Expression and Characterization of a New L-amino Acid Oxidase AAO Producing α-ketoglutaric Acid from L-glutamic Acid

An integrated cofactor and co-substrate recycling pathway for the biosynthesis of 1,5-pentanediol

BACKGROUND

1,5-pentanediol (1,5-PDO) is a linear diol with an odd number of methylene groups, which is an important raw material for polyurethane production. In recent years, the chemical methods have been predominantly employed for synthesizing 1,5-PDO. However, with the increasing emphasis on environmentally friendly production, it has been a growing interest in the biosynthesis of 1,5-PDO. Due to the limited availability of only three reported feasible biosynthesis pathways, we developed a new biosynthetic pathway to form a cell factory in Escherichia coli to produce 1,5-PDO.

RESULTS

In this study, we reported an artificial pathway for the synthesis of 1,5-PDO from lysine with an integrated cofactor and co-substrate recycling and also evaluated its feasibility in E.coli. To get through the pathway, we first screened aminotransferases originated from different organisms to identify the enzyme that could successfully transfer two amines from cadaverine, and thus GabT from E. coli was characterized. It was then cascaded with lysine decarboxylase and alcohol dehydrogenase from E. coli to achieve the whole-cell production of 1,5-PDO from lysine. To improve the whole-cell activity for 1,5-PDO production, we employed a protein scaffold of EutM for GabT assembly and glutamate dehydrogenase was also validated for the recycling of NADPH and α-ketoglutaric acid (α-KG). After optimizing the cultivation and bioconversion conditions, the titer of 1,5-PDO reached 4.03 mM.

CONCLUSION

We established a novel pathway for 1,5-PDO production through two consecutive transamination reaction from cadaverine, and also integrated cofactor and co-substrate recycling system, which provided an alternative option for the biosynthesis of 1,5-PDO.

Efficient production of α-ketoglutaric acid using an economical double-strain cultivation and catalysis system

The whole-cell catalysis strategy of alpha-ketoglutaric acid (α-KG) production from L-glutamic acid (L-Glu) using recombinant Escherichia coli, in which L-glutamate oxidase (LGox) was over-expressed, has replaced the traditional chemical synthesis strategy. However, large amounts of toxic by-product, H2O2, should be eliminated through co-expressing catalase (Cat), thus severely increasing burden in cells. To efficiently and economically produce α-KG, here, the genes SpLGox (from Streptomyces platensis NTU3304) and SlCat (from Streptomyces lividans TK24) were inserted into the low-dosage-IPTG (Isopropyl β-D-Thiogalactoside) inducible expression system, constructed in our previous work, in E. coli, respectively. Besides, a double-strain catalysis system was established and optimized to produce α-KG, and the productivity of α-KG was increased 97% compared with that through single strain catalysis. Finally, a double-strain cultivation strategy was designed and employed to simplify the scale-up fermentation. Using the optimized whole-cell biocatalyst conditions (pH 7.0, 35 °C), majority of the L-glutamic acid was transformed into α-KG and the titer reached 95.4 g/L after 6 h with the highest productivity at present. Therefore, this strategy may efficiently and cost-effectively produce α-KG, enhancing its potential for industrial applications. KEY POINTS: • SpLGox and SlCat were over-expressed to catalyze L-Glu to α-KG and eliminate by-product H2O2, respectively. • Double-strain cultivation and catalysis system can efficiently and cost-effectively produce α-KG from L-Glu.

Metabolic engineering and fermentation optimization strategies for producing organic acids of the tricarboxylic acid cycle by microbial cell factories

The organic acids of the tricarboxylic acid (TCA) pathway are important platform compounds and are widely used in many areas. The high-productivity strains and high-efficient and low-cost fermentation are required to satisfy a huge market size. The high metabolic flux of the TCA pathway endows microorganisms potential to produce high titers of these organic acids. Coupled with metabolic engineering and fermentation optimization, the titer of the organic acids has been significantly improved in recent years. Herein, we discuss and compare the recent advances in synthetic pathway engineering, cofactor engineering, transporter engineering, and fermentation optimization strategies to maximize the biosynthesis of organic acids. Such engineering strategies were mainly based on the TCA pathway and glyoxylate pathway. Furthermore, organic-acid-secretion enhancement and renewable-substrate-based fermentation are often performed to assist the biosynthesis of organic acids. Further strategies are also discussed to construct high-productivity and acid-resistant strains for industrial large-scale production.

Expression and Surface Display of an Acidic Cold-Active Chitosanase in Pichia pastoris Using Multi-Copy Expression and High-Density Cultivation

Chitosanase hydrolyzes β-(1,4)-linked glycosidic bonds are used in chitosan chains to release oligosaccharide mixtures. Here, we cloned and expressed a cold-adapted chitosanase (CDA, Genbank: MW094131) using multi-copy expression plasmids (CDA1/2/3/4) in Pichia pastoris. We identified elevated CDA expression levels in multi-copy strains, with strain PCDA4 selected for high-density fermentation and enzyme-activity studies. The high-density fermentation approach generated a CDA yield of 20014.8 U/mL, with temperature and pH optimization experiments revealing the highest CDA activity at 20 °C and 5.0, respectively. CDA was stable at 10 °C and 20 °C. Thus, CDA could be used at low temperatures. CDA was then displayed on P. pastoris using multi-copy expression plasmids. Then, multi-copy strains were constructed and labelled as PCDA(1-3)-AGα1. Further studies showed that the expression of CDA(1-3)-AGα1 in multi-copy strains was increased, and that strain PCDA3-AGα1 was chosen for high-density fermentation and enzyme activity studies. By using a multi-copy expression and high-density fermentation approach, we observed CDA-AGα1 expression yields of 102415 U/g dry cell weight. These data showed that the displayed CDA exhibited improved thermostability and was more stable over wider temperature and pH ranges than free CDA. In addition, displayed CDA could be reused. Thus, the data showed that displaying enzymes on P. pastoris may have applications in industrial settings.

High-level extracellular production and immobilisation of methyl parathion hydrolase from Plesiomonas sp. M6 expressed in Pichia pastoris

Methyl parathion hydrolase (MPH) hydrolyses methyl parathion efficiently and specifically. Herein, we produced MPH from Plesiomonas sp. M6 using a Pichia pastoris multi-copy expression system. The original signal peptide sequence of the target gene was removed, and a modified coding sequence was synthesised. Multi-copy expression plasmids containing MPH were constructed using pHBM905BDM, and used to generate recombinant strains containing 1, 2, 3 or 4 copies of the MPH gene. The results showed that a higher target gene copy number increased the production of recombinant MPH (MPH-R), as anticipated. The expression level of the recombinant strain containing four copies of the MPH gene was increased to 1.9 U/ml using 500 ml shake flasks, and the specific activity was 15.8 U/mg. High-density fermentation further increased the target protein yield to 18.4 U/ml. Several metal ions were tested as additives, and Ni²⁺, Co²⁺ and Mg²⁺ at a concentration of 1 mM enhanced MPH-R activity by 196%, 201% and 154%, respectively. Enzyme immobilisation was then applied to overcome the difficulties in recovery, recycling and long-term stability associated with the free enzyme. Immobilised MPH-R exhibited significantly enhanced thermal and long-term stability, as well as broad pH adaptability. In the presence of inhibitors and chelating agents such as sodium dodecyl sulphate (SDS), immobilised MPH-R displayed 2-fold higher activity than free MPH-R, demonstrating its potential for industrial application.