocatalytic method in Escherichia coli for the production of putrescine, using L-arginine as the substrate. L-arginine decarboxylase and agmatine ureohydrolase were co-expressed to produce putrescine from L-arginine. Ten plasmids with different copy numbers and ordering of genes were constructed to balance the expression of the two enzymes, and the best strain was pACYCDuet-speB-speA. The optimal concentration of L-arginine was determined to be 20 mM for this strain. The optimum pH of the biotransformation was 9.5, and the optimum temperature was 45 °C; under these conditions, the yield of putrescine was 98%. This whole-cell biocatalytic method appeared to have great potential for the production of putrescine.

Enhanced N-acetyl-D-neuraminic production from glycerol and N-acetyl-D-glucosamine by metabolically engineered Escherichia coli with a two-stage pH-shift control strategy

Typical N-acetyl-D-neuraminic acid (Neu5Ac) production uses N-acetyl-D-glucosamine (GlcNAc) and excess pyruvate as substrates in the enzymatic or whole-cell biocatalysis process. In a previous study, a Neu5Ac-producing biocatalytic process via engineered Escherichia coli SA-05/pDTrc-AB/pCDF-pck-ppsA was constructed without exogenous pyruvate. In this study, glycerol was found to be a good energy source compared with glucose for the catalytic system with resting cells, and Neu5Ac production increased to 13.97 ± 0.27 g L^-1. In addition, a two-stage pH shift strategy was carried out, and the Neu5Ac yield was improved to 14.61 ± 0.31 g L^-1. The GlcNAc concentration for Neu5Ac production was optimized. Finally, an integrated strategy was developed for Neu5Ac production, and the Neu5Ac yield reached as high as 18.17 ± 0.27 g L^-1. These results provide a new biocatalysis technology for Neu5Ac production without exogenous pyruvate.

Production of N-acetyl-D-neuraminic Acid by Recombinant Single Whole Cells Co-expressing N-acetyl-D-glucosamine-2-epimerase and N-acetyl-D-neuraminic Acid Aldolase

N-acetyl-D-neuraminic acid (Neu5Ac) is a costly precursor for many drugs such as anti-influenza antivirals. In a previous study, a whole-cell process for Neu5Ac production was developed using a combination of two Escherichia coli cells expressing Anabaena sp. CH1 N-acetyl-D-glucosamine-2-epimerase (bage) and E. coli N-acetyl-D-neuraminic acid aldolase (nanA), respectively. In this study, we constructed a bAGE and NanA co-expression system to improve Neu5Ac production. Two recombinant E. coli strains, E. coli BL21 (DE3) pET-bage-nanA (HA) and E. coli BL21 (DE3) pET-bage-2nanA (HAA), synchronously expressing bAGE and NanA were used as biocatalysts to generate Neu5Ac from N-acetyl-D-glucosamine (GlcNAc) and pyruvate. The HA biocatalysts produced 187.5 mM Neu5Ac within 8 h. The yield of GlcNAc was 15.6%, and the Neu5Ac production rate was 7.25 g/L/h. The most active HAA biocatalysts generated 412.6 mM Neu5Ac and a GlcNAc yield of 34.4%. HAA achieved a Neu5Ac production rate of 15.9 g/L/h, which surpassed those for all reported Neu5Ac production processes so far. The present study demonstrates that using recombinant E. coli cells synchronously expressing bAGE and NanA as biocatalysts could potentially be used in the industrial mass production of Neu5Ac.