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

Post-acidification of fermented milk and its molecular regulatory mechanism

The fermented milk products with lactic acid bacteria (LAB) are widely accepted by consumers. During the chilled-chain transportation and storage, LAB in the product keep producing lactic acid, and this will lead to post-acidification, which can affect the flavor, consumer acceptance and even shelf-life of the product. LAB is the determining factor affecting post-acidification. The acid production pathway in LAB and methods inhibiting post-acidification received widespread attention. This review will focus on the post-acidification from the perspective of fermentation starters, including acid production pathway in LAB, main factors and key enzymes affecting post-acidification. Lactobacillus delbrueckii subsp. bulgaricus is a key bacterial species responsible for post acidification in the fermented milk products. The different species and strains presented various differences in process like acid production, acid resistance and post-acidification. Furthermore, multiple factors, such as milk composition, fermentation temperature, and homogenization, also can influence post-acidification. Lactose transport and utilization pathways, as well as its subsequent products metabolic pathway directly influence the post-acidification. F0F1-ATPase, β-galactosidase, and lactate dehydrogenase are recognized as important enzymes related to post-acidification. The degree of post-acidification is mainly related to the acid production and acid resistance abilities of the fermentation starters, so the key enzymes related to post-acidification are mostly taking part in these two capacities. Recently, some new post-acidification related biomarker genes were found, providing a reference adjusting post-acidification without affecting fermentation rate and bacteria viability. To clarify the post-acidification mechanism at the molecular level will help control post- acidification.

Preparation of magnetic polyacrylamide hydrogel with chitosan for immobilization of glutamate decarboxylase to produce γ-aminobutyric acid

Gamma-aminobutyric acid (GABA) is an vital neurotransmitter, and the reaction to obtain GABA through biocatalysis requires coenzymes, which are therefore limited in the production of GABA. In this study, polyacrylamide hydrogels doped with chitosan and waste toner were synthesized for glutamate decarboxylase (GAD) and coenzyme co-immobilization to realize the production of GABA and the recovery of coenzymes. Enzymatic properties of immobilized GAD were discussed. The immobilized enzymes have significantly improved pH and temperature tolerance compared to free enzymes. In terms of reusability, after 10 repeated reuses of the immobilized GAD, the residual enzyme activity of immobilized GAD still retains 100% of the initial enzyme activity, and the immobilized coenzyme can also be kept at about 32%, with better stability and reusability. And under the control of no exogenous pH, immobilized GAD showed good performance in producing GABA. Therefore, in many ways, the new composite hydrogel provides another way for the utilization of waste toner and promises the possibility of industrial production of GABA.

Integrating Enzyme Evolution and Metabolic Engineering to Improve the Productivity of Γ-Aminobutyric Acid by Whole-Cell Biosynthesis in Escherichia Coli

γ-Aminobutyric acid (GABA) is used widely in various fields, such as agriculture, food, pharmaceuticals, and biobased chemicals. Based on glutamate decarboxylase (GadBM4) derived from our previous work, three mutants, GadM4-2, GadM4-8, and GadM4-31, were obtained by integrating enzyme evolution and high-throughput screening methods. The GABA productivity obtained through whole-cell bioconversion using recombinant Escherichia coli cells harboring mutant GadBM4-2 was enhanced by 20.27% compared to that of the original GadBM4. Further introduction of the central regulator GadE of the acid resistance system and the enzymes from the deoxyxylulose-5-phosphate-independent pyridoxal 5'-phosphate biosynthesis pathway resulted in a 24.92% improvement in GABA productivity, reaching 76.70 g/L/h without any cofactor addition with a greater than 99% conversion ratio. Finally, when one-step bioconversion was applied for the whole-cell catalysis in a 5 L bioreactor, the titer of GABA reached 307.5 ± 5.94 g/L with a productivity of 61.49 g/L/h by using crude l-glutamic acid (l-Glu) as the substrate. Thus, the biocatalyst constructed above combined with the whole-cell bioconversion method represents an effective approach for industrial GABA production.

Chromosomal editing of Corynebacterium glutamicum ATCC 13032 to produce gamma-aminobutyric acid

Corynebacterium glutamicum has been used as a sustainable microbial producer for various bioproducts using cheap biomass resources. In this study, a high GABA-producing C. glutamicum strain was constructed by chromosomal editing. Lactobacillus brevis-derived gadB2 was introduced into the chromosome of C. glutamicum ATCC 13032 to produce gamma-aminobutyric acid and simultaneously blocked the biosynthesis of lactate and acetate. GABA transport and degradation in C. glutamicum were also blocked to improve GABA production. As precursor of GABA, l-glutamic acid synthesis in C. glutamicum was enhanced by introducing E. coli gdhA encoding glutamic dehydrogenase, and the copy numbers of gdhA and gadB2 were also optimized for higher GABA production. The final C. glutamicum strain CGY705 could produce 33.17 g/L GABA from glucose in a 2.4-L bioreactor after 78 h fed-batch fermentation. Since all deletion and expression of genes were performed using chromosomal editing, fermentation of the GABA-producing strains constructed in this study does not need supplementation of any antibiotics and inducers. The strategy used in this study has potential value in the development of GABA-producing bacteria.

Recent advances in microbial production of diamines, aminocarboxylic acids, and diacids as potential platform chemicals and bio-based polyamides monomers

Recently, bio-based manufacturing processes of value-added platform chemicals and polymers in biorefineries using renewable resources have extensively been developed for sustainable and carbon dioxide (CO₂) neutral-based industry. Among them, bio-based diamines, aminocarboxylic acids, and diacids have been used as monomers for the synthesis of polyamides having different carbon numbers and ubiquitous and versatile industrial polymers and also as precursors for further chemical and biological processes to afford valuable chemicals. Until now, these platform bio-chemicals have successfully been produced by biorefinery processes employing enzymes and/or microbial host strains as main catalysts. In this review, we discuss recent advances in bio-based production of diamines, aminocarboxylic acids, and diacids, which has been developed and improved by systems metabolic engineering strategies of microbial consortia and optimization of microbial conversion processes including whole cell bioconversion and direct fermentative production.

Expanding the pH range of glutamate decarboxylase from L. pltarum LC84 by site-directed mutagenesis

Introduction: Glutamate decarboxylase is a class Ⅱ amino acid decarboxylase dependent onpyridoxal-5'-phosphate (PLP), which catalyzes the decarboxylation of substrateL-glutamate (L-Glu) to synthesize γ-aminobutyric acid (GABA). The low activity ofglutamic acid decarboxylase (GAD) and its ability to catalyze only under acidicconditions limit its application in biosynthesis of GABA. Methods: Taking glutamic acid decarboxylase from Lactobacillus plantarum, which produces GABA, as the research object, the mutation site was determined by amino acid sequence analysis of GAD, the mutation was introduced by primers, and the mutant was constructed by whole plasmid PCR and expressed in Escherichia coli. Then, the enzymatic properties of the mutant were analyzed. Finally, the three-dimensional structure of the mutant was simulated to support the experimental results. Results and Discussion: In this case, mutants E313S and Q347H of glutamate decarboxylase from L. pltarum LC84 (LpGAD) were constructed by targeted mutagenesis. Compared with the wild-type, their enzyme activity increased by 62.4% and 12.0% at the optimum pH 4.8, respectively. In the range of pH 4.0-7.0, their enzyme activity was higher than that of the wild-type, and enzyme activity of mutant E313S was 5 times that of the wild-type at pH 6.2. Visualization software PyMOL analyzed the 3D structure of the mutant predicted by homologous modeling, and the results showed that mutant E313S may broadened the reaction pH of LpGAD through the influence of surface charge, while mutant Q347H may broadened the reaction pH of LpGAD through the stacking effect of aromatic rings. In a word, mutants E313S and Q347H were improved the enzyme activity and were broadened the reaction pH of the enzyme, which made it possible for it to be applied in food industry and laid the foundation for the industrial production of GABA.

A comprehensive investigation into the production of gamma-aminobutyric acid by Limosilactobacillus fermentum NG16, a tuna gut isolate

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is widely known to have multiple physiological functions. The present study aimed to investigate the cultivation parameters for GABA production by a lactic acid bacteria (LAB) strain isolated from a tuna gut sample. Among 60 tuna gut LAB, only 7 Limosilactobacillus fermentum isolates, i.e. NG01, NG12, NG13, NG14, NG16, NG23, and NG27, were capable of GABA fermentation, with NG16 being the most potent GABA producer. The GABA production by isolate NG16 was therefore thoroughly characterised. The optimal batch culture conditions for GABA production were an initial cell density of 5×106 CFU mL−1, a monosodium glutamate concentration of 2%, an initial pH of 7, a fermentation temperature of 35 °C, and an incubation time of 96 h. Under this cultivation conditions, NG16 produced a maximum GABA yield of 25.52 ± 0.41 mM.

Optimization of Gamma Aminobutyric Acid Production Using High Pressure Processing (HPP) by Lactobacillus brevis PML1

In the present research, the production potential of gamma aminobutyric acid (GABA) using Lactobacillus brevis PML1 was investigated. In addition, the microorganism viability was examined in MAN, ROGOSA, and SHARPE (MRS) after undergoing high hydrostatic pressure at 100, 200, and 300 MPa for 5, 10, and 15 min. Response surface methodology (RSM) was applied to optimize the production conditions of GABA as well as the bacteria viability. Analysis of variance (ANOVA) indicated that both the independent variables (pressure and time) significantly influenced the dependent ones (GABA and bacteria viability) ( $P<0.05$ ). The optimum extraction conditions to maximize the production of GABA included the pressure of 300 MPa and the time of 15 min. The amount of the compound was quantified using thin-layer chromatography (TLC) and spectrophotometry. For the process optimization, a central composite design (CCD) was created using Design Expert with 5 replications at the center point, whereby the highest content of GABA was obtained to be 397.73 ppm which was confirmed by high performance liquid chromatography (HPLC). Moreover, scanning electron microscopy (SEM) was utilized to observe the morphological changes in the microorganism. The results revealed that not only did have Lactobacillus brevis PML1 the potential for the production of GABA under conventional conditions (control sample) but also the content of this bioactive compound could be elevated by optimizing the production parameters.

Reconstruction of the glutamate decarboxylase system in Lactococcus lactis for biosynthesis of food-grade γ-aminobutyric acid

Gamma-aminobutyric acid (GABA), an important bioactive compound, is synthesized through the decarboxylation of L-glutamate (L-Glu) by glutamate decarboxylase (GAD). The use of lactic acid bacteria (LAB) as catalysts opens interesting avenues for the biosynthesis of food-grade GABA. However, a key obstacle involved in the improvement of GABA production is how to resolve the discrepancy of optimal pH between the intracellular GAD activity and cell growth. In this work, a potential GAD candidate (LpGadB) from Lactobacillus plantarum was heterologously expressed in Escherichia coli. Recombinant LpGadB existed as a homodimer under the native conditions with a molecular mass of 109.6 kDa and exhibited maximal activity at 40°C and pH 5.0. The K_m value and catalytic efficiency (k_cat/K_m) of LpGadB for L-Glu was 21.33 mM and 1.19 mM^-1s^-1, respectively, with the specific activity of 26.67 μM/min/mg protein. Subsequently, four C-terminally truncated LpGadB mutants (GadB_ΔC10, GadB_ΔC11, GadB_ΔC12, GadB_ΔC13) were constructed based on homology modeling. Among them, the mutant GadB_ΔC11 with highest catalytic activity at near-neutral pH values was selected. In further, the GadB_ΔC11 and Glu/GABA antiporter (GadC) of Lactococcus lactis were co-overexpressed in the host L. lactis NZ3900. Finally, after 48 h of batch fermentation, the engineered strain L. lactis NZ3900/pNZ8149-gadB_ΔC11C yielded GABA concentration up to 33.52 g/L by applying a two-stage pH control strategy. Remarkably, this is the highest yield obtained to date for GABA from fermentation with L. lactis as a microbial cell factory.Key points• The GadB from L. plantarum was heterologously expressed in E. coli and biochemically characterized.• Deletion of the C-plug in GadB shifted its pH-dependent activity toward a higher pH.• Reconstructing the GAD system of L. lactis is an effective approach for improving its GABA production.

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.

Immobilization and enzymatic properties of glutamate decarboxylase from Enterococcus faecium by affinity adsorption on regenerated chitin

Glutamate decarboxylase (GAD, EC 4.1.1.15) is an important enzyme in gamma-aminobutyric acid biosynthesis and DL-glutamic acid resolution. In this study, the Enterococcus faecium-derived GAD was successfully immobilized by regenerated chitin (RC) via specific adsorption of cellulose-binding domain (CBD). The optimal binding buffer was 20 mmol/L phosphate buffer saline (pH 8.0), and the RC binding capacity was 1.77 ± 0.11 mg_cbd-gad/g_rc under this condition. The ratio of wet RC and crude enzyme solution used for immobilization was recommended to 3:50 (g/mL). To evaluate the effect of RC immobilization on GAD, properties of the immobilize GAD (RC-CBD-GAD) were investigated. Results indicated RC-CBD-GAD was relatively stable at pH 4.4-5.6 and temperature - 20-40 °C, and the optimal reaction pH value and temperature were pH 4.8 and 50 °C, respectively. When it was reacted with 5 mmol/L of follow chemical reagents respectively, the activity of RC-CBD-GAD was hardly affected by EDTA, KCl, and NaCl, and significantly inactivated by AgNO₃, MnSO₄, MgSO₄, CuSO₄, ZnSO₄, FeCl₂, FeCl₃, AlCl₃, CaCl_2, and Pb(CH₃COO)₂. The apparent K_m and V_max were 28.35 mmol/L and 147.06 μmol/(g_RC-CBD-GAD·min), respectively. The optimum time for a batch of catalytic reaction without exogenous pH control was 2 h. Under this reaction time, RC-CBD-GAD had a good reusability with a half-life of 23 cycles, indicating that it was very attractive for GABA industry. As a novel, efficient, and green CBD binding carrier, RC provides an alternative way to protein immobilization.

Production of Gamma-Aminobutyric Acid from Lactic Acid Bacteria: A Systematic Review

Gamma-aminobutyric acid (GABA) is widely distributed in nature and considered a potent bioactive compound with numerous and important physiological functions, such as anti-hypertensive and antidepressant activities. There is an ever-growing demand for GABA production in recent years. Lactic acid bacteria (LAB) are one of the most important GABA producers because of their food-grade nature and potential of producing GABA-rich functional foods directly. In this paper, the GABA-producing LAB species, the biosynthesis pathway of GABA by LAB, and the research progress of glutamate decarboxylase (GAD), the key enzyme of GABA biosynthesis, were reviewed. Furthermore, GABA production enhancement strategies are reviewed, from optimization of culture conditions and genetic engineering to physiology-oriented engineering approaches and co-culture methods. The advances in both the molecular mechanisms of GABA biosynthesis and the technologies of synthetic biology and genetic engineering will promote GABA production of LAB to meet people’s demand for GABA. The aim of the review is to provide an insight of microbial engineering for improved production of GABA by LAB in the future.

Enhancement of GAD Storage Stability with Immobilization on PDA-Coated Superparamagnetic Magnetite Nanoparticles

To improve the storage stability of glutamic acid decarboxylase (GAD), superparamagnetic magnetite (Fe3O4) nanoparticles were synthesized by co-precipitation method and coated with polydopamine (PDA) for GAD immobilization. Dynamic light scattering and transmission electron microscopy were used to determine size of the nanoparticles, which were approximately 10 nm, increasing to 15 nm after PDA-coating and to 20 nm upon GAD binding. Vibrational scanning measurements significantly represented the superparamagnetic behavior of the Fe3O4, and X-ray diffraction analysis confirmed that the crystalline structure before and after coating with PDA and the further immobilization of GAD remained the same. Thermogravimetric analysis and Fourier-transform infrared spectroscopy proved that the PDA-coating on Fe3O4 and further immobilization of GAD were successful. After immobilization, the enzyme can be used with a relative specific activity of 40.7% after five successive uses. The immobilized enzyme retained relative specific activity of about 50.5% after 15 days of storage at 4 °C, while free enzyme showed no relative specific activity after two days of storage. The GAD immobilization on PDA-coated magnetite nanoparticles was reported for the improvement of enzyme storage stability for the first time.

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.

Increasing thermal stability of glutamate decarboxylase from Escherichia. coli by site-directed saturation mutagenesis and its application in GABA production

Gamma-amino butyric acid (GABA) is an important bio-product used in pharmaceuticals, functional foods, and a precursor of the biodegradable plastic polyamide 4 (Nylon 4). Glutamate decarboxylase B (GadB) from Escherichia. coli is a highly active biocatalyst that can convert l-glutamate to GABA. However, its practical application is limited by the poor thermostability and only active under acidic conditions of GadB. In this study, we performed site-directed saturation mutagenesis of the N-terminal residues of GadB from Escherichia coli to improve its thermostability. A triple mutant (M6, Gln5Ile/Val6Asp/Thr7Gln) showed higher thermostability, with a 5.6 times (560%) increase in half-life value at 45 °C, 8.7 °C rise in melting temperature (Tm) and a 14.3 °C rise in the temperature at which 50% of the initial activity remained after 15 min incubation (T¹⁵₅₀), compared to wild-type enzyme. Protein 3D structure analysis showed that the induced new hydrogen bonds in the same polypeptide chain or between polypeptide chains in E. coli GadB homo-hexamer may be responsible for the improved thermostability. Increased thermostability contributed to increased GABA conversion ability. After 12 h conversion of 3 mol/L l-glutamate, GABA produced and mole conversion rate catalyzed by M6 whole cells was 297 g/L and 95%, respectively, while those by wild-type GAD was 273.5 g/L and 86.2%, respectively.

Enhanced productivity of gamma-amino butyric acid by cascade modifications of a whole-cell biocatalyst

We previously developed a gamma-amino butyric acid (GABA)-producing strain of Escherichia coli, leading to production of 614.15 g/L GABA at 45 °C from L-glutamic acid (L-Glu) with a productivity of 40.94 g/L/h by three successive whole-cell conversion cycles. However, the increase in pH caused by the accumulation of GABA resulted in inactivation of the biocatalyst and consequently led to relatively lower productivity. In this study, by overcoming the major problem associated with the increase in pH during the production process, a more efficient biocatalyst was obtained through cascade modifications of the previously reported E. coli strain. First, we introduced four amino acid mutations to the codon-optimized GadB protein from Lactococcus lactis to shift its decarboxylation activity toward a neutral pH, resulting in 306.65 g/L of GABA with 99.14 mol% conversion yield and 69.8% increase in GABA productivity. Second, we promoted transportation of L-Glu and GABA by removing the genomic region encoding the C-plug of GadC (a glutamate/GABA antiporter) to allow its transport path to remain open at a neutral pH, which improved the GABA productivity by 16.8% with 99.3 mol% conversion of 3 M L-Glu. Third, we enhanced the expression of soluble GadB by introducing the GroESL molecular chaperones, leading to 20.2% improvement in GABA productivity, with 307.40 g/L of GABA and a 61.48 g/L/h productivity obtained in one cycle. Finally, we inhibited the degradation of GABA by inactivation of gadA and gadB from the E. coli genome, which resulted in almost no GABA degradation after 40 h. After the cascade system modifications, the engineered recombinant E. coli strain achieved a 44.04 g/L/h productivity with a 99.6 mol% conversion of 3 M L-Glu in a 5-L bioreactor, about twofold increase in productivity compared to the starting strain. This increase represents the highest GABA productivity by whole-cell bioconversion using L-Glu as a substrate in one cycle observed to date, even better than the productivity obtained from the three successive conversion cycles.

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.

Biotechnological advances and perspectives of gamma-aminobutyric acid production

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that is widely distributed among various organisms. Since GABA has several well-known physiological functions, such as mediating neurotransmission and hypotensive activity, as well as having tranquilizer effects, it is commonly used as a bioactive compound in the food, pharmaceutical and feed industries. The major pathway of GABA biosynthesis is the irreversible decarboxylation of L-glutamate catalyzed by glutamate decarboxylase (GAD), which develops a safe, sustainable and environmentally friendly alternative in comparison with traditional chemical synthesis methods. To date, several microorganisms have been successfully engineered for high-level GABA biosynthesis by overexpressing exogenous GADs. However, the activity of almost all reported microbial GADs sharply decreases at physiological near-neutral pH, which in turn provokes negative effects on the application of these GADs in the recombinant strains for GABA production. Therefore, ongoing efforts in the molecular evolution of GADs, in combination with high-throughput screening and metabolic engineering of particular producer strains, offer fascinating new prospects for effective, environmentally friendly and economically viable GABA biosynthesis. In this review, we briefly introduce the applications in which GABA is used, and summarize the most important methods associated with GABA production. The major achievements and present challenges in the biotechnological synthesis of GABA, focusing on screening and enzyme engineering of GADs, as well as metabolic engineering strategy for one-step GABA biosynthesis, will be extensively discussed.

Common Distribution of gad Operon in Lactobacillus brevis and its GadA Contributes to Efficient GABA Synthesis toward Cytosolic Near-Neutral pH

Many strains of lactic acid bacteria (LAB) and bifidobacteria have exhibited strain-specific capacity to produce γ-aminobutyric acid (GABA) via their glutamic acid decarboxylase (GAD) system, which is one of amino acid-dependent acid resistance (AR) systems in bacteria. However, the linkage between bacterial AR and GABA production capacity has not been well established. Meanwhile, limited evidence has been provided to the global diversity of GABA-producing LAB and bifidobacteria, and their mechanisms of efficient GABA synthesis. In this study, genomic survey identified common distribution of gad operon-encoded GAD system in Lactobacillus brevis for its GABA production among varying species of LAB and bifidobacteria. Importantly, among four commonly distributed amino acid-dependent AR systems in Lb. brevis, its GAD system was a major contributor to maintain cytosolic pH homeostasis by consuming protons via GABA synthesis. This highlights that Lb. brevis applies GAD system as the main strategy against extracellular and intracellular acidification demonstrating its high capacity of GABA production. In addition, the abundant GadA retained its activity toward near-neutral pH (pH 5.5–6.5) of cytosolic acidity thus contributing to efficient GABA synthesis in Lb. brevis. This is the first global report illustrating species-specific characteristic and mechanism of efficient GABA synthesis in Lb. brevis.

GABA production and structure of gadB/gadC genes in Lactobacillus and Bifidobacterium strains from human microbiota

Gamma-amino butyric acid (GABA) is an active biogenic substance synthesized in plants, fungi, vertebrate animals and bacteria. Lactic acid bacteria are considered the main producers of GABA among bacteria. GABA-producing lactobacilli are isolated from food products such as cheese, yogurt, sourdough, etc. and are the source of bioactive properties assigned to those foods. The ability of human-derived lactobacilli and bifidobacteria to synthesize GABA remains poorly characterized. In this paper, we screened our collection of 135 human-derived Lactobacillus and Bifidobacterium strains for their ability to produce GABA from its precursor monosodium glutamate. Fifty eight strains were able to produce GABA. The most efficient GABA-producers were Bifidobacterium strains (up to 6 g/L). Time profiles of cell growth and GABA production as well as the influence of pyridoxal phosphate on GABA production were studied for L. plantarum 90sk, L. brevis 15f, B. adolescentis 150 and B. angulatum GT102. DNA of these strains was sequenced; the gadB and gadC genes were identified. The presence of these genes was analyzed in 14 metagenomes of healthy individuals. The genes were found in the following genera of bacteria: Bacteroidetes (Bacteroides, Parabacteroides, Alistipes, Odoribacter, Prevotella), Proteobacterium (Esherichia), Firmicutes (Enterococcus), Actinobacteria (Bifidobacterium). These data indicate that gad genes as well as the ability to produce GABA are widely distributed among lactobacilli and bifidobacteria (mainly in L. plantarum, L. brevis, B. adolescentis, B. angulatum, B. dentium) and other gut-derived bacterial species. Perhaps, GABA is involved in the interaction of gut microbiota with the macroorganism and the ability to synthesize GABA may be an important feature in the selection of bacterial strains - psychobiotics.

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.

Expression, characterization and mutagenesis of a novel glutamate decarboxylase from Bacillus megaterium

OBJECTIVES

To search for a novel glutamate decarboxylase (GAD) with an optimum pH towards near-neutrality in order to improve production of gamma-aminobutyric acid (GABA) in recombinant hosts.

RESULTS

A novel glutamate decarboxylase, BmGAD, from Bacillus megaterium was overexpressed and purified. BmGAD was approximately 53 kDa by SDS-PAGE analysis. Its optimum activity was at pH 5 and 50 °C. BmGAD had a specific activity of 59 ± 5.2 U mg(-1) at pH 6, which is the highest value reported so far. The apparent Km and Vmax values of BmGAD were 8 ± 0.5 mM and 150 ± 4.7 U mg(-1), respectively. Through site-directed mutagenesis, two BmGAD mutants (E294R and H467A) showed higher Vmax values than that of wild-type, with the values of 210 ± 6.9 and 180 ± 4.1 U mg(-1) at pH 5 and 50 °C, respectively.

CONCLUSIONS

The unusual high activity of BmGAD at pH 6 makes it an attractive GABA-producing candidate in industrial application.

Directed evolution and mutagenesis of glutamate decarboxylase from Lactobacillus brevis Lb85 to broaden the range of its activity toward a near-neutral pH

Glutamate decarboxylase (GAD) transforms l-glutamate into γ-aminobutyric acid (GABA) with the consumption of a proton. GAD derived from lactic acid bacteria exhibits optimum activity at pH 4.0-5.0 and significantly loses activity at near-neutral pH. To broaden the active range of the GAD GadB1 from Lactobacillus brevis Lb85 toward a near-neutral pH, irrational design using directed evolution and rational design using site-specific mutagenesis were performed. For directed evolution of GadB1, a sensitive high-throughput screening strategy based on a pH indicator was established. One improved mutant, GadB1(T17I/D294G/Q346H), was selected from 800 variants after one round of EP-PCR. It exhibited 3.9- and 25.0-fold increase in activity and catalytic efficiency, respectively at pH 6.0. Through site-specific mutagenesis, several improved mutants were obtained, with GadB1(E312S) being the best one. The combined mutant GadB1(T17I/D294G/E312S/Q346H) showed even higher catalytic efficiency, 13.1- and 43.2-fold that of wild-type GadB1 at pH 4.6 and 6.0, respectively. The amount of GABA produced in gadB1(T17I/D294G/Q346H)-, gadB1(E312S)- and gadB1(T17I/D294G/E312S/Q346H)-expressing Corynebacterium glutamicum ATCC 13032 from endogenous l-glutamate increased by 9.6%, 20.3% and 63.9%, respectively. These results indicate that these mutations have beneficial effects on expanding the active pH range and on GABA biosynthesis, suggesting these GadB1 variants as potent candidates for GABA production.

Thermostabilization of glutamate decarboxylase B from Escherichia coli by structure-guided design of its pH-responsive N-terminal interdomain

Glutamate decarboxylase B (GadB) from Escherichia coli is a highly active biocatalyst that can convert l-glutamate to γ-aminobutyrate (GABA), a precursor of 2-pyrrolidone (a monomer of Nylon 4). In contrast to vigorous studies of pH shifting of GadB, mesophilic GadB has not been stabilized by protein engineering. In this study, we improved the thermostability of GadB through structural optimization of its N-terminal interdomain. According to structural analysis, the N-terminal fourteen residues (1-14) of homo-hexameric GadB formed a triple-helix bundle interdomain at acidic pH and contributed to the thermostability of GadB in preliminary tests as the pH shifted from 7.6 to 4.6. GadB thermostabilization was achieved by optimization of hydrophobic and electrostatic interactions at the N-terminal interdomain. A triple mutant (GadB-TM: Gln5Asp/Val6Ile/Thr7Glu) showed higher thermostability than the wild-type (GadB-WT), i.e., 7.9 and 7.7°C increases in the melting temperature (Tm) and the temperature at which 50% of the initial activity remained after 10min incubation (T50(10)), respectively. The triple mutant showed no reduction of catalytic activity in enzyme kinetics. Molecular dynamics (MD) simulation at high temperature showed that reinforced interactions of the triple mutant rigidified the N-terminal interdomain compared to the wild-type, leading to GadB thermostabilization.