Cloning and expression of a full-length glutamate decarboxylase gene fromLactobacillus brevis BH2

Evaluation of Nano-Wall Material for Production of Novel Lyophilized-Probiotic Product

Lyophilization is one of the most used methods for bacterial preservation. In this process, the cryoprotectant not only largely decreases cellular damage but also plays an important part in the conservation of viability during freeze-drying. This study investigated using cryoprotectant and a mixture of the cryoprotectant to maintain probiotic activity. Seven probiotic strains were considered: (Limosilactobacillus reuteri KUKPS6103; Lacticaseibacillus rhamnosus KUKPS6007; Lacticaseibacillus paracasei KUKPS6201; Lactobacillus acidophilus KUKPS6107; Ligilactobacillus salivarius KUKPS6202; Bacillus coagulans KPSTF02; Saccharomyces cerevisiae subsp. boulardii KUKPS6005) for the production of a multi-strain probiotic and the complex medium for the lyophilized synbiotic production. Cholesterol removal, antioxidant activity, biofilm formation and gamma aminobutyric acid (GABA) production of the probiotic strains were analyzed. The most biofilm formation occurred in L. reuteri KUKPS6103 and the least in B. coagulans KPSTF02. The multi-strain probiotic had the highest cholesterol removal. All the probiotic strains had GABA production that matched the standard of γ-aminobutyric acid. The lyophilized synbiotic product containing complex medium as a cryoprotectant and wall material retained a high viability of 7.53 × 10⁸ CFU/g (8.89 log CFU/g) after 8 weeks of storage. We found that the survival rate of the multi-strain probiotic after freeze-drying was 15.37% in the presence of complex medium that was used as high performing wall material. Our findings provided a new type of wall material that is safer and more effective and, can be extensively applied in relevant food applications.

Expression, purification, and characterization of glutamate decarboxylase from human gut-originated Lactococcus garvieae MJF010

Human gut-originated lactic acid bacteria were cultivated, and high γ-aminobutyric acid (GABA)-producing Lactococcus garvieae MJF010 was identified. To date, despite the importance of GABA, no studies have investigated GABA-producing Lactococcus species, except for Lc. lactis. A recombinant glutamate decarboxylase of the strain MJF010 (rLgGad) was successfully expressed in Escherichia coli BL21(DE3) with a size of 53.9 kDa. rLgGad could produce GABA, which was verified using the silylation-derivative fragment ions of GABA. The purified rLgGad showed the highest GABA-producing activity at 35 °C and pH 5. rLgGad showed a melting temperature of 43.84 °C. At 30 °C, more than 80% of the activity was maintained even after 7 h; however, it rapidly decreased at 50 °C. The kinetic parameters, K_m, V_max, and k_cat, of rLgGad were 2.94 mM, 0.023 mM/min, and 12.3 min^- 1, respectively. The metal reagents of CaCl₂, MgCl₂, and ZnCl₂ significantly had positive effects on rLgGad activity. However, most coenzymes including pyridoxal 5'-phosphate showed no significant effects on enzyme activity. In conclusion, this is the first report of Gad from Lc. garvieae species and provides important enzymatic information related to GABA biosynthesis in the Lactococcus genus.

Cell factory for γ-aminobutyric acid (GABA) production using Bifidobacterium adolescentis

Background

Bifidobacteria are gram-positive, probiotic, and generally regarded as safe bacteria. Techniques such as transformation, gene knockout, and heterologous gene expression have been established for Bifidobacterium, indicating that this bacterium can be used as a cell factory platform. However, there are limited previous reports in this field, likely because of factors such as the highly anaerobic nature of this bacterium. Bifidobacterium adolescentis is among the most oxygen-sensitive Bifidobacterium species. It shows strain-specific gamma-aminobutyric acid (GABA) production. GABA is a potent bioactive compound with numerous physiological and psychological functions. In this study, we investigated whether B. adolesentis could be used for mass production of GABA.

Results

The B. adolescentis 4–2 strain isolated from a healthy adult human produced approximately 14 mM GABA. It carried gadB and gadC, which encode glutamate decarboxylase and glutamate GABA antiporter, respectively. We constructed pKKT427::Pori-gadBC and pKKT427::Pgap-gadBC plasmids carrying gadBC driven by the original gadB (ori) and gap promoters, respectively. Recombinants of Bifidobacterium were then constructed. Two recombinants with high production abilities, monitored by two different promoters, were investigated. GABA production was improved by adjusting the fermentation parameters, including the substrate concentration, initial culture pH, and co-factor supplementation, using response surface methodology. The optimum initial cultivation pH varied when the promoter region was changed. The ori promoter was induced under acidic conditions (pH 5.2:4.4), whereas the constitutive gap promoter showed enhanced GABA production at pH 6.0. Fed-batch fermentation was used to validate the optimum fermentation parameters, in which approximately 415 mM GABA was produced. The conversion ratio of glutamate to GABA was 92–100%.

Conclusion

We report high GABA production in recombinant B. adolescentis. This study provides a foundation for using Bifidobacterium as a cell factory platform for industrial production of GABA.

Screening of gamma-aminobutyric acid-producing lactic acid bacteria and the characteristic of glutamate decarboxylase from Levilactobacillus brevis F109-MD3 isolated from kimchi

AIMS

This study aimed to screen the γ-aminobutyric acid (GABA)-producing lactic acid bacteria (LAB) from kimchi, and investigate the glutamate decarboxylase (GAD) activity of the highest GABA-producing strain.

METHODS AND RESULTS

Seven strains of LAB were screened from kimchi with GABA-producing activity. Strain Levilactobacillus brevis F109-MD3 showed the highest GABA-producing ability. It produced GABA at a concentration of 520 mmol l^-1 with a 97.4% GABA conversion rate in MRS broth containing 10% monosodium glutamate for 72 h. The addition of pyridoxal 5'-phosphate had no significant effect on the GAD activity of L. brevis F109-MD3. The optimal pH range of GAD was 3.0-5.0 and the optimal temperature was 65°C. The D value of GAD at 50, 60 and 70°C was 7143, 971 and 124 min respectively and Z value was 11.36°C.

CONCLUSIONS

Seven strains isolated from kimchi, especially F109-MD3, showed high GABA-production ability even in the high concentrations of MSG at 7.5% and 10%. The GAD activity showed an effective broad pH range and higher optimal temperature.

SIGNIFICANCE AND IMPACT OF THE STUDY

These seven strains could be potentially useful for food-grade GABA production and the development of healthy foods.

Characterization of three glutamate decarboxylases from Bacillus spp. for efficient γ-aminobutyric acid production

BACKGROUND

Gamma-aminobutyric acid (GABA) is an important bio-product used in pharmaceuticals and functional foods and as a precursor of the biodegradable plastic polyamide 4. Glutamate decarboxylase (GAD) converts L-glutamate (L-Glu) into GABA via decarboxylation. Compared with other methods, develop a bioconversion platform to produce GABA is of considerable interest for industrial use.

RESULTS

Three GAD genes were identified from three Bacillus strains and heterologously expressed in Escherichia coli BL21 (DE3). The optimal reaction temperature and pH values for three enzymes were 40 °C and 5.0, respectively. Of the GADs, GADZ11 had the highest catalytic efficiency towards L-Glu (2.19 mM^- 1 s^- 1). The engineered E. coli strain that expressed GADZ11 was used as a whole-cell biocatalyst for the production of GABA. After repeated use 14 times, the cells produced GABA with an average molar conversion rate of 98.6% within 14 h.

CONCLUSIONS

Three recombinant GADs from Bacillus strains have been conducted functional identification. The engineered E. coli strain heterologous expressing GADZ1, GADZ11, and GADZ20 could accomplish the biosynthesis of L-Glu to GABA in a buffer-free reaction at a high L-Glu concentration. The novel engineered E. coli strain has the potential to be a cost-effective biotransformation platform for the industrial production of GABA.

Evaluation of GABA Production and Probiotic Activities of Enterococcus faecium BS5

Gamma-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter in the central nervous system and is produced by irreversible decarboxylation of glutamate. It possesses several physiological functions such as neurotransmission, diuretic, and tranquilizer effects and also regulates cardiovascular functions such as blood pressure and heart rate in addition to playing a role in the reduction of pain and anxiety. The objective of this study was to evaluate the GABA producing ability and probiotic capability of certain lactic acid bacteria strains isolated from dairy products. Around sixty-four bacterial isolates were collected and screened for their ability to produce GABA from monosodium glutamate, among which nine isolates were able to produce GABA. The most efficient GABA producer was Enterococcus faecium BS5. Further, assessment of several important and desirable probiotic properties showed that Ent. faecium BS5 was resistant to acid stress, bile salt, and antibiotics. Ent. faecium BS5 may potentially be used for large-scale industrial production of GABA and also for functional fermented product development.

Glutamate Decarboxylase from Lactic Acid Bacteria-A Key Enzyme in GABA Synthesis

Glutamate decarboxylase (l-glutamate-1-carboxylase, GAD; EC 4.1.1.15) is a pyridoxal-5’-phosphate-dependent enzyme that catalyzes the irreversible α-decarboxylation of l-glutamic acid to γ-aminobutyric acid (GABA) and CO₂. The enzyme is widely distributed in eukaryotes as well as prokaryotes, where it—together with its reaction product GABA—fulfils very different physiological functions. The occurrence of gad genes encoding GAD has been shown for many microorganisms, and GABA-producing lactic acid bacteria (LAB) have been a focus of research during recent years. A wide range of traditional foods produced by fermentation based on LAB offer the potential of providing new functional food products enriched with GABA that may offer certain health-benefits. Different GAD enzymes and genes from several strains of LAB have been isolated and characterized recently. GABA-producing LAB, the biochemical properties of their GAD enzymes, and possible applications are reviewed here.

Effect of DR1558, a Deinococcus radiodurans response regulator, on the production of GABA in the recombinant Escherichia coli under low pH conditions

BACKGROUND

Gamma aminobutyric acid (GABA) is an important platform chemical, which has been used as a food additive and drug. Additionally, GABA is a precursor of 2-pyrrolidone, which is used in nylon synthesis. GABA is usually synthesized from glutamate in a reaction catalyzed by glutamate decarboxylase (GAD). Currently, there are several reports on GABA production from monosodium glutamate (MSG) or glucose using engineered microbes. However, the optimal pH for GAD activity is 4, which is the limiting factor for the efficient microbial fermentative production of GABA as fermentations are performed at pH 7. Recently, DR1558, a response regulator in the two-component signal transduction system was identified in Deinococcus radiodurans. DR1558 is reported to confer cellular robustness to cells by binding the promoter regions of genes via DNA-binding domains or by binding to the effector molecules, which enable the microorganisms to survive in various environmental stress conditions, such as oxidative stress, high osmotic shock, and low pH.

RESULTS

In this study, the effect of DR1558 in enhancing GABA production was examined using two different strategies: whole-cell bioconversion of GABA from MSG and direct fermentative production of GABA from glucose under acidic culture conditions. In the whole-cell bioconversion, GABA produced by E. coli expressing GadBC and DR1558 (6.52 g/L GABA from 13 g/L MSG·H₂O) in shake flask culture at pH 4.5 was 2.2-fold higher than that by E. coli expressing only GadBC (2.97 g/L of GABA from 13 g/L MSG·H₂O). In direct fermentative production of GABA from glucose, E. coli ∆gabT expressing isocitrate dehydrogenase (IcdA), glutamate dehydrogenase (GdhA), GadBC, and DR1558 produced 1.7-fold higher GABA (2.8 g/L of GABA from 30 g/L glucose) than E. coli ∆gabT expressing IcdA, GdhA, and GadBC (1.6 g/L of GABA from 30 g/L glucose) in shake flask culture at an initial pH 7.0. The transcriptional analysis of E. coli revealed that DR1558 conferred acid resistance to E. coli during GABA production. The fed-batch fermentation of E. coli expressing IcdA, GdhA, GadBC, and DR1558 performed at pH 5.0 resulted in the final GABA titer of 6.16 g/L by consuming 116.82 g/L of glucose in 38 h.

CONCLUSION

This is the first report to demonstrate GABA production by acidic fermentation and to provide an engineering strategy for conferring acid resistance to the recombinant E. coli for GABA production.

A Brief Review on the Non-protein Amino Acid, Gamma-amino Butyric Acid (GABA): Its Production and Role in Microbes

Gamma-Aminobutyric acid (GABA) is a non-protein amino acid widely distributed in nature. It is produced through irreversible α-decarboxylation of glutamate by enzyme glutamate decarboxylase (GAD). GABA and GAD have been found in plants, animals, and microorganisms. GABA is distributed throughout the human body and it is involved in the regulation of cardiovascular conditions such as blood pressure and heart rate, and plays a role in the reduction of anxiety and pain. Although researchers had produced GABA by chemical method earlier it became less acceptable as it pollutes the environment. Researchers now use a more promising microbial method for the production of GABA. In the drug and food industry, demand for GABA is immense. So, large scale conversion of GABA by microbes has got much attention. So this review focuses on the isolation source, production, and functions of GABA in the microbial system. We also summarize the mechanism of action of GABA and its shunt pathway.

Effect of adding amino acids on the production of Gamma-Aminobutyric Acid (GABA) by mycelium of Lentinula edodes

This study was carried out to investigate the production of a health functional food component through the production of GABA by mycelium of Lentinula edodes (LE) cultured in a medium containing four different amino acids. To confirm the GABA content in the medium, the amount of GABA produced by adding 0.1 M of glutamic acid, alanine, glycine, or lysine to Potato Dextrose Agar (PDA) medium and Potato Dextrose Broth (PDB) medium was determined. The amount of mycelia in the PDB medium was 4.85 g/L in the amino acid-free medium, 5.12 g/L in the glutamic acid medium, 4.63 g/L in the alanine medium, 4.87 g/L in the glycine medium, and 4.18 g/L in the lysine medium. The amount of amino acid added to the medium did not interfere with the normal growth of LE because the amount of excess amino acid was not significantly different from that of the control. The GABA content was 10.35 mg/L in the control (amino acid-free), 30.29 mg/L in the glutamic acid supplemented medium, 11.70 mg/L in the alanine supplemented medium, 10.62 mg/L in the glycine supplemented medium and 3.96 mg/L in Lysine supplemented medium. These results show that the excess glutamic acid had the highest level of GABA in the mushroom culture medium. On the other hand, it was confirmed that the addition of excess alanine and glycine did not affect the GABA production compared to the control. These results suggest that continuous GABA production could not be achieved by using an ion exchange resin after the disruption of GABA production by biological methods, however, continuous GABA production using the mycelium of LE is possible in this study.

Identification of new glutamate decarboxylases from Streptomyces for efficient production of γ-aminobutyric acid in engineered Escherichia coli

Background

Gamma (γ)-Aminobutyric acid (GABA) as a bioactive compound is used extensively in functional foods, pharmaceuticals and agro-industry. It can be biosynthesized via decarboxylation of monosodium glutamate (MSG) or L-glutamic acid (L-Glu) by glutamate decarboxylase (GAD; EC4.1.1.15). GADs have been identified from a variety of microbial sources, such as Escherichia coli and lactic acid bacteria. However, no GADs from Streptomyces have been characterized. The present study is aimed to identify new GADs from Streptomyces strains and establish an efficient bioproduction platform for GABA in E. coli using these enzymes.

Results

By sequencing and analyzing the genomes of three Streptomyces strains, three putative GADs were discovered, including StGAD from Streptomyces toxytricini NRRL 15443, SsGAD from Streptomyces sp. MJ654-NF4 and ScGAD from Streptomyces chromofuscus ATCC 49982. The corresponding genes were cloned from these strains and heterologously expressed in E. coli BL21(DE3). The purified GAD proteins showed a similar molecular mass to GadB from E. coli BL21(DE3). The optimal reaction temperature is 37 °C for all three enzymes, while the optimum pH values for StGAD, SsGAD and ScGAD are 5.2, 3.8 and 4.2, respectively. The kinetic parameters including V_max, K_m, k_cat and k_cat/K_m values were investigated and calculated through in vitro reactions. SsGAD and ScGAD showed high biocatalytic efficiency with k_cat/K_m values of 0.62 and 1.21 mM^− 1·s^− 1, respectively. In addition, engineered E. coli strains harboring StGAD, SsGAD and ScGAD were used as whole-cell biocatalysts for production of GABA from L-Glu. E. coli/SsGAD showed the highest capability of GABA production. The cells were repeatedly used for 10 times, with an accumulated yield of 2.771 kg/L and an average molar conversion rate of 67% within 20 h.

Conclusions

Three new GADs have been functionally characterized from Streptomyces, among which two showed higher catalytic efficiency than previously reported GADs. Engineered E. coli harboring SsGAD provides a promising cost-effective bioconversion system for industrial production of GABA.

Electronic supplementary material

The online version of this article (10.1186/s13036-019-0154-7) contains supplementary material, which is available to authorized users.

Production of γ-aminobutyric acid in Escherichia coli by engineering MSG pathway

The compound γ-aminobutyric acid (GABA) has many important physiological functions. The effect of glutamate decarboxylases and the glutamate/GABA antiporter on GABA production was investigated in Escherichia coli. Three genes, gadA, gadB, and gadC were cloned and ligated alone or in combination into the plasmid pET32a. The constructed plasmids were transformed into Escherichia coli BL21(DE3). Three strains, E. coli BL21(DE3)/pET32a-gadA, E. coli BL21(DE3)/pET32a-gadAB and E. coli BL21(DE3)/pET32a-gadABC were selected and identified. The respective titers of GABA from the three strains grown in shake flasks were 1.25, 2.31, and 3.98 g/L. The optimal titer of the substrate and the optimal pH for GABA production were 40 g/L and 4.2, respectively. The highest titer of GABA was 23.6 g/L at 36 h in batch fermentation and was 31.3 g/L at 57 h in fed-batch fermentation. This study lays a foundation for the development and use of GABA.

Lactobacillus brevis CGMCC 1306 glutamate decarboxylase: Crystal structure and functional analysis

Glutamate decarboxylase (GAD), which is a unique pyridoxal 5-phosphate (PLP)-dependent enzyme, can catalyze α-decarboxylation of l-glutamate (L-Glu) to γ-aminobutyrate (GABA). The crystal structure of GAD in complex with PLP from Lactobacillus brevis CGMCC 1306 was successfully solved by molecular-replacement, and refined at 2.2 Å resolution to an R_work factor of 18.76% (R_free = 23.08%). The coenzyme pyridoxal 5-phosphate (PLP) forms a Schiff base with the active-site residue Lys279 by continuous electron density map, which is critical for catalysis by PLP-dependent decarboxylase. Gel filtration showed that the active (pH 4.8) and inactive (pH 7.0) forms of GAD are all dimer. The residues (Ser126, Ser127, Cys168, Ile211, Ser276, His278 and Ser321) play important roles in anchoring PLP cofactor inside the active site and supporting its catalytic reactivity. The mutant T215A around the putative substrate pocket displayed an 1.6-fold improvement in catalytic efficiency (k_cat/K_m) compared to the wild-type enzyme (1.227 mM^-1 S^-1 versus 0.777 mM^-1 S^-1), which was the highest activity among all variants tested. The flexible loop (Tyr308-Glu312), which is positioned near the substrate-binding site, is involved in the catalytic reaction, and the conserved residue Tyr308 plays a vital role in decarboxylation of L-Glu.

Expression and characterization of glutamate decarboxylase from Lactobacillus brevis HYE1 isolated from kimchi

A putative gene (gadlbhye1) encoding glutamate decarboxylase (GAD) was cloned from Lactobacillus brevis HYE1 isolated from kimchi, a traditional Korean fermented vegetable. The amino acid sequences of GADLbHYE1 showed 48% homology with the GadA family and 99% identity with the GadB family from L. brevis. The cloned GADLbHYE1 was functionally expressed in Escherichia coli using inducible expression vectors. The expressed recombinant GADLbHYE1 was successfully purified by Ni-NTA affinity chromatography, and had a molecular mass of 54 kDa with optimal hydrolysis activity at 55 °C and pH 4.0. Its thermal stability was determined to be higher than that of other GADs from L. brevis, based on its melting temperature (75.18 °C). Kinetic parameters including K_m and V_max values for GADLbHYE1 were 4.99 mmol/L and 0.224 mmol/L/min, respectively. In addition, the production of gamma-aminobutyric acid in E. coli BL21 harboring gadlbhye1/pET28a was increased by adding pyridoxine as a cheaper coenzyme.

High γ-aminobutyric acid production from lactic acid bacteria: Emphasis on Lactobacillus brevis as a functional dairy starter

γ-Aminobutyric acid (GABA) and GABA-rich foods have shown anti-hypertensive and anti-depressant activities as the major functions in humans and animals. Hence, high GABA-producing lactic acid bacteria (LAB) could be used as functional starters for manufacturing novel fermented dairy foods. Glutamic acid decarboxylases (GADs) from LAB are highly conserved at the species level based on the phylogenetic tree of GADs from LAB. Moreover, two functionally distinct GADs and one intact gad operon were observed in all the completely sequenced Lactobacillus brevis strains suggesting its common capability to synthesize GABA. Difficulties and strategies for the manufacture of GABA-rich fermented dairy foods have been discussed and proposed, respectively. In addition, a genetic survey on the sequenced LAB strains demonstrated the absence of cell envelope proteinases in the majority of LAB including Lb. brevis, which diminishes their cell viabilities in milk environments due to their non-proteolytic nature. Thus, several strategies have been proposed to overcome the non-proteolytic nature of Lb. brevis in order to produce GABA-rich dairy foods.

Potential of a lactic acid bacterial starter culture with gamma-aminobutyric acid (GABA) activity for production of fermented sausage

The ability of lactic acid bacterial starter cultures to produce gamma-aminobutyric acid (GABA) during sausage fermentation was studied. Among 305 strains of lactic acid bacteria isolated from kimchi samples, 11 strains were selected as starter candidates based on the following criteria: growth speed, pH lowering ability, and biogenic amine productivity including GABA-producing activity. During in vitro tests, the Y8 (Lactobacillus brevis), O52, and KA20 strains produced 39.00 ± 1.36, 49.73 ± 3.80, and 64.59 ± 0.61 mg/kg of GABA, respectively. Interestingly, although isolate Y8 showed low productivity in vitro, the GABA content it produced during in situ tests (61.30 ± 2.61 mg/kg) was similar to that produced by isolate PM3 (L. brevis) used as positive control (69.64 ± 2.20 mg/kg). Therefore, isolate Y8 was selected as the best functional starter culture for the production of fermented sausage because it exhibited rapid growth, safety, and abundant GABA productivity.

GABA-producing Bifidobacterium dentium modulates visceral sensitivity in the intestine

BACKGROUND

Recurrent abdominal pain is a common and costly health-care problem attributed, in part, to visceral hypersensitivity. Increasing evidence suggests that gut bacteria contribute to abdominal pain perception by modulating the microbiome-gut-brain axis. However, specific microbial signals remain poorly defined. γ-aminobutyric acid (GABA) is a principal inhibitory neurotransmitter and a key regulator of abdominal and central pain perception from peripheral afferent neurons. Although gut bacteria are reported to produce GABA, it is not known whether the microbial-derived neurotransmitter modulates abdominal pain.

METHODS

To investigate the potential analgesic effects of microbial GABA, we performed daily oral administration of a specific Bifidobacterium strain (B. dentiumATCC 27678) in a rat fecal retention model of visceral hypersensitivity, and subsequently evaluated pain responses.

KEY RESULTS

We demonstrate that commensal Bifidobacterium dentium produces GABA via enzymatic decarboxylation of glutamate by GadB. Daily oral administration of this specific Bifidobacterium (but not a gadB deficient) strain modulated sensory neuron activity in a rat fecal retention model of visceral hypersensitivity.

CONCLUSIONS & INFERENCES

The functional significance of microbial-derived GABA was demonstrated by gadB-dependent desensitization of colonic afferents in a murine model of visceral hypersensitivity. Visceral pain modulation represents another potential health benefit attributed to bifidobacteria and other GABA-producing species of the intestinal microbiome. Targeting GABAergic signals along this microbiome-gut-brain axis represents a new approach for the treatment of abdominal pain.

Increasing thermal stability and catalytic activity of glutamate decarboxylase in E. coli: An in silico study

Glutamate decarboxylase (GAD) is an enzyme that converts l-glutamate to gamma amino butyric acid (GABA) that is a widely used drug to treat mental disorders like Alzheimer's disease. In this study for the first time point mutation was performed virtually in the active site of the E. coli GAD in order to increase thermal stability and catalytic activity of the enzyme. Energy minimization and addition of water box were performed using GROMACS 5.4.6 package. PoPMuSiC 2.1 web server was used to predict potential spots for point mutation and Modeller software was used to perform point mutation on three dimensional model. Molegro virtual docker software was used for cavity detection and stimulated docking study. Results indicate that performing mutation separately at positions 164, 302, 304, 393, 396, 398 and 410 increase binding affinity to substrate. The enzyme is predicted to be more thermo- stable in all 7 mutants based on ΔΔG value.

Characterization of glutamate decarboxylase from Synechocystis sp. PCC6803 and its role in nitrogen metabolism

Glutamate decarboxylase (GAD) (EC 4.1.1.15), an enzyme responsible for the synthesis of γ-aminobutyric acid (GABA), from Synechocystis sp. PCC6803 was cloned and overexpressed in Escherichia coli BL21(DE3). The purified enzyme was expressed as a monomeric protein with a molecular mass of 53 and 55 kDa as determined by SDS-PAGE and gel filtration chromatography, respectively. The enzyme activity was pyridoxal-5'-phosphate dependent with an optimal activity at pH 6.0 and 30 °C. The catalytic properties of this enzyme were, Km = 19.6 mM; kcat = 100.7 s(-1); and kcat/Km = 5.1 mM(-1) s(-1). The transcription levels of genes involved in nitrogen metabolism were up-regulated in the Δgad strain. The mutant showed approximately 4- and 8-fold increases in the transcript levels of kgd and gabdh encoding a novel α-ketoglutarate decarboxylase and γ-aminobutanal dehydrogenase, respectively. Overall results suggested that in Synechocystis lacking a functional GAD, the γ-aminobutanal dehydrogenase might serve as an alternative catalytic pathway for GABA synthesis.

Enhanced production of recombinant Escherichia coli glutamate decarboxylase through optimization of induction strategy and addition of pyridoxine

This report describes the optimization of recombinant Escherichia coli glutamate decarboxylase (GAD) production from engineered E. coli BL21(DE3) in a 3-L fermentor. Investigation of different induction strategies revealed that induction was optimal when the temperature was maintained at 30°C, the inducer (lactose) was fed at a rate of 0.2 g L(-1)h(-1), and protein expression was induced when the cell density (OD600) reached 50. Under these conditions, the GAD activity of 1273.8 U mL(-1) was achieved. Because GAD is a pyridoxal 5'-phosphate (PLP)-dependent enzyme, the effect of supplementing the medium with pyridoxine hydrochloride (PN), a cheap and stable PLP precursor, on GAD production was also investigated. When the culture medium was supplemented with PN to a concentration of 2mM at the initiation of protein expression, and then again 10h later, the GAD activity reached 3193.4 U mL(-1), which represented the highest GAD production ever reported.

Efficient production of gamma-aminobutyric acid using Escherichia coli by co-localization of glutamate synthase, glutamate decarboxylase, and GABA transporter

Gamma-aminobutyric acid (GABA) is an important bio-product, which is used in pharmaceutical formulations, nutritional supplements, and biopolymer monomer. The traditional GABA process involves the decarboxylation of glutamate. However, the direct production of GABA from glucose is a more efficient process. To construct the recombinant strains of Escherichia coli, a novel synthetic scaffold was introduced. By carrying out the co-localization of glutamate synthase, glutamate decarboxylase, and GABA transporter, we redirected the TCA cycle flux to GABA pathway. The genetically engineered E. coli strain produced 1.08 g/L of GABA from 10 g/L of initial glucose. Thus, with the introduction of a synthetic scaffold, we increased GABA production by 2.2-fold. The final GABA concentration was increased by 21.8% by inactivating competing pathways.

Effect of γ-Aminobutyric Acid-producing Lactobacillus Strain on Laying Performance, Egg Quality and Serum Enzyme Activity in Hy-Line Brown Hens under Heat Stress

Heat-stress remains a costly issue for animal production, especially for poultry as they lack sweat glands, and alleviating heat-stress is necessary for ensuring animal production in hot environment. A high γ-aminobutyric acid (GABA)-producer Lactobacillus strain was used to investigate the effect of dietary GABA-producer on laying performance and egg quality in heat-stressed Hy-line brown hens. Hy-Line brown hens (n = 1,164) at 280 days of age were randomly divided into 4 groups based on the amount of freeze-dried GABA-producer added to the basal diet as follows: i) 0 mg/kg, ii) 25 mg/kg, iii) 50 mg/kg, and iv) 100 mg/kg. All hens were subjected to heat-stress treatment through maintaining the temperature and the relative humidity at 28.83±3.85°C and 37% to 53.9%, respectively. During the experiment, laying rate, egg weight and feed intake of hens were recorded daily. At the 30th and 60th day after the start of the experiment, biochemical parameters, enzyme activity and immune activity in serum were measured. Egg production, average egg weight, average daily feed intake, feed conversion ratio and percentage of speckled egg, soft shell egg and misshaped egg were significantly improved (p<0.05) by the increasing supplementation of the dietary GABA-producer. Shape index, eggshell thickness, strength and weight were increased linearly with increasing GABA-producer supplementation. The level of calcium, phosphorus, glucose, total protein and albumin in serum of the hens fed GABA-producing strain supplemented diet was significantly higher (p<0.05) than that of the hens fed the basal diet, whereas cholesterol level was decreased. Compared with the basal diet, GABA-producer strain supplementation increased serum level of glutathione peroxidase (p = 0.009) and superoxide dismutase. In conclusion, GABA-producer played an important role in alleviating heat-stress, the isolated GABA-producer strain might be a potential natural and safe probiotic to use to improve laying performance and egg quality in heat-stressed hens.

Mechanisms of acid tolerance in bacteria and prospects in biotechnology and bioremediation

Acidogenic and aciduric bacteria have developed several survival systems in various acidic environments to prevent cell damage due to acid stress such as that on the human gastric surface and in the fermentation medium used for industrial production of acidic products. Common mechanisms for acid resistance in bacteria are proton pumping by F1-F0-ATPase, the glutamate decarboxylase system, formation of a protective cloud of ammonia, high cytoplasmic urease activity, repair or protection of macromolecules, and biofilm formation. The field of synthetic biology has rapidly advanced and generated an ever-increasing assortment of genetic devices and biological modules for applications in biofuel and novel biomaterial productions. Better understanding of aspects such as overproduction of general shock proteins, molecular mechanisms, and responses to cell density adopted by microorganisms for survival in low pH conditions will prove useful in synthetic biology for potential industrial and environmental applications.

Production of gamma-aminobutyric acid from glucose by introduction of synthetic scaffolds between isocitrate dehydrogenase, glutamate synthase and glutamate decarboxylase in recombinant Escherichia coli

Escherichia coli were engineered for the direct production of gamma-aminobutyric acid from glucose by introduction of synthetic protein scaffold. In this study, three enzymes consisting GABA pathway (isocitrate dehydrogenase, glutamate synthase and glutamate decarboxylase) were connected via synthetic protein scaffold. By introduction of scaffold, 0.92g/L of GABA was produced from 10g/L of glucose while no GABA was produced in wild type E. coli. The optimum pH and temperature for GABA production were 4.5 and 30°C, respectively. When competing metabolic network was inactivated by knockout mutation, maximum GABA concentration of 1.3g/L was obtained from 10g/L glucose. The recombinant E. coli strain which produces GABA directly from glucose was successfully constructed by introduction of protein scaffold.

Production of γ-aminobutyric acid by microorganisms from different food sources

BACKGROUND

γ-Aminobutyric acid (GABA) is a potentially bioactive component of foods and pharmaceuticals. The aim of this study was screen lactic acid bacteria belonging to the Czech Collection of Microorganisms, and microorganisms (yeast and bacteria) from 10 different food sources for GABA production by fermentation in broth or plant and animal products.

RESULTS

Under an aerobic atmosphere, very low selectivity of GABA production (from 0.8% to 1.3%) was obtained using yeast and filamentous fungi, while higher selectivity (from 6.5% to 21.0%) was obtained with bacteria. The use of anaerobic conditions, combined with the addition of coenzyme (pyridoxal-5-phosphate) and salts (CaCl2 , NaCl), led to the detection of a low concentration of GABA precursor. Simultaneously, using an optimal temperature of 33 °C, a pH of 6.5 and bacteria from banana (Pseudomonadaceae and Enterobacteriaceae families), surprisingly, a high selectivity of GABA was obtained. A positive impact of fenugreek sprouts on the proteolytic process and GABA production from plant material as a source of GABA precursor was identified.

CONCLUSIONS

Lactic acid bacteria for the production of new plant and animal GABA-rich products from different natural sources containing GABA precursor can be used.

Overexpression and optimization of glutamate decarboxylase in Lactobacillus plantarum Taj-Apis362 for high gamma-aminobutyric acid production

Gamma-aminobutyric acid (GABA) is an important bioactive compound biosynthesized by microorganisms through decarboxylation of glutamate by glutamate decarboxylase (GAD). In this study, a full-length GAD gene was obtained by cloning the template deoxyribonucleic acid to pTZ57R/T vector. The open reading frame of the GAD gene showed the cloned gene was composed of 1410 nucleotides and encoded a 469 amino acids protein. To improve the GABA-production, the GAD gene was cloned into pMG36e-LbGAD, and then expressed in Lactobacillus plantarum Taj-Apis362 cells. The overexpression was confirmed by SDS-PAGE and GAD activity, showing a 53 KDa protein with the enzyme activity increased by sevenfold compared with the original GAD activity. The optimal fermentation conditions for GABA production established using response surface methodology were at glutamic acid concentration of 497.973 mM, temperature 36°C, pH 5.31 and time 60 h. Under the conditions, maximum GABA concentration obtained (11.09 mM) was comparable with the predicted value by the model at 11.23 mM. To our knowledge, this is the first report of successful cloning (clone-back) and overexpression of the LbGAD gene from L. plantarum to L. plantarum cells. The recombinant Lactobacillus could be used as a starter culture for direct incorporation into a food system during fermentation for production of GABA-rich products.

Gas release-based prescreening combined with reversed-phase HPLC quantitation for efficient selection of high-γ-aminobutyric acid (GABA)-producing lactic acid bacteria

High γ-aminobutyric acid (GABA)-producing lactobacilli are promising for the manufacture of GABA-rich foods and to synthesize GRAS (generally recognized as safe)-grade GABA. However, common chromatography-based screening is time-consuming and inefficient. In the present study, Korean kimchi was used as a model of lactic acid-based fermented foods, and a gas release-based prescreening of potential GABA producers was developed. The ability to produce GABA by potential GABA producers in de Man, Rogosa, and Sharpe medium supplemented with or without monosodium glutamate was further determined by HPLC. Based on the results, 9 isolates were regarded as high GABA producers, and were further genetically identified as Lactobacillus brevis based on the sequences of 16S rRNA gene. Gas release-based prescreening combined with reversed-phase HPLC confirmation was an efficient and cost-effective method to identify high-GABA-producing LAB, which could be good candidates for probiotics. The GABA that is naturally produced by these high-GABA-producing LAB could be used as a food additive.

Characterization and immobilization on nickel-chelated Sepharose of a glutamate decarboxylase A from Lactobacillus brevis BH2 and its application for production of GABA

A gene encoding glutamate decarboxylase A (GadA) from Lactobacillus brevis BH2 was expressed in a His-tagged form in Escherichia coli cells, and recombinant protein exists as a homodimer consisting of identical subunits of 53 kDa. GadA was absolutely dependent on the ammonium sulfate concentration for catalytic activity and secondary structure formation. GadA was immobilized on the metal affinity resin with an immobilization yield of 95.8%. The pH optima of the immobilized enzyme were identical with those of the free enzyme. However, the optimum temperature for immobilized enzyme was 5 °C higher than that for the free enzyme. The immobilized GadA retained its relative activity of 41% after 30 reuses of reaction within 30 days and exhibited a half-life of 19 cycles within 19 days. A packed-bed bioreactor with immobilized GadA showed a maximum yield of 97.8% GABA from 50 mM l-glutamate in a flow-through system under conditions of pH 4.0 and 55 °C.

Characterization of a glutamate decarboxylase (GAD) gene from Lactobacillus zymae

Lactic acid bacteria (LAB) were isolated from Kimchi, a Korean traditional fermented vegetable food. LAB accumulating GABA (γ-aminobutyric acid) in the culture media were screened by TLC analysis. One isolate, GU240, produced the highest amount of GABA among the 3,000 isolates and identified as a Lactobacillus zymae strain. Glutamate decarboxylase (GAD) gene was cloned and over-expressed in E. coli BL21(DE3) using pET26b(+). The recombinant GAD was purified by using a Ni-NTA column. Its size was 53 kDa by SDS-PAGE. Maximum GAD activity was at pH 4.5 and 41 °C and the activity was dependent on pyridoxal 5'-phosphate. Km and Vmax of LzGAD were 1.7 mM and 0.01 mM/min, respectively, when glutamate was used as a substrate.

gadA gene locus in Lactobacillus brevis NCL912 and its expression during fed-batch fermentation

Normally, Lactobacillus brevis has two glutamate decarboxylase (GAD) genes; gadA and gadB. Using PCR, we cloned the gadA gene from L. brevis strain NCL912, a high yield strain for the production of gamma-aminobutyric acid (GABA). However, despite using 61 different primer pairs, including degenerate primers from conserved regions, we were unable to use PCR to clone gadB from the NCL912 strain. Furthermore, we could not clone it by genomic walking over 3000 bp downstream of the aldo-keto reductase gene, a single-copy gene that is located 1003 bp upstream of gadB in L. brevis ATCC367. Altogether, the data suggest that L. brevis NCL912 does not contain a gadB gene. By genomic walking, we cloned regions upstream and downstream of the gadA gene to obtain a 4615 bp DNA fragment that included the complete gadA locus. The locus contained the GAD gene (gadA) and the glutamate:GABA antiporter gene (gadC), which appear to be transcribed in an operon (gadCA), and a transcriptional regulator (gadR) of gadCA. During whole fed-batch fermentation, the expression of gadR, gadC and gadA was synchronized and correlated well with GABA production. The gadA locus we cloned from NCL912 has reduced homology compared with gadA loci of other L. brevis strains, and these differences might explain the ability of NCL912 to produce higher levels of GABA in culture.

Efficient gamma-aminobutyric acid bioconversion by employing synthetic complex between glutamate decarboxylase and glutamate/GABA antiporter in engineered Escherichia coli

Gamma-aminobutyric acid (GABA) is a precursor of one of the most promising heat-resistant biopolymers, Nylon-4, and can be produced by the decarboxylation of monosodium glutamate (MSG). In this study, a synthetic protein complex was applied to improve the GABA conversion in engineered Escherichia coli. Complexes were constructed by assembling a single protein–protein interaction domain SH3 to the glutamate decarboxylase (GadA and GadB) and attaching a cognate peptide ligand to the glutamate/GABA antiporter (GadC) at the N-terminus, C-terminus, and the 233rd amino acid residue. When GadA and GadC were co-overexpressed via the C-terminus complex, a GABA concentration of 5.65 g/l was obtained from 10 g/l MSG, which corresponds to a GABA yield of 93 %. A significant increase of the GABA productivity was also observed where the GABA productivity increased 2.5-fold in the early culture period due to the introduction of the synthetic protein complex. The GABA pathway efficiency and GABA productivity were enhanced by the introduction of the complex between Gad and glutamate/GABA antiporter.

Improvement in antioxidant activity, angiotensin-converting enzyme inhibitory activity and in vitro cellular properties of fermented pepino milk by Lactobacillus strains containing the glutamate decarboxylase gene

BACKGROUND

The purpose of this study was to evaluate the functional potential of fermented pepino extract (PE) milk by Lactobacillus strains containing the glutamate decarboxylase (GAD) gene. Three Lactobacillus strains were selected, including L. brevis BCRC 12310, L. casei BCRC 14082 and L. salivarius subsp. salivarius BCRC 14759. The contents of free amino acids, total phenolics content, total carotenoids and the associated functional and antioxidant abilities were analyzed, including angiotensin-converting enzyme (ACE) inhibition activity, 1,1-diphenyl-2-picylhydrazyl (DPPH) radical-scavenging ability and oxygen radical absorbance capacity (ORAC). Cell proliferation of fermented PE milk was also evaluated by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay.

RESULTS

Compared to the unfermented PE, fermented PE milk from Lactobacillus strains with the GAD gene showed higher levels of total phenolics, γ-aminobutyric acid, ACE inhibitory activity, DPPH, and ORAC. The viability of human promyelocytic leukemia cells (HL-60) determined by the MTT method decreased significantly when the cells were incubated with the PE and the fermented PE milk extracts.

CONCLUSION

The consumption of fermented PE milk from Lactobacillus strains with the GAD gene is expected to benefit health. Further application as a health food is worthy of investigation. © 2012 Society of Chemical Industry.

Synthesis of nylon 4 from gamma-aminobutyrate (GABA) produced by recombinant Escherichia coli

In this study, we developed recombinant Escherichia coli strains expressing Lactococcus lactis subsp. lactis Il1403 glutamate decarboxylase (GadB) for the production of GABA from glutamate monosodium salt (MSG). Syntheses of GABA from MSG were examined by employing recombinant E. coli XL1-Blue as a whole cell biocatalyst in buffer solution. By increasing the concentration of E. coli XL1-Blue expressing GadB from the OD₆₀₀ of 2-10, the concentration and conversion yield of GABA produced from 10 g/L of MSG could be increased from 4.3 to 4.8 g/L and from 70 to 78 %, respectively. Furthermore, E. coli XL1-Blue expressing GadB highly concentrated to the OD₆₀₀ of 100 produced 76.2 g/L of GABA from 200 g/L of MSG with 62.4 % of GABA yield. Finally, nylon 4 could be synthesized by the bulk polymerization using 2-pyrrolidone that was prepared from microbially synthesized GABA by the reaction with Al₂O₃ as catalyst in toluene with the yield of 96 %.

C-terminal truncation of glutamate decarboxylase from Lactobacillus brevis CGMCC 1306 extends its activity toward near-neutral pH

Glutamate decarboxylase (GAD) from Lactobacillus brevis is a very promising candidate for biosynthesis of GABA and various other bulk chemicals that can be derived from GABA. However, no structure of GAD of this origin has been reported to date, which limits enzyme engineering strategy to improve its properties for better use in production of GABA. Bacterial GAD exhibits an acidic pH optimum and there is often a sharp pH dependence. In the present work, site-directed mutagenesis was performed to delete the C-terminal residues of GAD to generate a mutant, designated as GADΔC, which exhibited extended activity toward near-neutral pH compared to the wild type. Comparison of the UV-visible, fluorescence and Circular Dichroism spectra of the mutant with those of the wild type revealed that the microenvironment of the active site had been changed. Based on the homology model, we speculated that the substrate entrance was probably enlarged in GADΔC. These results provide evidence for the important role of C-terminal region in the pH-dependent regulation of enzyme activity, and the resulting mutant would be useful in a bioreactor for continuous production of GABA.

Synthesis of γ-aminobutyric acid by expressing Lactobacillus brevis-derived glutamate decarboxylase in the Corynebacterium glutamicum strain ATCC 13032

PURPOSE OF WORK

Purpose of this work is to synthesize γ-aminobutyric acid by glutamate-producing species expressing Lactobacillus brevis-derived glutamate decarboxylase genes, i.e. recombinant Corynebacterium glutamicum strains, which directly convert endogenous L-glutamate precursor into γ-aminobutyric acid (GABA) through single-step fermentation. To express exogenous glutamate decarboxylase (GAD) in an L-glutamate-producing strain, Lactobacillus brevis Lb85, which can produce GABA, was used. Two Lb85 GAD genes, gadB1 and gadB2, and the ancillary genes, gadC-gadB2 and gadR-gadC-gadB2, were cloned separately into pDXW-8 and transformed into C. glutamicum. All four recombinant strains produced GABA whereas the wild-type strain did not. GABA produced by the recombinant strains continually increased after 36 h of fermentation. Although the mRNA levels of LbgadB2 and LbgadC were similar among the corresponding recombinants, GABA production of pDXW-8/gadRCB2 at 72 h (2.15 g/l) was higher than that of pDXW-8/gadCB2 (1.25 g/l) and pDXW-8/gadB2 (0.88 g/l). Thus, by introducing Lbgad genes, C. glutamicum was genetically engineered to synthesize GABA using endogenous L-glutamate.