Autocatalytic processing of gamma-glutamyltranspeptidase

Characterization of three γ-glutamyltranspeptidases from Pseudomonas aeruginosa PAO1

The Pseudomonas aeruginosa strain, PAO1, has three putative γ-glutamyltranspeptidase (GGT) genes: ggtI, ggtII, and ggtIII. In this study, the expression of each of these genes in P. aeruginosa PAO1 was analyzed, and the properties of the corresponding GGT proteins were investigated. This is the first report on biochemical characterization of GGT paralogs from Pseudomonas species. The crude extracts prepared from P. aeruginosa PAO1 exhibited hydrolysis and transpeptidation activities of 17.3 and 65.0 mU/mg, respectively, and the transcription of each gene to mRNA was confirmed by RT-PCR. All genes were cloned, and the expression plasmids constructed were introduced into an Escherichia coli expression system. Enzyme activity of the expressed protein of ggtI (PaGGTI) was not detected in the system, while the enzyme activities of the expressed proteins derived from ggtII and ggtIII (PaGGTII and PaGGTIII, respectively) were detected. However, the enzyme activity of PaGGTII was very low and easily decreased. PaGGTII with C-terminal his-tag (PaGGTII25aa) showed increased activity and stability, and the purified enzyme consisted of a large subunit of 40 kDa and a small subunit of 28 kDa. PaGGTIII consisted of a large subunit of 37 kDa and a small subunit of 24 kDa. The maximum hydrolysis and transpeptidation activities of PaGGTII25aa were obtained at 40ºC-50ºC, and the maximum hydrolysis and transpeptidation activities of PaGGTIII were obtained at 50ºC-60ºC. These enzymes retained approximately 80% of their hydrolysis and transpeptidation activities after incubation at 50ºC for 10 min, reflecting good stability. Both PaGGTII25aa and PaGGTIII showed higher activities of hydrolysis and transpeptidation in the alkali range than in the acidic range. However, they were highly stable at a wide pH range (5-10.5).

Targeting gamma-glutamyl transpeptidase: A pleiotropic enzyme involved in glutathione metabolism and in the control of redox homeostasis

Gamma-glutamyl transpeptidase (GGT) is an enzyme located on the outer membrane of the cells where it regulates the metabolism of glutathione (GSH), the most abundant intracellular antioxidant thiol. GGT plays a key role in the control of redox homeostasis, by hydrolyzing extracellular GSH and providing the cell with the recovery of cysteine, which is necessary for de novo intracellular GSH and protein biosynthesis. Therefore, the upregulation of GGT confers to the cell greater resistance to oxidative stress and the advantage of growing fast. Indeed, GGT is upregulated in inflammatory conditions and in the progression of various human tumors and it is involved in many physiological disorders related to oxidative stress, such as cardiovascular disease and diabetes. Currently, increased GGT expression is considered a marker of liver damage, cancer, and low-grade chronic inflammation. This review addresses the current knowledge on the structure-function relationship of GGT, focusing on human GGT, and provides information on the pleiotropic biological role and relevance of the enzyme as a target of drugs aimed at alleviating oxidative stress-related diseases. The development of new GGT inhibitors is critically discussed, as are the advantages and disadvantages of their potential use in clinics. Considering its pleiotropic activities and evolved functions, GGT is a potential "moonlighting protein".

The glutathione import system satisfies the Staphylococcus aureus nutrient sulfur requirement and promotes interspecies competition

Sulfur is an indispensable element for bacterial proliferation. Prior studies demonstrated that the human pathogen Staphylococcus aureus utilizes glutathione (GSH) as a source of nutrient sulfur; however, mechanisms of GSH acquisition are not defined. Here, we identify a five-gene locus comprising a putative ABC-transporter and predicted γ-glutamyl transpeptidase (ggt) that promotes S. aureus proliferation in medium supplemented with either reduced or oxidized GSH (GSSG) as the sole source of nutrient sulfur. Based on these phenotypes, we name this transporter operon the glutathione import system (gisABCD). Ggt is encoded within the gisBCD operon, and we show that the enzyme is capable of liberating glutamate using either GSH or GSSG as substrates, demonstrating it is a bona fide γ-glutamyl transpeptidase. We also determine that Ggt is expressed in the cytoplasm, representing only the second example of cytoplasmic Ggt localization, the other being Neisseria meningitidis. Bioinformatic analyses revealed that Staphylococcus species closely related to S. aureus encode GisABCD-Ggt homologs. However, homologous systems were not detected in Staphylococcus epidermidis. Consequently, we establish that GisABCD-Ggt provides a competitive advantage for S. aureus over S. epidermidis in a GSH- and GSSG-dependent manner. Overall, this study describes the discovery of a nutrient sulfur acquisition system in S. aureus that targets GSSG in addition to GSH and promotes competition against other staphylococci commonly associated with the human microbiota.

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.

First Example of the Extracellular Surface Expression of Intrinsically Periplasmic Escherichia coli γ-Glutamyltranspeptidase, a Member of the N-Terminal Nucleophile Hydrolase Superfamily, and the Use of Cells as a Catalyst for γ-Glutamylvalylglycine Production

Although the purified Escherichia coli γ-glutamyltranspeptidase has much higher transpeptidation activity than hydrolysis activity, almost all γ-glutamyltranspeptidase activity is hydrolysis activity in vivo, that is when measured using the whole cells. By using the Met1 to Arg232 fragment of E. coli YiaT or the CapA of Bacillus subtilis subsp. Natto as an anchor protein, we succeeded in expressing E. coli γ-glutamyltranspeptidase on the extracellular surface of the cells, and these cells showed higher transpeptidation activity than hydrolysis activity in the presence of NaCl. Furthermore, E. coli cells overexpressing γ-glutamyltranspeptidase without an anchor from the T5 promoter maintained γ-glutamyltranspeptidase on the extracellular surface of the cells immediately after being harvested from the culture medium, but the enzyme was released from the extracellular surface of the cells subsequently in the absence of NaCl. Using these cells expressing γ-glutamyltranspeptidase on the extracellular surface, γ-Glu-Val-Gly, a kokumi compound, was successfully produced.

Protein Hydrolysates from Pleurotus geesteranus Modified by Bacillus amyloliquefaciens γ-Glutamyl Transpeptidase Exhibit a Remarkable Taste-Enhancing Effect

Long-term high salt intake exerts a negative impact on human health. The excessive use of sodium substitutes in the food industry can lead to decreased sensory quality of food. γ-Glutamyl peptides with pronounced taste-enhancing effects can offer an alternative approach to salt reduction. However, the content and yield of γ-glutamyl peptides in natural foods are relatively low. Enzyme-catalyzed synthesis of γ-glutamyl peptides provides a feasible solution. In this study, Pleurotus geesteranus was hydrolyzed by Flavourzyme to generate protein hydrolysates. Subsequently, they were modified by Bacillus amyloliquefaciens γ-glutamyl transpeptidase to generate γ-glutamyl peptides. The reaction conditions were optimized and their taste-enhancing effects were evaluated. Their peptide sequences were identified by parallel reaction monitoring with liquid chromatography-tandem mass spectrometry and analyzed using molecular docking. The optimal conditions for generation of γ-glutamyl peptides were a pH of 10.0, an enzyme condition of 1.2 U/g, and a reaction time of 2 h, which can elicit a strong kokumi taste. Notably, it exhibited a remarkable taste-enhancing effect for umami intensity (76.07%) and saltiness intensity (1.23-fold). Several novel γ-glutamyl peptide sequences were found by liquid chromatography-tandem mass spectrometry, whereas the binding to the calcium-sensing receptor was confirmed by molecular docking analysis. Overall, γ-glutamyl peptides from P. geesteranus could significantly enhance the umami and salt tastes, which can serve as promising taste enhancers.

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.

Expression of gamma-glutamyltransferase 1 in glioblastoma cells confers resistance to cystine deprivation-induced ferroptosis

Ferroptosis is an iron-dependent mode of cell death caused by excessive oxidative damage to lipids. Lipid peroxidation is normally suppressed by glutathione peroxidase 4, which requires reduced glutathione. Cystine is a major resource for glutathione synthesis, especially in cancer cells. Therefore, cystine deprivation or inhibition of cystine uptake promotes ferroptosis in cancer cells. However, the roles of other molecules involved in cysteine deprivation–induced ferroptosis are unexplored. We report here that the expression of gamma-glutamyltransferase 1 (GGT1), an enzyme that cleaves extracellular glutathione, determines the sensitivity of glioblastoma cells to cystine deprivation–induced ferroptosis at high cell density (HD). In glioblastoma cells expressing GGT1, pharmacological inhibition or deletion of GGT1 suppressed the cell density–induced increase in intracellular glutathione levels and cell viability under cystine deprivation, which were restored by the addition of cysteinylglycine, the GGT product of glutathione cleavage. On the other hand, cystine deprivation induced glutathione depletion and ferroptosis in GGT1-deficient glioblastoma cells even at an HD. Exogenous expression of GGT1 in GGT1-deficient glioblastoma cells inhibited cystine deprivation–induced glutathione depletion and ferroptosis at an HD. This suggests that GGT1 plays an important role in glioblastoma cell survival under cystine-limited and HD conditions. We conclude that combining GGT inhibitors with ferroptosis inducers may provide an effective therapeutic approach for treating glioblastoma.

An overall framework for the E. coli γ-glutamyltransferase-catalyzed transpeptidation reactions

γ-Glutamyl derivatives of proteinogenic or modified amino acids raise considerable interest as flavor enhancers or biologically active compounds. However, their supply, on a large scale and at reasonable costs, remains challenging. Enzymatic synthesis has been recognized as a possible affordable alternative with respect to both isolation procedures from natural sources, burdened by low-yield and by the requirement of massive amount of starting material, and chemical synthesis, inconvenient because of the need of protection/deprotection steps. The E. coli γ-glutamyltransferase (Ec-GGT) has already been proposed as a biocatalyst for the synthesis of various γ-glutamyl derivatives. However, enzymatic syntheses using this enzyme usually provide the desired products in limited yield. Hydrolysis and autotranspeptidation of the donor substrate have been identified as the side reactions affecting the final yield of the catalytic process. In addition, experimental conditions need to be specifically adjusted for each acceptor substrate. Substrate specificity and the fine characterization of the activities exerted by the enzyme over time has so far escaped rationalization. In this work, reactions catalyzed by Ec-GGT between the γ-glutamyl donor glutamine and several representative acceptor amino acids have been finely analyzed with the identification of single reaction products over time. This approach allowed to rationalize the effect of donor/acceptor molar ratio on the outcome of the transpeptidation reaction and on the distribution of the different byproducts, inferring a general scheme for Ec-GGT-catalyzed reactions. The propensity to react of the different acceptor substrates is in agreement with recent findings obtained using model substrates and further supported by x-ray crystallography and will contribute to characterize the still elusive acceptor binding site of the enzyme.

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.

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 essential for the metabolism of γ-glutamyl compounds in bacteria and its application

The enzymatic characteristics of γ-glutamyltranspeptidase were elucidated. The catalytic nucleophile of the enzymatic reaction of Escherichia coli γ-glutamyltranspeptidase was identified as the Oγ of the N-terminal Thr-residue of the small subunit. It was demonstrated that the inactive precursor of γ-glutamyltranspeptidase is processed autocatalytically and intramolecularly into the active heterodimeric mature enzyme via an ester intermediate. The catalytic nucleophile of this processing reaction was identified as the same Oγ atom of the N-terminal Thr-residue of the small subunit. These results were also supported by the three-dimensional structures of the γ-glutamyl enzyme intermediate and of the precursor-mimicked T391A nonprocessable mutant enzyme. Applications of transpeptidation and hydrolysis activities of bacterial γ-glutamyltranspeptidases were developed. Using transpeptidation activity, efficient enzymatic production of useful γ-glutamyl compounds, such as prodrug for Parkinson's disease, theanine and kokumi compound, was enabled. Hydrolysis activity was used as glutaminase and the mutant enzymes gaining glutaryl-7-aminocephalosporanic acid acylase activity were isolated.

Simulating Multiple Substrate-Binding Events by γ-Glutamyltransferase Using Accelerated Molecular Dynamics

γ-Glutamyltransferase (GGT) is an enzyme that uses γ-glutamyl compounds as substrates and catalyzes their transfer to a water molecule or an acceptor substrate with varied physiological function in bacteria, plants, and animals. Crystal structures of GGT are known for different species and in different states of the chemical reaction; however, the structural dynamics of the substrate binding to the catalytic site of GGT are unknown. Here, we modeled Escherichia coli GGT's glutamine binding by using a swarm of accelerated molecular dynamics (aMD) simulations. Characterization of multiple binding events identified three structural binding motifs composed of polar residues in the binding pocket that govern glutamine binding into the active site. Simulated open and closed conformations of a lid-loop protecting the binding cavity suggest its role as a gating element by allowing or blocking substrates entry into the binding pocket. Partially open states of the lid-loop are accessible within thermal fluctuations, while the estimated free energy cost of a complete open state is 2.4 kcal/mol. Our results suggest that both specific electrostatic interactions and GGT conformational dynamics dictate the molecular recognition of substrate-GGT complexes.

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.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

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.

In vitro synergistic effects of three enzymes from Bacillus subtilis CH-1 on keratin decomposition

Extracellular protease Vpr (Vpr), gamma-glutamyltranspeptidase (GGT; EC 2.3.2.2) and glyoxal/methylglyoxal reductase (YvgN; EC 1.1.1.21) are extracellular enzymes involved in feather degradation, which were identified by secretome analyses from an efficient feather-degrading strain Bacillus subtilis CH-1. The encoding sequences corresponding to the three secretory enzymes were cloned into vector pET22b for recombinant expression in Escherichia coli strain BL21 (DE3). Afterward, the proteins containing the C-terminal His-tag were purified using a Ni-IDA column. The optimal temperatures and pH values for protease activity of recombinant Vpr, GGT, and YvgN were identified as 45 °C/pH 7.0, 40 °C/pH 8.0, and 50 °C/pH 6.0 respectively when casein is the substrate. Furthermore, the synergistic effects of the three enzymes were studied using feather powder as substrate. Vpr was the core enzyme to hydrolyze keratin, while GGT and YvgN were coenzymes providing reducing activities for keratin decomposition. The keratinolytic activity was enhanced to about 1.4-folds when YvgN and Vpr applied together in comparison to Vpr alone. And the keratinolytic activity almost reached to 1.5-folds when all the three enzymes were combined to use. The study provides a novel perspective of the mechanism of keratin degradation by microorganisms, and thereby may also be of relevance for the design of an industrial process for enzymatic keratin degradation; however, additional experiments must be done to substantiate this conclusion.

Bacterial γ-glutamyltranspeptidases, physiological function, structure, catalytic mechanism and application

γ-Glutamyltranspeptidase (GGT) has been widely used as a marker enzyme of hepatic and biliary diseases and relations between various diseases and its activity have been studied extensively. Nevertheless, several of its fundamental enzymatic characteristics had not been elucidated. We obtained homogeneous preparation of GGTs from bacteria, characterized them, and elucidated its physiological function that is common to mammalian cells, using GGT-deficient E. coli. Prior to GGT of all living organisms, we also identified catalytic nucleophile of E. coli GGT and revealed the post-translational processing mechanism for its maturation, and also its crystal structure was determined. The reaction intermediate was trapped and the structure-based reaction mechanism was presented. As for its application, using its transferase activity, we developed the enzymatic synthesis of various γ-glutamyl compounds that are promising in food, nutraceutical and medicinal industries. We found GGT of Bacillus subtilis is salt-tolerant and can be used as a glutaminase, which is important in food industry, to enhance umami of food, such as soy sauce and miso. We succeeded in converting bacterial GGT to glutaryl-7-aminocephalosporanic acid acylase, which is an important enzyme in cephem antibiotics production, by site-directed and random mutagenesis.

Functional Characterization of the Grapevine γ-Glutamyl Transferase/Transpeptidase (E.C. 2.3.2.2) Gene Family Reveals a Single Functional Gene Whose Encoded Protein Product Is Not Located in Either the Vacuole or Apoplast

γ-glutamyl transferases/transpeptidases (E.C. 2.3.2.2, GGTs) are involved in the catabolism of many compounds that are conjugated to glutathione (GSH), which have a variety of roles. GSH can act as storage and transport vehicle for reduced sulfur; it is involved in the detoxification of xenobiotics and also acts as a redox buffer by utilizing its thiol residue to protect against reactive oxygen species, which accumulate in response to biotic and abiotic stress. Furthermore, many distinctive flavor and aroma compounds in Sauvignon blanc wines originate from odorless C5- and C6-GSH conjugates or their GGT catabolized derivatives. These precursors are then processed into their volatile forms by yeast during fermentation. In many plant species, two or more isoforms of GGTs exist that target GSH-conjugates to either the apoplast or the vacuole. A bioinformatics approach identified multiple GGT candidates in grapevine (Vitis vinifera). However, only a single candidate, VvGGT3, has all the conserved residues needed for GGT activity. This is intriguing given the variety of roles of GSH and GGTs in plant cells. Characterization of VvGGT3 from cv. Sauvignon blanc was then undertaken. The VvGGT3 transcript is present in roots, leaves, inflorescences, and tendril and at equal abundance in the skin, pulp, and seed of mature berries and shows steady accumulation over the course of whole berry development. In addition, the VvGGT3 transcript in whole berries is upregulated upon Botrytis cinerea infection as well as mechanical damage to leaf tissue. VvGGT3-GFP fusion proteins transiently over-expressed in onion cells were used to study subcellular localization. To confirm VvGGT3 activity and localization in vivo, the fluorescent γ-glutamyl-7-amido-4-methylcoumarin substrate was added to Nicotiana benthamiana leaves transiently over-expressing VvGGT3. In combination, these results suggest that the functional VvGGT3 is associated with membrane-like structures. This is not consistent with its closely related functionally characterized GGTs from Arabidopsis, radish and garlic.

Seasonal Theanine Accumulation and Related Gene Expression in the Roots and Leaf Buds of Tea Plants (Camellia Sinensis L.)

Theanine, a unique and abundant non-proteinogenic amino acid in tea, confers to the tea infusion its umami taste and multiple health benefits. Its content in new tea shoots is dynamic in winter and spring. However, its seasonal accumulation pattern and the underlying regulation mechanism of tea plants remain largely unknown. In this study, we measured the theanine contents in the roots and leaf buds of 13 tea cultivars at four time points from winter to spring (Dec. 12, Mar. 1, Mar. 23, and Apr. 13). We found theanine accumulated significantly in the roots to as high as ∼6% dry weight. We found theanine content in the roots was constant or slightly decreased on Mar. 1 compared with Dec.12 but increased consistently on Mar. 23 and then decreased on Apr. 13 in all 13 cultivars. In the leaf buds, theanine content kept increasing from Mar. 1 to Mar. 23 and decreasing from Apr. 13 in most of the 13 cultivars, meaning it was probably both season- and developmental stage-dependent. The expression of theanine biosynthesis and amino acid transporter genes in the roots and buds at the four time points was then examined. The correlation analyses between the gene expression and theanine content suggested the expression of theanine-biosynthesis genes was generally and negatively correlated with theanine content; however, the expression of amino acid transporter genes including CsLHT was generally and positively correlated with theanine contents. Finally, we showed that CsLHT has theanine transport activity. Taken together, this study provided insight into the seasonal regulation of theanine biosynthesis and transport in tea plants during winter and spring.

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.

Effect of the inserted active-site-covering lid loop on the catalytic activity of a mutant B. subtilis γ-glutamyltransferase (GGT)

γ-Glutamylpeptides are compounds derived from the acylation of an amino acid or a short peptide by the γ-carboxyl carbon of the side chain of glutamic acid. Due to their altered chemico-physical and organoleptic properties, they may be interesting substitutes or precursors of parent compounds used in pharmaceutical, dietetic and cosmetic formulations. Some of them are naturally occurring flavor enhancers or are endowed with biological activities. Enzymatic approaches to the synthesis of γ-glutamyl derivatives based on the use of γ-glutamyltransferases (GGTs, EC 2.3.2.2) have been proposed, which should be able to alleviate the problems connected with the troublesome and low-yielding extraction from natural sources or the non-economical chemical synthesis, which requires protection/deprotection steps. With the aim of overcoming the current limitations in the use of GGTs as biocatalysts, a mutant GGT was investigated. The mutant GGT was obtained by inserting the active-site-covering lid loop of the E. coli GGT onto the structure of B. subtilis GGT. With respect to the wild-type enzyme, the mutant showed a more demanding substrate specificity and a low hydrolase activity. These results represent an attempt to correlate the structural features of a GGT to its different activities. However, the ability of the mutant enzyme to catalyze the subsequent addition of several γ-glutamyl units, inherited by the parent B. subtilis GGT, still represents a limitation to its full application as a biocatalyst for preparative purposes.

A mutant γ-glutamyltransferase with improve transpeptidase activity was obtained by inserting the active site-covering lid loop on an enzyme naturally lacking it.

New Method To Produce Kokumi Seasoning from Protein Hydrolysates Using Bacterial Enzymes

In the present study, we demonstrate a novel use for a commercially available glutaminase that can be used as a γ-glutamyltranspeptidase in kokumi seasoning production. Soy protein and gluten were hydrolyzed using a protease isolated from Bacillus licheniformis. The resulting protein hydrolysates were γ-glutamylated with a γ-glutamyltranspeptidase, which is sold as a glutaminase from Bacillus amyloliquefaciens, to produce kokumi seasonings. For γ-glutamylation of soy protein hydrolysate, glutamine was added to the reaction mixture. On the other hand, reaction conditions for enzymatic proteolysis were optimized to liberate glutamine from gluten in large amounts, and the addition of glutamine was not required for γ-glutamylation of gluten hydrolysate. The soy protein and gluten hydrolysates as well as their γ-glutamylated products were subjected to taste evaluation. Soy protein hydrolysates were bitter. Although γ-glutamylation significantly reduced bitterness, the taste was still considered unfavorable. γ-Glutamylated gluten hydrolysate is the most preferable sample and had significantly enhanced thickness, kokumi, and umami tastes, with a moderate increase in saltiness.

Functional role of the conserved glycine residues, Gly481 and Gly482, of the γ-glutamyltranspeptidase from Bacillus licheniformis

Six mutants bearing single amino acid substitutions in the small subunit of Bacillus licheniformis γ-glutamyltranspeptudase (BlGGT) have been constructed by site-directed mutagenesis. The resultant enzymes were overexpressed in Escherichia coli and purified by affinity chromatography for biochemical and biophysical characterizations. Replacing Gly481 by either Ala or Glu did affect both autocatalytic processing and catalytic activity of the enzyme, but the substitution of this residue to arginine resulted in an unprocessed enzyme with insignificant catalytic activity. The replacement of another conserved glycine residue, Gly482, by either Ala or Glu caused a significant change in the functional integrity of the enzyme. Moreover, the mutation of Gly482 to arginine led to a marked reduction in the autocatalytic processing. Structural analyses revealed that the fluorescence and circular dichroism properties of mutant proteins were basically consistent with those of BlGGT. However, guanidine hydrochloride (GdnHCl)-induced transitions of most mutants were profoundly reduced in comparison with that of wild-type enzyme. Molecular modeling suggests that the conserved Gly481 and Gly482 residues of BlGGT are located at critical positions to create an environment suitable for both autoprocessing and catalytic reactions.

High Resolution X-ray Diffraction Dataset for Bacillus licheniformis Gamma Glutamyl Transpeptidase-acivicin complex: SUMO-Tag Renders High Expression and Solubility

Gamma glutamyl transpeptidase, (GGT) is a ubiquitous protein which plays a central role in glutathione metabolism and has myriad clinical implications. It has been shown to be a virulence factor for pathogenic bacteria, inhibition of which results in reduced colonization potential. However, existing inhibitors are effective but toxic and therefore search is on for novel inhibitors, which makes it imperative to understand the interactions of various inhibitors with the protein in substantial detail. High resolution structures of protein bound to different inhibitors can serve this purpose. Gamma glutamyl transpeptidase from Bacillus licheniformis is one of the model systems that have been used to understand the structure-function correlation of the protein. The structures of the native protein (PDB code 4OTT), of its complex with glutamate (PDB code 4OTU) and that of its precursor mimic (PDB code 4Y23) are available, although at moderate/low resolution. In the present study, we are reporting the preliminary analysis of, high resolution X-ray diffraction data collected for the co-crystals of B. licheniformis, Gamma glutamyl transpeptidase, with its inhibitor, Acivicin. Crystals belong to the orthorhombic space group P2₁2₁2₁ and diffract X-ray to 1.45 Å resolution. This is the highest resolution data reported for all GGT structures available till now. The use of SUMO fused expression system enhanced yield of the target protein in the soluble fraction, facilitating recovery of protein with high purity. The preliminary analysis of this data set shows clear density for the inhibitor, acivicin, in the protein active site.

γ-Glutamyltranspeptidase is an endogenous activator of Toll-like receptor 4-mediated osteoclastogenesis

Chronic inflammation-associated bone destruction, which is observed in rheumatoid arthritis (RA) and periodontitis, is mediated by excessive osteoclastogenesis. We showed previously that γ-glutamyltranspeptidase (GGT), an enzyme involved in glutathione metabolism, acts as an endogenous activator of such pathological osteoclastogenesis, independent of its enzymatic activity. GGT accumulation is clinically observed in the joints of RA patients, and, in animals, the administration of recombinant GGT to the gingival sulcus as an in vivo periodontitis model induces an increase in the number of osteoclasts. However, the underlying mechanisms of this process remain unclear. Here, we report that Toll-like receptor 4 (TLR4) recognizes GGT to activate inflammation-associated osteoclastogenesis. Unlike lipopolysaccharide, GGT is sensitive to proteinase K treatment and insensitive to polymyxin B treatment. TLR4 deficiency abrogates GGT-induced osteoclastogenesis and activation of NF-κB and MAPK signaling in precursor cells. Additionally, GGT does not induce osteoclastogenesis in cells lacking the signaling adaptor MyD88. The administration of GGT to the gingival sulcus induces increased osteoclastogenesis in wild-type mice, but does not induce it in TLR4-deficient mice. Our findings elucidate a novel mechanism of inflammation-associated osteoclastogenesis, which involves TLR4 recognition of GGT and subsequent activation of MyD88-dependent signaling.

Phosphonate-based irreversible inhibitors of human γ-glutamyl transpeptidase (GGT). GGsTop is a non-toxic and highly selective inhibitor with critical electrostatic interaction with an active-site residue Lys562 for enhanced inhibitory activity

γ-Glutamyl transpeptidase (GGT, EC 2.3.2.2) that catalyzes the hydrolysis and transpeptidation of glutathione and its S-conjugates is involved in a number of physiological and pathological processes through glutathione metabolism and is an attractive pharmaceutical target. We report here the evaluation of a phosphonate-based irreversible inhibitor, 2-amino-4-{[3-(carboxymethyl)phenoxy](methoyl)phosphoryl}butanoic acid (GGsTop) and its analogues as a mechanism-based inhibitor of human GGT. GGsTop is a stable compound, but inactivated the human enzyme significantly faster than the other phosphonates, and importantly did not inhibit a glutamine amidotransferase. The structure-activity relationships, X-ray crystallography with Escherichia coli GGT, sequence alignment and site-directed mutagenesis of human GGT revealed a critical electrostatic interaction between the terminal carboxylate of GGsTop and the active-site residue Lys562 of human GGT for potent inhibition. GGsTop showed no cytotoxicity toward human fibroblasts and hepatic stellate cells up to 1mM. GGsTop serves as a non-toxic, selective and highly potent irreversible GGT inhibitor that could be used for various in vivo as well as in vitro biochemical studies.

Biochemical and structural characterization of Klebsiella pneumoniae oxamate amidohydrolase in the uric acid degradation pathway

HpxW from the ubiquitous pathogen Klebsiella pneumoniae is involved in a novel uric acid degradation pathway downstream from the formation of oxalurate. Specifically, HpxW is an oxamate amidohydrolase which catalyzes the conversion of oxamate to oxalate and is a member of the Ntn-hydrolase superfamily. HpxW is autoprocessed from an inactive precursor to form a heterodimer, resulting in a 35.5 kDa α subunit and a 20 kDa β subunit. Here, the structure of HpxW is presented and the substrate complex is modeled. In addition, the steady-state kinetics of this enzyme and two active-site variants were characterized. These structural and biochemical studies provide further insight into this class of enzymes and allow a mechanism for catalysis consistent with other members of the Ntn-hydrolase superfamily to be proposed.

The maturation mechanism of γ-glutamyl transpeptidases: Insights from the crystal structure of a precursor mimic of the enzyme from Bacillus licheniformis and from site-directed mutagenesis studies

γ-Glutamyl transpeptidases (γ-GTs) are members of N-terminal nucleophile hydrolase superfamily. They are synthetized as single-chain precursors, which are then cleaved to form mature enzymes. Basic aspects of autocatalytic processing of these pro-enzymes are still unknown. Here we describe the X-ray structure of the precursor mimic of Bacillus licheniformis γ-GT (BlGT), obtained by mutating catalytically important threonine to alanine (T399A-BlGT), and report results of autoprocessing of mutants of His401, Thr415, Thr417, Glu419 and Arg571. Data suggest that Thr417 is in a competent position to activate the catalytic threonine (Thr399) for nucleophilic attack of the scissile peptide bond and that Thr415 plays a major role in assisting the process. On the basis of these new structural results, a possible mechanism of autoprocessing is proposed. This mechanism, which guesses the existence of a six-membered transition state involving one carbonyl and two hydroxyl groups, is in agreement with all the available experimental data collected on γ-GTs from different species and with our new Ala-scanning mutagenesis data.

Emerging regulatory paradigms in glutathione metabolism

One of the hallmarks of cancer is the ability to generate and withstand unusual levels of oxidative stress. In part, this property of tumor cells is conferred by elevation of the cellular redox buffer glutathione. Though enzymes of the glutathione synthesis and salvage pathways have been characterized for several decades, we still lack a comprehensive understanding of their independent and coordinate regulatory mechanisms. Recent studies have further revealed that overall central metabolic pathways are frequently altered in various tumor types, resulting in significant increases in biosynthetic capacity and feeding into glutathione synthesis. In this review, we will discuss the enzymes and pathways affecting glutathione flux in cancer and summarize current models for regulating cellular glutathione through both de novo synthesis and efficient salvage. In addition, we examine the integration of glutathione metabolism with other altered fates of intermediary metabolites and highlight remaining questions about molecular details of the accepted regulatory modes.

Structure of Bacillus subtilis γ-glutamyltranspeptidase in complex with acivicin: diversity of the binding mode of a classical and electrophilic active-site-directed glutamate analogue

γ-Glutamyltranspeptidase (GGT) is an enzyme that plays a central role in glutathione metabolism, and acivicin is a classical inhibitor of GGT. Here, the structure of acivicin bound to Bacillus subtilis GGT determined by X-ray crystallography to 1.8 Å resolution is presented, in which it binds to the active site in a similar manner to that in Helicobacter pylori GGT, but in a different binding mode to that in Escherichia coli GGT. In B. subtilis GGT, acivicin is bound covalently through its C3 atom with sp2 hybridization to Thr403 Oγ, the catalytic nucleophile of the enzyme. The results show that acivicin-binding sites are common, but the binding manners and orientations of its five-membered dihydroisoxazole ring are diverse in the binding pockets of GGTs.

Garlic γ-glutamyl transpeptidases that catalyze deglutamylation of biosynthetic intermediate of alliin

S-Alk(en)yl-L-cysteine sulfoxides are pharmaceutically important secondary metabolites produced by plants that belong to the genus Allium. Biosynthesis of S-alk(en)yl-L-cysteine sulfoxides is initiated by S-alk(en)ylation of glutathione, which is followed by the removal of glycyl and γ-glutamyl groups and S-oxygenation. However, most of the enzymes involved in the biosynthesis of S-alk(en)yl-L-cysteine sulfoxides in Allium plants have not been identified. In this study, we identified three genes, AsGGT1, AsGGT2, and AsGGT3, from garlic (Allium sativum) that encode γ-glutamyl transpeptidases (GGTs) catalyzing the removal of the γ-glutamyl moiety from a putative biosynthetic intermediate of S-allyl-L-cysteine sulfoxide (alliin). The recombinant proteins of AsGGT1, AsGGT2, and AsGGT3 exhibited considerable deglutamylation activity toward a putative alliin biosynthetic intermediate, γ-glutamyl-S-allyl-L-cysteine, whereas these proteins showed very low deglutamylation activity toward another possible alliin biosynthetic intermediate, γ-glutamyl-S-allyl-L-cysteine sulfoxide. The deglutamylation activities of AsGGT1, AsGGT2, and AsGGT3 toward γ-glutamyl-S-allyl-L-cysteine were elevated in the presence of the dipeptide glycylglycine as a γ-glutamyl acceptor substrate, although these proteins can act as hydrolases in the absence of a proper acceptor substrate, except water. The apparent K m values of AsGGT1, AsGGT2, and AsGGT3 for γ-glutamyl-S-allyl-L-cysteine were 86 μM, 1.1 mM, and 9.4 mM, respectively. Subcellular distribution of GFP-fusion proteins transiently expressed in onion cells suggested that AsGGT2 localizes in the vacuole, whereas AsGGT1 and AsGGT3 possess no apparent transit peptide for localization to intracellular organelles. The different kinetic properties and subcellular localizations of AsGGT1, AsGGT2, and AsGGT3 suggest that these three GGTs may contribute differently to the biosynthesis of alliin in garlic.

Human GGT2 does not autocleave into a functional enzyme: A cautionary tale for interpretation of microarray data on redox signaling

AIMS

Human γ-glutamyltranspeptidase 1 (hGGT1) is a cell-surface enzyme that is a regulator of redox adaptation and drug resistance due to its glutathionase activity. The human GGT2 gene encodes a protein that is 94% identical to the amino-acid sequence of hGGT1. Transcriptional profiling analyses in a series of recent publications have implicated the hGGT2 enzyme as a modulator of disease processes. However, hGGT2 has never been shown to encode a protein with enzymatic activity. The aim of this study was to express the protein encoded by hGGT2 and each of its known variants and to assess their stability, cellular localization, and enzymatic activity.

RESULTS

We discovered that the proteins encoded by hGGT2 and its variants are inactive propeptides. We show that hGGT2 cDNAs are transcribed with a similar efficiency to hGGT1, and the expressed propeptides are N-glycosylated. However, they do not autocleave into heterodimers, fail to localize to the plasma membrane, and do not metabolize γ-glutamyl substrates. Substituting the coding sequence of hGGT1 to conform to alterations in a CX3C motif encoded by hGGT2 mRNAs disrupted autocleavage of the hGGT1 propeptide into a heterodimer, resulting in loss of plasma membrane localization and catalytic activity.

INNOVATION AND CONCLUSIONS

This is the first study to evaluate hGGT2 protein. The data show that hGGT2 does not encode a functional enzyme. Microarray data which have reported induction of hGGT2 mRNA should not be interpreted as induction of a protein that has a role in the metabolism of extracellular glutathione and in maintaining the redox status of the cell.

Novel insights into eukaryotic γ-glutamyltranspeptidase 1 from the crystal structure of the glutamate-bound human enzyme

The enzyme γ-glutamyltranspeptidase 1 (GGT1) is a conserved member of the N-terminal nucleophile hydrolase family that cleaves the γ-glutamyl bond of glutathione and other γ-glutamyl compounds. In animals, GGT1 is expressed on the surface of the cell and has critical roles in maintaining cysteine levels in the body and regulating intracellular redox status. Expression of GGT1 has been implicated as a potentiator of asthma, cardiovascular disease, and cancer. The rational design of effective inhibitors of human GGT1 (hGGT1) has been delayed by the lack of a reliable structural model. The available crystal structures of several bacterial GGTs have been of limited use due to differences in the catalytic behavior of bacterial and mammalian GGTs. We report the high resolution (1.67 Å) crystal structure of glutamate-bound hGGT1, the first of any eukaryotic GGT. Comparisons of the active site architecture of hGGT1 with those of its bacterial orthologs highlight key differences in the residues responsible for substrate binding, including a bimodal switch in the orientation of the catalytic nucleophile (Thr-381) that is unique to the human enzyme. Compared with several bacterial counterparts, the lid loop in the crystal structure of hGGT1 adopts an open conformation that allows greater access to the active site. The hGGT1 structure also revealed tightly bound chlorides near the catalytic residue that may contribute to catalytic activity. These are absent in the bacterial GGTs. These differences between bacterial and mammalian GGTs and the new structural data will accelerate the development of new therapies for GGT1-dependent diseases.

Covalent Immobilization of Bacillus licheniformis γ-Glutamyl Transpeptidase on Aldehyde-Functionalized Magnetic Nanoparticles

This work presents the synthesis and use of surface-modified iron oxide nanoparticles for the covalent immobilization of Bacillus licheniformis γ-glutamyl transpeptidase (BlGGT). Magnetic nanoparticles were prepared by an alkaline solution of divalent and trivalent iron ions, and they were subsequently treated with 3-aminopropyltriethoxysilane (APES) to obtain the aminosilane-coated nanoparticles. The functional group on the particle surface and the amino group of BlGGT was then cross-linked using glutaraldehyde as the coupling reagent. The loading capacity of the prepared nanoparticles for BlGGT was 34.2 mg/g support, corresponding to 52.4% recovery of the initial activity. Monographs of transmission electron microscopy revealed that the synthesized nanoparticles had a mean diameter of 15.1 ± 3.7 nm, and the covalent cross-linking of the enzyme did not significantly change their particle size. Fourier transform infrared spectroscopy confirmed the immobilization of BlGGT on the magnetic nanoparticles. The chemical and kinetic behaviors of immobilized BlGGT are mostly consistent with those of the free enzyme. The immobilized enzyme could be recycled ten times with 36.2% retention of the initial activity and had a comparable stability respective to free enzyme during the storage period of 30 days. Collectively, the straightforward synthesis of aldehyde-functionalized nanoparticles and the efficiency of enzyme immobilization offer wide perspectives for the practical use of surface-bound BlGGT.