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We developed an efficient multi-enzyme cascade reaction to produce (R)- or (S)-3,4-Dihydroxyphenyllactic acid [(R)- or (S)-Danshensu, (R)- or (S)-DSS] from 3,4-Dihydroxyphenyl-l-alanine (l-DOPA) in Escherichia coli by introducing tyrosine aminotransferase (tyrB), glutamate dehydrogenase (cdgdh) and d-aromatic lactate dehydrogenase (csldhD) or l-aromatic lactate dehydrogenase (tcldhL). First, the genes in the pathway were overexpressed and fine-tuned for (R)- or (S)-DSS production. The resulting strain, E. coli TGL 2.1 and E. coli TGL 2.2, which overexpressed tyrB with the stronger T7 promoter and cdgdh, cs ldhD or tcldhL with the weaker Trc promoter, E. coli TGL 2.1 yielded 57% increase in (R)-DSS production: 59.8 ± 2.9 mM. Meanwhile, E. coli TGL 2.2 yielded 54% increase in (S)-DSS production: 52.2 ± 2.4 mM. The optimal concentration of L-glutamate was found to be 20 mM for production of (R)- or (S)-DSS. Finally, l-DOPA were transformed into (R)- or (S)-DSS with an excellent enantiopure form (enantiomeric excess > 99.99%) and productivity of 6.61 mM/h and 4.48 mM/h, respectively.

In this study, Pickering emulsion gels were prepared by the self-gel method based on kappa carrageenan (kC). The effects of particle stabilizers and polysaccharide concentrations on the microstructure, rheological characteristics, and texture of Pickering emulsion gels stabilized by xanthan gum/lysozyme nanoparticles (XG/Ly NPs) with kC were discussed. The viscoelasticity of Pickering emulsion gels increased significantly with the increase of kC and XG/Ly NPs. The results of temperature sweep showed that the gel formation mainly depended on the kC addition. The XG/Ly NPs addition could accelerate the formation of Pickering emulsion gels and increase its melting temperature (Tmelt), which is helpful to improve the thermal stability of emulsion gels. Cryo-scanning electron microscope (Cryo-SEM) images revealed that Pickering emulsion gel has a porous network structure, and the oil droplets were well wrapped in the pores. The hardness increased significantly with the increase of XG/Ly NPs and kC. In particular, the Pickering emulsion gel hardness was up to 2.9 Newton (N) when the concentration of kC and XG/Ly NPs were 2%. The results showed that self-gelling polysaccharides, such as kC, could construct and regulate the structure and characteristics of Pickering emulsion gel. This study provides theoretical support for potential new applications of emulsion gels as functional colloids and delivery systems in the food industry.

BACKGROUND Carboxyspermidine (C‐Spd) is a potentially valuable polyamine carboxylate compound and an excellent building block for spermidine synthesis, which is a critical polyamine with significant implications for human health and longevity. C‐Spd can also be used to prepare multivalent cationic lipids and modify nucleoside probes. Because of these positive effects on human health, C‐Spd is of considerable interest as a food additive and pharmaceutical target. RESULTS A putative gene afcasdh from Agrobacterium fabrum str. C58, encoding carboxyspermidine dehydrogenase with C‐Spd biosynthesis activity, was synthesized and transformed into Escherichia coli BL21 (DE3) for overexpression. The recombinant AfCASDH was purified and fully characterized. The optimum temperature and pH for the recombinant enzyme were 30 °C and 7.5, respectively. The coupled catalytic strategy of AfCASDH and various NADPH regeneration systems were developed to enhance the efficient production of C‐Spd compound. Finally, the maximum titer of C‐Spd production successfully achieved 1.82 mmol L−1 with a yield of 91% by optimizing the catalytic conditions. CONCLUSION A novel AfCASDH from A. fabrum str. C58 was characterized that could catalyze the formation of C‐Spd from putrescine and l‐aspartate‐β‐semialdehyde (L‐Asa). A whole‐cell catalytic strategy coupled with NADPH regeneration was established successfully for C‐Spd biosynthesis for the first time. The coupled system indicated that AfCASDH might provide a feasible method for the industrial production of C‐Spd. © 2021 Society of Chemical Industry.

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The efficiency of whole‐cell biotransformation is often affected by the genetic instability of plasmid‐based expression systems, which require selective pressure to maintain the stability of the plasmids. To circumvent this shortcoming, we constructed a chromosome engineering strain for the synthesis of phenylpyruvic acid (PPA) from l‐phenylalanine. First, l‐amino acid deaminase (pmLAAD) from Proteus myxofaciens was incorporated into Escherichia coli BL21 (DE3) chromosome and the copy numbers of pmLAAD were increased by chemically induced chromosomal evolution (CIChE). Fifty‐nine copies of pmLAAD were obtained in E. coli BL8. The PPA titer of E. coli BL8 reached 2.22 g/L at 6 h. Furthermore, the deletion of lacI improved PPA production. In the absence of isopropyl‐β‐d‐thiogalactopyranoside, the resulting strain, E. coli BL8△recA△lacI, produced 2.65 g/L PPA at 6 h and yielded a 19.37% increase in PPA production compared to E. coli BL8△recA. Finally, the engineered E. coli BL8△recA△lacI strain achieved 19.14 g/L PPA at 24 h in a 5‐L bioreactor.