Contribution of tyrosine 6 to the catalytic mechanism of isoenzyme 3-3 of glutathione S-transferase

Human glutathione transferases catalyze the reaction between glutathione and nitrooleic acid

Nitroalkene fatty acids (NO2-FAs) are formed endogenously. They regulate cell signaling pathways and are being developed clinically to treat inflammatory diseases. NO2-FAs are electrophilic and form thioether adducts with glutathione (GSH), which are exported from cells. Glutathione transferases (GSTs), a superfamily of enzymes, contribute to the cellular detoxification of hydrophobic electrophiles by catalyzing their conjugation to GSH. Herein, we evaluated the capacity of five human GSTs (M1-1, M2-2, M4-4, A4-4, and P1-1) to catalyze the reaction between nitrooleic acid (NO2-OA) and GSH. The reaction was monitored by HPLC-ESI-MS/MS, and catalytic activity was detected with hGSTs M1-1 and A4-4. Using stopped-flow spectrophotometry, a 1400- and 7500-fold increase in the apparent second-order rate constant was observed for hGST M1-1 and hGST A4-4, respectively, compared to the uncatalyzed reaction (pH 7.4, 25 °C). The acceleration was in part due to a higher availability of the thiolate. The crystal structure of hGST M1-1 in complex with the adduct was solved at 2.55 Å resolution, revealing that the ligand was bound within the active site, and establishing a foundation to build a model of hGST A4-4 in complex with the adduct. A larger number of interactions between the enzyme and the fatty acid were observed for hGST A4-4 compared to hGST M1-1, probably contributing to the increased catalysis. Altogether, these results show, for the first time, that hGSTs can catalyze the reaction between GSH and NO2-FAs, likely affecting the signaling actions of these metabolites and expanding the repertoire of GST substrates.

Glutathione S-transferase: A versatile and dynamic enzyme

The dynamic and versatile group of enzymes referred to as glutathione S-transferases (GSTs) play diverse roles in cellular detoxification, safeguarding hosts from oxidative damage, and performing various other functions. This review explores different classes of GST, existence of polymorphisms in GST, functions of GST and utilizations of GST inhibitors in treatment of human diseases. The study indicates that the cytosolic GSTs, mitochondrial GSTs, microsomal GSTs, and bacterial proteins that provide resistance to Fosfomycin are the major classes. Given a GST, variation in its expression and function among individuals is due to the presence of polymorphic alleles that encode it. Genetic polymorphism might result in the modification of GST activity, thereby increasing individuals' vulnerability to harmful chemical compounds. GSTs have been demonstrated to play a regulatory function in cellular signalling pathways through kinases, S-Glutathionylation, and in detoxification processes. Various applications of bacterial GSTs and their potential roles in plants were examined. Targeting GSTs, especially GSTP1-1, is considered a potential therapeutic strategy for treating cancer and diseases linked to abnormal cell proliferation. Their role in cancer cell growth, differentiation, and resistance to anticancer agents makes them promising targets for drug development, offering prospects for the future.

Low-molecular-weight thiol transferases in redox regulation and antioxidant defence

Low-molecular-weight (LMW) thiols are produced in all living cells in different forms and concentrations. Glutathione (GSH), coenzyme A (CoA), bacillithiol (BSH), mycothiol (MSH), ergothioneine (ET) and trypanothione T(SH)2 are the main LMW thiols in eukaryotes and prokaryotes. LMW thiols serve as electron donors for thiol-dependent enzymes in redox-mediated metabolic and signaling processes, protect cellular macromolecules from oxidative and xenobiotic stress, and participate in the reduction of oxidative modifications. The level and function of LMW thiols, their oxidized disulfides and mixed disulfide conjugates in cells and tissues is tightly controlled by dedicated oxidoreductases, such as peroxiredoxins, glutaredoxins, disulfide reductases and LMW thiol transferases. This review provides the first summary of the current knowledge of structural and functional diversity of transferases for LMW thiols, including GSH, BSH, MSH and T(SH)2. Their role in maintaining redox homeostasis in single-cell and multicellular organisms is discussed, focusing in particular on the conjugation of specific thiols to exogenous and endogenous electrophiles, or oxidized protein substrates. Advances in the development of new research tools, analytical methodologies, and genetic models for the analysis of known LMW thiol transferases will expand our knowledge and understanding of their function in cell growth and survival under oxidative stress, nutrient deprivation, and during the detoxification of xenobiotics and harmful metabolites. The antioxidant function of CoA has been recently discovered and the breakthrough in defining the identity and functional characteristics of CoA S-transferase(s) is soon expected.

Profiling the Site of Protein CoAlation and Coenzyme A Stabilization Interactions

Coenzyme A (CoA) is a key cellular metabolite known for its diverse functions in metabolism and regulation of gene expression. CoA was recently shown to play an important antioxidant role under various cellular stress conditions by forming a disulfide bond with proteins, termed CoAlation. Using anti-CoA antibodies and liquid chromatography tandem mass spectrometry (LC-MS/MS) methodologies, CoAlated proteins were identified from various organisms/tissues/cell-lines under stress conditions. In this study, we integrated currently known CoAlated proteins into mammalian and bacterial datasets (CoAlomes), resulting in a total of 2093 CoAlated proteins (2862 CoAlation sites). Functional classification of these proteins showed that CoAlation is widespread among proteins involved in cellular metabolism, stress response and protein synthesis. Using 35 published CoAlated protein structures, we studied the stabilization interactions of each CoA segment (adenosine diphosphate (ADP) moiety and pantetheine tail) within the microenvironment of the modified cysteines. Alternating polar-non-polar residues, positively charged residues and hydrophobic interactions mainly stabilize the pantetheine tail, phosphate groups and the ADP moiety, respectively. A flexible nature of CoA is observed in examined structures, allowing it to adapt its conformation through interactions with residues surrounding the CoAlation site. Based on these findings, we propose three modes of CoA binding to proteins. Overall, this study summarizes currently available knowledge on CoAlated proteins, their functional distribution and CoA-protein stabilization interactions.

Expanding the family of Mu-class glutathione transferases in the cestode parasite Echinococcus granulosus sensu lato

Glutathione transferases (GSTs) perform catalytic and non-catalytic activities, mostly involved in stress-response and cell detoxification. Helminth parasites express several GSTs of multiple classes that are involved in the neutralization of potentially harmful oxidants, and in the inactivation or removal of xenobiotics. Additionally, GSTs participate in immunomodulatory processes that facilitate the parasite establishment and survival within its host. In Echinococcus granulosus sensu lato (s.l.) -the cestode parasite responsible for cystic echinococcosis- only one Mu-class GST has been reported. In the present work, by using bioinformatic and proteomic approaches we searched for novel Mu-class GSTs potentially involved in the parasite oxidative-stress metabolism. In the genome of E. granulosus s.l., 6 GST-related sequences were found to constitute a strongly conserved phylogenetical clade with Mu-class members. Among them, 5 displayed conserved gene structure (exon/intron), as well as specific residues and motifs characteristic of Mu-class enzymes. By proteomic analysis, 3 Mu-GSTs were identified to be expressed in the protoscolex parasite stage, 2 of them being firstly described as Mu-class GSTs here. The existence of more than one productive Mu-GST gene expands the parasite xenobiotic phase II metabolism, which might have beneficial roles on E. granulosus s.l. ability to successfully infect its host.

Genetic Incorporation of Selenotyrosine Significantly Improves Enzymatic Activity of Agrobacterium radiobacter Phosphotriesterase

Tyrosine plays important roles in many enzymes. To facilitate enzyme design, mechanistic studies and minimize structural perturbation in the active site, here we report the genetic incorporation of a novel unnatural amino acid selenotyrosine (SeHF), which has single-atom replacement in comparison to tyrosine. The arPTE-(Agrobacterium radiobacter Phosphotriesterase) Tyr309SeHF mutant exhibits a significant 12-fold increase in kcat and 3.2-fold enhancement in kcat /KM at pH 7.0. Molecular dynamics simulations show that the SeHF309 mutation results in a conformational switch which opens up the product release pocket and increases the product release rate, thereby elevating the overall enzyme activity. Significant improvement of the catalytic efficiency at neutral pH by single unnatural amino acid (UAA) mutation broadens the application of this enzyme, and provides valuable insights to the mechanism. Our method represents a new approach for designing enzymes with enhanced activity.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

The thiol of human serum albumin: Acidity, microenvironment and mechanistic insights on its oxidation to sulfenic acid

Human serum albumin (HSA) has a single reduced cysteine residue, Cys34, whose acidity has been controversial. Three experimental approaches (pH-dependence of reactivity towards hydrogen peroxide, ultraviolet titration and infrared spectroscopy) are used to determine that the pK_a value in delipidated HSA is 8.1±0.2 at 37°C and 0.1M ionic strength. Molecular dynamics simulations of HSA in the sub-microsecond timescale show that while sulfur exposure to solvent is limited and fluctuating in the thiol form, it increases in the thiolate, stabilized by a persistent hydrogen-bond (HB) network involving Tyr84 and bridging waters to Asp38 and Gln33 backbone. Insight into the mechanism of Cys34 oxidation by H₂O₂ is provided by ONIOM(QM:MM) modeling including quantum water molecules. The reaction proceeds through a slightly asynchronous S_N2 transition state (TS) with calculated Δ^‡G and Δ^‡H barriers at 298K of respectively 59 and 54kJmol^-1 (the latter within chemical accuracy from the experimental value). A post-TS proton transfer leads to HSA-SO^- and water as products. The structured reaction site cages H₂O₂, which donates a strong HB to the thiolate. Loss of this HB before reaching the TS modulates Cys34 nucleophilicity and contributes to destabilize H₂O₂. The lack of reaction-site features required for differential stabilization of the TS (positive charges, H₂O₂ HB strengthening) explains the striking difference in kinetic efficiency for the same reaction in other proteins (e.g. peroxiredoxins). The structured HB network surrounding HSA-SH with sequestered waters carries an entropic penalty on the barrier height. These studies contribute to deepen the understanding of the reactivity of HSA-SH, the most abundant thiol in human plasma, and in a wider perspective, provide clues on the key aspects that modulate thiol reactivity against H₂O₂.

Yeast 3',5'-bisphosphate nucleotidase: an affinity tag for protein purification

Affinity chromatography is one of the most popular methods for protein purification. Each tag method has its advantages and disadvantages, and combination of different tags and developing of new tags had been proposed and performed. Yeast 3',5'-bisphosphate nucleotidase, also known as HAL2, hydrolyzes 3'-phosphoadenosine 5'-phosphate (PAP) with submicromolar Km, which indicated the tight interactions between HAL2 and PAP. In order to explore the feasibility of HAL2 as a protein purification affinity tag, HAL2 was further characterized with PAP as substrate. Results demonstrated that KmPAP and kcatPAP were ∼0.3μM and ∼11s(-)(1), respectively. Kd for PAP was 0.008μM in the presence of Ca(2+). pH was also found to affect interactions between HAL2 and PAP, with tightest binding (Kd∼8nM) at pH 7.5 and 8. The purification protocol was rationally designed based on nanomolar affinity to PAP agarose in the presence of Ca(2+), which could satisfy the metal requirement for PAP binding, prevent hydrolysis of immobilized PAP and could be chelated by ethylene glycol tetraacetic acid (EGTA) for elution. A series of expression vectors were further constructed and Escherichia coli adenosine 5'-phosphosulfate kinase (APSK) was prokaryotically expressed, purified and characterized. Ready to use expression vector with eight commonly used restriction enzyme recognition sites in multiple cloning site was subsequently constructed. By comparing with current popular tags, HAL2 was found to be an efficient and economical tag for prokaryotic protein expression and purification.

Low-molecular-weight thiols in thiol-disulfide exchange

SIGNIFICANCE

Oxidative stress is widely invoked in inflammation, aging, and complex diseases. To avoid unwanted oxidations, the redox environment of cellular compartments needs to be tightly controlled. The complementary action of oxidoreductases and of high concentrations of low-molecular-weight (LMW) nonprotein thiols plays an essential role in maintaining the redox potential of the cell in balance.

RECENT ADVANCES

While LMW thiols are central players in an extensive range of redox regulation/metabolism processes, not all organisms use the same thiol cofactors to this effect, as evidenced by the recent discovery of mycothiol (MSH) and bacillithiol (BSH) among different gram-positive bacteria.

CRITICAL ISSUES

LMW thiol-disulfide exchange processes and their cellular implications are often oversimplified, as only the biology of the free thiols and their symmetrical disulfides is considered. In bacteria under oxidative stress, especially where concentrations of different LMW thiols are comparable [e.g., BSH, coenzyme A (CoA), and cysteine (Cys) in many low-G+C gram-positive bacteria (Firmicutes)], mixed disulfides (e.g., CoASSB and CySSCoA) must surely be major thiol-redox metabolites that need to be taken into consideration.

FUTURE DIRECTIONS

There are many microorganisms whose LMW thiol-redox buffers have not yet been identified (either bioinformatically or experimentally). Many elements of BSH and MSH redox biochemistry remain to be explored. The fundamental biophysical properties, thiol pK(a) and redox potential, have not yet been determined, and the protein interactome in which the biothiols MSH and BSH are involved needs further exploration.

Understanding the pK(a) of redox cysteines: the key role of hydrogen bonding

Many cellular functions involve cysteine chemistry via thiol-disulfide exchange pathways. The nucleophilic cysteines of the enzymes involved are activated as thiolate. A thiolate is much more reactive than a neutral thiol. Therefore, determining and understanding the pK(a)s of functional cysteines are important aspects of biochemistry and molecular biology with direct implications for redox signaling. Here, we describe the experimental and theoretical methods to determine cysteine pK(a) values, and we examine the factors that control these pK(a)s. Drawing largely on experience gained with the thioredoxin superfamily, we examine the roles of solvation, charge-charge, helix macrodipole, and hydrogen bonding interactions as pK(a)-modulating factors. The contributions of these factors in influencing cysteine pK(a)s and the associated chemistry, including the relevance for the reaction kinetics and thermodynamics, are discussed. This analysis highlights the critical role of direct hydrogen bonding to the cysteine sulfur as a key factor modulating the equilibrium between thiol S-H and thiolate S(-). This role is easily understood intuitively and provides a framework for biochemical functional insights.

Pre-steady-state kinetic characterization of thiolate anion formation in human leukotriene C₄ synthase

Human leukotriene C₄ synthase (hLTC4S) is an integral membrane protein that catalyzes the committed step in the biosynthesis of cysteinyl-leukotrienes, i.e., formation of leukotriene C₄ (LTC₄). This molecule, together with its metabolites LTD₄ and LTE₄, induces inflammatory responses, particularly in asthma, and thus, the enzyme is an attractive drug target. During the catalytic cycle, glutathione (GSH) is activated by hLTC4S that forms a nucleophilic thiolate anion that will attack LTA₄, presumably according to an S(N)2 reaction to form LTC₄. We observed that GSH thiolate anion formation is rapid and occurs at all three monomers of the homotrimer and is concomitant with stoichiometric release of protons to the medium. The pK(a) (5.9) for enzyme-bound GSH thiol and the rate of thiolate formation were determined (k(obs) = 200 s⁻¹). Taking advantage of a strong competitive inhibitor, glutathionesulfonic acid, shown here by crystallography to bind in the same location as GSH, we determined the overall dissociation constant (K(d((GS) = 14.3 μM). The release of the thiolate was assessed using a GSH release experiment (1.3 s⁻¹). Taken together, these data establish that thiolate anion formation in hLTC4S is not the rate-limiting step for the overall reaction of LTC₄ production (k(cat) = 26 s⁻¹), and compared to the related microsomal glutathione transferase 1, which displays very slow GSH thiolate anion formation and one-third of the sites reactivity, hLTC4S has evolved a different catalytic mechanism.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Investigation of the role of conserved residues Ser13, Asn48 and Pro49 in the catalytic mechanism of the tau class glutathione transferase from Glycine max

Plant glutathione transferases (GSTs) play a key role in the metabolism of various xenobiotics. In this report, the catalytic mechanism of the tau class GSTU4-4 isoenzyme from Glycine max (GmGSTU4-4) was investigated by site-directed mutagenesis and steady-state kinetic analysis. The catalytic properties of the wild-type enzyme and three mutants of strictly conserved residues (Ser13Ala, Asn48Ala and Pro49Ala) were studied in 1-chloro-2,4-dinitrobenzene (CDNB) conjugation reaction. The results showed that the mutations significantly affect substrate binding and specificity. The effect of Ser13Ala mutation on the catalytic efficiency of the enzyme could be explained by assuming the direct involvement of Ser13 to the reaction chemistry and the correct positioning of GSH and CDNB in the ternary catalytic complex. Asn48 and Pro49 were found to have a direct role on the structural integrity of the GSH-binding site (G-site). Moreover, mutation of Asn48 and Pro49 residues may bring about secondary effects altering the thermal stability and the catalytic activity (k(cat)) of the enzyme without affecting the nature of the rate-limiting step of the catalytic reaction.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

A selenium-dependent glutathione peroxidase (Se-GPx) and two glutathione S-transferases (GSTs) from Chinese shrimp (Fenneropenaeus chinensis)

Glutathione S-transferases (GSTs) and glutathione peroxidases (GPxs) are essential components of cellular detoxification systems that defend cells against reactive oxygen species (ROSs). Two GSTgenes have recently been cloned from Fenneropenaeus chinensis and BLAST P analysis shows that one GST, designated FcMuGST, is similar to members of MuGST while the other has similarities to ThetaGST (FcThetaGST). A selenium-dependent glutathione peroxidase (Se-GPx) has also been cloned from F. chinensis. The alignment of the deduced GST and GPx amino acid sequences with those from other species showed that the residues essential for enzymatic function of these three proteins are highly conserved. Tissue distribution and response to pathogens for the three genes was investigated by RT-PCR analysis, which showed that the transcript of FcMuGST and FcGPx increased in response to Vibrio anguillarum infection, while FcThetaGST showed little change at the transcript level. GPx activity in gill tissues quickly increased at 6 h after V. anguillarum challenge and maintained at a relatively high level from 6 h to 24 h. Total GST activity in hepatopancreas and intestines of the bacterial challenged shrimp was increased at 6 h, and gradually recovered from 12 and 24 h to the normal level. These three genes were all predicted to play an important role in detoxification defense reactions. FcMuGST primarily scavenges excess ROS produced after bacterial infection, while clearance of endogenous hydrophobic electrophile molecules was mainly dependent on activities of FcThetaGST.

Cysteine 10 is critical for the activity of Ochrobactrum anthropi glutathione transferase and its mutation to alanine causes the preferential binding of glutathione to the H-site

The role of the evolutionarily conserved residue Cys10 in Ochrobactrum anthropi glutathione transferase (OaGST) has been examined by replacing it with an alanine. A double mutant C10A/S11A was also prepared. The effect of the replacements on the coniugating and thiotransferase activities, and on the thermal and chemical stability of the enzyme was analyzed. Our data support the view that in OaGST, in contrast with other beta class GSTs that display significant differences in the glutathione-binding site, Cys10 is a key residue for glutathione coniugating activity. Furthermore, analysis of the OaGST-Cys10Ala structure, crystallized in the presence of glutathione, reveals that this mutation causes a switch between the high-affinity G-site and a low-affinity H-site where hydrophobic cosubstrates bind and where we observe the presence of an unexpected glutathione.

Role of invariant tyrosines in a crustacean mu-class glutathione S-transferase from shrimp Litopenaeus vannamei: site-directed mutagenesis of Y7 and Y116

Y6 and Y115 are key amino acids involved in enzyme-substrate interactions in mu-class glutathione S-transferase (GST). They provide electrophilic assistance and stabilize substrates through their hydroxyl groups. Two site-directed mutants (Y7F and Y116F) and the wild-type shrimp GSTs were expressed in Escherichia coli, and the steady-state kinetic parameters were determined using CDNB as the second substrate. The mutants were modeled based on a crystal structure of a mu-class GST to obtain further insights about the changes at the active site. The Y116F mutant had an increase in kcat contrary to Y7F compared to the wild type. Molecular modeling showed that the shrimp GST has a H108 residue that may contribute to compensate and lead to a less deleterious change when conserved tyrosine residues are mutated. This work indicates that shrimp GST is a useful model to understand the catalysis mechanisms in this critical enzyme.

Molecular cloning and expression study of pi-class glutathione S-transferase (pi-GST) and selenium-dependent glutathione peroxidase (Se-GPx) transcripts in the freshwater bivalve Dreissena polymorpha

Glutathione S-transferases (GST) and glutathione peroxidases (GPx) are essential components of cellular detoxification systems. We identified GST and GPx transcripts in the freshwater bivalve Dreissena polymorpha, their full-length coding sequences were obtained by reverse-transcription PCR using degenerated primers followed by 5' and 3' RACE-PCR (rapid amplification of cDNA ends-PCR). The cDNA identified encoded proteins of 205 and 243 amino acids corresponding respectively to a pi-class GST and a selenium-dependent GPx. The comparison of the deduced amino acid sequences with GST and GPx from other species showed that the residues essential to the enzymatic function of these two proteins are highly conserved. We studied their expression pattern in the digestive gland, the gills and the excretory system of D. polymorpha. The results showed that pi-GST mRNA expression is higher in the digestive gland than in the gills or the excretory system. Se-GPx transcripts are expressed at high, medium and very low levels in the digestive gland, the excretory system and the gills, respectively.

A functionally conserved basic residue in glutathione transferases interacts with the glycine moiety of glutathione and is pivotal for enzyme catalysis

The present study characterized conserved residues in a GST (glutathione transferase) in the active-site region that interacts with glutathione. This region of the active site is near the glycine moiety of glutathione and consists of a hydrogen bond network. In the GSTD (Delta class GST) studied, adGSTD4-4, the network consisted of His(38), Met(39), Asn(47), Gln(49), His(50) and Cys(51). In addition to contributing to glutathione binding, this region also had major effects on enzyme catalysis, as shown by changes in kinetic parameters and substrate-specific activity. The results also suggest that the electron distribution of this network plays a role in stabilization of the ionized thiol of glutathione as well as impacting on the catalytic rate-limiting step. This area constitutes a second glutathione active-site network involved in glutathione ionization distinct from a network previously observed interacting with the glutamyl end of glutathione. This second network also appears to be functionally conserved in GSTs. In the present study, His(50) is the key basic residue stabilized by this network, as shown by up to a 300-fold decrease in k(cat) and 5200-fold decrease in k(cat)/K(m) for glutathione. Although these network residues have a minor role in structural integrity, the replaced residues induced changes in active-site topography as well as generating positive co-operativity towards glutathione. Moreover, this network at the glycine moiety of GSH (glutathione) also contributed to the 'base-assisted deprotonation model' for GSH ionization. Taken together, the results indicate a critical role for the functionally conserved basic residue His(50) and this hydrogen bond network in the active site.

Combined supplementation of soy and garlic modulate biochemical parameters of 7,12-dimethylbenz[alpha]anthracene induced mammary cancer in female albino rats

The study was designed to investigate the chemoprotective effect of Combined Supplementation of soy and garlic on 7,12-dimethylbenz[alpha]anthrance (DMBA) induced mammary cancer in female Albino rats. Animals (eighty rats) where equally divided into four groups, (twenty rats each). Group I: each rat received 1 mL of 0.1% saline daily for twenty days. Group II: received 1 mL of 0.1% saline given orally once a day for twenty days after DMBA infusion (30 mg kg-1) and these rats served as controls (cancer control) for groups III and IV. Group III: were post treated with garlic after Infusion with DMBA. Group IV: Treated with Combined Supplementation of Soy and Garlic after DMBA infusion. Biochemical measurement were carried out on liver tissues and serum of experimental animals. Circulating levels of lactate dehydrogenase (LDH), glutamate oxaloacetate transaminase (GOT) and glutamate pyrovate transaminase (GPT) were elevated, while the activities of glutathione-s-transferase (GST) and superoxide dismutase (SOD) and the levels of reduced glutathione (GSH) were reduced in liver tissues in DMBA infused rats as compared to normal. Combined Supplementation of Soy and Garlic treatment altered the above mentioned biochemical parameters towards normal values. Present findings indicate that combined supplementation of soy and garlic got more marked effect on DMBA infused rats as compared to a group of rats received only garlic treatment.

A model for glutathione binding and activation in the fosfomycin resistance protein, FosA

The genomically encoded fosfomycin resistance protein from Pseudomonas aeruginosa (FosA(PA)) utilizes Mn(II) and K(+) to catalyze the addition of glutathione (GSH) to C1 of the antibiotic rendering it inactive. Although this protein has been structurally and kinetically characterized with respect to the substrate, fosfomycin, questions remain regarding how the enzyme binds the thiol substrate, GSH. Computational studies have revealed a potential GSH binding site in FosA(PA) that involves six electrostatic or hydrogen-bonding interactions with protein side-chains as well as six additional residues that contribute van der Waals interactions. A strategically placed tyrosine residue, Y39, appears to be involved in the ionization of GSH during catalysis. The Y39F mutant exhibits a 13-fold reduction of catalytic activity (k(cat)=14+/-2s(-1)), suggesting a role in the ionization of GSH. Mutation of five other residues (W34, Q36, S50, K90, and R93) implicated in ionic of hydrogen-bonding interactions resulted in enzymes with reduced catalytic efficiency, affinity for GSH, or both. The mutant enzymes were also found to be less effective resistant proteins in the biological context of Escherichia coli. The more conservative W34H mutant has native-like catalytic efficiency suggesting that the imidazole NH group can replace the indole group of W34 that is important for GSH binding. In the absence of co-crystal structural data with the thiol substrate, these results provide important insights into the role of GSH in catalysis.

Role of Ser11 in the stabilization of the structure of Ochrobactrum anthropi glutathione transferase

GSTs (glutathione transferases) are a multifunctional group of enzymes, widely distributed and involved in cellular detoxification processes. In the xenobiotic-degrading bacterium Ochrobactrum anthropi, GST is overexpressed in the presence of toxic concentrations of aromatic compounds such as 4-chlorophenol and atrazine. We have determined the crystal structure of the GST from O. anthropi (OaGST) in complex with GSH. Like other bacterial GSTs, OaGST belongs to the Beta class and shows a similar binding pocket for GSH. However, in contrast with the structure of Proteus mirabilis GST, GSH is not covalently bound to Cys10, but is present in the thiolate form. In our investigation of the structural basis for GSH stabilization, we have identified a conserved network of hydrogen-bond interactions, mediated by the presence of a structural water molecule that links Ser11 to Glu198. Partial disruption of this network, by mutagenesis of Ser11 to alanine, increases the K(m) for GSH 15-fold and decreases the catalytic efficiency 4-fold, even though Ser11 is not involved in GSH binding. Thermal- and chemical-induced unfolding studies point to a global effect of the mutation on the stability of the protein and to a central role of these residues in zippering the terminal helix of the C-terminal domain to the starting helix of the N-terminal domain.

Probing the Mechanism of GSH Activation in Schistosoma haematobium Glutathione-S-transferase by Site-directed Mutagenesis and X-ray Crystallography

During turnover, the catalytic tyrosine residue (Tyr10) of the sigma class Schistosoma haematobium wild-type glutathione-S-transferase is expected to switch alternately in and out of the reduced glutathione-binding site (G-site). The Tyrout10 conformer forms a pi-cation interaction with the guanidinium group of Arg21. As in other similar glutathione-S-transferases, the catalytic Tyr has a low pKa of 7.2. In order to investigate the catalytic role of Tyr10, and the structural and functional roles of Arg21, we carried out structural studies on two Arg21 mutants (R21L and R21Q) and a Tyr10 mutant, Y10F. Our crystallographic data for the two Arg21 mutants indicate that only the Tyrout10 conformation is populated, thereby excluding a role of Arg21 in the stabilisation of the out conformation. However, Arg21 was confirmed to be catalytically important and essential for the low pKa of Tyr10. Upon comparison with structural data generated for reduced glutathione-bound and inhibitor-bound wild-type enzymes, it was observed that the orientations of Tyr10 and Arg35 are concerted and that, upon ligand binding, minor rearrangements occur within conserved residues in the active site loop. These rearrangements are coupled to quaternary rigid-body movements at the dimer interface and alterations in the localisation and structural order of the C-terminal domain.

Contribution of the mu loop to the structure and function of rat glutathione transferase M1-1

The "mu loop," an 11-residue loop spanning amino acid residues 33-43, is a characteristic structural feature of the mu class of glutathione transferases. To assess the contribution of the mu loop to the structure and function of rat GST M1-1, amino acid residues 35-44 (35GDAPDYDRSQ44) were excised by deletion mutagenesis, resulting in the "Deletion Enzyme." Kinetic studies reveal that the Km values of the Deletion Enzyme are markedly increased compared with those of the wild-type enzyme: 32-fold for 1-chloro-2,4-dinitrobenzene, 99-fold for glutathione, and 880-fold for monobromobimane, while the Vmax value for each substrate is increased only modestly. Results from experiments probing the structure of the Deletion Enzyme, in comparison with that of the wild-type enzyme, suggest that the secondary and quaternary structures have not been appreciably perturbed. Thermostability studies indicate that the Deletion Enzyme is as stable as the wild-type enzyme at 4 degrees C and 10 degrees C, but it rapidly loses activity at 25 degrees C, unlike the wild-type enzyme. In the temperature range of 4 degrees C through 25 degrees C, the loss of activity of the Deletion Enzyme is not the result of a change in its structure, as determined by circular dichroism spectroscopy and sedimentation equilibrium centrifugation. Collectively, these results indicate that the mu loop is not essential for GST M1-1 to maintain its structure nor is it required for the enzyme to retain some catalytic activity. However, it is an important determinant of the enzyme's affinity for its substrates.

Crystal structure of a new class of glutathione transferase from the model human hookworm nematode Heligmosomoides polygyrus

The crystal structure of GST Nu2-2 (HpolGSTN2-2) from the model hookworm nematode Heligmosomoides polygyrus has been solved by the molecular replacement method and refined to a resolution of 1.71 A, providing the first structural data from a class of nematode-specific GSTs. By structural alignment with two Sigma class GSTs, glutathione could be rationally docked into the G-site of the enzyme. By comparing with all mammalian GST classes, a novel, long, and deep cleft was identified at the H-site, providing a potential site for ligand binding. This new GST class may support the establishment of infection parasitic nematodes by passively neutralizing chemical toxins derived from host environment. The structure serves as a starting point for structure-based drug/inhibitor design that would aim to selectively disrupt nematode chemical defenses.

Plasmodium falciparum glutathione S-transferase--structural and mechanistic studies on ligand binding and enzyme inhibition

Glutathione S-transferase of the malarial parasite Plasmodium falciparum (PfGST) represents a novel class of GST isoenzymes. Since the architecture of the PfGST substrate binding site differs significantly from its human counterparts and there is only this one isoenzyme present in the parasite, PfGST is considered a highly attractive target for antimalarial drug development. Here we report the mechanistic, kinetic, and structural characterization of PfGST as well as its interaction with different ligands. Our data indicate that in solution PfGST is present as a tetramer that dissociates into dimers in the presence of glutathione (GSH). Fluorescence spectroscopy shows that in the presence of GSH GST serves as ligandin for parasitotoxic ferriprotoporphyrin IX with a high- and a low-affinity binding site. This is supported by a clear uncompetitive inhibition type. Site-directed mutagenesis studies demonstrate that neither Cys 86 nor Cys 101 contribute to the peroxidase activity of the enzyme, which is thus performed GSH-dependently at the active site. Tyr 9 is responsible for the deprotonation of GSH and Lys 15, but also Gln 71 are involved in GSH binding. We furthermore report the 2.4 A resolution X-ray structure of PfGST cocrystallized with the inhibitor S-hexylglutathione. In comparison with a previously reported structure obtained by crystal soaking, differences occur at the C-terminal end of helix alpha4 and at the S-hexylmoiety of the inhibitor. We furthermore show that, in contrast to previous reports, the antimalarial drug artemisinin is not metabolized by PfGST.

Trypanosoma cruzi: Sequence polymorphism of the gene encoding the Tc52 immunoregulatory-released factor in relation to the phylogenetic diversity of the species

We have previously identified a Trypanosoma cruzi gene encoding a protein named Tc52 sharing structural and functional properties with the thioredoxin and glutaredoxin family involved in thiol-disulfide redox reactions. Gene targeting strategy and immunological studies allowed showing that Tc52 is among T. cruzi virulence factors. Taking into account that T. cruzi has a genetic variability that might be important determinant that governs the different behaviour of T. cruzi clones in vitro and in vivo, we thought it was of interest to analyse the sequence polymorphism of Tc52 gene in several reference clones. The DNA sequences of 12 clones which represent the whole genetic diversity of T. cruzi allowed showing that 40 amino-acid positions over 400 analysed are targets for mutations. A number of residues corresponding to putative amino-acids playing a role in GSH binding and/or enzymatic function and others located nearby are subject to mutations. Although the immunological analysis showed that Tc52 is present in parasite extracts from different clones, it is possible that the amino-acid differences could affect the enzymatic and/or the immunomodulatory function of Tc52 variants and therefore the parasite phenotype.

An electron-sharing network involved in the catalytic mechanism is functionally conserved in different glutathione transferase classes

In Anopheles dirus glutathione transferase D3-3, there are electrostatic interactions between the negatively charged glutamyl alpha-carboxylate group of glutathione, the positively charged Arg-66, and the negatively charged Asp-100. This ionic interaction is stabilized by a network of hydrogen bonds from Ser-65, Thr-158, Thr-162, and a conserved water-mediated contact. This alternating ionic bridge interaction between negatively and positively charged residues stabilized by a network of hydrogen bonding we have named an electron-sharing network. We show that the electron-sharing network assists the glutamyl alpha-carboxylate of glutathione to function as a catalytic base accepting the proton from the thiol group forming an anionic glutathione, which is a crucial step in the glutathione transferase (GST) catalysis. Kinetic studies demonstrate that the mutation of electron-sharing network residues results in a decreased ability to lower the pKa of the thiol group of glutathione. Although the residues that contribute to the electron-sharing network are not conserved in the primary sequence, structural characterizations indicate that the presence of the network can be mapped to the same region in all GST classes. A structural diversification but functional conservation suggests a significant role for the electron-sharing network in catalysis as the purpose was maintained during the divergent evolution of GSTs. This network appears to be a functionally conserved motif that contributes to the "base-assisted deprotonation" model suggested to be essential for the glutathione ionization step of the catalytic mechanism.

The crystal structure of the reduced, Zn2+-bound form of the B. subtilis Hsp33 chaperone and its implications for the activation mechanism

The bacterial heat shock protein Hsp33 is a redox-regulated chaperone activated by oxidative stress. In response to oxidation, four cysteines within a Zn2+ binding C-terminal domain form two disulfide bonds with concomitant release of the metal. This leads to the formation of the biologically active Hsp33 dimer. The crystal structure of the N-terminal domain of the E. coli protein has been reported, but neither the structure of the Zn2+ binding motif nor the nature of its regulatory interaction with the rest of the protein are known. Here we report the crystal structure of the full-length B. subtilis Hsp33 in the reduced form. The structure of the N-terminal, dimerization domain is similar to that of the E. coli protein, although there is no domain swapping. The Zn2+ binding domain is clearly resolved showing the details of the tetrahedral coordination of Zn2+ by four thiolates. We propose a structure-based activation pathway for Hsp33.

Functional analysis of active site residues of the fosfomycin resistance enzyme FosA from Pseudomonas aeruginosa

The metalloglutathione transferase FosA catalyzes the conjugation of glutathione to carbon-1 of the antibiotic fosfomycin, rendering it ineffective as an antibacterial drug. Codon randomization and selection for the ability of resulting clones to confer fosfomycin resistance to Escherichia coli were used to identify residues critical for FosA function. Of the 24 codons chosen for randomization, 16 were found to be essential because only the wild type amino acid was selected. These included ligands to the Mn(2+) and the K(+), residues that furnish hydrogen bonds to fosfomycin, and residues located in a putative glutathione/fosfomycin-binding site. The remaining eight positions randomized were tolerant to substitutions. Site-directed mutagenesis of some of the essential and tolerant amino acids to alanine was performed, and the activity of the purified proteins was determined. Mutation of the residues that are within hydrogen bonding distance to the oxirane or phosphonate oxygens of fosfomycin resulted in variants with very low or no activity. Mutation of Ser(94), which bridges one of the phosphonate oxygens with a potassium ion, resulted in insoluble protein. The Y39A mutation in the putative glutathione-binding site resulted in a 4-fold increase in the apparent K(m) for glutathione. Only two of the amino acids in the substrate-binding site are conserved in the related fosfomycin resistance proteins FosB and FosX, whereas no amino acids in the putative glutathione-binding site are conserved.

cDNA cloning and expression pattern of pi-class glutathione S-transferase in the freshwater bivalves Unio tumidus and Corbicula fluminea

Glutathione S-transferases (GSTs) are enzymes involved in major detoxification reactions of xenobiotics in many organisms. The aim of this work was the identification of GST transcripts in the freshwater bivalves Unio tumidus and Corbicula fluminea. We used degenerated primers designed in the highly conserved regions of GST to amplify the corresponding mRNA. Full-length coding sequences were obtained by 5' and 3' rapid amplification of cDNA ends. In the two species, the GST cDNAs identified encoded a protein of 205 amino acids. The comparison of the deduced amino acid sequences with GSTs from other species showed that the enzymes belong to the pi-class and the amino acids defining the binding sites of glutathione (G-site) and for xenobiotic substrates (H-site) are highly conserved. Specific amplifications of the GST mRNA from U. tumidus and C. fluminea were performed on the digestive gland, the excretory system and the gills. For each mussel, the results revealed that the pi-class GSTs are expressed at the same level in the three tissues.

Molecular cloning, expression and characterization of glutathione S-transferase from Mytilus edulis

The gene coding for glutathione S-transferase (GST) has been isolated from the Mytilus edulis hepatopancreas. Open reading frame analysis indicated that the M. edulis GST (meGST) gene encodes a protein of 206 amino acid residues with a calculated molecular mass of 23.68 kDa. The deduced amino acid sequence showed high sequence similarity with the sequence of the pi class GST. The meGST was expressed in Escherichia coli, and the recombinant meGST was purified by affinity chromatography and characterized. The recombinant meGST exhibited high activity towards the substrates ethacrynic acid (ECA) and 1-chloro-2,4-dinitrobenzene (CDNB). Kinetic analysis with respect to CDNB as substrate gave a K(m) of 0.68 mM and a V(max) of 0.10 mmol/min per mg protein. The recombinant meGST had a maximum activity at approximately pH 8.5, and its optimum temperature was 39 degrees C. The predicted three-dimensional structure of the meGST revealed the N-terminal domain possesses a thioredoxin fold and the six helices of the C-terminal domain make a alpha-helical bundle. These features indicate that the meGST belongs to pi class GST.

A gene from Aspergillus nidulans with similarity to URE2 of Saccharomyces cerevisiae encodes a glutathione S-transferase which contributes to heavy metal and xenobiotic resistance

Aspergillus nidulans is a saprophytic ascomycete that utilizes a wide variety of nitrogen sources. We identified a sequence from A. nidulans similar to the glutathione S-transferase-like nitrogen regulatory domain of Saccharomyces cerevisiae Ure2. Cloning and sequencing of the gene, designated gstA, revealed it to be more similar to URE2 than the S. cerevisiae glutathione S-transferases. However, creation and analysis of a gstA deletion mutant revealed that the gene does not participate in nitrogen metabolite repression. Instead, it encodes a functional theta class glutathione S-transferase that is involved in resistance to a variety of xenobiotics and metals and confers susceptibility to the systemic fungicide carboxin. Northern analysis showed that gstA transcription is strongly activated upon exposure to 1-chloro-2,4-dinitrobenzene and weakly activated by oxidative stress or growth on galactose as a carbon source. These results suggest that nitrogen metabolite repression in A. nidulans does not involve a homolog of the S. cerevisiae URE2 gene and that the global nitrogen regulatory system differs significantly in these two fungi.

GSTB1-1 from Proteus mirabilis: a snapshot of an enzyme in the evolutionary pathway from a redox enzyme to a conjugating enzyme

The native form of the bacterial glutathione transferase B1-1 (EC ) is characterized by one glutathione (GSH) molecule covalently linked to Cys-10. This peculiar disulfide, only found in the Beta and Omega class glutathione S-transferases (GSTs) but absent in all other GSTs, prompts questions about its role and how GSH can be activated and utilized in the reaction normally performed by GSTs. Stopped-flow and spectroscopic experiments suggest that, in the native enzyme (GSTB1-1ox), a second GSH molecule is present, albeit transiently, in the active site. This second GSH binds to the enzyme through a bimolecular interaction followed by a fast thiol-disulfide exchange with the covalently bound GSH. The apparent pK(a) of the non-covalently bound GSH is lowered from 9.0 to 6.4 +/- 0.2 in similar fashion to other GSTs. The reduced form of GSTB1-1 (GSTB1-1red) binds GSH 100-fold faster and also induces a more active deprotonation of the substrate with an apparent pK(a) of 5.2 +/- 0.1. Apparently, the absence of the mixed disulfide does not affect k(cat) and K(m) values in the GST conjugation activity, which is rate-limited by the chemical step both in GSTB1-1red and in GSTB1-1ox. However, GSTB1-1ox follows a steady-state random sequential mechanism whereas a rapid-equilibrium random sequential mechanism is adopted by GSTB1-1red. Remarkably, GSTB1-1ox and GSTB1-1red are equally able to catalyze a glutaredoxin-like catalysis using cysteine S-sulfate and hydroxyethyl disulfide as substrates. Cys-10 is an essential residue in this redox activity, and its replacement by alanine abolishes this enzymatic activity completely. It appears that GSTB1-1 behaves like an "intermediate enzyme" between the thiol-disulfide oxidoreductase and the GST superfamilies.

Inactivation of glutathione S-transferases by nitric oxide-derived oxidants: exploring a role for tyrosine nitration

Reactive intermediates derived from nitric oxide ((*)NO) are thought to play a contributing role in disease states associated with inflammation and infection. We show here that glutathione S-transferases (GSTs), principal enzymes responsible for detoxification of endogenous and exogenous electrophiles, are susceptible to inactivation by reactive nitrogen species (RNS). Treatment of isolated GSTs or rat liver homogenates with either peroxynitrite, the myeloperoxidase/hydrogen peroxide/nitrite system, or tetranitromethane, resulted in loss of GST activity with a concomitant increase in the formation of protein-associated 3-nitrotyrosine (NO(2)Tyr). This inactivation was only partially (<25%) reversible by dithiothreitol, and exposure of GSTs to hydrogen peroxide or S-nitrosoglutathione was only partially inhibitory (<25%) and did not result in protein nitration. Thus, irreversible modifications such as tyrosine nitration may have contributed to GST inactivation by RNS. Since all GSTs contain a critical, highly conserved, active-site tyrosine residue, we postulated that this Tyr residue might present a primary target for nitration by RNS, thus leading to enzyme inactivation. To directly investigate this possibility, we analyzed purified mouse liver GST-mu, following nitration by several RNS, by trypsin digestion, HPLC separation, and matrix-assisted laser desorption/ionization-time of flight analysis, to determine the degree of tyrosine nitration of individual Tyr residues. Indeed, nitration was found to occur preferentially on several tyrosine residues located in and around the GST active site. However, RNS concentrations that resulted in near complete GST inactivation only caused up to 25% nitration of even preferentially targeted tyrosine residues. Hence, nitration of active-site tyrosine residues may contribute to GST inactivation by RNS, but is unlikely to fully account for enzyme inactivation. Overall, our studies illustrate a potential mechanism by which RNS may promote (oxidative) injury by environmental pollutants in association with inflammation.

The role of glutathione in the isomerization of delta 5-androstene-3,17-dione catalyzed by human glutathione transferase A1-1

Human glutathione transferase (GST) A1-1 efficiently catalyzes the isomerization of Delta(5)-androstene-3,17-dione (AD) into Delta(4)-androstene-3,17-dione. High activity requires glutathione, but enzymatic catalysis occurs also in the absence of this cofactor. Glutathione alone shows a limited catalytic effect. S-Alkylglutathione derivatives do not promote the reaction, and the pH dependence of the isomerization indicates that the glutathione thiolate serves as a base in the catalytic mechanism. Mutation of the active-site Tyr(9) into Phe significantly decreases the steady-state kinetic parameters, alters their pH dependence, and increases the pK(a) value of the enzyme-bound glutathione thiol. Thus, Tyr(9) promotes the reaction via its phenolic hydroxyl group in protonated form. GST A2-2 has a catalytic efficiency with AD 100-fold lower than the homologous GST A1-1. Another Alpha class enzyme, GST A4-4, is 1000-fold less active than GST A1-1. The Y9F mutant of GST A1-1 is more efficient than GST A2-2 and GST A4-4, both having a glutathione cofactor and an active-site Tyr(9) residue. The active sites of GST A2-2 and GST A1-1 differ by only four amino acid residues, suggesting that proper orientation of AD in relation to the thiolate of glutathione is crucial for high catalytic efficiency in the isomerization reaction. The GST A1-1-catalyzed steroid isomerization provides a complement to the previously described isomerase activity of 3beta-hydroxysteroid dehydrogenase.