Biophysical characterization of Bacillus licheniformis and Escherichia coli γ-glutamyltranspeptidases: A comparative analysis

Effects of Sodium Dodecyl Sulfate on the Enzyme Catalysis and Conformation of a Recombinant γ-Glutamyltranspeptidase from Bacillus licheniformis

The study of interactions between proteins and surfactants is of relevance in a diverse range of applications including food, enzymatic detergent formulation, and drug delivery. In spite of sodium dodecyl sulfate (SDS)-induced unfolding has been studied in detail at the protein level, deciphering the conformation-activity relationship of a recombinant γ-glutamyltranspeptidase (BlrGGT) from Bacillus licheniformis remains important to understand how the transpeptidase activity is related to its conformation. In this study, we examined the enzyme catalysis and conformational transition of BlrGGT in the presence of SDS. Enzymatic assays showed that the transpeptidase activity of BlrGGT was greatly affected by SDS in a concentration-dependent manner with approximately 90% inactivation at 6 mM. Native polyacrylamide gel electrophoresis of SDS-treated samples clearly revealed that the heterodimeric enzyme was apparently dissociated into two different subunits at concentrations above 2 mM. The study of enzyme kinetics showed that SDS can act as a mixed-type inhibitor to reduce the catalytic efficiency of BlrGGT. Moreover, the t_1/2 value of the enzyme at 55 °C was greatly reduced from 495.1 min to 7.4 min in the presence of 1 mM SDS. The I₃/I₁ ratio of pyrene excimer fluorescence emission changed around 3.7 mM SDS in the absence of BlrGGT and the inflection point of enzyme samples was reduced to less than 2.7 mM. The Far-UV CD spectrum of the native enzyme had two negative peaks at 208 and 222 nm, respectively; however, both negative peaks increased in magnitude with increasing SDS concentration and reached maximal values at above 4.0 mM. The intrinsic fluorescence spectra of tryptophan further demonstrated that the SDS-induced enzyme conformational transition occurred at approximately 5.1 mM. Tween 20 significantly suppressed the interaction of BlrGGT with SDS by forming mixed micelles at a molar ratio of 1.0. Taken together, this study definitely promotes our better understanding of the relationship between the conformation and catalysis of BlrGGT.

Increase of Kokumi γ-Glutamyl Peptides in Porcine Hemoglobin Hydrolysate Using Bacterial γ-Glutamyltransferase

The kokumi sensation of protein hydrolysates could be enhanced by γ-glutamylation through forming a series of γ-glutamyl di- and tri-peptides. In this study, porcine hemoglobin hydrolysate was γ-glutamylated using enzymes from Bacillus amyloliquefaciens (Ba) or Bacillus licheniformis (Bl), which are sold as glutaminases but identified as γ-glutamyltransferases (GGTs). To yield more γ-glutamyl peptides, reaction conditions were optimized in terms of GGT source (BaGGT and BlGGT), substrate concentration (10, 20, and 40%), reaction time (3, 6, 12, and 24 h), and glutamine supplementation (20, 40, and 80 mM). Results showed that both the GGTs had the highest transpeptidase activity at similar pH values but different temperatures. In addition, BaGGT had stronger catalytic ability to form γ-glutamyl dipeptides, while BlGGT was more capable to generate γ-Glu-Val-Gly. Adding glutamine was more efficient to obtain more target peptides than adjusting the hydrolysate concentration and reaction time. This study contributes to the valorization of animal side streams.

Isolation and characterization of a salt-tolerant γ-glutamyl transpeptidase from xerophilic Aspergillus sydowii

Xerophilic Aspergillus molds isolated from halo-alkaliphilic and dry environments are attractive genetic resources for obtaining salt- and osmo-adaptive enzymes. A. sydowii MA0196 secreted the largest amount of γ-glutamyl transpeptidase (GGT) during solid-state fermentation at a low initial water activity (a w = 0.85). Gel filtration analysis revealed that the molecular mass of the purified native enzyme (MA0196 GGT) was 120 kDa. SDS-PAGE analysis showed that MA0196 GGT consists of two subunits with molecular masses of 56.4 and 33 kDa, indicating production from a proenzyme via autoproteolysis. Deglycosylation of the subunits by N-glycosidase F yielded 40.9 and 19.6 kDa species. MA0196 GGT retained transpeptidase and hydrolysis activities and their catalytic efficiency (k cat/K m) under high salt and low water activity. The enzyme displayed broad substrate specificity toward γ-glutamyl acceptors such as amino acids and the imidazole dipeptides, carnosine and anserine. Carnosine and L-glutamine were converted into γ-glutamyl-β-alanyl-L-histidine by MA0196 GGT with a 32.9% yield in the presence of 2% (v/v) dimethyl sulfoxide. Phylogenetic analysis indicated that MA0196 GGT forms a distinct lineage from A. oryzae and A. sojae GGTs. These excellent properties indicate that MA0196 GGT can be used in salted fermentation and for producing bioactive peptides.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03259-3.

Activation and thermal stabilization of a recombinant γ-glutamyltranspeptidase from Bacillus licheniformis ATCC 27811 by monovalent cations

The regulation of enzyme activity through complexation with certain metal ions plays an important role in many biological processes. In addition to divalent metals, monovalent cations (MVCs) frequently function as promoters for efficient biocatalysis. Here, we examined the effect of MVCs on the enzymatic catalysis of a recombinant γ-glutamyltranspeptidase (BlrGGT) from Bacillus licheniformis ATCC 27,811 and the application of a metal-activated enzyme to L-theanine synthesis. The transpeptidase activity of BlrGGT was enhanced by Cs⁺ and Na⁺ over a broad range of concentrations with a maximum of 200 mM. The activation was essentially independent of the ionic radius, but K⁺ contributed the least to enhancing the catalytic efficiency. The secondary structure of BlrGGT remained mostly unchanged in the presence of different concentrations of MVCs, but there was a significant change in its tertiary structure under the same conditions. Compared with the control, the half-life (t_1/2) of the Cs⁺-enriched enzyme at 60 and 65 °C was shown to increase from 16.3 and 4.0 min to 74.5 and 14.3 min, respectively. The simultaneous addition of Cs⁺ and Mg²⁺ ions exerted a synergistic effect on the activation of BlrGGT. This was adequately reflected by an improvement in the conversion of substrates to L-theanine by 3.3-15.1% upon the addition of 200 mM MgCl₂ into a reaction mixture comprising the freshly desalted enzyme (25 μg/mL), 250 mM L-glutamine, 600 mM ethylamine, 200 mM each of the MVCs, and 50 mM borate buffer (pH 10.5). Taken together, our results provide interesting insights into the complexation of MVCs with BlrGGT and can therefore be potentially useful to the biocatalytic production of naturally occurring γ-glutamyl compounds. KEY POINTS: • The transpeptidase activity of B. licheniformis γ-glutamyltranspeptidase can be activated by monovalent cations. • The thermal stability of the enzyme was profoundly increased in the presence of 200 mM Cs⁺. • The simultaneous addition of Cs⁺and Mg²⁺ions to the reaction mixture improves L-theanine production.

Improvement of the Stability and Activity of an LPMO Through Rational Disulfide Bonds Design

Lytic polysaccharide monooxygenases (LPMOs) oxidatively break down the glycosidic bonds of crystalline polysaccharides, significantly improving the saccharification efficiency of recalcitrant biomass, and have broad application prospects in industry. To meet the needs of industrial applications, enzyme engineering is needed to improve the catalytic performance of LPMOs such as enzyme activity and stability. In this study, we engineered the chitin-active CjLPMO10A from Cellvibrio japonicus through a rational disulfide bonds design. Compared with the wild-type, the variant M1 (N78C/H116C) exhibited a 3-fold increase in half-life at 60°C, a 3.5°C higher T₅₀¹⁵, and a 7°C rise in the apparent Tm. Furthermore, the resistance of M1 to chemical denaturation was significantly improved. Most importantly, the introduction of the disulfide bond improved the thermal and chemical stability of the enzyme without causing damage to catalytic activity, and M1 showed 1.5 times the specific activity of the wild-type. Our study shows that the stability and activity of LPMOs could be improved simultaneously by selecting suitable engineering sites reasonably, thereby improving the industrial adaptability of the enzymes, which is of great significance for applications.

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.

Magnetic Cross-Linked Enzyme Aggregates of a Transpeptidase-Specialized Variant (N450D) of Bacillus licheniformis γ-Glutamyl Transpeptidase: An Efficient and Stable Biocatalyst for l-Theanine Synthesis

γ-Glutamyl transpeptidase (GGT) catalyzes the transfer of glutathione’s γ-glutamyl group and related γ-glutamyl amides to water, amino acids or peptides, and utilizes a conserved Thr residue to process its own polypeptide chain into a large and a small subunit that then assemble to produce a catalytically competent enzyme. In this study, the magnetic cross-linked enzyme aggregates (mCLEAs) of a transpeptidase-specialized variant (N450D) of Bacillus licheniformis GGT were successfully prepared with optimized process parameters viz.1.25:1 (v/v) of isopropanol to N450D (0.3 mg/mL) ratio/0.02:1 (w/w) of enzyme to 3-aminopropyl triethoxysilane (APTES)-coated magnetic nanoparticle ratio/20 mM of glutaraldehyde. The prepared magnetic nanoparticles and immobilized enzyme (N450D-mCLEAs) were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscope integrated with energy dispersive X-ray spectroscopy (FESEM/EDS), and superparamagnetic analysis. As compared with free enzyme, N450D-mCLEAs displayed significantly higher heat resistance at temperatures of 55 and 60 °C, and had a greater stability over a storage period of one month. The immobilized enzyme could also be reused for 10 consecutive biocatalytic cycles with no significant reduction in the percent yield of l-theanine. Conclusively, this immobilization strategy surely provides a meaningful glance of developing N450D-mediated biocatalysis for the production of physiologically important γ-glutamyl compounds.

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.

Mutational Analysis of a Highly Conserved PLSSMXP Sequence in the Small Subunit of Bacillus licheniformis γ-Glutamyltranspeptidase

A highly conserved ⁴⁵⁸PLSSMXP⁴⁶⁴ sequence in the small subunit (S-subunit) of an industrially important Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT) was identified by sequence alignment. Molecular structures of the precursor mimic and the mature form of BlGGT clearly reveal that this peptide sequence is in close spatial proximity to the self-processing and catalytic sites of the enzyme. To probe the role of this conserved sequence, ten mutant enzymes of BlGGT were created through a series of deletion and alanine-scanning mutagenesis. SDS-PAGE and densitometric analyses showed that the intrinsic ability of BlGGT to undergo autocatalytic processing was detrimentally affected by the deletion-associated mutations. However, loss of self-activating capacity was not obviously observed in most of the Ala-replacement mutants. The Ala-replacement mutants had a specific activity comparable to or greater than that of the wild-type enzyme; conversely, all deletion mutants completely lost their enzymatic activity. As compared with BlGGT, S460A and S461S showed greatly enhanced k_cat/K_m values by 2.73- and 2.67-fold, respectively. The intrinsic tryptophan fluorescence and circular dichroism spectral profiles of Ala-replacement and deletion mutants were typically similar to those of BlGGT. However, heat and guanidine hydrochloride-induced unfolding transitions of the deletion-associated mutant proteins were severely reduced as compared with the wild-type enzyme. The predictive mutant models suggest that the microenvironments required for both self-activation and catalytic reaction of BlGGT can be altered upon mutations.

Specific biochemical amniotic fluid pattern of fetal isolated esophageal atresia

BACKGROUND

Perinatal care of esophageal atresia (EA) may be improved by prenatal diagnosis. Ultrasound findings (polyhydramnios and/or nonvisualization of fetal stomach) lead to a detection rate of ~50%. An amniotic fluid (AF) biochemical pattern characterized by high total protein, γ-glutamyl transpeptidase (GGTP), and normal l-leucine-aminopeptidase (AMP) leads to a 100% detection rate. The aim of this study was to explain this specific pattern.

METHODS

On the basis of enzyme activities assay, the following four objectives were sought: (i) comparing AF markers between EA and other digestive tract atresias, (ii) determining local GGTP synthesis in the esophagus (immunohistobiochemistry), (iii) determining the presence of a specific AF-AMP activity inhibitor, and (iv) comparing AF-AMP and AF-GGTP half-lives.

RESULTS

The AF-EA pattern was similar to that observed in upper duodenal atresia (above the Oddi sphincter). No local synthesis of GGTP was observed in the esophagus. No AF-AMP activity inhibitor was found. AF-GGTP had a longer half-life than AF-AMP.

CONCLUSION

Due to the swallowing anomaly observed in EA, GGTP and AMP values physiologically observed at 18 wk will decrease on the basis of the half-lives of markers, with a flat slope for GGTP and a sharp slope for AMP, therefore explaining the differences observed in the AF-EA pattern.

Heterogeneous nucleation helps the search for initial crystallization conditions of γ-glutamyl transpeptidase from Bacillus licheniformis

Here, the crystallization and preliminary X-ray diffraction studies of Bacillus licheniformis γ-glutamyl transpeptidase (BlGT) are reported. The serendipitous finding of heterogeneous nucleants in the initial experiments provided the first crystallization conditions for the protein. Crystals were grown by hanging-drop vapour diffusion using a precipitant solution consisting of 20%(w/v) PEG 3350, 0.2 M magnesium chloride hexahydrate, 0.1 M Tris-HCl pH 8.2. The protein crystallized in the orthorhombic space group P2(1)2(1)2(1), with one heterodimer per asymmetric unit and unit-cell parameters a = 60.90, b = 61.97, c = 148.24 Å. The BlGT crystals diffracted to 2.95 Å resolution.

γ-Glutamyltranspeptidases: sequence, structure, biochemical properties, and biotechnological applications

γ-Glutamyltranspeptidases (γ-GTs) are ubiquitous enzymes that catalyze the hydrolysis of γ-glutamyl bonds in glutathione and glutamine and the transfer of the released γ-glutamyl group to amino acids or short peptides. These enzymes are involved in glutathione metabolism and play critical roles in antioxidant defense, detoxification, and inflammation processes. Moreover, γ-GTs have been recently found to be involved in many physiological disorders, such as Parkinson's disease and diabetes. In this review, the main biochemical and structural properties of γ-GTs isolated from different sources, as well as their conformational stability and mechanism of catalysis, are described and examined with the aim of contributing to the discussion on their structure-function relationships. Possible applications of γ-glutamyltranspeptidases in different fields of biotechnology and medicine are also discussed.

Experimental evidence for the involvement of amino acid residue Glu398 in the autocatalytic processing of Bacillus licheniformis γ-glutamyltranspeptidase

The role of glutamate 398 in the autocatalytic processing of Bacillus licheniformis γ-glutamyltranspeptidase (BlGGT) was explored by site-directed mutagenesis. This glutamate was substituted by either alanine, aspartate, arginine or glutamine and the expressed mutant enzymes were purified to apparent homogeneity with metal-affinity chromatography. SDS–PAGE analysis showed that E398A, E398D and E398K were unable to process themselves into a large and a small subunit. However, E398Q was not only able to process itself, but also had a catalytic activity comparable to that of BlGGT. As compared with the wild-type enzyme, no significant change in circular dichroism spectra was observed for the mutant proteins. Thermal unfolding of BlGGT, E398A, E398D, E398K and E398Q followed the two-state unfolding process with a transition point (T_m) of 47.7–69.4 °C. Tryptophan fluorescence spectra of the mutant enzymes were different from the wild-type protein in terms of fluorescence intensity. Native BlGGT started to unfold beyond ∼1.92 M guanidine hydrochloride (GdnHCl) and reached an unfolded intermediate, [GdnHCl]_{0.5, N–U}, at 3.07 M equivalent to free energy change (Δ⁢GN−UH2⁢O) of 14.53 kcal/mol for the N → U process, whereas the denaturation midpoints for the mutant enzymes were 1.31–2.99 M equivalent to Δ⁢GN−UH2⁢O of 3.29–12.05 kcal/mol. Taken together, these results strongly suggest that the explored glutamate residue is indeed important for the autocatalytic processing of BlGGT.

Exploring the unfolding mechanism of γ-glutamyltranspeptidases: the case of the thermophilic enzyme from Geobacillus thermodenitrificans

γ-glutamyltranspeptidases (γ-GTs) are ubiquitous enzymes that catalyze the hydrolysis of γ-glutamyl bonds in glutathione and glutamine and the transfer of the released γ-glutamyl group to amino acids or short peptides. These enzymes are generally synthesized as precursor proteins, which undergo an intra-molecular autocatalytic cleavage yielding a large and a small subunit. In this study, circular dichroism and intrinsic fluorescence measurements have been used to investigate the structural features and the temperature- and guanidinium hydrochloride (GdnHCl)-induced unfolding of the mature form of the γ-GT from Geobacillus thermodenitrificans (GthGT) and that of its T353A mutant, which represents a mimic of the precursor protein. Data indicate that a) the mutant and the mature GthGT have a different secondary structure content and a slightly different exposure of hydrophobic regions, b) the thermal unfolding processes of both GthGT forms occur through a three-state model, characterized by a stable intermediate species, whereas chemical denaturations proceed through a single transition, c) both GthGT forms exhibit remarkable stability against temperature, but they do not display a strong resistance to the denaturing action of GdnHCl. These findings suggest that electrostatic interactions significantly contribute to the protein stability and that both the precursor and the mature form of GthGT assume compact and stable conformations to resist to the extreme temperatures where G. thermodenidrificans lives. Owing to its thermostability and unique catalytic properties, GthGT is an excellent candidate to be used as a glutaminase in food industry.