γ-aminobutyric acid can be produced by a one-step enzymatic reaction catalyzed by glutamic acid decarboxylase. The reaction system is simple and environmentally friendly. However, the majority of GAD enzymes catalyze the reaction under acidic pH at a relatively narrow range. Thus, inorganic salts are usually needed to maintain the optimal catalytic environment, which adds additional components to the reaction system. In addition, the pH of solution will gradually rise along with the production of γ-aminobutyric acid, which is not conducive for GAD to function continuously. In this study, we cloned the glutamate decarboxylase LpGAD from a Lactobacillus plantarum capable of efficiently producing γ-aminobutyric acid, and rationally engineered the catalytic pH range of LpGAD based on surface charge. A triple point mutant LpGAD^{S24R/D88R/Y309K} was obtained from different combinations of 9 point mutations. The enzyme activity at pH 6.0 was 1.68 times of that of the wild type, suggesting the catalytic pH range of the mutant was widened, and the possible mechanism underpinning this increase was discussed through kinetic simulation. Furthermore, we overexpressed the Lpgad and Lpgad^{S24R/D88R/Y309K} genes in Corynebacterium glutamicum E01 and optimized the transformation conditions. An optimized whole cell transformation process was conducted under 40 ℃, cell mass (OD₆₀₀) 20, 100 g/L l-glutamic acid substrate and 100 μmol/L pyridoxal 5-phosphate. The γ-aminobutyric acid titer of the recombinant strain reached 402.8 g/L in a fed-batch reaction carried out in a 5 L fermenter without adjusting pH, which was 1.63 times higher than that of the control. This study expanded the catalytic pH range of and increased the enzyme activity of LpGAD. The improved production efficiency of γ-aminobutyric acid may facilitate its large-scale production.
Glutamate Decarboxylase/genetics* ; Lactobacillus plantarum/genetics* ; Catalysis ; gamma-Aminobutyric Acid ; Hydrogen-Ion Concentration ; Glutamic Acid

L-glutamic acid is the world's largest bulk amino acid product that is widely used in the food, pharmaceutical and chemical industries. Using Corynebacterium glutamicum G01 as the starting strain, the fermentation by-product alanine content was firstly reduced by knocking out the gene encoding alanine aminotransferase (alaT), a major by-product related to alanine synthesis. Secondly, since the α-ketoglutarate node carbon flow plays an important role in glutamate synthesis, the ribosome-binding site (RBS) sequence optimization was used to reduce the activity of α-ketoglutarate dehydrogenase and enhance the glutamate anabolic flow. The endogenous conversion of α-ketoglutarate to glutamate was also enhanced by screening different glutamate dehydrogenase. Subsequently, the glutamate transporter was rationally desgined to improve the glutamate efflux capacity. Finally, the fermentation conditions of the strain constructed using the above strategy were optimized in 5 L fermenters by a gradient temperature increase combined with a batch replenishment strategy. The glutamic acid production reached (135.33±4.68) g/L, which was 41.2% higher than that of the original strain (96.53±2.32) g/L. The yield was 55.8%, which was 11.6% higher than that of the original strain (44.2%). The combined strategy improved the titer and the yield of glutamic acid, which provides a reference for the metabolic modification of glutamic acid producing strains.
Glutamic Acid ; Corynebacterium glutamicum/genetics* ; Ketoglutaric Acids ; Metabolic Engineering ; Alanine

As a model industrial host and microorganism with the generally regarded as safe (GRAS) status, Corynebacterium glutamicum not only produces amino acids on a large scale in the fermentation industry, but also has the potential to produce various new products. C. glutamicum usually encounters various stresses in the process of producing compounds, which severely affect cell viability and production performance. The development of synthetic biology provides new technical means for improving the robustness of C. glutamicum. In this review, we discuss the tolerance mechanisms of C. glutamicum to various stresses in the fermentation process. At the same time, we highlight new synthetic biology strategies for boosting C. glutamicum robustness, including discovering new stress-resistant elements, modifying transcription factors, and using adaptive evolution strategies to mine stress-resistant functional modules. Finally, prospects of improving the robustness of engineered C. glutamicum strains ware provided, with an emphasis on biosensor, screening and design of transcription factors, and utilizing the multiple regulatory elements.
Amino Acids/metabolism* ; Corynebacterium glutamicum/metabolism* ; Fermentation ; Metabolic Engineering ; Synthetic Biology

L-asparaginase hydrolyzes L-asparagine to produce L-aspartic acid and ammonia. It is widely distributed in microorganisms, plants and serum of some rodents, and has important applications in the pharmaceutical and food industries. However, the poor thermal stability, low catalytic efficiency and low yield hampered the further application of L-asparaginase. In this paper, rational design and 5' untranslated region (5'UTR) design strategies were used to increase the specific enzyme activity and protein expression of L-asparaginase derived from Rhizomucor miehei (RmAsnase). The results showed that among the six mutants constructed through homology modeling combined with sequence alignment, the specific enzyme activity of the mutant A344E was 1.5 times higher than the wild type. Subsequently, a food-safe strain Bacillus subtilis 168/pMA5-A344E was constructed, and the UTR strategy was used for the construction of recombinant strain B. subtilis 168/pMA5 UTR-A344E. The enzyme activity of B. subtilis 168/pMA5 UTR-A344E was 7.2 times higher than that of B. subtilis 168/pMA5-A344E. The recombinant strain B. subtilis 168/pMA5 UTR-A344E was scaled up in 5 L fermenter, and the final yield of L-asparaginase was 489.1 U/mL, showing great potential for industrial application.
Asparaginase/genetics* ; Bacillus subtilis/genetics* ; Industrial Microbiology ; Protein Engineering ; Rhizomucor/enzymology* ; Sequence Alignment

2-Hydroxybutyric acid (2-HBA) is an important intermediate for synthesizing biodegradable materials and various medicines. Chemically synthesized racemized 2-HBA requires deracemization to obtain optically pure enantiomers for industrial application. In this study, we designed a cascade biosynthesis system in Escherichia coli BL21 by coexpressing L-threonine deaminase (TD), NAD-dependent L-lactate dehydrogenase (LDH) and formate dehydrogenase (FDH) for production of optically pure (S)-2-HBA from bulk chemical L-threonine (L-Thr). To coordinate the production rate and the consumption rate of the intermediate 2-oxobutyric acid in the multi-enzyme cascade catalytic reactions, we explored promoter engineering to regulate the expression levels of TD and FDH, and developed a recombinant strain P21285FDH-T7V7827 with a tunable system to achieve a coordinated multi-enzyme expression. The recombinant strain P21285FDH-T7V7827 was able to efficiently produce (S)-2-HBA with the highest titer of 143 g/L and a molar yield of 97% achieved within 16 hours. This titer was approximately 1.83 times than that of the highest yield reported to date, showing great potential for industrial application. Our results indicated that constructing a multi-enzyme-coordinated expression system in a single cell significantly contributed to the biosynthesis of hydroxyl acids.
Escherichia coli/genetics* ; Formate Dehydrogenases ; Hydroxybutyrates ; Threonine Dehydratase

Leucine dehydrogenase (LDH) is the key rate-limiting enzyme in the production of L-2-aminobutyric acid (L-2-ABA). In this study, we modified the C-terminal Loop region of this enzyme to improve the specific enzyme activity and stability for efficient synthesis of L-2-ABA. Using molecular dynamics simulation of LDH, we analyzed the change of root mean square fluctuation (RMSF), rationally designed the Loop region with greatly fluctuated RMSF, and obtained a mutant EsLDHD2 with a specific enzyme activity 23.2% higher than that of the wild type. Since the rate of the threonine deaminase-catalyzed reaction converting L-threonine into 2-ketobutyrate was so fast, the multi-enzyme cascade catalysis system became unbalanced. Therefore, the LDH and the formate dehydrogenase were double copied in a new construct E. coli BL21/pACYCDuet-RM. Compared with E. coli BL21/pACYCDuet-RO, the molar conversion rate of L-2-ABA increased by 74.6%. The whole cell biotransformation conditions were optimized and the optimal pH, temperature and substrate concentration were 7.5, 35 °C and 80 g/L, respectively. Under these conditions, the molar conversion rate was higher than 99%. Finally, 80 g and 40 g L-threonine were consecutively fed into a 1 L reaction mixture under the optimal conversion conditions, producing 97.9 g L-2-ABA. Thus, this strategy provides a green and efficient synthesis of L-2-ABA, and has great industrial application potential.
Aminobutyrates ; Escherichia coli/genetics* ; Leucine Dehydrogenase/genetics* ; Threonine Dehydratase

5-aminolevulinic acid (5-ALA) plays an important role in the fields of medicine and agriculture. 5-ALA can be produced by engineered Escherichia coli and Corynebacterium glutamicum. We systematically engineered the C4 metabolic pathway of C. glutamicum to further improve its ability to produce 5-ALA. Firstly, the hemA gene encoding 5-ALA synthase (ALAS) from Rhodobacter capsulatus and Rhodopseudomonas palustris were heterologously expressed in C. glutamicum, respectively. The RphemA gene of R. palustris which showed relatively high enzyme activity was selected. Screening of the optimal ribosome binding site sequence RBS5 significantly increased the activity of RphemA. The ALAS activity of the recombinant strain reached (221.87±3.10) U/mg and 5-ALA production increased by 14.3%. Subsequently, knocking out genes encoding α-ketoglutarate dehydrogenase inhibitor protein (odhI) and succinate dehydrogenase (sdhA) increased the flux of succinyl CoA towards the production of 5-ALA. Moreover, inhibiting the expression of hemB by means of sRNA reduced the degradation of 5-ALA, while overexpressing the cysteine/O-acetylserine transporter eamA increased the output efficiency of intracellular 5-ALA. Shake flask fermentation using the engineered strain C. glutamicum 13032/∆odhI/∆sdhA-sRNAhemB- RBS5RphemA-eamA resulted in a yield of 11.90 g/L, which was 57% higher than that of the original strain. Fed-batch fermentation using the engineered strain in a 5 L fermenter produced 25.05 g/L of 5-ALA within 48 h, which is the highest reported-to-date yield of 5-ALA from glucose.
Aminolevulinic Acid/metabolism* ; Corynebacterium glutamicum/metabolism* ; Fermentation ; Metabolic Engineering ; Rhodobacter capsulatus/enzymology* ; Rhodopseudomonas/enzymology*

Glutamic acid is an important amino acid with wide range of applications and huge market demand. Therefore, by performing transcriptome sequencing and re-sequencing analysis on Corynebacterium glutamicum E01 and high glutamate-producing strain C. glutamicum G01, we identified and selected genes with significant differences in transcription and gene levels in the central metabolic pathway that may have greatly influenced glutamate synthesis and further increased glutamic acid yield. The oxaloacetate node and α-ketoglutarate node play an important role in glutamate synthesis. The oxaloacetate node and α-ketoglutarate node were studied to explore effect on glutamate production. Based on the integrated strain constructed from the above experimental results, the growth rate in a 5-L fermenter was slightly lower than that of the original strain, but the glutamic acid yield after 48 h reached (136.1±5.53) g/L, higher than the original strain (93.53±4.52) g/L, an increase by 45.5%; sugar-acid conversion rate reached 58.9%, an increase of 13.7% compared to 45.2% of the original strain. The application of the above experimental strategy improved the glutamic acid yield and the sugar-acid conversion rate, and provided a theoretical basis for the metabolic engineering of Corynebacterium glutamicum.
Citric Acid Cycle ; Corynebacterium glutamicum/metabolism* ; Glutamic Acid/metabolism* ; Metabolic Engineering ; Metabolic Networks and Pathways/genetics*

In this study, Escherichia coli BL21 (DE3) was used as the host to construct 2 recombinant E. coli strains that co-expressed leucine dehydrogenase (LDH, Bacillus cereus)/formate dehydrogenase (FDH, Ancylobacter aquaticus), or leucine dehydrogenase (LDH, Bacillus cereus)/alcohol dehydrogenase (ADH, Rhodococcus), respectively. L-2-aminobutyric acid was then synthesized by L-threonine deaminase (L-TD) with LDH-FDH or LDH-ADH by coupling with two different NADH regeneration systems. LDH-FDH process and LDH-ADH process were optimized and compared with each other. The optimum reaction pH of LDH-FDH process was 7.5, and the optimum reaction temperature was 35 °C. After 28 h, the concentration of L-2-aminobutyric acid was 161.8 g/L with a yield of 97%, when adding L-threonine in batches for controlling 2-ketobutyric acid concentration less than 15 g/L and using 50 g/L ammonium formate, 0.3 g/L NAD+, 10% LDH-FDH crude enzyme solution (V/V) and 7 500 U/L L-TD. The optimum reaction pH of LDH-ADH process was 8.0, and the optimum reaction temperature was 35 °C. After 24 h, the concentration of L-2-aminobutyric acid was 119.6 g/L with a yield of 98%, when adding L-threonine and isopropanol (1.2 times of L-threonine) in batches for controlling 2-ketobutyric acid concentration less than 15 g/L, removing acetone in time and using 0.3 g/L NAD⁺, 10% LDH-ADH crude enzyme solution (V/V) and 7 500 U/L L-TD. The process and results used in this paper provide a reference for the industrialization of L-2-aminobutyric acid.
Aminobutyrates ; metabolism ; Escherichia coli ; genetics ; Formate Dehydrogenases ; metabolism ; Leucine Dehydrogenase ; metabolism ; NAD ; metabolism

The trehalose synthase (ScTreS) gene from Streptomyces coelicolor was successfully cloned and heterologously expressed in Escherichia coli BL21(DE3). The protein purified by Ni-NTA affinity column showed an apparent molecular weight (MW) of 62.3 kDa analyzed by SDS-PAGE. The optimum temperature of the enzyme was 35 °C and the optimum pH was 7.0; the enzyme was sensitive to acidic conditions. By homologous modeling and sequence alignment, the enzyme was modified by site-directed mutagenesis. The relative activities of the mutant enzymes K246A and A165T were 1.43 and 1.39 times that of the wild type, an increased conversion rate of 14% and 10% respectively. To optimize the synthesis conditions of trehalose, the mutant strain K246A was cultivated in a 5-L fermentor and used for whole-cell transformation. The results showed that with the substrate maltose concentration of 300 g/L at 35 °C and pH 7.0, the highest conversion rate reached 71.3%, and the yield of trehalose was 213.93 g/L. However, when maltose concentration was increased to 700 g/L, the yield of trehalose can reach 465.98 g/L with a conversion rate of 66%.
Biocatalysis ; Cloning, Molecular ; Escherichia coli ; Glucosyltransferases ; Streptomyces coelicolor ; Trehalose