Heterologous expression and enzymatic characterization of γ-glutamyltranspeptidase from Bacillus amyloliquefaciens

Efficient expression of γ-glutamyl transpeptidase in Bacillus subtilis via CRISPR/Cas9n and its immobilization

In this study, we successfully applied the strategy of combining tandem promoters and tandem signal peptides with overexpressing signal peptidase to efficiently express and produce γ-glutamyl peptidase (GGT) enzymes (BsGGT, BaGGT, and BlGGT) from Bacillus subtilis, Bacillus amyloliquefaciens, and Bacillus licheniformis in Bacillus subtilis ATCC6051Δ5. In order to avoid the problem of instability caused by duplicated strong promoters, we assembled tandem promoters of different homologous genes from different species. To achieve resistance marker-free enzyme in the food industry, we first removed the replication origin and corresponding resistance marker of Escherichia coli from the expression vector. The plasmid was then transformed into the B. subtilis host, and the Kan resistance gene in the expression plasmid was directly edited and silenced using the CRISPR/Cas9n-AID base editing system. As a result, a recombinant protein expression carrier without resistance markers was constructed, and the enzyme activity of the BlGGT strain during shake flask fermentation can reach 53.65 U/mL. The recombinant BlGGT was immobilized with epoxy resin and maintained 82.8% enzyme activity after repeated use for 10 times and 87.36% enzyme activity after storage at 4 °C for 2 months. The immobilized BlGGT enzyme was used for the continuous synthesis of theanine with a conversion rate of 65.38%. These results indicated that our approach was a promising solution for improving enzyme production efficiency and achieving safe production of enzyme preparations in the food industry. KEY POINTS: • Efficient expression of recombinant proteins by a combination of dual promoter and dual signal peptide. • Construction of small vectors without resistance markers in B. subtilis using CRISPR/Cas9n-AID editing system. • The process of immobilizing BlGGT with epoxy resin was optimized.

Biochemical Characterization of γ-Glutamyl Transpeptidase from Bacillus altitudinis IHB B1644 and Its Application in the Synthesis of l-Theanine

An extracellular γ-glutamyl transpeptidase (GGT) produced from Bacillus altitudinis IHB B1644 was purified to homogeneity employing ion-exchange chromatography. GGT comprised two subunits of 40 and 22 kDa determined by SDS-PAGE. The maximum enzyme activity was optimal at pH 9 and 37 °C. The purified enzyme was stable from pH 5-10 and <50 °C. Steady-state kinetic studies revealed a K_m value of 0.538 mM against γ-GpNA. For substrate specificity, GGT showed highest affinity for l-methionine. The inhibitors' effect demonstrated that serine or threonine and tryptophan residues are essential for enzyme activity. l-Theanine production was optimized by employing a one-variable-at-a-time approach with 60-65% conversion rate. The final reaction consisted of 20 mM l-glutamine, 200 mM ethylamine hydrochloride, and 10 U mL^-1 enzyme concentration at 37 °C in Tris-Cl (50 mM, pH 9) for 5 h. l-Theanine was purified using a Dowex 50W X 8 hydrogen form resin and confirmed by HPLC and ¹H NMR spectroscopies.

Bacterial Gamma-Glutamyl Transpeptidase, an Emerging Biocatalyst: Insights Into Structure-Function Relationship and Its Biotechnological Applications

Gamma-glutamyl transpeptidase (GGT) enzyme is ubiquitously present in all life forms and plays a variety of roles in diverse organisms. Higher eukaryotes mainly utilize GGT for glutathione degradation, and mammalian GGTs have implications in many physiological disorders also. GGTs from unicellular prokaryotes serve different physiological functions in Gram-positive and Gram-negative bacteria. In the present review, the physiological significance of bacterial GGTs has been discussed categorizing GGTs from Gram-negative bacteria like Escherichia coli as glutathione degraders and from pathogenic species like Helicobacter pylori as virulence factors. Gram-positive bacilli, however, are considered separately as poly-γ-glutamic acid (PGA) degraders. The structure-function relationship of the GGT is also discussed mainly focusing on the crystallization of bacterial GGTs along with functional characterization of conserved regions by site-directed mutagenesis that unravels molecular aspects of autoprocessing and catalysis. Only a few crystal structures have been deciphered so far. Further, different reports on heterologous expression of bacterial GGTs in E. coli and Bacillus subtilis as hosts have been presented in a table pointing toward the lack of fermentation studies for large-scale production. Physicochemical properties of bacterial GGTs have also been described, followed by a detailed discussion on various applications of bacterial GGTs in different biotechnological sectors. This review emphasizes the potential of bacterial GGTs as an industrial biocatalyst relevant to the current switch toward green chemistry.

γ-Glutamyltranspeptidase from Bacillus amyloliquefaciens: transpeptidation activity enhancement and L-theanine production

L-theanine, a unique amino acid in green tea with health benefits, can be enzymatically synthesized by γ-glutamyltranspeptidase (γ-GT; EC 2.3.2.2). Here, a salt-tolerant γ-glutamyltranspeptidase from a marine bacterium Bacillus amyloliquefaciens was expressed in Escherichia. coli BL21 (DE3) and was shown to be optimally active at 55 °C, pH 8.5 and alkali stable. A mutant, with higher transpeptidation activity, was obtained following two rounds of directed evolution using error-prone PCR and site-saturation mutagenesis. The mutation increased the ratio of transpeptidation to hydrolysis from 1.6 to 35.6. Additionally, Kinetic analysis exhibited 17.5% decrease of K_m, 13.0-fold increase of K_cat, and 16.3-fold increase of K_cat/K_m in mutant V319A/S437 G versus the wild-type. The 3-D modelling analysis revealed a tighter binding pocket in mutant V319A/S437 G. The frequency of hydrogen bond between donor substrate and two residues in the catalytic pocket (Gly437 and Thr375) was enhanced, which stabilized the ligand binding and thus improved the catalytic efficiency. The optimal conditions for the biocatalytic synthesis were determined as pH 10.0, 20 μg mL^-1BaGT, 200 mM L-glutamine, 2 M ethylamine, and a reaction time of 5 h. The V319A/S437 G mutant was shown to increase the percentage yield of L-theanine from 58% to 83%. These results indicate the great potential of V319A/S437 G in L-theanine production after further study.

A chemical screening method for menaquinone-producing strains based on HPLC-UV technology

Despite menaquinones (MKs)-4 and - 7 being known to have extensive biological activities and applications, less attention has been paid to the other MKs. Thus, to obtain a range of MKs to further explore their pharmacological activities, structure-activity relationships, and applications, a chemical screening method for MK-producing strains was established based on high-performance liquid chromatography-ultraviolet (HPLC-UV) technology. Considering that Bacillus strains have proven to be an important MK-producing bioresource, twenty-nine putative Bacillus isolates previously sought from a fermented soybean sample were used for the validation of the chemical screening method, which ultimately led to the discovery of sixteen MK-producing strains. Among them, Bacillus subtilis DC-1 presented excellent ability to produce MKs. Another, a purchased strain of B. amyloliquefaciens was discovered to be an MK-producing strain. These results indicated this screening method was simple and highly efficient for the discovery of MK-producing strains, especially those producing a range of MK structures.

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.

γ-Glutamyl Cysteine Ligase of Lactobacillus reuteri Synthesizes γ-Glutamyl Dipeptides in Sourdough

Kokumi-active γ-glutamyl dipeptides (γ-GPs) accumulate in fermented food. γ-Glutamyl transferase, glutaminase, glutathione synthetase, and γ-glutamyl cysteine ligase (GCL) may synthesize γ-GPs. The genome of Lactobacillus reuteri encodes GCL but not glutathione synthetase or glutamyl transferase; therefore, this study investigated the role of GCL in γ-GP synthesis by L. reuteri LTH5448. Phylogenomic analysis of gcl in lactobacilli demonstrated that three genes coding for GCL are present in L. reuteri; two of these are present in L. reuteri LTH5448. Two deletion mutants of L. reuteri LTH5448, L. reuteri LTH5448Δ gcl1 and LTH5448Δ gcl1Δ gcl2, were constructed by double crossover mutagenesis. Growth and oxygen resistance of the mutants were comparable to the wild type. γ-Glu-Glu, γ-Glu-Leu, γ-Glu-Ile, γ-Glu-Val, and γ-Glu-Cys were quantified in buffer and sourdough fermentations by liquid chromatography-mass spectrometry. The wild type and L. reuteri Δ gcl1 but not Δ gcl1Δ gcl2 converted amino acids to γ-Glu-Cys. γ-Glu-Ile accumulation was reduced in both mutants; however, the disruption of gcl did not alter the biosynthesis of the other γ-GPs. In conclusion, gcl1 in L. reuteri mediates γ-Glu-Ile synthesis, gcl2 mediates γ-Glu-Cys synthesis, but neither gene affected synthesis of other γ-GPs. This study facilitates selection of starter cultures that synthesize γ-Glu peptides with kokumi activity and, thus, improve the taste of fermented foods.

Inference of sigma factor controlled networks by using numerical modeling applied to microarray time series data of the germinating prokaryote

A computational model of gene expression was applied to a novel test set of microarray time series measurements to reveal regulatory interactions between transcriptional regulators represented by 45 sigma factors and the genes expressed during germination of a prokaryote Streptomyces coelicolor. Using microarrays, the first 5.5 h of the process was recorded in 13 time points, which provided a database of gene expression time series on genome-wide scale. The computational modeling of the kinetic relations between the sigma factors, individual genes and genes clustered according to the similarity of their expression kinetics identified kinetically plausible sigma factor-controlled networks. Using genome sequence annotations, functional groups of genes that were predominantly controlled by specific sigma factors were identified. Using external binding data complementing the modeling approach, specific genes involved in the control of the studied process were identified and their function suggested.