Multiple unfolded states of glutathione transferase bbGSTP1-1 by guanidinium chloride

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.

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.

Structural, functional and unfolding characteristics of glutathione S-transferase of Plasmodium vivax

Glutathione S-transferases (GSTs) of Plasmodium parasites are potential targets for antimalarial drug and vaccine development. We investigated the equilibrium unfolding, functional activity regulation and stability characteristics of the unique GST of Plasmodium vivax (PvGST). Despite high sequence, structural, functional, and evolutionary similarity, the unfolding behavior of PvGST was significantly different from Plasmodium falciparum GST (PfGST). The unfolding pathway of PvGST was non-cooperative with stabilization of an inactive dimeric intermediate. The absence of any compact, folded monomeric intermediate during the unfolding transition suggests that inter-subunit interactions play an important role in stabilizing the protein. Presence of salts effectively inhibited PvGST enzymatic activity by quenching the nucleophilicity of the thiolate anion of GSH. Based on the present findings, together with our previous studies on PfGST, we propose that the regulation of GST enzymatic activity through a dimer-tetramer transition via GSH binding is an exclusive feature of Plasmodium.

Characterization of the activity and folding of the glutathione transferase from Escherichia coli and the roles of residues Cys(10) and His(106)

GSTs (glutathione transferases) are an important class of enzymes involved in cellular detoxification. GSTs are found in all classes of organisms and are implicated in resistance towards drugs, pesticides, herbicides and antibiotics. The activity, structure and folding, particularly of eukaryotic GSTs, have therefore been widely studied. The crystal structure of EGST (GST from Escherichia coli) was reported around 10 years ago and it suggested Cys(10) and His(106) as potential catalytic residues. However, the role of these residues in catalysis has not been further investigated, nor have the folding properties of the protein been described. In the present study we investigated the contributions of residues Cys(10) and His(106) to the activity and stability of EGST. We found that EGST shows a complex equilibrium unfolding profile, involving a population of at least two partially folded intermediates, one of which is dimeric. Mutation of residues Cys(10) and His(106) leads to stabilization of the protein and affects the apparent steady-state kinetic parameters for enzyme catalysis. The results suggest that the imidazole ring of His(106) plays an important role in the catalytic mechanism of the enzyme, whereas Cys(10) is involved in binding of the substrate, glutathione. Engineering of the Cys(10) site can be used to increase both the stability and GST activity of EGST. However, in addition to GST activity, we discovered that EGST also possesses thiol:disulfide oxidoreductase activity, for which the residue Cys(10) plays an essential role. Further, tryptophan quenching experiments indicate that a mixed disulfide is formed between the free thiol group of Cys(10) and the substrate, glutathione.

Multiple unfolding states of glutathione transferase from Physa acuta (Gastropada: Physidae)

The equilibrium unfolding of the major Physa acuta glutathione transferase isoenzyme (P. acuta GST(3)) has been performed using guanidinium chloride (GdmCl), urea, and acid denaturation to investigate the unfolding intermediates. Protein transitions were monitored by intrinsic fluorescence. The results indicate that unfolding of P. acuta GST(3) using GdmCl (0-3.0M) is a multistep process, i.e., three intermediates coexist in equilibrium. The first intermediate, a partially dissociated dimer, exists at low GdmCl concentration (approximately at 0.7M). At 1.2M GdmCl, a dimeric intermediate with a compact structure was observed. This intermediate undergoes dissociation into structural monomers at 1.75M of GdmCl. The monomeric intermediate started to be completely unfolding at higher GdmCl concentrations (>1.8M). Unfolding using urea (0-7.0M) and acid-induced structures as well as the fluorescence of 8-anilino-1-naphthalenesulfonate in the presence of different GdmCl concentrations confirmed that the unfolding is a multistep process. At concentrations of GdmCl or urea less than the midpoints or at the midpoint pH (pH 4.2-4.6), the unfolding transition is protein concentration independent and involved a change in the subunit tertiary structure yielding a partially active dimeric intermediate. The binding of glutathione to the enzyme active site stabilizes the native dimeric state.

Purification and catalytic properties of glutathione transferase from the hepatopancreas of crayfishmacrobrachium vollenhovenii (herklots)

Glutathione transferase from the hepatopancreas of fresh water crayfish Macrobrachium vollenhovenii was purified to apparent homogeneity by ion-exchange chromatography on DEAE-cellulose and by gel filtration on Sephadex G-100. The enzyme appeared to be a homodimer with molecular weight (Mr) of 46.0 +/- 1.4 kDa and a subunit Mr of 24.1 +/- 0.35 kDa. Chromatofocusing of the apparently pure enzyme revealed microheterogeneity and resolved it into two isozymic peaks, which were eluted at pH 8.36 and 8.22 respectively. Inhibition studies showed that the I50 value for cibacron blue, S-hexylglutathione, hematin, and N-ethylmaleimide (NEM) were 0.01 microM, 340 microM, 5 microM and 33 mM respectively. Out of the several substrates tested, only 1-chloro-2,4-dinitrobenzene (CDNB) and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole could be conjugated with glutathione. Chemical modification studies with DTNB revealed that two sulphydryl groups per dimer were essential to the activity of the enzymes. On the basis of structural and catalytic characteristics, M. vollenhovenii GST seems close, tentatively, to the omega and zeta classes of GST. Initial-velocity studies of the enzyme are consistent with a steady-state random kinetic mechanism. Denaturation and renaturation studies with guanidine HCl (Gdn-HCl) revealed that though low Gdn-HCl concentrations (less than 0.5 M) denatured the enzyme, the enzyme was able to renature completely (100%). At higher concentration of the denaturant (0.5-4 M), refolding studies indicated that complete renaturation was not achieved. The extent of renaturation was however a function of protein concentration. Our results are consistent with a three-state unfolding process.

Heterodimers of glutathione S-transferase can form between isoenzyme classes pi and mu

Glutathione S-transferases constitute a family of enzymes that detoxify xenobiotics by conjugating glutathione with a range of electrophilic substrates. The cytosolic glutathione S-transferase dimeric isoenzymes are currently divided into at least eight classes on the basis of their physical and chemical properties. Previously, heterodimers have only been detected within a given class of isoenzymes; however, here we describe for the first time the generation of a heterodimer between a pi class and mu class glutathione S-transferase. The heterodimer forms under mild conditions (dialysis against phosphate buffer, pH 6.5) and is best detected when one of the isoenzyme subunits is in excess. The activity of the pi-mu heterodimer toward several substrates indicates that interaction between these two dissimilar subunits influences the catalytic activity of this dimer. The production of this new heterodimer provides a new approach in glutathione S-transferase research to study the influence of one subunit on the catalytic activity of its partner subunit and to identify those amino acid residues which contribute to subunit interactions.