A hydrolytic γ-glutamyl transpeptidase from thermo-acidophilic archaeon Picrophilus torridus: binding pocket mutagenesis and transpeptidation

Expression and characterization of C-terminal truncated mutants of γ-glutamyltranspeptidase II (PaGGTII) from Pseudomonas aeruginosa PAO1

The gene encoding γ-glutamyltranspeptidase II (PaGGTII) from Pseudomonas aeruginosa PAO1 was cloned in Escherichia coli. Recombinant PaGGTII showed a weak activity (0.0332 U/mg), and it can be easily inactivated. Multiple alignment of microbial GGTs showed the redundancy of the C-terminal of the small subunit of PaGGTII in length. The truncation of eight amino acid residues at the C-terminal of PaGGTII remarkably improved the activity and stability of the enzyme (PaGGTIIΔ8; 0.388 U/mg). Further truncation at the C-terminal also provided the enzyme relatively higher activity (PaGGTIIΔ9, -Δ10, -Δ11, and -Δ12). Among C-terminal truncated mutants, we focused on PaGGTIIΔ8 and examined the effect of C-terminal amino acid residues on the properties of PaGGTIIΔ8 because the activity of PaGGTII was found to be greatly improved when 8 amino acid residues were truncated. Various mutant enzymes with different C-terminal amino acid residues were constructed. They were expressed in E. coli and purified to homogeneity by ion-exchange chromatography. The properties of PaGGTIIΔ8 and the mutants obtained from mutation at E569 were characterized. K_m and k_cat of PaGGTIIΔ8 for γ-glutamyl-p-nitroanilide (γ-GpNA) were 8.05 mM and 15.49 s^-1, respectively. PaGGTIIΔ8E569Y showed the highest catalytic efficiency for γ-GpNA with a k_cat/Km of 12.55 mM^-1 s^-1. Mg²⁺, Ca²⁺, and Mn²⁺ exhibited positive effects on the catalytic activity for PaGGTIIΔ8 and its ten E569 mutants.

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.

Improvement in salt-tolerance of Aspergillus oryzae γ-glutamyl transpeptidase via protein chimerization with Aspergillus sydowii homolog

γ-Glutamyl transpeptidase is one of the key enzymes involved in glutamate production during high-salt fermentation of soy sauce and miso by koji mold, Aspergillus oryzae. However, the activity of γ-glutamyl transpeptidase from A. oryzae (AOggtA) is markedly reduced in the presence of NaCl, thus classifying it as a non-salt-tolerant enzyme. In contrast, the homologous protein from the xerophilic mold, A. sydowii (ASggtA) maintains its activity under high-salt conditions. Therefore, in this study, a chimeric enzyme, ASAOggtA, was designed and engineered to improve salt-tolerance in AOggtA by swapping the N-terminal region, based on sequence and structure comparisons between salt-tolerant ASggtA and non-salt-tolerant AOggtA. The parental AOggtA and ASggtA and their chimera, ASAOggtA, were heterologously expressed in A. oryzae and purified. The chimeric enzyme inherited the superior activity and stability from each of the two parent enzymes. ASAOggtA showed > 2-fold greater tolerance than AOggtA in the presence of 18% NaCl. In addition, the chimera showed a broader range of pH stability and greater thermostability than ASggtA. AOggtA and ASAOggtA were sy over the range pH 3.0 to pH 10.5. Thermal stability was found to be in the order AOggtA (57.5 °C, t_1/2 = 32.5 min) > ASAOggtA (55 °C, t_1/2 = 20.5 min) > ASggtA (50 °C, t_1/2 = 12.5 min). The catalytic and structural characteristics indicated that non-salt-tolerant AOggtA would not undergo irreversible structural changes in the presence of NaCl, but rather a temporary conformational change, which might result in reducing the substrate binding and catalytic activity, on the basis of kinetic properties. In addition, the chimeric enzyme showed hydrolytic activity toward L-glutamine that was as high as that of AOggtA. The newly-designed chimeric ASAOggtA might have potential applications in high-salt fermentation, such as miso and shoyu, to increase the content of the umami-flavor amino acid, L-glutamate.

Effects of pH and NaCl on hydrolysis and transpeptidation activities of a salt-tolerant γ-glutamyltranspeptidase from Bacillus amyloliquefaciens S0904

Bacillus amyloliquefaciens S0904 was selected as a hyperproducer of a glutamine-hydrolyzing enzyme which was identified as a γ-glutamyltranspeptidase catalyzing both hydrolysis and transpeptidation of glutamyl substrates. The signal peptide-truncated recombinant enzyme (rBAGGT) showed two-fold enhanced specific activity for hydrolysis and optimum pH shift to pH 7 from pH 6 compared with the wild type. The hydrolysis activity of rBAGGT was tolerant against NaCl up to 2.5 M, whereas the transpeptidation activity decreased by NaCl. At pH 6, the addition of 1.5 M NaCl not only enhanced the hydrolysis activity but also inhibited the transpeptidation activity to be ignorable. By contrast, at pH 9 in the absence of NaCl, the alkaline pH-favored transpeptidation activity was 99% of the maximum with only 15% of the maximum hydrolysis activity. In conclusion, the hydrolysis and transpeptidation activities of the recombinant BAGGT is controllable by changing pH and whether or not to add NaCl.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-021-00928-6.

γ-Glutamyl transpeptidase-activatable near-infrared nanoassembly for tumor fluorescence imaging-guided photothermal therapy

Rationale: Precise treatment of tumors is attracting increasing attention. Molecular probes simultaneously demonstrating the diagnostic signal and pharmacological effect in response to tumor microenvironment are highly desired. γ-glutamyl transpeptidase (GGT) is a biomarker with significantly up-regulated expression in the tumor area. We developed a GGT responsive near-infrared (NIR) nanoassembly for tumor-specific fluorescence imaging-guided photothermal therapy. Methods: The GGT responsive NIR probe was constructed by conjugating GGT-specific substrate γ-glutamic acid (γ-Glu) with cyanine fluorophore (NRh-NH2) via amide reaction. The resulting NRh-G spontaneously assembled into nanoparticles (NRh-G-NPs) around 50 nm. The NPs were characterized and the properties evaluated in the presence or absence of GGT. Subsequently, we studied fluorescence imaging and photothermal therapy of NRh-G-NPs in vitro and in vivo. Results: NRh-G-NPs, upon specific reaction with GGT, turned into NRh-NH2-NPs, showing a ~180-fold fluorescence enhancement and excellent photothermal effect recovery. NRh-G-NPs could selectively light up U87MG tumor cells while their fluorescence was weak in L02 human normal liver cells. The NPs also showed excellent tumor cell ablation upon laser irradiation. After intravenous injection into tumor-bearing mice, NRh-G-NPs could arrive in the tumor area and specifically light up the tumor. Following laser irradiation, the tumor could be completely erased with no tumor reoccurrence for up to 40 days. Conclusions: NRh-G-NPs were specifically responsive to GGT overexpressed in U87MG tumor cells and selectively lit up the tumor for imaging-guided therapy. Besides, the recovery of photothermal property in the tumor area could improve cancer therapy precision and decreased side effects in normal tissues.

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.

Glutathione Degradation

SIGNIFICANCE

Glutathione degradation has for long been thought to occur only on noncytosolic pools. This is because there has been only one enzyme known to degrade glutathione (γ-glutamyl transpeptidase) and this localizes to either the plasma membrane (mammals, bacteria) or the vacuolar membrane (yeast, plants) and acts on extracellular or vacuolar pools. The last few years have seen the discovery of several new enzymes of glutathione degradation that function in the cytosol, throwing new light on glutathione degradation. Recent Advances: The new enzymes that have been identified in the last few years that can initiate glutathione degradation include the Dug enzyme found in yeast and fungi, the ChaC1 enzyme found among higher eukaryotes, the ChaC2 enzyme found from bacteria to man, and the RipAY enzyme found in some bacteria. These enzymes play roles ranging from housekeeping functions to stress responses and are involved in processes such as embryonic neural development and pathogenesis.

CRITICAL ISSUES

In addition to delineating the pathways of glutathione degradation in detail, a critical issue is to find how these new enzymes impact cellular physiology and homeostasis.

FUTURE DIRECTIONS

Glutathione degradation plays a far greater role in cellular physiology than previously envisaged. The differential regulation and differential specificities of various enzymes, each acting on distinct pools, can lead to different consequences to the cell. It is likely that the coming years will see these downstream effects being unraveled in greater detail and will lead to a better understanding and appreciation of glutathione degradation. Antioxid. Redox Signal. 27, 1200-1216.

Development of transgenic Brassica juncea lines for reduced seed sinapine content by perturbing phenylpropanoid pathway genes

Sinapine is a major anti-nutritive compound that accumulates in the seeds of Brassica species. When ingested, sinapine imparts gritty flavuor in meat and milk of animals and fishy odor to eggs of brown egg layers, thereby compromising the potential use of the valuable protein rich seed meal. Sinapine content in Brassica juncea germplasm ranges from 6.7 to 15.1 mg/g of dry seed weight (DSW) which is significantly higher than the prescribed permissible level of 3.0 mg/g of DSW. Due to limited natural genetic variability, conventional plant breeding approach for reducing the sinapine content has largely been unsuccessful. Hence, transgenic approach for gene silencing was adopted by targeting two genes-SGT and SCT, encoding enzymes UDP- glucose: sinapate glucosyltransferase and sinapoylglucose: choline sinapoyltransferase, respectively, involved in the final two steps of sinapine biosynthetic pathway. These two genes were isolated from B. juncea and eight silencing constructs were developed using three different RNA silencing approaches viz. antisense RNA, RNAi and artificial microRNA. Transgenics in B. juncea were developed following Agrobacterium-mediated transformation. From a total of 1232 independent T0 transgenic events obtained using eight silencing constructs, 25 homozygous lines showing single gene inheritance were identified in the T2 generation. Reduction of seed sinapine content in these lines ranged from 15.8% to 67.2%; the line with maximum reduction had sinapine content of 3.79 mg/g of DSW. The study also revealed that RNAi method was more efficient than the other two methods used in this study.

Evolving transpeptidase and hydrolytic variants of γ-glutamyl transpeptidase from Bacillus licheniformis by targeted mutations of conserved residue Arg109 and their biotechnological relevance

γ-Glutamyl transpeptidase (GGT) catalyzes the transfer of the γ-glutamyl moiety from donor compounds such as l-glutamine (Gln) and glutathione (GSH) to an acceptor. During the biosynthesis of various γ-glutamyl-containing compounds using GGT enzyme, auto-transpeptidation reaction leads to the formation of unwanted byproducts. Therefore, in order to alter the auto-transpeptidase activity of the GGT enzyme, the binding affinity of Gln should be modified. Structural studies of the Bacillus licheniformis GGT (BlGT) complexed with the glutamic acid has shown that glutamic acid has strong ionic interactions through its α-carboxlic group with the guanidine moiety of Arg109. This interaction appears to be an important contributor for the binding affinity of Gln. In view of this, six mutants of Bacillus licheniformis ER15 GGT (BlGGT) viz. Arg109Lys, Arg109Ser, Arg109Met, Arg109Leu, Arg109Glu and Arg109Phe were prepared. As seen from the structure of BlGT, the mutation of Arg109 to Lys109 may reduce the affinity for Gln to some extent, whereas the other mutations are expected to lower the affinity much more. Biophysical characterization and functional studies revealed that Arg109Lys mutant has increased transpeptidation activity and catalytic efficiency than the other mutants. The Arg109Lys mutant showed high conversion rates for l-theanine synthesis as well. Moreover, the Arg109Met mutant showed increased hydrolytic activity as it completely altered the binding of Gln at the active site. Also, the salt stability of the enzyme was significantly improved on replacing Arg109 by Met109 which is required for hydrolytic applications of GGTs in food industries.

Characterization of group II chaperonins from an acidothermophilic archaeon Picrophilus torridus

Chaperonins are a type of molecular chaperone that assist in the folding of proteins. Group II chaperonins play an important role in the proteostasis in the cytosol of archaea and eukarya. In this study, we expressed, purified, and characterized group II chaperonins from an acidothermophilic archaeon Picrophilus torridus. Two genes exist for group II chaperonins, and both of the gene products assemble to form double‐ring complexes similar to other archaeal group II chaperonins. One of the Picrophilus chaperonins, PtoCPNα, was able to refold denatured GFP at 50 °C. As expected, PtoCPNα exhibited an ATP‐dependent conformational change that is observed by the change in fluorescence and diffracted X‐ray tracking (DXT). In contrast, PtoCPNα lost its protein folding ability at moderate temperatures, becoming unable to interact with unfolded proteins. At lower temperatures, the release rate of the captured GFP from PtoCPNα was accelerated, and the affinity of denatured protein to PtoCPNα was weakened at the lower temperatures. Unexpectedly, in the DXT experiment, the fine motions were enhanced at the lower temperatures. Taken together, the results suggest that the fine tilting motions of the apical domain might correlate with the affinity of group II chaperonins for denatured proteins.

"Phylogenetic and evolutionary analysis of functional divergence among Gamma glutamyl transpeptidase (GGT) subfamilies"

Background

γ-glutamyltranspeptidase (GGT) is a bi-substrate enzyme conserved in all three domains of life. It catalyzes the cleavage and transfer of γ-glutamyl moiety of glutathione to either water (hydrolysis) or substrates like peptides (transpeptidation). GGTs exhibit great variability in their enzyme kinetics although the mechanism of catalysis is conserved. Recently, GGT has been shown to be a virulence factor in microbes like Helicobacter pylori and Bacillus anthracis. In mammalian cells also, GGT inhibition prior to chemotherapy has been shown to sensitize tumors to the therapy. Therefore, lately both bacterial and eukaryotic GGTs have emerged as potential drug targets, but the efforts directed towards finding suitable inhibitors have not yielded any significant results yet. We propose that delineating the residues responsible for the functional diversity associated with these proteins could help in design of species/clade specific inhibitors.

Results

In the present study, we have carried out phylogenetic analysis on a set of 47 GGT-like proteins to address the functional diversity. These proteins segregate into various subfamilies, forming separate clades on the tree. Sequence conservation and motif prediction studies show that even though most of the highly conserved residues have been characterized biochemically in previous studies, a significant number of novel putative sites and motifs are discovered that vary in a clade specific manner. Many of the putative sites predicted during the functional divergence type I and type II analysis, lie close to the known catalytic residues and line the walls of the substrate binding cavity, reinforcing their role in modulating the substrate specificity, catalytic rates and stability of this protein.

Conclusion

The study offers interesting insights into the evolution of GGT-like proteins in pathogenic vs. non-pathogenic bacteria, archaea and eukaryotes. Our analysis delineates residues that are highly specific to each GGT subfamily. We propose that these sites not only explain the differences in stability and catalytic variability of various GGTs but can also aid in design of specific inhibitors against particular GGTs. Thus, apart from the commonly used in-silico inhibitor screening approaches, evolutionary analysis identifying the functional divergence hotspots in GGT proteins could augment the structure based drug design approaches.

Reviewers

This article was reviewed by Andrei Osterman, Christine Orengo, and Srikrishna Subramanian. For complete reports, see the Reviewers’ reports section

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-015-0080-7) contains supplementary material, which is available to authorized users.

L-theanine synthesis using γ-glutamyl transpeptidase from Bacillus licheniformis ER-15

Recombinant γ-glutamyl transpeptidase (rBLGGT) from Bacillus licheniformis ER-15 was purified to homogeneity by ion-exchange chromatography. Molecular masses of large and small subunits were 42 and 22 kDa, respectively. The enzyme was optimally active at pH 9.0 and 60 °C and was alkali stable. K(m) and V(max) for γ-glutamyl-p-nitroanilide hydrochloride were 45 μM and 0.34 mM/min, respectively. L-Theanine synthesis was standardized using a one variable at a time approach followed by response surface methodology, which resulted in approximately 85-87% conversion of L-glutamine to L-theanine within 4 h. The standardized reaction contained 80 mM L-glutamine, 600 mM ethylamine, and 1.0 U/mL rBLGGTin 50 mM Tris-Cl (pH 9.0) at 37 °C. Similar conversions were also obtained with the enzyme immobilized in calcium alginate. Using immobilized enzyme, 35.2 g of L-theanine was obtained in three cycles of 1 L each. The product was purified by Dowex 50W X 8 hydrogen form resin and was confirmed by HPLC and proton NMR spectroscopy.

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.