Effects of pH and dissolved oxygen on the synthesis of γ-glutamyltranspeptidase from Bacillus subtilis SK 11.004

Statistical optimization of culture conditions of mesophillic gamma-glutamyl transpeptidase from Bacillus altitudinis IHB B1644

Microbial gamma-glutamyl transpeptidase (GGT) is a key enzyme in production of several γ-glutamyl compounds with food and pharmaceutical applications. Bacterial GGTs are not commercially available in the market owing to their low production from various sources. Thus, the study was focused on achieving the higher GGT production from B. altitudinis IHB B1644 by optimizing the culture conditions using one-variable-at-a-time (OVAT) strategy. A mesophillic temperature of 28 °C, agitation 200 rpm and neutral pH 7 were found to be optimal for higher GGT titre. Among the medium components, the monosaccharide glucose served as the best carbon source over disaccharides, and yeast extract was the preferred organic nitrogen source over inorganic nitrogen sources. The statistical approaches (Plakett-Burman and response surface methodology) were further employed for the optimization of medium components. Medium composition: 0.1% w/v glucose, 0.3% w/v yeast extract, 0.03% w/v magnesium sulphate, 0.20% w/v potassium dihydrogen phosphate and 2.5% w/v sodium chloride with inoculum size (1% v/v) was suitable for higher GGT titres (449 U ml^-1). Time kinetics showed the stability of enzyme up to 96 h of incubation suggesting its application in the industrial use. The proposed strategy resulted in 2.6-fold increase in the GGT production compared to that obtained in the unoptimized medium. The results demonstrated that RSM was fitting to identify the optimum production conditions and this finding should be of great importance for commercial GGT production.

Development of a highly efficient and specific L-theanine synthase

γ-Glutamylcysteine synthetase (γ-GCS) from Escherichia coli, which catalyzes the formation of L-glutamylcysteine from L-glutamic acid and L-cysteine, was engineered into an L-theanine synthase using L-glutamic acid and ethylamine as substrates. A high-throughput screening method using a 96-well plate was developed to evaluate the L-theanine synthesis reaction. Both site-saturation mutagenesis and random mutagenesis were applied. After three rounds of directed evolution, 13B6, the best-performing mutant enzyme, exhibited 14.6- and 17.0-fold improvements in L-theanine production and catalytic efficiency for ethylamine, respectively, compared with the wild-type enzyme. In addition, the specific activity of 13B6 for the original substrate, L-cysteine, decreased to approximately 14.6% of that of the wild-type enzyme. Thus, the γ-GCS enzyme was successfully switched to a specific L-theanine synthase by directed evolution. Furthermore, an ATP-regeneration system was introduced based on polyphosphate kinases catalyzing the transfer of phosphates from polyphosphate to ADP, thus lowering the level of ATP consumption and the cost of L-theanine synthesis. The final L-theanine production by mutant 13B6 reached 30.4 ± 0.3 g/L in 2 h, with a conversion rate of 87.1%, which has great potential for industrial applications.

Secretion of Bacillus amyloliquefaciens γ-Glutamyltranspeptidase from Bacillus subtilis and Its Application in Enzymatic Synthesis of l-Theanine

In this study, the gene of γ-glutamyltranspeptidase (GGT) from Bacillus amyloliquefaciens (BaGGT) controlled by the P_lac promoter was cloned into Bacillus subtilis to construct two recombinant vectors with either one or two signal peptides to drive extracellular secretion. After optimization, 90 ± 0.2 mg/L BaGGT was obtained when the inducing conditions were 24 h and 80 μM (IPTG). The properties of BaGGT were measured, showing that the optimal reaction conditions were 40 °C and pH 9.0 with 55.0 ± 0.5 U/mg enzymatic activity. K_m and V_max were 0.214 mM and 88.13 μmol/min/mg. BaGGT could be stored for 72 h with 90% of the initial activity at 40 °C and retained more than 50% of the initial activity after being maintained at different pH values for 24 h. Finally, enzymatic synthesis of l-theanine was performed with the optimal conditions: 20 mM l-Gln, 100 mM ethylamine HCl, 0.5 U/mL BaGGT, incubated at 40 °C for 6 h, 200 rpm.

Hyperproduction of γ-glutamyl transpeptidase from Bacillus licheniformis ER15 in the presence of high salt concentration

BACKGROUND

Microbial γ-glutamyl transpeptidases (GGTs) have been exploited in biotechnological, pharmaceutical, and food sectors for the synthesis of various γ-glutamyl compounds. But, till date, no bacterial GGTs are commercially available in the market because of lower levels of production from various sources. In the current study, production of GGT from Bacillus licheniformis ER15 was investigated to achieve high GGT titers.

RESULTS

Hyperproduction of GGT from B. licheniformis ER15 was achieved with 6.4-fold enhancement (7921.2 ± 198.7 U/L) by optimization of culture medium following one-variable-at-a-time strategy and statistical approaches. Medium consisting of Na₂HPO₄: 0.32% (w/v); KH₂PO₄: 0.15% (w/v); starch: 0.1% (w/v); soybean meal: 0.5% (w/v); NaCl: 4.0% (w/v), and MgCl₂: 5 mM was found to be optimal for maximum GGT titers. Maximum GGT titers were obtained, in the optimized medium at 37°C and 200 rpm, after 40 h. It was noteworthy that GGT production was a linear function of sodium chloride concentration, as observed during response surface methodology. While investigating the role of NaCl on GGT production, it was found that NaCl drastically decreased subtilisin concentration and indirectly increasing GGT recovery.

CONCLUSION

B. licheniformis ER15 is proved to be a potential candidate for large-scale production of GGT enzyme and its commercialization.