Catalytic and structural contributions for glutathione-binding residues in a Delta class glutathione S-transferase

Functional and Structural Diversity of Insect Glutathione S-transferases in Xenobiotic Adaptation

As a superfamily of multifunctional enzymes that is mainly associated with xenobiotic adaptation, glutathione S-transferases (GSTs) facilitate insects' survival under chemical stresses in their environment. GSTs confer xenobiotic adaptation through direct metabolism or sequestration of xenobiotics, and/or indirectly by providing protection against oxidative stress induced by xenobiotic exposure. In this article, a comprehensive overview of current understanding on the versatile functions of insect GSTs in detoxifying chemical compounds is presented. The diverse structures of different classes of insect GSTs, specifically the spatial localization and composition of their amino acid residues constituted in their active sites are also summarized. Recent availability of whole genome sequences of numerous insect species, accompanied by RNA interference, X-ray crystallography, enzyme kinetics and site-directed mutagenesis techniques have significantly enhanced our understanding of functional and structural diversity of insect GSTs.

SiteMotif: A graph-based algorithm for deriving structural motifs in Protein Ligand binding sites

Studying similarities in protein molecules has become a fundamental activity in much of biology and biomedical research, for which methods such as multiple sequence alignments are widely used. Most methods available for such comparisons cater to studying proteins which have clearly recognizable evolutionary relationships but not to proteins that recognize the same or similar ligands but do not share similarities in their sequence or structural folds. In many cases, proteins in the latter class share structural similarities only in their binding sites. While several algorithms are available for comparing binding sites, there are none for deriving structural motifs of the binding sites, independent of the whole proteins. We report the development of SiteMotif, a new algorithm that compares binding sites from multiple proteins and derives sequence-order independent structural site motifs. We have tested the algorithm at multiple levels of complexity and demonstrate its performance in different scenarios. We have benchmarked against 3 current methods available for binding site comparison and demonstrate superior performance of our algorithm. We show that SiteMotif identifies new structural motifs of spatially conserved residues in proteins, even when there is no sequence or fold-level similarity. We expect SiteMotif to be useful for deriving key mechanistic insights into the mode of ligand interaction, predict the ligand type that a protein can bind and improve the sensitivity of functional annotation.

A large number of biological functions are orchestrated by proteins. The function of proteins is governed by its structure and its interacting ligand. However, it is known that not all residues are involved in ligand recognition. More specifically, residues that are located within 4.5 Å of ligand atoms are considered to be ’binding sites’. Here, we have developed an algorithm called SiteMotif that efficiently aligns multiple binding sites into a common frame. This process enables us to derive conservation among the binding site residues in a sequence order independent manner. The algorithm was validated extensively across five different levels and measured binding site similarities in each of them. Previous research has found multiple instances where different proteins have comparable binding sites and hence perform the same function. We present the ability of our method to detect such scenarios. Finally, As a use case, we applied SiteMotif to a set of glutathione binding proteins and derived a site based sequence motif characteristic of all glutathione binding proteins.

Non-synonymous substitution of evolutionarily conserved residue in Tau class glutathione transferases alters structural and catalytic features

Plant-specific tau glutathione transferases (GSTs) are basically involved in catalysing γ-glutathione (GSH)-dependent conjugation reactions with pesticides and herbicides, which play an important role in the detoxification of pollutants. Given the lack of systematic biochemical and structural information on tau GSTs, the study of their mediated defence mechanisms against toxic compounds has been greatly hindered. Here, we reveal the importance of the Ile residue closely interacting with GSH for the structural stability and catalytic function of GST. Evolutionary conservation analysis indicated that the crucial G-site Ile55 in the SbGSTU6 was converted to Thr53 of SbGSTU7. The comparative biochemical data on SbGSTU6, SbGSTU7 and their mutants showed that the substitution of Ile by Thr caused significant decrease in the affinity and catalytic efficiency of the GSTs. The unfavourable structural flexibility and pKa distribution of the active cavity residues were also demonstrated. Crystallography studies and molecular dynamics simulations showed that the conversion resulted in the hydrogen bond recombination with GSH and conformational rearrangement of GST active cavity, in which the Ile residue was more conducive to the formation of enzyme substrate complexes. The extensive biochemical and structural data not only reveal the critical role of the conserved G-site Ile residue in catalysing GSH-conjugate reactions but also provide valuable resources for the development of GST engineering in analytical and agricultural biotechnology.

Delta class glutathione S-transferase (TuGSTd01) from the two-spotted spider mite Tetranychus urticae is inhibited by abamectin

GSTs (Glutathione S-transferases) are known to catalyze the nucleophilic attack of the sulfhydryl group of reduced glutathione (GSH) on electrophilic centers of xenobiotic compounds, including insecticides and acaricides. Genome analyses of the polyphagous spider mite herbivore Tetranychus urticae (two-spotted spider mite) revealed the presence of a set of 32 genes that code for secreted proteins belonging to the GST family of enzymes. To better understand the role of these proteins in T. urticae, we have functionally characterized TuGSTd01. Moreover, we have modeled the structure of the enzyme in apo form, as well as in the form with bound inhibitor. We demonstrated that this protein is a glutathione S-transferase that can conjugate glutathione to 1-chloro-2,4-dinitrobenzene (CDNB). We have tested TuGSTd01 activity with a range of potential substrates such as cinnamic acid, cumene hydroperoxide, and allyl isothiocyanate; however, the enzyme was unable to process these compounds. Using mutagenesis, we showed that putative active site variants S11A, E66A, S67A, and R68A mutants, which were residues predicted to interact directly with GSH, have no measurable activity, and these residues are required for the enzymatic activity of TuGSTd01. There are several reports that associate some T. urticae acaricide resistance with increased activity of GSTs . However, we found that TuGSTd01 is not able to detoxify abamectin; in fact, the acaricide inhibits the enzyme with K_i = 101 μM. Therefore, we suggest that the increased GST activity observed in abamectin resistant T. urticae field populations is a part of the compensatory feedback loop. In this case, the increased production of GSTs and relatively high concentration of GSH in cells allow GSTs to maintain physiological functions despite the presence of the acaricide.

Virtual screening and docking of lead like molecules against Glutathione-S-Transferase protein from Brugia malayi

Glutathione-S-transferase(s) (GST) is an important chemotherapeutic target in lymphatic filarasis caused by Brugia malayi and Wuchereria bancrofti. It has been playing an important role as major detoxification enzyme and help in intracellular transportation of hydrophobic substrates. Therefore, it is of interest to screen GST from Brugia malayi with millions of known ligands at the ZINC database using AUTODOCK for the identification of potential inhibitors with improved binding characteristics. We report two potent inhibitors ZINC00179016 and ZINC08385519 which are the molecules of pyrrolidinedione and benzimidazole families respectively as potential inhibitors of GST from Brugia malayi with suitable binding properties.

Monomeric Camelus dromedarius GSTM1 at low pH is structurally more thermostable than its native dimeric form

Glutathione S‒transferases (GSTs) are multifunctional enzymes that play an important role in detoxification, cellular signalling, and the stress response. Camelus dromedarius is well-adapted to survive in extreme desert climate and it has GSTs, for which limited information is available. This study investigated the structure-function and thermodynamic properties of a mu-class camel GST (CdGSTM1) at different pH. Recombinant CdGSTM1 (25.7 kDa) was expressed in E. coli and purified to homogeneity. Dimeric CdGSTM1 dissociated into stable but inactive monomeric subunits at low pH. Conformational and thermodynamic changes during the thermal unfolding pathway of dimeric and monomeric CdGSTM1 were characterised via a thermal shift assay and dynamic multimode spectroscopy (DMS). The thermal shift assay based on intrinsic tryptophan fluorescence revealed that CdGSTM1 underwent a two-state unfolding pathway at pH 1.0-10.0. Its Tm value varied with varying pH. Another orthogonal technique based on far-UV CD also exhibited two-state unfolding in the dimeric and monomeric states. Generally, proteins tend to lose structural integrity and stability at low pH; however, monomeric CdGSTM1 at pH 2.0 was thermally more stable and unfolded with lower van't Hoff enthalpy. The present findings provide essential information regarding the structural, functional, and thermodynamic properties of CdGSTM1 at pH 1.0-10.0.

Characterization of a Drosophila glutathione transferase involved in isothiocyanate detoxification

Glutathione transferases (GSTs) are ubiquitous key enzymes that catalyse the conjugation of glutathione to xenobiotic compounds in the detoxification process. GSTs have been proposed to play a dual role in the signal termination of insect chemodetection by modifying odorant and tasting molecules and by protecting the chemosensory system. Among the 40 GSTs identified in Drosophila melanogaster, the Delta and Epsilon groups are insect-specific. GSTs Delta and Epsilon may have evolved to serve in detoxification, and have been associated with insecticide resistance. Here, we report the heterologous expression and purification of the D. melanogaster GST Delta 2 (GSTD2). We investigated the capacity of GSTD2 to bind tasting molecules. Among them, we found that isothiocyanates (ITC), insecticidal compounds naturally present in cruciferous plant and perceived as bitter, are good substrates for GSTD2. The X-ray structure of GSTD2 was solved, showing the absence of the classical Ser catalytic residue, conserved in the Delta and Epsilon GSTs. Using molecular dynamics, the interaction of ITC with the GSTD2 three-dimensional structure is analysed and discussed. These findings allow us to consider a biological role for GSTD2 in chemoperception, considering GSTD2 expression in the chemosensory organs and the potential consequences of insect exposure to ITC.

Recent advances in protein engineering and biotechnological applications of glutathione transferases

Glutathione transferases (GSTs, EC 2.5.1.18) are a widespread family of enzymes that play a central role in the detoxification, metabolism, and transport or sequestration of endogenous or xenobiotic compounds. During the last two decades, delineation of the important structural and catalytic features of GSTs has laid the groundwork for engineering GSTs, involving both rational and random approaches, aiming to create new variants with new or altered properties. These approaches have expanded the usefulness of native GSTs, not only for understanding the fundamentals of molecular detoxification mechanisms, but also for the development medical, analytical, environmental, and agricultural applications. This review article attempts to summarize successful examples and current developments on GST engineering, highlighting in parallel the recent knowledge gained on their phylogenetic relationships, structural/catalytic features, and biotechnological applications.

Expression profiles of glutathione S-transferase superfamily in Spodoptera litura tolerated to sublethal doses of chlorpyrifos

Chlorpyrifos (CPF) is a broad-spectrum organophosphate insecticide. Glutathione S-transferases (GSTs) in insects are a family of detoxification enzymes and they play critical roles in CPF detoxification. Spodoptera litura is one of the most destructive agricultural pests in tropical and subtropical areas in the world. In this study, 37 Slgsts from 46 unique transcripts of gsts in S. litura transcriptome data, including eight previously reported GSTs, were identified and their expression patterns in susceptible and 12-generation-CPF-treated strains were analyzed to understand the roles of these Slgsts in sublethal doses of CPF tolerance. The results indicate that the members of the S. litura GST superfamily could be distinguished into three major groups: one group, including six cytosolic Slgsts (SlGSTe1, SlGSTe3, SlGSTe10, SlGSTe15, SlGSTo2 and SlGSTs5) and two microsomal Slgsts (SlMGST1-2 and SlMGST1-3), was directly responsible for CPF induction in both 12-generation-treated and susceptible strains; the second group, including three cytosolic Slgsts (SlGSTe13, SlGSTt1 and SlGSTz1) and one microsomal Slgst (SlMGST1-1), was induced only in the 12-generation-treated strain; the third group, including eight cytosolic Slgsts (two epsilon, three delta, one omega, one zeta and one unclassified Slgst), was expressed 1.52-5.15-fold higher in the 12-generation-treated strain than in the susceptible strain.

Catalysis of Silver catfish Major Hepatic Glutathione Transferase proceeds via rapid equilibrium sequential random Mechanism

Fish hepatic glutathione transferases are connected with the elimination of intracellular pollutants and detoxification of organic micro-pollutants in their aquatic ecosystem. The two-substrate steady state kinetic mechanism of Silver catfish (Synodontis eupterus) major hepatic glutathione transferases purified to apparent homogeneity was explored. The enzyme was dimeric enzyme with a monomeric size of 25.6 kDa. Initial-velocity studies and Product inhibition patterns by methyl glutathione and chloride with respect to GSH-CDNB; GSH-ρ-nitrophenylacetate; and GSH-Ethacrynic acid all conforms to a rapid equilibrium sequential random Bi Bi kinetic mechanism rather than steady state sequential random Bi Bi kinetic. α was 2.96 ± 0.35 for the model. The pH profile of V_max/K_M (with saturating 1-chloro-2,4-dinitrobenzene and variable GSH concentrations) showed apparent pKa value of 6.88 and 9.86. Inhibition studies as a function of inhibitor concentration show that the enzyme is a homodimer and near neutral GST. The enzyme poorly conjugates 4-hydroxylnonenal and cumene hydroperoxide and may not be involved in oxidative stress protection. The seGST is unique and overwhelmingly shows characteristics similar to those of homodimeric class Pi GSTs, as was indicated by its kinetic mechanism, substrate specificity and inhibition studies. The rate- limiting step, probably the product release, of the reaction is viscosity-dependent and is consequential if macro-viscosogen or micro-viscosogen.

Epsilon glutathione transferases possess a unique class-conserved subunit interface motif that directly interacts with glutathione in the active site

Epsilon class glutathione transferases (GSTs) have been shown to contribute significantly to insecticide resistance. We report a new Epsilon class protein crystal structure from Drosophila melanogaster for the glutathione transferase DmGSTE6. The structure reveals a novel Epsilon clasp motif that is conserved across hundreds of millions of years of evolution of the insect Diptera order. This histidine-serine motif lies in the subunit interface and appears to contribute to quaternary stability as well as directly connecting the two glutathiones in the active sites of this dimeric enzyme.

Structural contributions of Delta class glutathione transferase active-site residues to catalysis

GST (glutathione transferase) is a dimeric enzyme recognized for biotransformation of xenobiotics and endogenous toxic compounds. In the present study, residues forming the hydrophobic substrate-binding site (H-site) of a Delta class enzyme were investigated in detail for the first time by site-directed mutagenesis and crystallographic studies. Enzyme kinetics reveal that Tyr111 indirectly stabilizes GSH binding, Tyr119 modulates hydrophobic substrate binding and Phe123 indirectly modulates catalysis. Mutations at Tyr111 and Phe123 also showed evidence for positive co-operativity for GSH and 1-chloro-2,4-dinitrobenzene respectively, strongly suggesting a role for these residues in manipulating subunit–subunit communication. In the present paper we report crystal structures of the wild-type enzyme, and two mutants, in complex with S-hexylglutathione. This study has identified an aromatic ‘zipper’ in the H-site contributing a network of aromatic π–π interactions. Several residues of the cluster directly interact with the hydrophobic substrate, whereas others indirectly maintain conformational stability of the dimeric structure through the C-terminal domain (domain II). The Y119E mutant structure shows major main-chain rearrangement of domain II. This reorganization is moderated through the ‘zipper’ that contributes to the H-site remodelling, thus illustrating a role in co-substrate binding modulation. The F123A structure shows molecular rearrangement of the H-site in one subunit, but not the other, explaining weakened hydrophobic substrate binding and kinetic co-operativity effects of Phe123 mutations. The three crystal structures provide comprehensive evidence of the aromatic ‘zipper’ residues having an impact upon protein stability, catalysis and specificity. Consequently, ‘zipper’ residues appear to modulate and co-ordinate substrate processing through permissive flexing.

Expression and characterization of three new glutathione transferases, an epsilon (AcGSTE2-2), omega (AcGSTO1-1), and theta (AcGSTT1-1) from Anopheles cracens (Diptera: Culicidae), a major Thai malaria vector

Glutathione transferases (GSTs) (E.C.2.5.1.18) are multifunctional enzymes involved in the detoxification of many exogenous and endogenous compounds. This study aimed to characterize several new GSTs from Anopheles cracens, a major Thai malaria vector formerly known as Anopheles dirus. The three recombinant enzymes obtained were from the epsilon, theta and omega classes. They showed 80-93% identity to orthologous An. gambiae GSTs. AcGSTE2-2 possessed peroxidase activity that cannot be detected for the An. gambiae AgGSTE2-2. AcGSTT1-1 had high activity toward several substrates that are specific for mammalian theta class. The AcGSTO1-1 can use 1-chloro-2,4-dinitrobenzene, dichloroacetic acid, and hydroxyethyl disulfide substrates. The enzymes bound but did not metabolize the organophosphate temephos. The epsilon AcGSTE2-2 functioned as a peroxidase and DDT metabolizing enzyme. The theta AcGSTT1-1 functioned not only as peroxidase but also acted as a binding protein for organophosphates. The omega GST had thiol transferase activity suggesting a role in oxidative stress response.

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

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

Functional residues serve a dominant role in mediating the cooperativity of the protein ensemble

Conformational fluctuations in proteins have emerged as a potentially important aspect of biological function, although the precise relationship and the implications have yet to be fully explored. Numerous studies have reported that the binding of ligand can influence fluctuations. However, the role of the binding site in mediating these fluctuations is not known. Of particular interest is whether in addition to serving as structural scaffolds for recognition and catalysis, active-site residues may also play a role in modulating the cooperative network. To address this question, we employ an experimentally validated ensemble-based description of proteins to elucidate the extent to which perturbations at different sites can influence the cooperative network in the protein. Applying this method to a database of test proteins, it is found statistically that binding sites are located in regions most able to affect the cooperative network, even for cooperative interactions between residues distant to the binding sites. This indicates that the conformational manifold under native conditions is determined by the network of cooperative interactions within the protein and suggests that proteins have evolved to use these conformational fluctuations in carrying out their functions. Furthermore, because the energetic coupling pattern calculated for each protein is robust and relatively insensitive to sequence, these studies further suggest that binding sites evolved in regions of the protein that are inherently poised to take advantage of the fluctuations in the native structure.

Glutamate-64, a newly identified residue of the functionally conserved electron-sharing network contributes to catalysis and structural integrity of glutathione transferases

In Anopheles dirus glutathione transferase D3-3, position 64 is occupied by a functionally conserved glutamate residue, which interacts directly with the gamma-glutamate moiety of GSH (glutathione) as part of an electron-sharing network present in all soluble GSTs (glutathione transferases). Primary sequence alignment of all GST classes suggests that Glu64 is one of a few residues that is functionally conserved in the GST superfamily. Available crystal structures as well as consideration of the property of the equivalent residue at position 64, acidic or polar, suggest that the GST electron-sharing motif can be divided into two types. Electrostatic interaction between the GSH glutamyl and carboxylic Glu64, as well as with Arg66 and Asp100, was observed to extend the electron-sharing motif identified previously. Glu64 contributes to the catalytic function of this motif and the 'base-assisted deprotonation' that are essential for GSH ionization during catalysis. Moreover, this residue also appears to affect multiple steps in the enzyme catalytic strategy, including binding of GSH, nucleophilic attack by thiolate at the electrophilic centre and product formation, probably through active-site packing effects. Replacement with non-functionally-conserved amino acids alters initial packing or folding by favouring aggregation during heterologous expression. Thermodynamic and reactivation in vitro analysis indicated that Glu64 also contributes to the initial folding pathway and overall structural stability. Therefore Glu64 also appears to impact upon catalysis through roles in both initial folding and structural maintenance.

An intersubunit lock-and-key 'clasp' motif in the dimer interface of Delta class glutathione transferase

Structural investigations of a GST (glutathione transferase), adGSTD4-4, from the malaria vector Anopheles dirus show a novel lock-and-key 'Clasp' motif in the dimer interface of the Delta class enzyme. This motif also appears to be highly conserved across several insect GST classes, but differs from a previously reported mammalian lock-and-key motif. The aromatic 'key' residue not only inserts into a hydrophobic pocket, the 'lock', of the neighbouring subunit, but also acts as part of the 'lock' for the other subunit 'key'. The 'key' residues from both subunits show aromatic ring stacking with each other in a pi-pi interaction, generating a 'Clasp' in the middle of the subunit interface. Enzyme catalytic and structural characterizations revealed that single amino acid replacements in this 'Clasp' motif impacted on catalytic efficiencies, substrate selectivity and stability. Substitutions to the 'key' residue create strong positive co-operativity for glutathione binding, with a Hill coefficient approaching 2. The lock-and-key motif in general and especially the 'Clasp' motif with the pi-pi interaction appear to play a pivotal role in subunit communication between active sites, as well as in stabilizing the quaternary structure. Evidence of allosteric effects suggests an important role for this particular intersubunit architecture in regulating catalytic activity through conformational transitions of subunits. The observation of co-operativity in the mutants also implies that glutathione ligand binding and dimerization are linked. Quaternary structural changes of all mutants suggest that subunit assembly or dimerization basically manipulates subunit communication.

The structural roles of a conserved small hydrophobic core in the active site and an ionic bridge in domain I of Delta class glutathione S-transferase

GSTs (glutathione S-transferases; E.C.2.5.1.18) are a supergene family of dimeric multifunctional enzymes that have a major role in detoxification pathways. Using a GST from the mosquito Anopheles dirus (adGSTD4-4), we have characterized the enzymatic and physical properties of Leu-6, Thr-31, Leu-33, Ala-35, Glu-37, Lys-40 and Glu-42. These residues generate two motifs located in the N-terminal domain (domain I) that are functionally conserved across GST classes. The aim of this study was to understand the function of these two motifs. The first motif is a small hydrophobic core in the G-site (glutathione-binding site) wall, and the second motif contains an ionic bridge at the N-terminus of the alpha2 helix and is also part of the G-site. The mutations in the small hydrophobic core appear to have structural effects, as shown by the thermal stability, refolding rate and intrinsic fluorescence differences. In the Delta class GST, interactions form an ionic bridge motif located at the beginning of the alpha2 helix. The data suggest that electrostatic interactions in the alpha2 helix are involved in alpha-helix stabilization, and disruption of this ionic bridge interaction changes the movement of the alpha2-helix region, thereby modulating the interaction of the enzyme with substrates. These results show that the small hydrophobic core and ionic bridge have a major impact on structural stabilization, as well as being required to maintain structural conformation of the enzyme. These structural effects are also transmitted to the active site to influence substrate binding and specificity. Therefore changes in the conformation of the G-site wall in the active site appear to be capable of exerting influences on the tertiary structural organization of the whole GST protein.

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

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

Catalytic properties of glutathione-binding residues in a tau class glutathione transferase (PtGSTU1) from Pinus tabulaeformis

Glutathione transferases (GSTs) play important roles in stress tolerance and detoxification in plants. However, there is extremely little information on the molecular characteristics of GSTs in gymnosperms. In a previous study, we cloned a tau class GST (PtGSTU1) from a gymnosperm (Pinus tabulaeformis) for the first time. Based on the N-terminal amino acid sequence identity to the available crystal structures of plant tau GSTs, Ser13, Lys40, Ile54, Glu66 and Ser67 of PtGSTU1 were proposed as glutathione-binding (G-site) residues. The importance of Ser13 as a G-site residue was investigated previously. The functions of Lys40, Ile54, Glu66 and Ser67 of PtGSTU1 are examined in this study through site-directed mutagenesis. Enzyme assays and thermal stability measurements on the purified recombinant PtGSTU1 showed that substitution at each of these sites significantly affects the enzyme's substrate specificity and affinity for GSH, and these residues are essential for maintaining the stability of PtGSTU1. The results of protein expression and refolding analyses suggest that Ile54 is involved in the protein folding process. The findings demonstrate that the aforementioned residues are critical components of active sites that contribute to the enzyme's catalytic activity and structural stability.