The binding of substrates and a product of the enzymatic reaction to glutathione S-transferase A

Repurposing Glutathione Transferases: Directed Evolution Combined with Chemical Modification for the Creation of a Semisynthetic Enzyme with High Hydroperoxidase Activity

Glutathione peroxidases (GPXs) are antioxidant selenoenzymes, which catalyze the reduction of hydroperoxides via glutathione (GSH), providing protection to cells against oxidative stress metabolites. The present study aims to create an efficient semisynthetic GPX based on the scaffold of tau class glutathione transferase (GSTU). A library of GSTs was constructed via DNA shuffling, using three homologue GSTUs from Glycine max as parent sequences. The DNA library of the shuffled genes was expressed in E. coli and the catalytic activity of the shuffled enzymes was screened using cumene hydroperoxide (CuOOH) as substrate. A chimeric enzyme variant (named Sh14) with 4-fold enhanced GPX activity, compared to the wild-type enzyme, was identified and selected for further study. Selenocysteine (Sec) was substituted for the active-site Ser13 residue of the Sh14 variant via chemical modification. The GPX activity (kcat) and the specificity constant (kcat/Κm) of the evolved seleno-Sh14 enzyme (SeSh14) was increased 177- and 2746-fold, respectively, compared to that of the wild-type enzyme for CuOOH. Furthermore, SeSh14 effectively catalyzed the reduction of hydrogen peroxide, an activity that is completely undetectable in all GSTs. Such an engineered GPX-like biocatalyst based on the GSTU scaffold might serve as a catalytic bioscavenger for the detoxification of hazardous hydroperoxides. Furthermore, our results shed light on the evolution of GPXs and their structural and functional link with GSTs.

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.

Catalytic and structural diversity of the fluazifop-inducible glutathione transferases from Phaseolus vulgaris

Plant glutathione transferases (GSTs) comprise a large family of inducible enzymes that play important roles in stress tolerance and herbicide detoxification. Treatment of Phaseolus vulgaris leaves with the aryloxyphenoxypropionic herbicide fluazifop-p-butyl resulted in induction of GST activities. Three inducible GST isoenzymes were identified and separated by affinity chromatography. Their full-length cDNAs with complete open reading frame were isolated using RACE-RT and information from N-terminal amino acid sequences. Analysis of the cDNA clones showed that the deduced amino acid sequences share high homology with GSTs that belong to phi and tau classes. The three isoenzymes were expressed in E. coli and their substrate specificity was determined towards 20 different substrates. The results showed that the fluazifop-inducible glutathione transferases from P. vulgaris (PvGSTs) catalyze a broad range of reactions and exhibit quite varied substrate specificity. Molecular modeling and structural analysis was used to identify key structural characteristics and to provide insights into the substrate specificity and the catalytic mechanism of these enzymes. These results provide new insights into catalytic and structural diversity of GSTs and the detoxifying mechanism used by P. vulgaris.

Marine glutathione S-transferases

The aquatic environment is generally affected by the presence of environmental xenobiotic compounds. One of the major xenobiotic detoxifying enzymes is glutathione S-transferase (GST), which belongs to a family of multifunctional enzymes involved in catalyzing nucleophilic attack of the sulfur atom of glutathione (gamma-glutamyl-cysteinylglycine) to an electrophilic group on metabolic products or xenobiotic compounds. Because of the unique nature of the aquatic environment and the possible pollution therein, the biochemical evolution in terms of the nature of GSTs could by uniquely expressed. The full complement of GSTs has not been studied in marine organisms, as very few aquatic GSTs have been fully characterized. The focus of this article is to present an overview of the GST superfamily and their critical role in the survival of organisms in the marine environment, emphasizing the critical roles of GSTs in the detoxification of marine organisms and the unique characteristics of their GSTs compared to those from non-marine organisms.

Identification of lanthionine synthase C-like protein-1 as a prominent glutathione binding protein expressed in the mammalian central nervous system

Proteomic experiments were performed to identify novel glutathione (GSH) binding proteins expressed in the mammalian central nervous system. Bovine brain lysate was affinity purified using an immobilized glutathione-Sepharose column. Proteins that bound the immobilized glutathione were eluted with free glutathione and identified by one- and two-dimensional electrophoresis coupled with mass spectrometric analysis of tryptic fragments. Major proteins purified by this technique were glutathione S-transferase-mu (GST-mu) and GST-pi and lanthionine synthase C-like protein-1 (LanCL1). LanCL1 is a mammalian homologue of a prokaryotic enzyme responsible for the synthesis of thioether (lanthionine) cross-links within nascent polypeptide chains, yielding macrocyclic proteins with potent microbicidal activity. An antibody against LanCL1 was generated and applied to immunochemical studies of spinal cord tissue from SOD1G93A transgenic mice, a model for amyotrophic lateral sclerosis (ALS), wherein LanCL1 expression was found to be increased at presymptomatic stages of the disease. These results indicate LanCL1 is a glutathione binding protein possibly significant to neurodegenerative disease.

Development of a microscopic platform for real-time monitoring of biomolecular interactions

We developed a new microscopic platform for the real-time analysis of molecular interactions by combining microbead-tagging techniques with total internal reflection fluorescent microscopy (TIRFM). The optical manipulation of probe microbeads, followed by photo immobilization on a solid surface, enabled us to generate arrays with extremely high density (>100 microbeads in a 25 microm x 25 microm area), and TIRFM made it possible to monitor the binding reactions of fluorescently labeled targets onto probe microbeads without removal of free targets. We demonstrated the high performance of this platform through analyses of interactions between antigen and antibody and between small compounds and proteins. Then, recombinant protein levels in total cellular lysates of Escherichia coli were quantified from the association kinetics using antibody-immobilized microbead arrays, which served as a model for a protein-profiling array. Furthermore, in combination with in vitro synthesis-coupled protein labeling, we could kinematically analyze the interaction of nuclear factor kappaB (p50) with DNA. These results demonstrated that this platform enabled us to: (1) monitor binding processes of fluorescently labeled targets to multiple probes in real-time without removal of free targets, (2) determine concentrations of free targets only from the association kinetics at an early phase, and (3) greatly reduce the required volume of the target solution, in principle to subnanoliter, for molecular interaction analysis. The unique features of this microbead-based microarray system open the way to explore molecular interactions with a wide range of affinities in extremely small volumes of target solutions, such as extracts from single cells.

Characterization of bromosulphophthalein binding to human glutathione S-transferase A1-1: thermodynamics and inhibition kinetics

In addition to their catalytic functions, GSTs (glutathione S-transferases) bind a wide variety of structurally diverse non-substrate ligands. This ligandin function is known to result in the inhibition of catalytic function. The interaction between hGSTA1-1 (human class Alpha GST with two type 1 subunits) and a non-substrate anionic ligand, BSP (bromosulphophthalein), was studied by isothermal titration calorimetry and inhibition kinetics. The binding isotherm is biphasic, best described by a set of two independent sites: a high-affinity site and a low-affinity site(s). The binding stoichiometries for these sites are 1 and 3 molecules of BSP respectively. BSP binds to the high-affinity site 80 times more tightly (K(d)=0.12 microM) than it does to the low-affinity site(s) (K(d)=9.1 microM). Binding at these sites is enthalpically and entropically favourable, with no linkage to protonation events. Temperature- and salt-dependent studies indicate the significance of hydrophobic interactions in the binding of BSP, and that the low-affinity site(s) displays low specificity towards the anion. Binding of BSP results in the release of ordered water molecules at these hydrophobic sites, which more than offsets unfavourable entropic changes during binding. BSP inhibition studies show that the binding of BSP to its high-affinity site does not inhibit hGSTA1-1. This site, located near Trp-20, may be related to the buffer-binding site observed in GSTP1-1. The low-affinity-binding site(s) for BSP is most probably located at or near the active site of hGSTA1-1. Binding to this site(s) results in non-competitive inhibition with respect to CDNB (1-chloro-2,4-dinitrobenzene) (K(i)(BSP)=16.8+/-1.9 microM). Given the properties of the H site and the relatively small size of the electrophilic substrate CDNB, it is plausible that the active site of the enzyme can simultaneously accommodate both BSP and CDNB. This would explain the non-competitive behaviour of certain inhibitors that bind the active site (e.g. BSP).

Crystal structure of the 28 kDa glutathione S-transferase from Schistosoma haematobium

Schistomiasis is a debilitating parasitic disease which affects 200 million people, causing life-threatening complications in 10% of the patients. This paper reports the crystal structure of the Schistosoma haematobium 28 kDa glutathione S-transferase, a multifunctional enzyme involved in host-parasite interactions and presently considered as a promising vaccine candidate against schistosomiasis. The structures of the GSH-free enzyme, as well as the partially (approximately 40%) and almost fully (approximately 80%) GSH-saturated enzyme, exhibit a unique feature, absent in previous GST structures, concerning the crucial and invariant Tyr10 side chain which occupies two alternative positions. The canonical conformer, which allows an H-bond to be formed between the side chain hydroxyl group and the activated thiolate of GSH, is somewhat less than 50% occupied. The new conformer, with the phenoxyl ring on the opposite side of the mobile loop connecting strand 1 and helix 1, is stabilized by a polar interaction with the guanidinium group of the conserved Arg21 side chain. The presence of two conformers of Tyr10 may provide a clue about clarifying the multiple catalytic functions of Sh28GST and might prove to be relevant for the design of specific antischistosomal drugs. The K(d) for GSH binding was determined by equilibrium fluorescence titrations to be approximately 3 microM and by stopped-flow rapid mixing experiments to be approximately 9 microM. The relatively tight binding of GSH by Sh28GST explains the residually bound GSH in the crystal and supports a possible role of GSH as a tightly bound cofactor involved in the catalytic mechanism for prostaglandin D(2) synthase activity.

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.

Enzymes harboring unnatural amino acids: mechanistic and structural analysis of the enhanced catalytic activity of a glutathione transferase containing 5-fluorotryptophan

The catalytic characteristics and structure of the M1-1 isoenzyme of rat glutathione (GSH) transferase in which all four tryptophan residues in each monomer are replaced with 5-fluorotryptophan are described. The fluorine-for-hydrogen substitution does not change the interaction of the enzyme with GSH even though two tryptophan residues (Trp7 and Trp45) are involved in direct hydrogen-bonding interactions with the substrate. The rate constants for association and dissociation of the peptide, measured by stopped-flow spectrometry, remain unchanged by the unnatural amino acid. The 5-FTrp-substituted enzyme exhibits a kcat of 73 s-1 as compared to 18 s-1 for the native enzyme toward 1-chloro-2,4-dinitrobenzene. That the increase in the turnover number is due to an enhanced rate of product release in the mutant is confirmed by the kinetics of the approach to equilibrium for binding of the product. The crystal structure of the 5-FTrp-containing enzyme was solved at a resolution of 2.0 A by difference Fourier techniques. The structure reveals local conformational changes in the structural elements that define the approach to the active site which are attributed to steric interactions of the fluorine atoms associated with 5-FTrp146 and 5-FTrp214 in domain II. These changes appear to result in the enhanced rate of product release. This structure represents the first of a protein substituted with 5-fluorotryptophan.

Catalytic mechanism and role of hydroxyl residues in the active site of theta class glutathione S-transferases. Investigation of Ser-9 and Tyr-113 in a glutathione S-transferase from the Australian sheep blowfly, Lucilia cuprina

Spectroscopic and kinetic studies have been performed on the Australian sheep blowfly Lucilia cuprina glutathione S-transferase (Lucilia GST; EC 2.5.1.18) to clarify its catalytic mechanism. Steady state kinetics of Lucilia GST are non-Michaelian, but the quite hyperbolic isothermic binding of GSH suggests that a steady state random sequential Bi Bi mechanism is consistent with the anomalous kinetics observed. The rate-limiting step of the reaction is a viscosity-dependent physical event, and stopped-flow experiments indicate that product release is rate-limiting. Spectroscopic and kinetic data demonstrate that Lucilia GST is able to lower the pKa of the bound GSH from 9.0 to about 6.5. Based on crystallographic suggestions, the role of two hydroxyl residues, Ser-9 and Tyr-113, has been investigated. Removal of the hydroxyl group of Ser-9 by site-directed mutagenesis raises the pKa of bound GSH to about 7.6, and a very low turnover number (about 0.5% of that of wild type) is observed. This inactivation may be explained by a strong contribution of the Ser-9 hydroxyl group to the productive binding of GSH and by an involvement in the stabilization of the ionized GSH. This serine residue is highly conserved in the Theta class GSTs, so the present findings may be applicable to all of the family members. Tyr-113 appears not to be essential for the GSH activation. Stopped-flow data indicate that removal of the hydroxyl group of Tyr-113 does not change the rate-limiting step of reaction but causes an increase of the rate constants of both the formation and release of the GSH conjugate. Tyr-113 resides on alpha-helix 4, and its hydroxyl group hydrogen bonds directly to the hydroxyl of Tyr-105. This would reduce the flexibility of a protein region that contributes to the electrophilic substrate binding site; segmental motion of alpha-helix 4 possibly modulates different aspects of the catalytic mechanism of the Lucilia GST.

Binding of the aflatoxin-glutathione conjugate to mouse glutathione S-transferase A3-3 is saturated at only one ligand per dimer

The binding of two different reaction products (p-nitrobenzyl glutathione and the aflatoxin-glutathione conjugate) to mouse glutathione S-transferase A3-3 (mGSTA3-3) has been measured using equilibrium dialysis and a direct fluorescence quenching technique. As expected, p-nitrobenzyl glutathione was found to bind with a stoichiometry of 2.24 +/- 0.17 mol/mol of dimeric enzyme. However, the much larger aflatoxin-glutathione conjugate, 8, 9-dihydro-8-(S-glutathionyl)-9-hydroxyl-aflatoxin B1 (AFB-GSH), was found to bind with a stoichiometry of 1.12 +/- 0.08 mol/mol of dimeric enzyme. p-Nitrobenzyl glutathione bound mGSTA3-3 with a dissociation constant (Kd) of 59 +/- 17 microM while the aflatoxin-glutathione conjugate bound the enzyme with a Kd of 0.86 +/- 0.19 microM. Glutathione competitively inhibited binding of AFB-GSH to mGSTA3-3 with a Ki of 1.5 mM, suggesting that AFB-GSH was binding to the enzyme active site. Although AFB-GSH bound to mGSTA3-3 with a stoichiometry of 1 mol/mol of dimeric enzyme, AFB-GSH completely inhibited activity toward 1-chloro-2, 4-dinitrobenzene, indicating that AFB-GSH binding to one active site alters affinity for 1-chloro-2,4-dinitrobenzene in the active site of the other subunit. To our knowledge, this is the first report of a glutathione S-transferase reaction product which binds to the enzyme with a stoichiometry of 1 mol/mol of dimer.

Structural studies of a human pi class glutathione S-transferase. Photoaffinity labeling of the active site and target size analysis

The glutathione S-transferases (GSTs; EC 2.5.1.18) are a family of dimeric proteins that catalyze reactions between glutathione (GSH) and various electrophiles. A partial cDNA for human GST pi was obtained and the open reading frame completed. The completed cDNA was cloned, and GST pi protein was expressed in bacteria. Cloned enzyme was purified and had the same kinetic constants, molecular mass, pI value, and N-terminal sequence as placental GST pi except that some of the polypeptides had N-terminal methionines. A radiolabeled azido derivative of GSH, S-(p-azidophenacyl)-[3H]glutathione, was used to photoaffinity-label the active site of the cloned enzyme. Labeled enzyme did not bind to a GSH-agarose affinity column. Labeling was prevented in the presence of S-hexylglutathione, and noncovalently-bound azido affinity label was a competitive inhibitor towards 1-chloro-2,4-dinitrobenzene and GSH. These results suggest that the azido label was binding at the active site of the enzyme. Photoaffinity-labeled enzyme was trypsinized, and two labeled peptides were purified and sequenced. One peptide corresponded to residues 183-188, whereas the other corresponded to residues 183-186. These residues appear to form part of the hydrophobic (H-site) binding region of human GST pi that has not been shown previously. Cloned enzyme was subjected to radiation inactivation to assess the importance of subunit interactions in the maintenance of catalytic activity. The target size of enzymatic activity (23 kDa) was not significantly different from that of the protein monomer (24 kDa). Therefore, each subunit of human GST pi appears to be capable of independent catalytic activity.

Involvement of the carboxyl groups of glutathione in the catalytic mechanism of human glutathione transferase A1-1

The present study proposes the participation of both carboxylate groups of the glutathione molecule as functional entities in the catalytic apparatus of human glutathione transferase (GST) A1-1. Functional studies in combination with structural data provide evidence for the alpha-carboxylate of the Glu residue of glutathione acting as a proton acceptor in the catalytic mechanism. The Glu carboxylate is hydrogen-bonded to a protein hydroxyl group and a main-chain NH, as well as to a water molecule of low mobility in the active site region. The Glu alpha-carboxylate of glutathione is bound in a similar manner to the active sites of mammalian glutathione transferases of classes Alpha, Mu, and Pi, for which three-dimensional structures are known. Mutation of the hydroxyl group that is hydrogen-bonded to the alpha-carboxylate of the Glu residue of glutathione (Thr68->Val) caused a shift of the pH dependence of the enzyme-catalyzed reaction, suggesting that the acidic limb of the pH-activity profile reflects the ionization of the carboxylate of the Glu residue of glutathione. The second carboxylate group of glutathione, which is part of its Gly residue, interacts with two Arg side chains in GST A1-1. One of these residues (Arg45) may influence an ionic interaction (Arg221/Asp42), which appears to contribute to binding of the second substrate by fixing the C-terminal alpha-helix as a lid over the active site. Removal of the Gly residue from the glutathione molecule caused a 13-fold increase in the KM value for the electrophilic substrate. Thus, the Gly carboxylate of glutathione, by way of influencing the topology of the active site, contributes to the binding of the second substrate of the enzyme. Consequently, the glutathione molecule has several functions in the glutathione transferase catalyzed reactions, not only as a substrate providing the thiol group for different types of chemical reactions but also as a substrate contributing a carboxylate that acts as a proton acceptor in the catalytic mechanism and a carboxylate that modulates binding of the second substrate to the enzyme.

Site-directed mutagenesis of human glutathione transferase P1-1. Mutation of Cys-47 induces a positive cooperativity in glutathione transferase P1-1

Glutathione transferase P1-1 (EC 2.5.1.18) is a dimeric enzyme composed of identical subunits each containing one binding site for GSH and a second for the co-substrate e.g. 1-chloro-2,4-dinitrobenzene. Steady-state kinetics are strictly hyperbolic toward both these substrates. Replacement of Cys-47 with alanine or serine decreases the affinity for GSH and triggers a positive kinetic cooperativity with respect to the substrate. Hill coefficients were 1.31 and 1.43 for the C47A and C47S mutants. C47A/C101S and C47S/C101S double mutants display lower affinity for GSH and higher Hill coefficients (1.57 and 1.56, respectively) when compared with C47A and C47S single mutants. Conversely, replacement of Cys-101 with alanine or serine does not yield any cooperativity and any marked change of kinetic parameters. Fluorometric experiments gave sigmoidal isothermic GSH binding curves for all the Cys-47 mutants, with Hill coefficients similar to that obtained by the kinetic approach. These data, together with the activation experiments performed in the presence of S-hexylglutathione, suggest that the substitution of Cys-47 yields a dimeric low-affinity enzyme which may be revealed by the lack of a peculiar electrostatic bond between the thiolate form of Cys-47 and the protonated amino group of Lys-54.

X-ray crystal structures of cytosolic glutathione S-transferases. Implications for protein architecture, substrate recognition and catalytic function

Crystal structures of cytosolic glutathione S-transferases (EC 2.5.1.18), complexed with glutathione or its analogues, are reviewed. The atomic models define protein architectural relationships between the different gene classes in the superfamily, and reveal the molecular basis for substrate binding at the two adjacent subsites of the active site. Considerable progress has been made in understanding the mechanism whereby the thiol group of glutathione is destabilized (lowering its pKa) at the active site, a rate-enhancement strategy shared by the soluble glutathione S-transferases.

Effect of subunit interactions on enzymatic activity of glutathione S-transferases: a radiation inactivation study

The glutathione S-transferases are a family of dimeric enzymes. Three isozymes from the alpha family, termed YaYa, YaYc, and YcYc, and three from the mu family, termed Yb1Yb1, Yb1Yb2, and Yb2Yb2, were purified from rat liver. Binding studies were performed by equilibrium dialysis using a radiolabeled product, S(-)[14C](dinitrophenyl)glutathione. Each isozyme contained two independent binding sites which had equal affinity for the ligand. The presence of two independent active sites per enzyme dimer suggests that each subunit contains a complete active site. This conclusion was examined further using radiation inactivation which also allowed for assessment of the importance of subunit interactions in catalytic activity. The activity target size of YaYa (47 kDa) was significantly larger than the protein monomer target size (31 kDa); similarly the activity target size of YaYc was that of the dimer (54 kDa). In contrast, the activity target sizes of Yb1Yb1 and Yb2Yb2 were the same, being 35 and 29 kDa, respectively, and the protein monomer target size of Yb1Yb1 also was similar, being 32 kDa. These data indicate that interactions between subunits are critical for the maintenance of enzymatic activity of alpha class enzymes whereas each subunit of the two mu class proteins is capable of independent catalytic activity.

Identification of Tyr115 labeled by S-(4-bromo-2,3-dioxobutyl)glutathione in the hydrophobic substrate binding site of glutathione S-transferase, isoenzyme 3-3

Incubation of S-(4-bromo-2,3-dioxobutyl)glutathione (S-BDB-G), a reactive analogue of glutathione, with the 3-3 isoenzyme of rat liver glutathione S-transferase at pH 6.5 and 25 degrees C results in a time-dependent inactivation of the enzyme. The kobs exhibits a nonlinear dependence on S-BDB-G concentration from 50 to 900 microM, with a kmax of 0.073 min-1 and KI = 120 microM. The addition of 5 mM S-hexylglutathione, a competitive inhibitor with respect to glutathione, completely protects against inactivation by S-BDB-G. About 2.0 mol of [3H]S-BDB-G/mol of enzyme subunit is incorporated concomitant with 100% inactivation, whereas only 0.96 mol of reagent/mol subunit is incorporated in the presence of S-hexylglutathione when activity is fully retained. Modified enzyme, prepared by incubating glutathione S-transferase with [3H]S-BDB-G in the absence or in the presence of S-hexylglutathione, was reduced with NaBH4, reacted with N-ethylmaleimide, and digested with trypsin. Analysis of the tryptic digests, fractionated by reverse-phase high-performance liquid chromatography, revealed Tyr115 as the amino acid whose reaction with S-BDB-G correlates with inactivation. Examination of the stability of S-(4-bromo-2,3-dioxobutyl)glutathione and modified enzyme in the absence and presence of dithiothreitol and under acidic conditions suggests that for stable linkage to peptides, the carbonyl moieties of the reagent should be reduced immediately after modification of a protein. Comparison of results from the 4-4 and 3-3 isoenzymes of rat liver glutathione S-transferase (both of the mu gene class) indicates: the 4-4 isoenzyme exhibits a greater affinity for S-BDB-G; Cys86 is labeled by [3H]S-BDB-G in both isoenzymes but is nonessential for activity; in the 3-3 isoenzyme, Cys86 is more accessible to S-BDB-G; and Tyr115 is an important residue in the hydrophobic binding site of both enzymes.

Characterization and localization of glutathione binding sites on cultured astrocytes

Glutathione (GSH) binding sites found in brain white matter in a previous study using biotinylated GSH (Third IBRO World Congress Neurosci. Abstr., 1991, P59.17) suggested that there might GSH receptors on glial cells. In the present study, radioligand receptor assays were performed on cultured astrocytes using [35S]GSH. Scatchard analyses of saturation binding of [35S]GSH revealed two binding sites: Kd1 = 2.0 +/- 0.1 nM, Bmax1 = 89.5 +/- 1.5 fmole/2.2 x 10(5) cells and Kd2 = 12.8 +/- 0.4 nM, Bmax2 = 187.7 +/- 2.4 fmol/2.2 x 10(5) cells. The saturable and displacible high affinity [35S]GSH binding we have observed suggests that this binding is not due to GSH sequestration by uptake sites or to the association of GSH with GSH S-transferases or GSH peroxidases which have Kds in the microM range. Colloidal gold and immunofluorescence double labelling were used to visualize the binding sites at the cellular level. Positive colloidal gold decoration further suggests that these labelled binding sites are membrane receptors on astrocytes.

Age-related alterations of enzyme activities and subunits of hepatic glutathione S-transferases in male and female Fischer-344 rats

Enzyme activities of glutathione S-transferases (GSTs) toward five different substrates (benzalacetone (PBO), styrene oxide (STOX), sulfobromophthalein (BSP), 1,2-dichloro-4-nitrobenzene (DCNB) and 1-chloro-2,4-dinitrobenzene (CDNB)) as well as concentrations of four subunits of GST isozymes (1, 2, 3 and 4) were determined using cytosol fractions obtained from livers of young (6 months) and old (26 months) Fischer-344 rats of both sexes. Values for enzyme activities for three substrates (DCNB, BSP and PBO) in young male rats were significantly higher than the corresponding values in female rats. In old male rats, values were generally lower than the corresponding values in young male rats, becoming close to corresponding values in young female rats. Old female rats, however, exhibited values close to those in young female rats, except for DCNB and STOX values, which were slightly lower in old female rats. GST subunits 3 and 4, as determined by high-performance liquid chromatography after purification by affinity chromatography using S-hexyl-glutathione, were predominant in young males, whereas concentrations of subunits 1 and 2 were higher in females than in males. In male rat livers, concentrations of subunits 3 and 4 decreased considerably with age while those of subunits 1 and 2 increased, so that the subunit pattern in old male rats tended to be similar to that of young female rats. In old females, a decrease in the concentration of subunits 3 and 4 and an increase in the concentration of subunit 1 were also observed as in old male rats, while the subunit 2 concentration tended to decline. Furthermore, the elution pattern of affinity chromatography changed with age, yielding an earlier elution of most subunits in old male rats and of subunit 1 in old female rats. The results suggest that age-related changes that occur with GSTs in livers of male rats are essentially a feminization of the isozyme pattern. However, despite rather unremarkable changes in enzyme activities with age in females, considerable changes of subunit pattern (a general decrease in concentration of subunits 2, 3 and 4 and an increase in the concentration of subunit 1) were also observed in female rats, and these were much greater than could be predicted from enzyme activity changes with age in this sex.

Inactivation of human placenta glutathione S-transferase by SH/SS exchange reaction with biological disulfides

The oxidized glutathione inhibited the activity of glutathione S-transferase purified from human placenta just through competitive inhibition. On the other hand, cystine and cystamine inactivated the activity by pseudo first-order in low concentrations, accompanying the stoichiometric incorporation of the radioactivity of [14C]-cystine to the enzyme protein until a half mole per one subunit. This and the protective effect of glutathione analogues suggested that the SH/SS exchange reaction occurred between the disulfide and the SH group near the glutathione binding site of the enzyme to form a mixed disulfide.

Explanation of the non-hyperbolic kinetics of the glutathione S-transferases by the simplest steady-state random sequential Bi Bi mechanism

We have demonstrated that the simplest steady-state random sequential Bi Bi mechanism is sufficient to explain the previously reported non-hyperbolic kinetics of glutathione S-transferase 3-3 [Pabst MJ et al., J Biol Chem 249: 7140-7150, 1974; Jakobson I et al., Biochem J 177: 861-868, 1979]. The metabolism of 1-chloro-2,4-dinitrobenzene by rat liver glutathione S-transferase isoenzymes 2-2 and 3-3 and of 1,2-dichloro-4-nitrobenzene by isoenzyme 3-4 was shown to exhibit non-hyperbolic kinetics, which are best fit by the simplest steady-state random sequential Bi Bi mechanism. Neither more complex steady-state mechanisms nor the superimposition of product inhibition or enzyme memory on the simplest steady-state mechanism was necessary to generate non-hyperbolic kinetics for the glutathione S-transferases.

Construction, expression, and preliminary characterization of chimeric class mu glutathione S-transferases with altered catalytic properties

An expression plasmid for isoenzyme 3-3 of rat liver glutathione S-transferase has been constructed from the cDNA clone pGTA/C44 and the pAS expression vector pMG27NS, and used for the efficient production of the enzyme in the Escherichia coli strain M5219. The plasmid has also been manipulated, through the use of synthetic linkers, to encode chimeric polypeptides in which short sequences of the closely related isoenzyme 4-4 have been substituted into the N-terminal and C-terminal variable domains of isoenzyme 3-3. The chimeric polypeptides designated 4(9)3(208), 3(209)4(8), and 4(9)3(200)4(8) are expressed with varying degrees of efficiency in E. coli. The active dimeric holoenzymes 3-3, (4(9)3(208]2, (3(209)4(8]2, and (4(9)3(200)4(8]2 can be isolated. The spectroscopic and kinetic properties of the chimeric enzymes are significantly different than the native enzyme.

Characterization of a variant rat glutathione S-transferase by cDNA expression in Escherichia coli

We have isolated a glutathione S-transferase Yb1 subunit cDNA from a lambda gt11 cDNA collection constructed from rat testis poly(A) RNA enriched for glutathione S-transferase mRNA activities. This Yb1 cDNA, designated pGTR201, is identical to our liver Yb1 cDNA clone pGTR200 except for a shorter 5'-untranslated sequence. Active glutathione S-transferase is expressed from this Yb1 cDNA driven by the tac promoter on the plasmid construct pGTR201-KK. The expressed glutathione S-transferase protein begins with the third codon (Met) of the cDNA, and is missing the N-terminal proline of rat liver glutathione S-transferase 3-3. Therefore, our Escherichia coli expressed glutathione S-transferase protein represents a variant form of glutathione S-transferase 3-3 (Yb1Yb1), designated GST 3-3(-1). The expressed Yb1 subunits are assembled into a dimer as purified from sonicated E. coli crude extracts. In the absence of dithiothreitol three active isomers can be resolved by ion-exchange chromatography. The pure protein has an extinction coefficient of 9.21 x 10(4) M-1 cm-1 at 280 nm or E0.1% 280 = 1.78 and a pI at 8.65. It has a substrate specificity pattern similar to that of the authentic glutathione S-transferase 3-3. The GST 3-3(-1) has a KM of 202 microM for reduced GSH and of 36 microM for 1-chloro-2,4-dinitrobenzene. The turnover number for this conjugation reaction is 57 s-1. Results of kinetic studies of this reaction with GST 3-3(-1) are consistent with a sequential substrate binding mechanism. We conclude that the first amino acid proline of glutathione S-transferase 3-3 is not essential for enzyme activities.

S-(4-azidophenacyl)[35S]glutathione photoaffinity labeling of rat liver plasma membrane-associated proteins

A method for the synthesis of the glutathione conjugate S-(4-azidophenacyl)[35S]glutathione is described. The compound was used for photoaffinity labeling of proteins present in canalicular membrane vesicles (CMV), sinusoidal membrane vesicles (SMV), mitochondria and microsomes from rat liver. Most of the radioactivity introduced by photoaffinity labeling of CMV appeared in the 25-29 kDa range. Further labeled proteins were observed in bands at 37, 105 and about 120 kDa. 79% of the 25-29 kDa associated radioactivity was recovered in the supernatant after extensive revesiculation (washing) of the vesicles, together with the 37 kDa protein. CMV and SMV contained glutathione S-transferase (GST) activity which in CMV was decreased by 75% by washing. Photolabeling of a mixture of purified basic GST subunits from rat liver resulted in a band pattern at 25-29 kDa similar to that in the membrane preparations. Isoelectric focusing of the CMV indicated the presence of basic soluble GST subunits. S-Hexylglutathione-Sepharose affinity chromatography showed reversible binding of photolabeled proteins at 25-29 kDa. Difference photoaffinity labeling with GSSG, S-hexylglutathione, taurocholate and phenylmethylsulfonyl fluoride decreased the radioactivity bound by GST, but not that introduced into the 105 kDa protein band present in CMV. It is concluded that membrane-associated basic GST isoenzymes are present in standard membrane vesicle preparations. In the cell, the function may be transport of GST-bound compounds across the membrane and protection of the membranes against electrophiles.

Spectroscopic and kinetic evidence for the thiolate anion of glutathione at the active site of glutathione S-transferase

Ultraviolet difference spectroscopy of the binary complex of isozyme 4-4 of rat liver glutathione S-transferase with glutathione (GSH) and the enzyme alone or as the binary complex with the oxygen analogue, gamma-L-glutamyl-L-serylglycine (GOH), at neutral pH reveals an absorption band at 239 nm (epsilon = 5200 M-1 cm-1) that is assigned to the thiolate anion (GS-) of the bound tripeptide. Titration of this difference absorption band over the pH range 5-8 indicates that the thiol of enzyme-bound GSH has a pKa = 6.6, which is about 2.4 pK units less than that in aqueous solution and consistent with the kinetically determined pKa previously reported [Chen et al. (1988) Biochemistry 27, 647]. The observed shift in the pKa between enzyme-bound and free GSH suggests that about 3.3 kcal/mol of the intrinsic binding energy of the peptide is utilized to lower the pKa into the physiological pH range. Apparent dissociation constants for both GSH and GOH are comparable and vary by a factor of less than 2 over the same pH range. Site occupancy data and spectral band intensity reveal large extinction coefficients at 239 nm (epsilon = 5200 M-1 cm-1) and 250 nm (epsilon = 1100 M-1 cm-1) that are consistent with the existence of either a glutathione thiolate (E.GS-) or ion-paired thiolate (EH+.GS-) in the active site. The observation that GS- is likely the predominant tripeptide species bound at the active site suggested that the carboxylate analogue of GSH, gamma-L-glutamyl-(D,L-2-aminomalonyl)glycine, should bind more tightly than GSH.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of iduronic acid and glucuronic acid in glycosaminoglycans after stoichiometric reduction and depolymerization using high-performance liquid chromatography and ultraviolet detection

The reduction of uronic acids in glycosaminoglycans (GAGs) prior to depolymerization reactions is one way in which the uronic acid content of polysaccharides can be studied without major losses. The obtained monosaccharides can be recovered from the subsequent depolymerization with a yield better than 95%. Following reduction, depolymerization, and lyophilization, D-glucuronic acid is converted to D-Glc and L-iduronic acid to 1,6-anhydro-idose. Per-O-benzoyl derivatives of these monosaccharides can be separated and detected in nanogram amounts using reversed phase HPLC. A linear detector response was obtained for injections up to 22 nmol (4 micrograms) of Glc and 1,6-anhydro-idose and the detection limit was 5 and 7 pmol, respectively. Reduction, depolymerization, and derivatization with subsequent chromatography of various GAGs can be readily performed in the 1- to 30-micrograms range.

Glutathione transferases--structure and catalytic activity

The glutathione transferases are recognized as important catalysts in the biotransformation of xenobiotics, including drugs as well as environmental pollutants. Multiple forms exist, and numerous transferases from mammalian tissues, insects, and plants have been isolated and characterized. Enzymatic properties, reactions with antibodies, and structural characteristics have been used for classification of the glutathione transferases. The cytosolic mammalian enzymes could be grouped into three distinct classes--Alpha, Mu, and Pi; the microsomal glutathione transferase differs greatly from all the cytosolic enzymes. Members of each enzyme class have been identified in human, rat, and mouse tissues. Comparison of known primary structures of representatives of each class suggests a divergent evolution of the enzyme proteins from a common precursor. Products of oxidative metabolism such as organic hydroperoxides, epoxides, quinones, and activated alkenes are possible "natural" substrates for the glutathione transferases. Particularly noteworthy are 4-hydroxyalkenals, which are among the best substrates found. Homologous series of substrates give information about the properties of the corresponding binding site. The catalytic mechanism and the active-site topology have been probed also by use of chiral substrates. Steady-state kinetics have provided evidence for a "sequential" mechanism.

A rapid purification procedure for camel kidney glutathione S-transferase

Glutathione S-transferase was isolated from supernatant of camel kidney homogenate centrifugation at 37,000 xg by glutathione agarose affinity chromatography. The enzyme preparation has a specific activity of 44 mumol/min/mg protein and recovery was more than 85% of the enzyme activity in the crude extract. Glutathione agarose affinity chromatography resulted in a purification factor of about 49 and chromatofocusing resolved the purified enzyme into two major isoenzymes (pI 8.7 and 7.9) and two minor isoenzymes (pI 8.3 and 6.9). The homogeneity of the purified enzyme was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration on Sephadex G-100. The different isoenzymes were composed of a binary combination of two subunits with molecular weight of 29,000 D and 26,000 D to give a native molecular weight of 55,000 D. The substrate specificities of the major camel kidney glutathione S-transferase isoenzymes were determined towards a range of substrates. 1-chloro-2,4-dinitrobenzene was the preferred substrate for all the isoenzymes. Isoenzyme III (pI 7.9) had higher specific activity for ethacrynic acid and isoenzyme II (pI 8.3) was the only isoenzyme that exhibited peroxidase activity. Ouchterlony double-diffusion analysis with rabbit antiserum prepared against the camel kidney enzyme showed fusion of precipitation lines with the enzymes from camel brain, liver and lung and no cross reactivity was observed with enzymes from kidneys of sheep, cow, rat, rabbit and mouse. Different storage conditions have been found to affect the enzyme activity and the loss in activity was marked at room temperature and upon repeated freezing and thawing.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.

Stereoselective product inhibition of glutathione S-transferase

Isozymes 3-3 and 4-4 of rat liver glutathione S-transferase are stereoselectively inhibited by the diastereomers of 9,10-dihydro-9-glutathionyl-10-hydroxyphenanthrene, 1. The conformation of the biphenyl moiety is the same in the enzyme -1 complex as in aqueous solution with the glutathionyl and hydroxy groups in the axial positions. Isozyme 4-4 is also inhibited by the four diastereomers of 1,2-diphenyl-1-(S-glutathionyl)-2-hydroxyethane. The stereoselectivity of inhibition is modest in all cases and is manifest in both the type of inhibition as well as the magnitude of Ki.

Synthesis and characterization of the oxygen and desthio analogues of glutathione as dead-end inhibitors of glutathione S-transferase

The oxygen analogue, gamma-L-Glu-L-SerGly (GOH) and desthio analogue, gamma-L-Glu-L-AlaGly (GH) have been synthesized by a simple three step procedure involving active ester coupling of N-t-BOC-alpha-(4-nitrophenyl)-L-glutamate to L-SerGly and L-AlaGly, respectively. The two peptides are excellent dead-end inhibitors of isozymes 3-3 and 4-4 of rat liver glutathione S-transferase. At low fixed concentrations of 1-chloro-2,4-dinitrobenzene (CDNB) GOH and GH are linear competitive inhibitors of isozyme 3-3 vs glutathione with KI values of 13.0 and 116 microM, respectively. Both peptides are non-competitive (mixed-type) inhibitors vs CDNB when glutathione is the fixed substrate. Similar results are obtained with both peptides and isozyme 4-4. The results rule out ordered or ping-pong kinetic mechanisms where the electrophile adds first.