Purification, properties and cDNA cloning of glutamate decarboxylase in germinated faba bean (Vicia faba L.)

Unraveling the Potential of γ-Aminobutyric Acid: Insights into Its Biosynthesis and Biotechnological Applications

γ-Aminobutyric acid (GABA) is a widely distributed non-protein amino acid that serves as a crucial inhibitory neurotransmitter in the brain, regulating various physiological functions. As a result of its potential benefits, GABA has gained substantial interest in the functional food and pharmaceutical industries. The enzyme responsible for GABA production is glutamic acid decarboxylase (GAD), which catalyzes the irreversible decarboxylation of glutamate. Understanding the crystal structure and catalytic mechanism of GAD is pivotal in advancing our knowledge of GABA production. This article provides an overview of GAD's sources, structure, and catalytic mechanism, and explores strategies for enhancing GABA production through fermentation optimization, metabolic engineering, and genetic engineering. Furthermore, the effects of GABA on the physiological functions of animal organisms are also discussed. To meet the increasing demand for GABA, various strategies have been investigated to enhance its production, including optimizing fermentation conditions to facilitate GAD activity. Additionally, metabolic engineering techniques have been employed to increase the availability of glutamate as a precursor for GABA biosynthesis. By fine-tuning fermentation conditions and utilizing metabolic and genetic engineering techniques, it is possible to achieve higher yields of GABA, thus opening up new avenues for its application in functional foods and pharmaceuticals. Continuous research in this field holds immense promise for harnessing the potential of GABA in addressing various health-related challenges.

Mass spectrometry imaging enables visualization of the localization of glutamate decarboxylase activity in germinating legume seeds

Visualizing the distribution of enzymes is vital for understanding physiological phenomena. Enzyme histochemistry is a technique used to investigate the localization of enzyme activity. However, the target is restricted to enzymes with easy-to-design artificial substrates that can develop color through reactions. Mass spectrometry imaging (MSI)-based enzyme histochemistry has been developed as a novel method to visualize enzyme localization. It can be applied to enzyme histochemistry as it detects products from the supplied substrate using enzymes present on the tissue sections. However, enzyme histochemistry using MSI has not been applied to plant tissue samples yet. Glutamate decarboxylase (GAD, EC: 4.1.1.15) is an enzyme that catalyzes the decarboxylation reaction of l-glutamic acid to produce γ-aminobutyric acid (GABA). GABA biosynthesis is important both in the field of food chemistry and plant physiology. This study focused on GAD during the legume germination process and successfully visualized GAD activity in legume seeds using MSI for the first time. Furthermore, the localization of GAD activity in the embryonic axis of germinated soybean seeds and alfalfa seeds could be visualized.

Polyamines Metabolism Interacts with γ-Aminobutyric Acid, Proline and Nitrogen Metabolisms to Affect Drought Tolerance of Creeping Bentgrass

Due to increased global warming and climate change, drought has become a serious threat to horticultural crop cultivation and management. The purpose of this study was to investigate the effect of spermine (Spm) pretreatment on metabolic alterations of polyamine (PAs), γ-aminobutyric acid (GABA), proline (Pro), and nitrogen associated with drought tolerance in creeping bentgrass (Agrostis stolonifera). The results showed that drought tolerance of creeping bentgrass could be significantly improved by the Spm pretreatment, as demonstrated by the maintenance of less chlorophyll loss and higher photosynthesis, gas exchange, water use efficiency, and cell membrane stability. The Spm pretreatment further increased drought-induced accumulation of endogenous PAs, putrescine, spermidine, and Spm, and also enhanced PAs metabolism through improving arginine decarboxylases, ornithine decarboxylase, S-adenosylmethionine decarboxylase, and polyamine oxidase activities during drought stress. In addition, the Spm application not only significantly improved endogenous GABA content, glutamate content, activities of glutamate decarboxylase and α-ketoglutarase, but also alleviated decline in nitrite nitrogen content, nitrate reductase, glutamine synthetase, glutamate synthetase, and GABA aminotransferase activities under drought stress. The Spm-pretreated creeping bentgrass exhibited significantly lower ammonia nitrogen content and nitrite reductase activity as well as higher glutamate dehydrogenase activity than non-pretreated plants in response to drought stress. These results indicated beneficial roles of the Spm on regulating GABA and nitrogen metabolism contributing towards better maintenance of Tricarboxylic acid (TCA) cycle in creeping bentgrass. Interestingly, the Spm-enhanced Pro metabolism rather than more Pro accumulation could be the key regulatory mechanism for drought tolerance in creeping bentgrass. Current findings provide a comprehensive understanding of PAs interaction with other metabolic pathways to regulate drought tolerance in grass species.

Sodium chloride stimulates the biomass and astaxanthin production by Haematococcus pluvialis via a two-stage cultivation strategy

A major challenge facing by astaxanthin industrialization is the low productivity and high production costs. This study established a two-stage cultivation strategy based on the application of NaCl to improve the production of biomass and astaxanthin by Haematococcus pluvialis. During the first growth stage, 12.5 mg L^-1 NaCl led to a remarkable enhancement in biomass, which was 1.28 times compared with the control. Moreover, 2 g L^-1 NaCl stimulated the astaxanthin content from 12.18 mg g^-1 to 25.92 mg g^-1 during the second induction stage. Simultaneously, salinity stress application increased the lipids and GABA contents, as well as the levels of Ca²⁺ and carotenogenic genes' expression, but suppressed the contents of carbohydrate and protein and high-light induced-ROS. This study proposed a simple and convenient strategy for efficient coproduction of biomass and astaxanthin and provides insights into the underlying mechanism of astaxanthin biosynthesis in H. pluvialis induced by salinity stress.

Characterization of two tea glutamate decarboxylase isoforms involved in GABA production

Tea (Camellia sinensis) contains two active glutamate decarboxylases (CsGADs), whose unclear properties were examined here. CsGAD1 was 4-fold higher than CsGAD2 in activity. Their K_m values for L-glutamate were around 5 mM. CsGAD1 and CsGAD2 performed best at 55 and 40 °C, respectively, and were both stimulated by calcium/calmodulin (Ca²⁺/CaM). Over 40 °C, their calmodulin-binding domains degraded. CsGADs were most active at pH 5.6, and were stimulated by Ca²⁺/CaM at pH 5.6-6.6, but inactivated at pH 3.6. Ca²⁺/CaM restored the CsGAD1 activity suppressed by inhibitors. CsGADs and CsCaM were localized to the cytosol. CsGAD1 was more highly expressed in most tissues, while CsGAD2 expression was more induced under stresses. The characteristics we first elucidated here revealed that CsGAD1 is the predominant isoform in tea plant, with CsGAD2 exhibiting a supplementary role under certain conditions. The information will contribute to regulation of GABA tea quality.

Ca2+ involved in GABA signal transduction for phenolics accumulation in germinated hulless barley under NaCl stress

In this study, in order to investigate the role of Ca2+ in GABA signal transduction involved in phenolics accumulation in barley seedlings under NaCl stress, the seedlings were treated with exogenous GABA and its synthesis inhibitor, 3-mercaplopropionic acid (3-MP), as well as Ca2+ channel blockers La3+, Ca2+ chelator EGTA, and Ca2+ release channel inhibitor 2-aminoethoxydiphenyl borate (2-APB). The results showed that GABA significantly enhanced phenolics, calcium and calmodulin content. It also induced Ca2+ influx in barley root tips cells, and altered the distribution of Ca2+, making calcium precipitates more uniform and intensive. While, 3-MP treatment led to opposite changes, which suggested that GABA was essential for calcium content increase. In addition, accumulation of phenolics was inhibited by LaCl3, EGTA and 2-APB treatments, and this inhibition could be alleviated partly by exogenous GABA. Taken together, Ca2+ was involved in GABA signal transduction for phenolics accumulation in barley seedlings under NaCl stress.

Effects of the inhibitor of glutamate decarboxylase on the development and GABA accumulation in germinating fava beans under hypoxia-NaCl stress

Glutamate decarboxylase (GAD) is the key enzyme in GABA shunt, which catalyzes the α-decarboxylation of glutamate to produce GABA. A specific inhibitor for GAD is convenient to study the dynamic balances of GABA metabolism in plants. The inhibitor of GAD in germinated fava beans was screened, and its inhibitory effect on the growth and GABA accumulation in fava beans during germination under hypoxia-NaCl stress was investigated. The inhibitory effect of aminoxyacetate for fava bean GAD was better than those of other chemicals, and it increased with the increase in concentration in vivo. After aminoxyacetate (5 mM) application for 4 days during germination, the GAD activity in germinating fava beans was significantly inhibited by more than 90% in both organs. Meanwhile, the growth of fava bean sprouts was also slightly suppressed. Moreover, the GABA contents decreased by 43.9% and 81.5% in a 4 day-old cotyledon and embryo, respectively, under aminoxyacetate treatment compared with that in the control. In summary, these results showed that aminoxyacetate can serve as a specific inhibitor of GAD in plants. At least 43.9% and 81.5% of GABA in germinating fava beans under hypoxia-NaCl stress were synthesized via GABA shunt.

Effects of aminoxyacetate, a specific inhibitor of glutamate decarboxylase, on GABA accumulation in germinating fava beans under hypoxia-NaCl stress.

Evolutionary Trails of Plant Group II Pyridoxal Phosphate-Dependent Decarboxylase Genes

Type II pyridoxal phosphate-dependent decarboxylase (PLP_deC) enzymes play important metabolic roles during nitrogen metabolism. Recent evolutionary profiling of these genes revealed a sharp expansion of histidine decarboxylase genes in the members of Solanaceae family. In spite of the high sequence homology shared by PLP_deC orthologs, these enzymes display remarkable differences in their substrate specificities. Currently, limited information is available on the gene repertoires and substrate specificities of PLP_deCs which renders their precise annotation challenging and offers technical challenges in the immediate identification and biochemical characterization of their full gene complements in plants. Herein, we explored their evolutionary trails in a comprehensive manner by taking advantage of high-throughput data accessibility and computational approaches. We discussed the premise that has enabled an improved reconstruction of their evolutionary lineage and evaluated the factors offering constraints in their rapid functional characterization, till date. We envisage that the synthesized information herein would act as a catalyst for the rapid exploration of their biochemical specificity and physiological roles in more plant species.

Accumulation of γ-aminobutyric acid in soybean by hypoxia germination and freeze-thawing incubation

BACKGROUND

γ-Aminobutyric acid (GABA) can be synthesised by the GABA shunt and polyamine degradation pathway in plants under hypoxia stress and lower temperature. The hypoxia germination freeze-thawing incubation was used as a new technique for accumulating more GABA in soybean.

RESULTS

Results showed that glutamate decarboxylase (GAD) and diamine oxidase (DAO) activity as well as GABA content increased during germination within 24 h under hypoxia. However, the contents of dry matters and protein decreased. When the hypoxia-treated sprouts were frozen at -18 °C for 12 h and thawed at 25 °C for 6 h, GABA content increased drastically to 7.21-fold of the non-frozen sprouts. Subsequently, the freeze-thawing sprouts were ground into homogenates and incubated. GABA content was 14.20-fold of the only-soaked seeds when homogenates was incubated at 45 °C for 80 min within 400 µmol L(-1) pyridoxine (VB6) (pH 6.5).

CONCLUSION

The hypoxia germination freeze-thawing incubation was an effective method for accumulating GABA in soybean. During incubation, DAO was more important for GABA formation in homogenate of germinated soybean compared with GAD.

Effects of ABA and CaCl2 on GABA accumulation in fava bean germinating under hypoxia-NaCl stress

Effects of exogenous abscisic acid (ABA) and CaCl2 on γ-aminobutyric acid (GABA) accumulation of germinated fava bean under hypoxia-NaCl stress were investigated. Exogenous ABA resulted in the enhancement of glutamate decarboxylase (GAD) and diamine oxidase (DAO) activity as well as GABA content in cotyledon and shoot. CaCl2 increased both enzyme activities in shoot and GABA content in cotyledon and shoot. ABA downregulated GAD expression in cotyledon and radicle, while upregulated that in shoot; it also upregulated DAO expression in each organ. CaCl2 upregulated GAD expression in cotyledon, while downregulated that in radicle. However, it upregulated DAO expression in shoot, downregulated that in radicle. ABA inhibitor fluridon and ethylenediaminetetraacetic acid inhibited GAD and DAO activities significantly so that inhibited GABA accumulation through reducing ABA biosynthesis and chelating Ca2+, respectively. However, they upregulated GAD and DAO expression in varying degrees. These results indicate that ABA and Ca2+ participate in GABA biosynthesis in fava bean during germination under hypoxia-NaCl stress.

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.

Maximising the phytochemical content and antioxidant activity of Ecuadorian brown rice sprouts through optimal germination conditions

Germinated brown rice (GBR) is considered a healthy alternative to white rice in the fight against chronic diseases. As the functional quality of GBR depends on genotype and germination conditions, the objectives were to identify suitable Ecuadorian brown rice cultivars and optimal germination time and temperature to maximise γ-aminobutyric acid (GABA), total phenolics compounds (TPC) and antioxidant activity of GBR. Regression models for the prediction of phytochemical composition and antioxidant activity in GBR were also obtained. Germination improved GABA, TPC and antioxidant activity in all cultivars. Maximum GABA and antioxidant activity were attained at 34 °C for 96 h, while the highest TPC was found at 28 °C for 96 h in all cultivars. GBR cv. GO displayed the highest antioxidant activity and cv. 15 was the most effective at accumulating GABA and TPC in the optimal germination conditions. Therefore, Ecuadorian GBR could be used for the preparation of functional foods serving as preventative strategies in combating chronic diseases.