Temperature adaptation of glutathione S-transferase P1-1. A case for homotropic regulation of substrate binding

Kinetic Behavior of Glutathione Transferases: Understanding Cellular Protection from Reactive Intermediates

Glutathione transferases (GSTs) are the primary catalysts protecting from reactive electrophile attack. In this review, the quantitative levels and distribution of glutathione transferases in relation to physiological function are discussed. The catalytic properties (random sequential) tell us that these enzymes have evolved to intercept reactive intermediates. High concentrations of enzymes (up to several hundred micromolar) ensure efficient protection. Individual enzyme molecules, however, turn over only rarely (estimated as low as once daily). The protection of intracellular protein and DNA targets is linearly proportional to enzyme levels. Any lowering of enzyme concentration, or inhibition, would thus result in diminished protection. It is well established that GSTs also function as binding proteins, potentially resulting in enzyme inhibition. Here the relevance of ligand inhibition and catalytic mechanisms, such as negative co-operativity, is discussed. There is a lack of knowledge pertaining to relevant ligand levels in vivo, be they exogenous or endogenous (e.g., bile acids and bilirubin). The stoichiometry of active sites in GSTs is well established, cytosolic enzyme dimers have two sites. It is puzzling that a third of the site's reactivity is observed in trimeric microsomal glutathione transferases (MGSTs). From a physiological point of view, such sub-stoichiometric behavior would appear to be wasteful. Over the years, a substantial amount of detailed knowledge on the structure, distribution, and mechanism of purified GSTs has been gathered. We still lack knowledge on exact cell type distribution and levels in vivo however, especially in relation to ligand levels, which need to be determined. Such knowledge must be gathered in order to allow mathematical modeling to be employed in the future, to generate a holistic understanding of reactive intermediate protection.

Erythrocyte glutathione transferase. A sensitive Up-Down biomarker of environmental and industrial pollution

Erythrocyte glutathione transferase is a well-known biomarker of environmental pollution. Examination of the extensive scientific literature discovers an atypical and very interesting property of this enzyme which may reveal a chronic exposition to many contaminants but in some cases even an acute and short-term dangerous contamination. This review also underlines the peculiar molecular and kinetic properties of this enzyme which makes it unique in the panorama of enzymes used as biomarker for environmental contamination.

Glutathione Transferase P1-1 an Enzyme Useful in Biomedicine and as Biomarker in Clinical Practice and in Environmental Pollution

Glutathione transferase P1-1 (GSTP1-1) is expressed in some human tissues and is abundant in mammalian erythrocytes (here termed e-GST). This enzyme is able to detoxify the cell from endogenous and exogenous toxic compounds by using glutathione (GSH) or by acting as a ligandin. This review collects studies that propose GSTP1-1 as a useful biomarker in different fields of application. The most relevant studies are focused on GSTP1-1 as a biosensor to detect blood toxicity in patients affected by kidney diseases. In fact, this detoxifying enzyme is over-expressed in erythrocytes when unusual amounts of toxins are present in the body. Here we review articles concerning the level of GST in chronic kidney disease patients, in maintenance hemodialysis patients and to assess dialysis adequacy. GST is also over-expressed in autoimmune disease like scleroderma, and in kidney transplant patients and it may be used to check the efficiency of transplanted kidneys. The involvement of GSTP in the oxidative stress and in other human pathologies like cancer, liver and neurodegenerative diseases, and psychiatric disorders is also reported. Promising applications of e-GST discussed in the present review are its use for monitoring human subjects living in polluted areas and mammals for veterinary purpose.

Evolution of Negative Cooperativity in Glutathione Transferase Enabled Preservation of Enzyme Function

Negative cooperativity in enzyme reactions, in which the first event makes subsequent events less favorable, is sometimes well understood at the molecular level, but its physiological role has often been obscure. Negative cooperativity occurs in human glutathione transferase (GST) GSTP1-1 when it binds and neutralizes a toxic nitric oxide adduct, the dinitrosyl-diglutathionyl iron complex (DNDGIC). However, the generality of this behavior across the divergent GST family and its evolutionary significance were unclear. To investigate, we studied 16 different GSTs, revealing that negative cooperativity is present only in more recently evolved GSTs, indicating evolutionary drift in this direction. In some variants, Hill coefficients were close to 0.5, the highest degree of negative cooperativity commonly observed (although smaller values of n_H are theoretically possible). As DNDGIC is also a strong inhibitor of GSTs, we suggest negative cooperativity might have evolved to maintain a residual conjugating activity of GST against toxins even in the presence of high DNDGIC concentrations. Interestingly, two human isoenzymes that play a special protective role, safeguarding DNA from DNDGIC, display a classical half-of-the-sites interaction. Analysis of GST structures identified elements that could play a role in negative cooperativity in GSTs. Beside the well known lock-and-key and clasp motifs, other alternative structural interactions between subunits may be proposed for a few GSTs. Taken together, our findings suggest the evolution of self-preservation of enzyme function as a novel facility emerging from negative cooperativity.

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.

Phosphorylation of Glutathione S-Transferase P1 (GSTP1) by Epidermal Growth Factor Receptor (EGFR) Promotes Formation of the GSTP1-c-Jun N-terminal kinase (JNK) Complex and Suppresses JNK Downstream Signaling and Apoptosis in Brain Tumor Cells

Under normal physiologic conditions, the glutathione S-transferase P1 (GSTP1) protein exists intracellularly as a dimer in reversible equilibrium with its monomeric subunits. In the latter form, GSTP1 binds to the mitogen-activated protein kinase, JNK, and inhibits JNK downstream signaling. In tumor cells, which frequently are characterized by constitutively high GSTP1 expression, GSTP1 undergoes phosphorylation by epidermal growth factor receptor (EGFR) at tyrosine residues 3, 7, and 198. Here we report on the effect of this EGFR-dependent GSTP1 tyrosine phosphorylation on the interaction of GSTP1 with JNK, on the regulation of JNK downstream signaling by GSTP1, and on tumor cell survival. Using in vitro and in vivo growing human brain tumors, we show that tyrosine phosphorylation shifts the GSTP1 dimer-monomer equilibrium to the monomeric state and facilitates the formation of the GSTP1-JNK complex, in which JNK is functionally inhibited. Targeted mutagenesis and functional analysis demonstrated that the increased GSTP1 binding to JNK results from phosphorylation of the GSTP1 C-terminal Tyr-198 by EGFR and is associated with a >2.5-fold decrease in JNK downstream signaling and a significant suppression of both spontaneous and drug-induced apoptosis in the tumor cells. The findings define a novel mechanism of regulatory control of JNK signaling that is mediated by the EGFR/GSTP1 cross-talk and provides a survival advantage for tumors with activated EGFR and high GSTP1 expression. The results lay the foundation for a novel strategy of dual EGFR/GSTP1 for treating EGFR+ve, GSTP1 expressing GBMs.

Inactivation of human salivary glutathione transferase P1-1 by hypothiocyanite: a post-translational control system in search of a role

Glutathione transferases (GSTs) are a superfamily of detoxifying enzymes over-expressed in tumor tissues and tentatively proposed as biomarkers for localizing and monitoring injury of specific tissues. Only scarce and contradictory reports exist about the presence and the level of these enzymes in human saliva. This study shows that GSTP1-1 is the most abundant salivary GST isoenzyme, mainly coming from salivary glands. Surprisingly, its activity is completely obscured by the presence of a strong oxidizing agent in saliva that causes a fast and complete, but reversible, inactivation. Although salivary α-defensins are also able to inhibit the enzyme causing a peculiar half-site inactivation, a number of approaches (mass spectrometry, site directed mutagenesis, chromatographic and spectrophotometric data) indicated that hypothiocyanite is the main salivary inhibitor of GSTP1-1. Cys47 and Cys101, the most reactive sulfhydryls of GSTP1-1, are mainly involved in a redox interaction which leads to the formation of an intra-chain disulfide bridge. A reactivation procedure has been optimized and used to quantify GSTP1-1 in saliva of 30 healthy subjects with results of 42±4 mU/mg-protein. The present study represents a first indication that salivary GSTP1-1 may have a different and hitherto unknown function. In addition it fulfills the basis for future investigations finalized to check the salivary GSTP1-1 as a diagnostic biomarker for diseases.

Synthesis and structure--activity relationship of new cytotoxic agents targeting human glutathione-S-transferases

The 6-((7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thio)hexan-1-ol (NBDHEX, 1), a "suicide inhibitor" of the glutathione-S-transferase GSTP1-1, showed pro-apoptotic properties in tumor cells, but in vivo studies were limited by poor bioavailability and high affinity towards GSTM2-2, expressed in many non-cancerous tissues. Here we describe the synthesis and biological characterization of new 1 analogs (2-40), in which the hydroxyhexyl portion at the C4-sulfur atom has been replaced with phenyl-containing moieties as well as substituted alkyl chains. Some of the new compounds displayed 10-100 times increased water-solubility (8, 11, 17, 26-28, 34, 35), and most of them showed higher GSTP1-1 selectivity (2-20, 23-26, 31-33, 35) than 1. The presence of a phenyl ring with polar substituents is in general associated, with some exceptions (23, 24) to low cytotoxicity in osteosarcoma U-2OS cells. Differently, some alkyl derivatives possess cytotoxicity comparable (26, 34, 35) or higher (30, 32) than 1. Among the novel compounds, selected ones (26, 27, 34, and 35) deserve further investigation for their anticancer potential.

Precisely controlled molecular imprinting of glutathione-s-transferase by orientated template immobilization using specific interaction with an anchored ligand on a gold substrate

We demonstrate a novel synthetic route for molecularly imprinted polymer (MIP) thin films using a bottom-up approach utilizing protein–ligand specific interactions.

S-nitrosation of glutathione transferase p1-1 is controlled by the conformation of a dynamic active site helix

S-Nitrosation is a post-translational modification of protein cysteine residues, which occurs in response to cellular oxidative stress. Although it is increasingly being linked to physiologically important processes, the molecular basis for protein regulation by this modification remains poorly understood. We used transient kinetic methods to determine a minimal mechanism for spontaneous S-nitrosoglutathione (GSNO)-mediated transnitrosation of human glutathione transferase (GST) P1-1, a major detoxification enzyme and key regulator of cell proliferation. Cys(47) of GSTP1-1 is S-nitrosated in two steps, with the chemical step limited by a pre-equilibrium between the open and closed conformations of helix α2 at the active site. Cys(101), in contrast, is S-nitrosated in a single step but is subject to negative cooperativity due to steric hindrance at the dimer interface. Despite the presence of a GSNO binding site at the active site of GSTP1-1, isothermal titration calorimetry as well as nitrosation experiments using S-nitrosocysteine demonstrate that GSNO binding does not precede S-nitrosation of GSTP1-1. Kinetics experiments using the cellular reductant glutathione show that Cys(101)-NO is substantially more resistant to denitrosation than Cys(47)-NO, suggesting a potential role for Cys(101) in long term nitric oxide storage or transfer. These results constitute the first report of the molecular mechanism of spontaneous protein transnitrosation, providing insight into the post-translational control of GSTP1-1 as well as the process of protein transnitrosation in general.

Human glutathione transferases catalyzing the conjugation of the hepatoxin microcystin-LR

Many cyanobacterial species are able to produce cyanotoxins as secondary metabolites. Among them, microcystins (MC) are a group of around 80 congeners of toxic cyclic heptapeptides. MC-LR is the most studied MC congener, in view of its high acute hepatotoxicity and tumor promoting activity. Humans may be exposed to cyanotoxins through several routes, the oral one being the most important. The accepted pathway for MC-LR detoxication and excretion in the urine is GSH conjugation. The GSH adduct (GS-MCLR) formation has been shown to occur spontaneously and enzymatically, catalyzed by glutathione transferases (GSTs). The enzymatic reaction has been reported but not characterized both in vitro and in vivo in animal and plant species. No data are available on humans. In the present work, the MC-LR conjugation with GSH catalyzed by five recombinant human GSTs (A1-1, A3-3, M1-1, P1-1, and T1-1) has been characterized for the first time. All GSTs are able to catalyze the reaction; kinetic parameters K(m), k(cat), and their relative specific activities to form GS-MCLR were derived (T1-1 > A1-1 > M1-1 > A3-3 ≫ P1-1). In the range of MC tested concentrations used (0.25-50 μM) GSTT1-1 and A1-1 showed a typical saturation curve with similar affinity for MC-LR (≈80 μM; k(cat) values 0.18 and 0.10 min(-1), respectively), A3-3 and M1-1 were linear, whereas GSTP1-1 showed a temperature-dependent sigmoidal allosteric curve with a k(cat) = 0.11 min(-1). The enzymes mainly expressed in the liver and gastrointestinal tract, GSTA1-1, T1-1, and M1-1, seemed to be mainly involved in the MC-LR detoxification after oral exposure, whereas P1-1 kinetics and location in the skin suggest a role related to dermal exposure. Considering the high frequency of some GST polymorphism, especially M1 and T1 gene deletion, with complete loss in activity, this information could be the first step to identify groups of individual at higher risk associated with MC exposure.

Class Pi glutathione transferase unfolds via a dimeric and not monomeric intermediate: functional implications for an unstable monomer

Cytosolic class pi glutathione transferase P1-1 (GSTP1-1) is associated with drug resistance and proliferative pathways because of its catalytic detoxification properties and ability to bind and regulate protein kinases. The native wild-type protein is homodimeric, and whereas the dimeric structure is required for catalytic functionality, a monomeric and not dimeric form of class pi GST is reported to mediate its interaction with and inhibit the activity of the pro-apoptotic enzyme c-Jun N-terminal kinase (JNK) [Adler, V., et al. (1999) EMBO J. 18, 1321-1334]. Thus, the existence of a stable monomeric form of wild-type class pi GST appears to have physiological relevance. However, there are conflicting accounts of the subunit's intrinsic stability since it has been reported to be either unstable [Dirr, H., and Reinemer, P. (1991) Biochem. Biophys. Res. Commun. 180, 294-300] or stable [Aceto, A., et al. (1992) Biochem. J. 285, 241-245]. In this study, the conformational stability of GSTP1-1 was re-examined by equilibrium folding and unfolding kinetics experiments. The data do not demonstrate the existence of a stable monomer but that unfolding of hGSTP1-1 proceeds via an inactive, nativelike dimeric intermediate in which the highly dynamic helix 2 is unfolded. Furthermore, molecular modeling results indicate that a dimeric GSTP1-1 can bind JNK. According to the available evidence with regard to the stability of the monomeric and dimeric forms of GSTP1-1 and the modality of the GST-JNK interaction, formation of a complex between GSTP1-1 and JNK most likely involves the dimeric form of the GST and not its monomer as is commonly reported.

Monomer-dimer equilibrium in glutathione transferases: a critical re-examination

Glutathione transferases (GSTs) are dimeric enzymes involved in cell detoxification versus many endogenous toxic compounds and xenobiotics. In addition, single monomers of GSTs appear to be involved in particular protein-protein interactions as in the case of the pi class GST that regulates the apoptotic process by means of a GST-c-Jun N-terminal kinase complex. Thus, the dimer-monomer transition of GSTs may have important physiological relevance, but many studies reached contrasting conclusions both about the modality and extension of this event and about the catalytic competence of a single subunit. This paper re-examines the monomer-dimer question in light of novel experiments and old observations. Recent papers claimed the existence of a predominant monomeric and active species among pi, alpha, and mu class GSTs at 20-40 nM dilution levels, reporting dissociation constants (K(d)) for dimeric GST of 5.1, 0.34, and 0.16 microM, respectively. However, we demonstrate here that only traces of monomers could be found at these concentrations since all these enzymes display K(d) values of <<1 nM, values thousands of times lower than those reported previously. Time-resolved and steady-state fluorescence anisotropy experiments, two-photon fluorescence correlation spectroscopy, kinetic studies, and docking simulations have been used to reach such conclusions. Our results also indicate that there is no clear evidence of the existence of a fully active monomer. Conversely, many data strongly support the idea that the monomeric form is scarcely active or fully inactive.

Tyrosine phosphorylation of the human glutathione S-transferase P1 by epidermal growth factor receptor

Epidermal growth factor receptor (EGFR) gene amplification, mutations, and/or aberrant activation are frequent abnormalities in malignant gliomas and other human cancers and have been associated with an aggressive clinical course and a poor therapeutic outcome. Elevated glutathione S-transferase P1 (GSTP1), a major drug-metabolizing and stress response signaling protein, is also associated with drug resistance and poor clinical outcome in gliomas and other cancers. Here, we provide evidence that GSTP1 is a downstream EGFR target and that EGFR binds to and phosphorylates tyrosine residues in the GSTP1 protein in vitro and in vivo. Mass spectrometry and mutagenesis analyses in a cell-free system and in gliomas cells identified Tyr-7 and Tyr-198 as major EGFR-specific phospho-acceptor residues in the GSTP1 protein. The phosphorylation increased GSTP1 enzymatic activity significantly, and computer-based modeling showed a corresponding increase in electronegativity of the GSTP1 active site. In human glioma and breast cancer cells, epidermal growth factor stimulation rapidly increased GSTP1 tyrosine phosphorylation and decreased cisplatin sensitivity. Lapatinib, a clinically active EGFR inhibitor, significantly reversed the epidermal growth factor-induced cisplatin resistance. These data define phosphorylation and activation of GSTP1 by EGFR as a novel, heretofore unrecognized component of the EGFR signaling network and a novel mechanism of tumor drug resistance, particularly in tumors with elevated GSTP1 and/or activated EGFR.

Modulating catalytic activity by unnatural amino acid residues in a GSH-binding loop of GST P1-1

The loop following helix alpha2 in glutathione transferase P1-1 has two conserved residues, Cys48 and Tyr50, important for glutathione (GSH) binding and catalytic activity. Chemical modification of Cys48 thwarts the catalytic activity of the enzyme, and mutation of Tyr50 generally decreases the k(cat) value and the affinity for GSH in a differential manner. Cys48 and Tyr50 were targeted by site-specific mutations and chemical modifications in order to investigate how the alpha2 loop modulates GSH binding and catalysis. Mutation of Cys48 into Ala increased K(M)(GSH) 24-fold and decreased the binding energy of GSH by 1.5 kcal/mol. Furthermore, the protein stability against thermal inactivation and chemical denaturation decreased. The crystal structure of the Cys-free variant was determined, and its similarity to the wild-type structure suggests that the mutation of Cys48 increases the flexibility of the alpha2 loop rather than dislocating the GSH-interacting residues. On the other hand, replacement of Tyr50 with Cys, producing mutant Y50C, increased the Gibbs free energy of the catalyzed reaction by 4.8 kcal/mol, lowered the affinity for S-hexyl glutathione by 2.2 kcal/mol, and decreased the thermal stability. The targeted alkylation of Cys50 in Y50C increased the affinity for GSH and protein stability. Characterization of the most active alkylated variants, S-n-butyl-, S-n-pentyl-, and S-cyclobutylmethyl-Y50C, indicated that the affinity for GSH is restored by stabilizing the alpha2 loop through positioning of the key residue into the lock structure of the neighboring subunit. In addition, k(cat) can be further modulated by varying the structure of the key residue side chain, which impinges on the rate-limiting step of catalysis.

Glutathione transferases in the genomics era: new insights and perspectives

In the last decade the tumultuous development of "omics" greatly improved our ability to understand protein structure, function and evolution, and to define their roles and networks in complex biological processes. This fast accumulating knowledge holds great potential for biotechnological applications, from the development of biomolecules with novel properties of industrial and medical importance, to the creation of transgenic organisms with new, favorable characteristics. This review focuses on glutathione transferases (GSTs), an ancient protein superfamily with multiple roles in all eukaryotic organisms, and attempts to give an overview of the new insights and perspectives provided by omics into the biology of these proteins. Among the aspects considered are the redefinition of GST subfamilies, their evolution in connection with structurally related families, present and future biotechnological outcomes.

An empirical extremum principle for the hill coefficient in ligand-protein interactions showing negative cooperativity

The Hill coefficient (nH) is a central parameter in the study of ligand-protein interactions, which measures the degree of cooperativity between subunits that bind the ligand in multisubunit proteins. The most common usage of nH is as an estimate of the minimal number of interacting binding sites in positively cooperating systems. In the present study, a statistical interpretation of nH for a generalized system of multiple identical binding sites is developed. This interpretation is then applied to the derivation of an empirical extremum principle for nH in negatively cooperating systems of identical binding sites, which can be used for the estimation of the minimal number of interacting sites in such systems.

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.

S-(2,3-dichlorotriazinyl)glutathione. A new affinity label for probing the structure and function of glutathione transferases

S-(2,3-Dichlorotriazinyl)glutathione (SDTG) was synthesized and shown to be an effective alkylating affinity label for recombinant maize glutathione S-transferase I (GST I). Inactivation of GST I by SDTG at pH 6.5 followed biphasic pseudo-first-order saturation kinetics. The biphasic kinetics can be described in terms of a fast initial phase of inactivation followed by a slower phase, leading to 42 +/- 3% residual activity. The rate of inactivation for both phases exhibits nonlinear dependence on SDTG concentration, consistent with the formation of a reversible complex with the enzyme (K(d) 107.9 +/- 2.1 micro m for the fast phase, and 224.5 +/- 4.2 micro m for the slow phase) before irreversible modification with maximum rate constants of 0.049 +/- 0.002 min(-1) and 0.0153 +/- 0.001 min(-1) for the fast and slow phases, respectively. Protection from inactivation was afforded by substrate analogues, demonstrating the specificity of the reaction. When the enzyme was inactivated (42% residual activity), approximately 1 mol SDTG per mol dimeric enzyme was incorporated. Amino-acid analysis, molecular modelling, and site-directed mutagenesis studies suggested that the modifying residue is Met121, which is located at the end of alpha-helix H"'(3) and forms part of the xenobiotic-binding site. The results reveal an unexpected structural communication between subunits, which consists of mutually exclusive modification of Met residues across enzyme subunits. Thus, modification of Met121 on one subunit prevents modification of Met121 on the other subunit. This communication is governed by Phe51, which is located at the dimer interface and forms part of the hydrophobic lock-and-key intersubunit motif. The ability of SDTG to inactivate other glutathione-binding enzymes and GST isoenzymes was also investigated, and it was concluded that this new reagent may have general applicability as an affinity reagent for other enzymes with glutathione-binding sites.

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.

Functional role of the lock and key motif at the subunit interface of glutathione transferase p1-1

The glutathione transferases (GSTs) represent a superfamily of dimeric proteins. Each subunit has an active site, but there is no evidence for the existence of catalytically active monomers. The lock and key motif is responsible for a highly conserved hydrophobic interaction in the subunit interface of pi, mu, and alpha class glutathione transferases. The key residue, which is either Phe or Tyr (Tyr(50) in human GSTP1-1) in one subunit, is wedged into a hydrophobic pocket of the other subunit. To study how an essentially inactive subunit influences the activity of the neighboring subunit, we have generated the heterodimer composed of subunits from the fully active human wild-type GSTP1-1 and the nearly inactive mutant Y50A obtained by mutation of the key residue Tyr(50) to Ala. Although the key residue is located far from the catalytic center, the k(cat) value of mutant Y50A decreased about 1300-fold in comparison with the wild-type enzyme. The decrease of the k(cat) value of the heterodimer by about 27-fold rather than the expected 2-fold in comparison with the wild-type enzyme indicates that the two active sites of the dimeric enzyme work synergistically. Further evidence for cooperativity was found in the nonhyperbolic GSH saturation curves. A network of hydrogen-bonded water molecules, found in crystal structures of GSTP1-1, connects the two active sites and the main chain carbonyl group of Tyr(50), thereby offering a mechanism for communication between the two active sites. It is concluded that a subunit becomes catalytically competent by positioning the key residue of one subunit into the lock pocket of the other subunit, thereby stabilizing the loop following the helix alpha2, which interacts directly with GSH.

Thermodynamic description of the effect of the mutation Y49F on human glutathione transferase P1-1 in binding with glutathione and the inhibitor S-hexylglutathione

The thermodynamics of binding of both the substrate glutathione (GSH) and the competitive inhibitor S-hexylglutathione to the mutant Y49F of human glutathione S-transferase (hGST P1-1), a key residue at the dimer interface, has been investigated by isothermal titration calorimetry and fluorescence spectroscopy. Calorimetric measurements indicated that the binding of these ligands to both the Y49F mutant and wild-type enzyme is enthalpically favorable and entropically unfavorable over the temperature range studied. The affinity of these ligands for the Y49F mutant is lower than those for the wild-type enzyme due mainly to an entropy change. Therefore, the thermodynamic effect of this mutation is to decrease the entropy loss due to binding. Calorimetric titrations in several buffers with different ionization heat amounts indicate a release of protons when the mutant binds GSH, whereas protons are taken up in binding S-hexylglutathione at pH 6.5. This suggests that the thiol group of GSH releases protons to buffer media during binding and a group with low pKa (such as Asp98) is responsible for the uptake of protons. The temperature dependence of the free energy of binding, DeltaG0, is weak because of the enthalpy-entropy compensation caused by a large heat capacity change. The heat capacity change is -199.5 +/- 26.9 cal K-1 mol-1 for GSH binding and -333.6 +/- 28.8 cal K-1 mol-1 for S-hexylglutathione binding. The thermodynamic parameters are consistent with the mutation Tyr49 --> Phe, producing a slight conformational change in the active site.

Glutathione transferase P1-1: self-preservation of an anti-cancer enzyme

Self-preservation is a typical property of living organisms, observed in the simplest prokaryotic cell as well as in the more complex pluricellular organisms. Surprisingly we found a self-preservation mechanism operating at the level of a single enzyme. Human glutathione transferase P1-1 operates in such a way towards either killer compounds (competitive and irreversible inhibitors) or physical factors (temperature and UV-rays), which could suppress its detoxicating and anti-cancer activity in the cell. This property, here termed 'co-operative self-preservation', is based on a structural intersubunit communication, by which one subunit, as a consequence of an inactivating modification, triggers a defence arrangement in the other subunit. Paradoxically this ability, developed during evolution for the survival of the cell, may not always be advantageous for us. In fact, glutathione transferase P1-1 is overexpressed in most tumour cells and pharmacological attempts to inhibit this enzyme in vivo, to prevent the drug resistance phenomenon during chemotherapy, may be thwarted by such self-preservation.

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.

Synthesis and activity of novel glutathione analogues containing an urethane backbone linkage

The new GSH analogues H-Glo(-Ser-Gly-OH)-OH (5), its O-benzyl derivative 4, and H-Glo(-Asp-Gly-OH)-OH (9), characterized by the replacement of central cysteine with either serine or aspartic acid, and containing an urethanic fragment as isosteric substitution of the scissile gamma-glutamylic junction, have been synthesized and characterized. Their ability to inhibit human GST P1-1 (hGST P1-1) in comparison with H-Glu(-Ser-Gly-OH)-OH and H-Glu(-Asp-Gly-OH)-OH, which are potent competitive inhibitors of rat GST 3-3 and 4-4, has been evaluated. In order to further investigate the effect of the isosteric substitution on the binding abilities of the new GSH analogues 4, 5 and 9, the previously reported cysteinyl-containing analogue H-Glo(-Cys-Gly-OH)-OH has been also evaluated as a co-substrate for hGSTP1-1.

Human glutathione transferase P1-1 and nitric oxide carriers; a new role for an old enzyme

S-Nitrosoglutathione and the dinitrosyl-diglutathionyl iron complex are involved in the storage and transport of NO in biological systems. Their interactions with the human glutathione transferase P1-1 may reveal an additional physiological role for this enzyme. In the absence of GSH, S-nitrosoglutathione causes rapid and stable S-nitrosylation of both the Cys(47) and Cys(101) residues. Ion spray ionization-mass spectrometry ruled out the possibility of S-glutathionylation and confirms the occurrence of a poly-S-nitrosylation in GST P1-1. S-Nitrosylation of Cys(47) lowers the affinity 10-fold for GSH, but this negative effect is minimized by a half-site reactivity mechanism that protects one Cys(47)/dimer from nitrosylation. Thus, glutathione transferase P1-1, retaining most of its original activity, may act as a NO carrier protein when GSH depletion occurs in the cell. The dinitrosyl-diglutathionyl iron complex, which is formed by S-nitrosoglutathione decomposition in the presence of physiological concentrations of GSH and traces of ferrous ions, binds with extraordinary affinity to one active site of this dimeric enzyme (K(i) < 10(-12) m) and triggers negative cooperativity in the vacant subunit (K(i) = 10(-9) m). The complex bound to the enzyme is stable for hours, whereas in the free form and at low concentrations, its life time is only a few minutes. ESR and molecular modeling studies provide a reasonable explanation of this strong interaction, suggesting that Tyr(7) and enzyme-bound GSH could be involved in the coordination of the iron atom. All of the observed findings suggest that glutathione transferase P1-1, by means of an intersubunit communication, may act as a NO carrier under different cellular conditions while maintaining its well known detoxificating activity toward dangerous compounds.

Disorder-to-order transition of the active site of human class Pi glutathione transferase, GST P1-1

Glutathione transferases comprise a large family of cellular detoxification enzymes that function by catalyzing the conjugation of glutathione (GSH) to electron-deficient centers on carcinogens and other toxins. NMR methods have been used to characterize the structure and dynamics of a human class pi enzyme, GST P1-1, in solution. Resonance assignments have been obtained for the unliganded enzyme and the GSH and S-hexylglutathione (GS-hexyl) complexes. Differences in chemical shifts between the GSH and GS-hexyl complexes suggest more extensive structural differences between these two enzyme-ligand complexes than detected by previous crystallographic methods. The NMR studies reported here clearly show that an alpha-helix (alpha2) within the GSH binding site exists in multiple conformations at physiological temperatures in the absence of ligand. A single conformation of alpha2 is induced by the presence of either GSH or GS-hexyl or a reduction in temperature to below 290 K. The large enthalpy of the transition ( approximately 150 kJ/mol) suggests a considerable structural rearrangement of the protein. The Gibbs free energy for the transition to the unfolded form is on the order of -4 to -6 kJ/mol at physiological temperatures (37 degrees C). This order-to-disorder transition contributes substantially to the overall thermodynamics of ligand binding and should be considered in the design of selective inhibitors of class pi glutathione transferases.

Human glutathione transferase A1-1 demonstrates both half-of-the-sites and all-of-the-sites reactivity

A study of the kinetics of a heterodimeric variant of glutathione transferase (GST) A1-1 has led to the conclusion that, although the wild-type enzyme displays all-of-the-sites reactivity in nucleophilic aromatic substitution reactions, it demonstrates half-of-the-sites reactivity in addition reactions. The heterodimer, designed to be essentially catalytically inactive in one subunit due to a single point mutation (D101K), and the two parental homodimers were analyzed with seven different substrates, exemplifying three types of reactions catalyzed by glutathione transferases (nucleophilic aromatic substitution, addition, and double-bond isomerization reactions). Stopped-flow kinetic results suggested that the wild-type GST A1-1 behaved with half-of-the-sites reactivity in a nucleophilic aromatic substitution reaction, but steady-state kinetic analyses of the GST A1-D101K heterodimer revealed that this was presumably due to changes to the extinction coefficient of the enzyme-bound product. In contrast, steady-state kinetic analysis of the heterodimer with three different substrates of addition reactions provided evidence that the wild-type enzyme displayed half-of-the-sites reactivity in association with these reactions. The half-of-the-sites reactivity was shown not to be dependent on substrate size, the level of saturation of the enzyme with glutathione, or relative catalytic rate.

Thermodynamic analysis of the binding of glutathione to glutathione S-transferase over a range of temperatures

The binding properties of a glutathione S-transferase (EC 2.5.1.18) from Schistosoma japonicum to substrate glutathione (GSH) has been investigated by intrinsic fluorescence and isothermal titration calorimetry (ITC) at pH 6.5 over a temperature range of 15-30 degrees C. Calorimetric measurements in various buffer systems with different ionization heats suggest that protons are released during the binding of GSH at pH 6.5. We have also studied the effect of pH on the thermodynamics of GSH-GST interaction. The behaviour shown at different pHs indicates that at least three groups must participate in the exchange of protons. Fluorimetric and calorimetric measurements indicate that GSH binds to two sites in the dimer of 26-kDa glutathione S-transferase from Schistosoma japonicum (SjGST). On the other hand, noncooperativity for substrate binding to SjGST was detected over a temperature range of 15-30 degrees C. Among thermodynamic parameters, whereas DeltaG degrees remains practically invariant as a function of temperature, DeltaH and DeltaS degrees both decrease with an increase in temperature. While the binding is enthalpically favorable at all temperatures studied, at temperatures below 25 degrees C, DeltaG degrees is also favoured by entropic contributions. As the temperature increases, the entropic contributions progressively decrease, attaining a value of zero at 24.3 degrees C, and then becoming unfavorable. During this transition, the enthalpic contributions become progressively favorable, resulting in an enthalpy-entropy compensation. The temperature dependence of the enthalpy change yields the heat capacity change (DeltaCp degrees ) of -0.238 +/- 0.04 kcal per K per mol of GSH bound.

The conserved Asn49 of maize glutathione S-transferase I modulates substrate binding, catalysis and intersubunit communication

The functional and structural role of the conserved Asn49 of theta class maize glutathione S-transferase was investigated by site-directed mutagenesis. Asn49 is located in the type I beta turn formed by residues 49-52, and is involved in extensive hydrogen-bonding interactions between alpha helix 2 and the rest of the N-terminal domain. The substitution of Asn49 with Ala induces positive cooperativity for 1-chloro-2,4-dinitrobenzene (CDNB) binding as reflected by a Hill coefficient of 1.9 (S(0.5)CDNB = 0.43 mm). The positive cooperativity is also confirmed by following the isothermic binding of 1-hydroxyl-2,4-dinitrobenzene (HDNB) by UV-difference spectroscopy. In addition, the mutated enzyme exhibits: (a) an increase in the Km(GSH) value of about 6.5-fold, and decrease in kcat value of about fourfold; (b) viscosity-independent kinetic parameters; (c) lower thermostability, and (d) increased susceptibility to proteolytic attack by trypsin, when compared to the wild-type enzyme. It is concluded that Asn49 affects the rate-limiting step of the catalytic reaction, and contributes significantly to the structural and binding characteristics of both the glutathione binding site (G-site) and the electrophile substrate binding site (H-site) by affecting the structural integrity of a type I beta turn (comprising residues 49-52) and probably the flexibility of the highly mobile short 310 helical segment of alpha helix 2 (residues 35-46). These structural perturbations are probably transmitted, via Phe51 and Phe65, to alpha helix H3" of the adjacent subunit which contains key residues that interact with the electrophile substrate and contribute to the monomer-monomer contact region. This may accounts for the positive cooperativity observed.

Molecular basis for the polymerization of octopus lens S-crystallin

S-Crystallin from octopus lens has a tertiary structure similar to sigma-class glutathione transferase (GST). However, after isolation from the lenses, S-crystallin was found to aggregate more easily than sigma-GST. In vitro experiments showed that the lens S-crystallin can be polymerized and finally denatured at increasing concentration of urea or guanidinium chloride (GdmCl). In the intermediate concentrations of urea or GdmCl, the polymerized form of S-crystallin is aggregated, as manifested by the increase in light scattering and precipitation of the protein. There is a delay time for the initiation of polymerization. Both the delay time and rate of polymerization depend on the protein concentration. The native protein showed a maximum fluorescence emission spectrum at 341 nm. The GdmCl-denatured protein exhibited two fluorescence maxima at 310 nm and 358 nm, respectively, whereas the urea-denatured protein showed a fluorescence peak at 358 nm with a small peak at 310 nm. The fluorescence intensity was quenched. Monomers, dimers, trimers, and polymers of the native protein were observed by negative-stain electron microscopic analysis. The aggregated form, however, showed irregular structure. The aggregate was solubilized in high concentrations of urea or GdmCl. The redissolved denatured protein showed an identical fluorescence spectrum to the protein solution that was directly denatured with high concentrations of urea or GdmCl. The denatured protein was readily refolded to its native state by diluting with buffer solution. The fluorescence spectrum of the renatured protein solution was similar to that of the native form. The phase diagrams for the S-crystallin in urea and GdmCl were constructed. Both salt concentration and pH value of the solution affect the polymerization rate, suggesting the participation of ionic interactions in the polymerization. Comparison of the molecular models of the S-crystallin and sigma-GST suggests that an extra ion-pair between Asp-101 and Arg-14 in S-crystallin contributes to stabilizing the protomer. Furthermore, the molecular surface of S-crystallin has a protruding Lys-208 on one side and a complementary patch of aspartate residues (Asp-90, Asp-94, Asp-101, Asp-102, Asp-179, and Asp-180) on the other side. We propose a molecular model for the S-crystallin polymer in vivo, which involves side-by-side associations of Lys-208 from one protomer and the aspartate patch from another protomer that allows the formation of a polymeric structure spontaneously into a liquid crystal structure in the lens.

Active site serine promotes stabilization of the reactive glutathione thiolate in rat glutathione transferase T2-2. Evidence against proposed sulfatase activity of the corresponding human enzyme

Ser(11) in rat glutathione transferase T2-2 is important for stabilization of the reactive enzyme-bound glutathione thiolate in the reaction with 1-menaphthyl sulfate. The S11A mutation increased the pK(a) value for the pH dependence of the rate constant for pre-steady-state product formation, from 5.7 to 7.9. This pH dependence is proposed to reflect titration of enzyme-bound glutathione thiol. Further, the mutation lowered the k(cat) value but not because of the impaired stabilization of the glutathione thiolate. In fact, several steps on the reaction pathway were affected by the S11A mutation, and the cause of the decreased k(cat) for the mutant was found to be a slower product release. The data presented here contradict the hypothesis that glutathione transferase T2-2 could act as a sulfatase that is not dependent on Ser(11) for the catalytic activity, as proposed for the corresponding human enzyme (Tan, K.-L., Chelvanayagam, G., Parker, M. W., and Board, P. G. (1996) Biochem. J. 319, 315-321; Rossjohn, J., McKinstry, W. J., Oakley, A. J., Verger, D., Flanagan, J., Chelvanayagam, G., Tan, K.-L., Board, P. G., and Parker, M. W. (1998) Structure 6, 309-322). On the contrary, Ser(11) governs both chemical and physical steps of the catalyzed reaction.

Probing subunit interactions in alpha class rat liver glutathione S-transferase with the photoaffinity label glutathionyl S-[4-(succinimidyl)benzophenone]

Glutathionyl S-[4-(succinimidyl)benzophenone] (GS-Succ-BP), an analogue of the product of glutathione and electrophilic substrate, acts as a photoaffinity label of dimeric rat liver glutathione S-transferase (GST), isoenzyme 1-1. A time-dependent loss of enzyme activity is observed upon irradiation of the enzyme with long wavelength UV light in the presence of the reagent. The initial rate of inactivation exhibits nonlinear dependence on the concentration of the reagent, characterized by an apparent dissociation constant of the enzyme-reagent complex (K(R)) of 99 +/- 2 microM and k(max) of 0.082 +/- 0.005 min(-1). Protection against this inactivation is provided by the electrophilic substrate (ethacrynic acid), electrophilic substrate analogue (dinitrophenol), and product analogues (S-hexylglutathione and p-nitrobenzylglutathione) but not by steroids (Delta(5)-androstene-3,17-dione and 17beta-estradiol-3, 17-disulfate). These results suggest that GS-Succ-BP binds and reacts with the enzyme within the xenobiotic substrate binding site, and this reaction site is distinct from the substrate and nonsubstrate steroid binding sites of the enzyme. About 1 mol of reagent is incorporated into 1 mol of enzyme dimer when the enzyme is completely inactivated. Met-208 is the only amino acid target of the reagent, and modification of this residue in one enzyme subunit of the GST 1-1 dimer completely abolishes the enzyme activity of both subunits. In order to evaluate the role of subunit interactions in the Alpha class glutathione S-transferases, inactive GS-Succ-BP-modified GST 1-1 was mixed with unlabeled, active GST 2-2. The enzyme subunits were dissociated in dilute trifluoroacetic acid and then renatured at pH 7.8 and separated by chromatofocusing into GST 1-1, 1-2, and 2-2. The specific activities of the heterodimer toward several substrates indicate that the loss of catalytic activity in the unmodified subunit of the modified GST 1-1 is the indirect result of the interaction between the two enzyme subunits and that this subunit interaction is absent in the heterodimer GST 1-2.

Anticooperative ligand binding properties of recombinant ferric Vitreoscilla homodimeric hemoglobin: a thermodynamic, kinetic and X-ray crystallographic study

Thermodynamics and kinetics for cyanide, azide, thiocyanate and imidazole binding to recombinant ferric Vitreoscilla sp. homodimeric hemoglobin (Vitreoscilla Hb) have been determined at pH 6.4 and 7.0, and 20.0 degrees C, in solution and in the crystalline state. Moreover, the three-dimensional structures of the diligated thiocyanate and imidazole derivatives of recombinant ferric Vitreoscilla Hb have been determined by X-ray crystallography at 1.8 A (Rfactor=19.9%) and 2.1 A (Rfactor=23.8%) resolution, respectively. Ferric Vitreoscilla Hb displays an anticooperative ligand binding behaviour in solution. This very unusual feature can only be accounted for by assuming ligand-linked conformational changes in the monoligated species, which lead to the observed 300-fold decrease in the affinity of cyanide, azide, thiocyanate and imidazole for the monoligated ferric Vitreoscilla Hb with respect to that of the fully unligated homodimer. In the crystalline state, thermodynamics for azide and imidazole binding to ferric Vitreoscilla Hb may be described as a simple process with an overall ligand affinity for the homodimer corresponding to that for diligation in solution. These data suggest that the ligand-free homodimer, observed in the crystalline state, is constrained in a low affinity conformation whose ligand binding properties closely resemble those of the monoligated species in solution. From the kinetic viewpoint, anticooperativity is reflected by the 300-fold decrease of the second-order rate constant for cyanide and imidazole binding to the monoligated ferric Vitreoscilla Hb with respect to that for ligand association to the ligand-free homodimer in solution. On the other hand, values of the first-order rate constant for cyanide and imidazole dissociation from the diligated and monoligated derivatives of ferric Vitreoscilla Hb in solution are closely similar. As a whole, ligand binding and structural properties of ferric Vitreoscilla Hb appear to be unique among all Hbs investigated to date.