Human glutathione transferase T2-2 discloses some evolutionary strategies for optimization of substrate binding to the active site of glutathione transferases

Genetic variants affecting chemical mediated skin immunotoxicity

The skin is an immune-competent organ and this function may be impaired by exposure to chemicals, which may ultimately result in immune-mediated dermal disorders. Interindividual variability to chemical-induced skin immune reactions is associated with intrinsic individual characteristics and their genomes. In the last 30-40 years, several genes influencing susceptibility to skin immune reactions were identified. The aim of this review is to provide information regarding common genetic variations affecting skin immunotoxicity. The polymorphisms selected for this review are related to xenobiotic-metabolizing enzymes (CYPA1 and CYPB1 genes), antioxidant defense (GSTM1, GSTT1, and GSTP1 genes), aryl hydrocarbon receptor signaling pathway (AHR and ARNT genes), skin barrier function transepidermal water loss (FLG, CASP14, and SPINK5 genes), inflammation (TNF, IL10, IL6, IL18, IL31, and TSLP genes), major histocompatibility complex (MHC) and neuroendocrine system peptides (CALCA, TRPV1, ACE genes). These genes present variants associated with skin immune responses and diseases, as well as variants associated with protecting skin immune homeostasis following chemical exposure. The molecular and association studies focusing on these genetic variants may elucidate their functional consequences and contribution in the susceptibility to skin immunotoxicity. Providing information on how genetic variations affect the skin immune system may reduce uncertainties in estimating chemical hazards/risks for human health in the future.

Characterisation of a Teladorsagia circumcincta glutathione transferase

A 615 bp full length cDNA encoding a Teladorsagia circumcincta glutathione transferase (TcGST) was cloned, expressed in Escherichia coli and the recombinant protein purified and its kinetic properties determined. The predicted protein consisted of 205 amino acids and was present as a single band of about 24 kDa on SDS-PAGE. Multiple alignments of the protein sequence of TcGST with homologues from other helminths showed that the highest identity of 53-68% with haem-binding nematode proteins designated as members of the nu class of GSTs. Substrate binding sites and conserved regions were identified and were generally conserved. The predicted 3-dimensional structures of TcGST and HcGST revealed highly open binding cavities typical of this class of GST, considered to allow greater accessibility to diverse ligands compared with other classes of GST. At 25 °C, the optimum pH for TcGST activity was pH 7, the V_max was 1535 ± 33 nmoles.min^-1. mg^-1 protein and the apparent K_m for the substrate 1-chloro-2,4-dinitrobenzene (CDNB) was 0.22 ± 0.01 mM (mean ± SD, n = 2). Antibodies in both serum and saliva from field-immune, but not nematode-naïve, sheep, recognised recombinant TcGST in enzyme-linked immunosorbent assays. The recognition of the recombinant protein by antibodies generated by exposure of sheep to the native enzyme indicates similar antigenicity of the two proteins. These findings could aid in the design of novel drugs and vaccine antigens for economically important parasites of livestock.

Association of glutathione S-transferase polymorphisms with the severity of mustard lung

Introduction: Glutathione S-transferase (GST) is one of the major detoxifiers in alveoli. Polymorphism in GST genes can influence the ability of individuals to suppress oxidative stress and inflammation. The present study was aimed to explore the hypothesis that the genetic polymorphisms of GST T1, M1 and P1 are associated with the severity of the mustard lung in the sulfur mustard-exposed individuals.

Methods: Blood samples were taken from 185 sulfur mustard-exposed and 57 unexposed subjects. According to the stage of the mustard lung, sulfur mustard-exposed patients were categorized in the mild/moderate and severe/very severe groups. A multiplex PCR method was conducted to identify GSTM1 and GSTT1 null genotypes. To determine the polymorphisms of GSTP1 in exon 5 (Ile105Val) and exon 6 (Ala114Val), RFLP-PCR method was performed.

Results: The frequency of GSTM1 homozygous deletion was significantly higher in the severe/very severe patients compared with the mild/moderate subjects (66.3% vs. 48%, P = 0.013). The GSTM1 null genotype was associated with the severity of mustard lung (adjusted odds ratio [OR], 2.257; 95% CI, 1.219-4.180). There was no significant association between GSTT1 and GSTP1 polymorphisms with the severity of the mustard lung.

Conclusion: The different distribution of GSTM1 null genotype in severe/very severe and mild/moderate groups indicated that the severity of the mustard lung might be associated with the genetic polymorphism(s).

Structure of a Highly Active Cephalopod S-crystallin Mutant: New Molecular Evidence for Evolution from an Active Enzyme into Lens-Refractive Protein

Crystallins are found widely in animal lenses and have important functions due to their refractive properties. In the coleoid cephalopods, a lens with a graded refractive index provides good vision and is required for survival. Cephalopod S-crystallin is thought to have evolved from glutathione S-transferase (GST) with various homologs differentially expressed in the lens. However, there is no direct structural information that helps to delineate the mechanisms by which S-crystallin could have evolved. Here we report the structural and biochemical characterization of novel S-crystallin-glutathione complex. The 2.35-Å crystal structure of a S-crystallin mutant from Octopus vulgaris reveals an active-site architecture that is different from that of GST. S-crystallin has a preference for glutathione binding, although almost lost its GST enzymatic activity. We’ve also identified four historical mutations that are able to produce a “GST-like” S-crystallin that has regained activity. This protein recapitulates the evolution of S-crystallin from GST. Protein stability studies suggest that S-crystallin is stabilized by glutathione binding to prevent its aggregation; this contrasts with GST-σ, which do not possess this protection. We suggest that a tradeoff between enzyme activity and the stability of the lens protein might have been one of the major driving force behind lens evolution.

Directed evolution of Tau class glutathione transferases reveals a site that regulates catalytic efficiency and masks co-operativity

A library of Tau class GSTs (glutathione transferases) was constructed by DNA shuffling using the DNA encoding the Glycine max GSTs GmGSTU2-2, GmGSTU4-4 and GmGSTU10-10. The parental GSTs are >88% identical at the sequence level; however, their specificity varies towards different substrates. The DNA library contained chimaeric structures of alternated segments of the parental sequences and point mutations. Chimaeric GST sequences were expressed in Escherichia coli and their enzymatic activities towards CDNB (1-chloro-2,4-dinitrobenzene) and the herbicide fluorodifen (4-nitrophenyl α,α,α-trifluoro-2-nitro-p-tolyl ether) were determined. A chimaeric clone (Sh14) with enhanced CDNB- and fluorodifen-detoxifying activities, and unusual co-operative kinetics towards CDNB and fluorodifen, but not towards GSH, was identified. The structure of Sh14 was determined at 1.75 Å (1 Å=0.1 nm) resolution in complex with S-(p-nitrobenzyl)-glutathione. Analysis of the Sh14 structure showed that a W114C point mutation is responsible for the altered kinetic properties. This was confirmed by the kinetic properties of the Sh14 C114W mutant. It is suggested that the replacement of the bulky tryptophan residue by a smaller amino acid (cysteine) results in conformational changes of the active-site cavity, leading to enhanced catalytic activity of Sh14. Moreover, the structural changes allow the strengthening of the two salt bridges between Glu(66) and Lys(104) at the dimer interface that triggers an allosteric effect and the communication between the hydrophobic sites.

The impact of nitric oxide toxicity on the evolution of the glutathione transferase superfamily: a proposal for an evolutionary driving force

Glutathione transferases (GSTs) are protection enzymes capable of conjugating glutathione (GSH) to toxic compounds. During evolution an important catalytic cysteine residue involved in GSH activation was replaced by serine or, more recently, by tyrosine. The utility of these replacements represents an enigma because they yield no improvements in the affinity toward GSH or in its reactivity. Here we show that these changes better protect the cell from nitric oxide (NO) insults. In fact the dinitrosyl·diglutathionyl·iron complex (DNDGIC), which is formed spontaneously when NO enters the cell, is highly toxic when free in solution but completely harmless when bound to GSTs. By examining 42 different GSTs we discovered that only the more recently evolved Tyr-based GSTs display enough affinity for DNDGIC (KD < 10(-9) M) to sequester the complex efficiently. Ser-based GSTs and Cys-based GSTs show affinities 10(2)-10(4) times lower, not sufficient for this purpose. The NO sensitivity of bacteria that express only Cys-based GSTs could be related to the low or null affinity of their GSTs for DNDGIC. GSTs with the highest affinity (Tyr-based GSTs) are also over-represented in the perinuclear region of mammalian cells, possibly for nucleus protection. On the basis of these results we propose that GST evolution in higher organisms could be linked to the defense against NO.

Arginine 104 is a key catalytic residue in leukotriene C4 synthase

Human leukotriene C(4) synthase (hLTC(4)S) is an integral membrane enzyme that conjugates leukotriene (LT) A(4) with glutathione to form LTC(4), a precursor to the cysteinyl leukotrienes (LTC(4), LTD(4), and LTE(4)) that are involved in the pathogenesis of human bronchial asthma. From the crystal structure of hLTC(4)S, Arg-104 and Arg-31 have been implicated in the conjugation reaction. Here, we used site-directed mutagenesis, UV spectroscopy, and x-ray crystallography to examine the catalytic role of Arg-104 and Arg-31. Exchange of Arg-104 with Ala, Ser, Thr, or Lys abolished 94.3-99.9% of the specific activity against LTA(4). Steady-state kinetics of R104A and R104S revealed that the K(m) for GSH was not significantly affected. UV difference spectra of the binary enzyme-GSH complex indicated that GSH ionization depends on the presence of Arg-104 because no thiolate signal, with λ(max) at 239 nm, could be detected using R104A or R104S hLTC(4)S. Apparently, the interaction of Arg-104 with the thiol group of GSH reduces its pK(a) to allow formation of a thiolate anion and subsequent nucleophilic attack at C6 of LTA(4). On the other hand, exchange of Arg-31 with Ala or Glu reduced the catalytic activity of hLTC(4)S by 88 and 70%, respectively, without significantly affecting the k(cat)/K(m) values for GSH, and a crystal structure of R31Q hLTC(4)S (2.1 Å) revealed a Gln-31 side chain pointing away from the active site. We conclude that Arg-104 plays a critical role in the catalytic mechanism of hLTC(4)S, whereas a functional role of Arg-31 seems more elusive. Because Arg-104 is a conserved residue, our results pertain to other homologous membrane proteins and represent a structure-function paradigm probably common to all microsomal GSH transferases.

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.

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.

Cooperativity and Pseudo-cooperativity in the Glutathione S-Transferase from Plasmodium falciparum

Binding and catalytic properties of glutathione S-transferase from Plasmodium falciparum (PfGST) have been studied by means of fluorescence, steady state and pre-steady state kinetic experiments, and docking simulations. This enzyme displays a peculiar reversible low-high affinity transition, never observed in other GSTs, which involves the G-site and shifts the apparent K(D) for glutathione (GSH) from 200 to 0.18 mM. The transition toward the high affinity conformation is triggered by the simultaneous binding of two GSH molecules to the dimeric enzyme, and it is manifested as an uncorrected homotropic behavior, termed "pseudo-cooperativity." The high affinity enzyme is able to activate GSH, lowering its pK(a) value from 9.0 to 7.0, a behavior similar to that found in all known GSTs. Using 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, this enzyme reveals a potential optimized mechanism for the GSH conjugation but a low catalytic efficiency mainly due to a very low affinity for this co-substrate. Conversely, PfGST efficiently binds one molecule of hemin/monomer. The binding is highly cooperative (n(H) = 1.8) and occurs only when GSH is bound to the enzyme. The thiolate of GSH plays a crucial role in the intersubunit communication because no cooperativity is observed when S-methylglutathione replaces GSH. Docking simulations suggest that hemin binds to a pocket leaning into both the G-site and the H-site. The iron is coordinated by the amidic nitrogen of Asn-115, and the two carboxylate groups are in electrostatic interaction with the epsilon-amino group of Lys-15. Kinetic and structural data suggest that PfGST evolved by optimizing its binding property with the parasitotoxic hemin rather than its catalytic efficiency toward toxic electrophilic compounds.

Binding and kinetic mechanisms of the Zeta class glutathione transferase

The Zeta class of glutathione transferases (GSTs) has only recently been discovered and hence has been poorly characterized. Here we investigate the substrate binding and kinetic mechanisms of the human Zeta class GSTZ1c-1c by means of pre-steady state and steady-state experiments and site-directed mutagenesis. Binding of GSH occurs at a very low rate compared with that observed for the more recently evolved GSTs (Alpha, Mu, and Pi classes). Moreover, the single step binding mechanism observed in this enzyme is reminiscent of that found for the Theta class enzyme, whereas the Alpha, Mu, and Pi classes have adopted a multistep binding mechanism. Replacement of Cys16 with Ala increases the rate of GSH release from the active site causing a 10-fold decrease of affinity toward GSH. Cys16 also plays a crucial role in co-substrate binding; the mutant enzyme is unable to bind the carcinogenic substrate dichloroacetic acid in the absence of GSH. However, both substrate binding and GSH activation are not rate-limiting in catalysis. A peculiarity of the hGSTZ1c-1c is the half-site activation of bound GSH. This suggests a primitive monomer-monomer interaction that, in the recently diverged GSTP1-1, gives rise to a sophisticated cooperative mechanism that preserves the catalytic efficiency of this GST under stress conditions.

The specific interaction of dinitrosyl-diglutathionyl-iron complex, a natural NO carrier, with the glutathione transferase superfamily: suggestion for an evolutionary pressure in the direction of the storage of nitric oxide

The interaction of dinitrosyl-diglutathionyl-iron complex (DNDGIC), a natural carrier of nitric oxide, with representative members of the human glutathione transferase (GST) superfamily, i.e. GSTA1-1, GSTM2-2, GSTP1-1, and GSTT2-2, has been investigated by means of pre-steady and steady state kinetics, fluorometry, electron paramagnetic resonance, and radiometric experiments. This complex binds with extraordinary affinity to the active site of all these dimeric enzymes; GSTA1-1 shows the strongest interaction (KD congruent with 10-10 m), whereas GSTM2-2 and GSTP1-1 display similar and slightly lower affinities (KD congruent with 10-9 m). Binding of the complex to GSTA1-1 triggers structural intersubunit communication, which lowers the affinity for DNDGIC in the vacant subunit and also causes a drastic loss of enzyme activity. Negative cooperativity is also found in GSTM2-2 and GSTP1-1, but it does not affect the catalytic competence of the second subunit. Stopped-flow and fluorescence data fit well to a common minimal binding mechanism, which includes an initial interaction with GSH and a slower bimolecular interaction of DNDGIC with one high and one low affinity binding site. Interestingly, the Theta class GSTT2-2, close to the ancestral precursor of GSTs, shows very slow binding kinetics and hundred times lowered affinity (KD congruent with 10-7 m), whereas the bacterial GSTB1-1 is not inhibited by DNDGIC. Molecular modeling and EPR data reveal structural details that may explain the observed kinetic data. The optimized interaction with this NO carrier, developed in the more recently evolved GSTs, may be related to the acquired capacity to utilize NO as a signal messenger.

Glutathione transferase superfamily behaves like storage proteins for dinitrosyl-diglutathionyl-iron complex in heterogeneous systems

Electron paramagnetic resonance and kinetics experiments have been made to determine the formation, stability, and fate of the natural nitric oxide carrier, dinitrosyl-diglutathionyl-iron complex (DNDGIC), in heterogeneous systems approaching in vivo conditions. Both in human placenta and rat liver homogenates DNDGIC is formed spontaneously from GSH, S-nitroso-glutathione, and trace amounts of ferrous ions. DNDGIC is unstable in homogenates depleted of glutathione S-transferase (GST); an initial phase of rapid decomposition is followed by a slower decay, which is inversely proportional to the concentration. In the crude human placenta homogenate, GSTP1-1, which represents 90% of the cytosolic GST isoenzymes, is the preferential target for DNDGIC. It binds the complex almost stoichiometrically and stabilizes it for several hours (t1/2 = 8 h). In the presence of an excess of DNDGIC, negative cooperativity in GSTP1-1 opposes the complete loss of the usual detoxicating activity of this enzyme. In the rat liver homogenate, multiple endogenous GSTs (mainly Alpha and Mu class isoenzymes) bind the complex quantitatively and stabilize it (t1/2 = 4.5 h); negative cooperativity is also seen for these GSTs. Thus, the entire pool of cytosolic GSTs, with the exception of the Theta GST, represents a target for stoichiometric amounts of DNDGIC and may act as storage proteins for nitric oxide. These results confirm the existence of a cross-link between NO metabolism and the GST superfamily.

Evolution of supergene families associated with insecticide resistance

The emergence of insecticide resistance in the mosquito poses a serious threat to the efficacy of many malaria control programs. We have searched the Anopheles gambiae genome for members of the three major enzyme families- the carboxylesterases, glutathione transferases, and cytochrome P450s-that are primarily responsible for metabolic resistance to insecticides. A comparative genomic analysis with Drosophila melanogaster reveals that a considerable expansion of these supergene families has occurred in the mosquito. Low gene orthology and little chromosomal synteny paradoxically contrast the easily identified orthologous groups of genes presumably seeded by common ancestors. In A. gambiae, the independent expansion of paralogous genes is mainly a consequence of the formation of clusters among locally duplicated genes. These expansions may reflect the functional diversification of supergene families consistent with major differences in the life history and ecology of these organisms. These data provide a basis for identifying the resistance-associated enzymes within these families. This will enable the resistance status of mosquitoes, flies, and possibly other holometabolous insects to be monitored. The analyses also provide the means for identifying previously unknown molecules involved in fundamental biological processes such as development.

Identification and characterization of GSTT3, a third murine Theta class glutathione transferase

A novel Theta class glutathione transferase (GST) isoenzyme from mouse termed mGSTT3 has been identified by analysis of the expressed sequence tag database. The gene encoding mGSTT3 is clustered with the mGSTT1 and mGSTT2 genes on chromosome 10 and has an exon/intron structure that is similar to that of the other Theta class genes. mGSTT3 is expressed strongly in the liver and to a decreasing extent in the kidney and testis. Recombinant mGSTT3-3 expressed in Escherichia coli had a substrate-specificity profile that differed significantly from that of GSTT1-1 and GSTT2-2 isoenzymes. A molecular model of mGSTT3 suggested that, in comparison with GSTT2, a decrease in volume of the hydrophobic substrate-binding site and the loss of the sulphate-binding pocket prevents its use of the GSTT2 substrate 1-menaphthyl sulphate.

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.

Human glutathione transferase T2-2 discloses some evolutionary strategies for optimization of the catalytic activity of glutathione transferases

Steady state, pre-steady state kinetic experiments, and site-directed mutagenesis have been used to dissect the catalytic mechanism of human glutathione transferase T2-2 with 1-menaphthyl sulfate as co-substrate. This enzyme is close to the ancestral precursor of the more recently evolved glutathione transferases belonging to Alpha, Pi, and Mu classes. The enzyme displays a random kinetic mechanism with very low k(cat) and k(cat)/K(m)((GSH)) values and with a rate-limiting step identified as the product release. The chemical step, which is fast and causes product accumulation before the steady state catalysis, strictly depends on the deprotonation of the bound GSH. Replacement of Arg-107 with Ala dramatically affects the fast phase, indicating that this residue is crucial both in the activation and orientation of GSH in the ternary complex. All pre-steady state and steady state kinetic data were convincingly fit to a kinetic mechanism that reflects a quite primordial catalytic efficiency of this enzyme. It involves two slowly interconverting or not interconverting enzyme populations (or active sites of the dimeric enzyme) both able to bind and activate GSH and strongly inhibited by the product. Only one population or subunit is catalytically competent. The proposed mechanism accounts for the apparent half-site behavior of this enzyme and for the apparent negative cooperativity observed under steady state conditions. These findings also suggest some evolutionary strategies in the glutathione transferase family that have been adopted for the optimization of the catalytic activity, which are mainly based on an increased flexibility of critical protein segments and on an optimal orientation of the substrate.