A conserved N-capping motif contributes significantly to the stabilization and dynamics of the C-terminal region of class Alpha glutathione S-transferases

Efficient Adsorption and Extraction of Glutathione S-Transferases with Glutathione-Functionalized Graphene Oxide-Polyhedral Oligomeric Silsesquioxane Composite

Glutathione S-transferases (GSTs) are important type-II detoxification enzymes that protect DNA and proteins from damage and are often used as protein tags for the expression of fusion proteins. In the present work, octa-aminopropyl caged polyhedral oligomeric silsesquioxane (OA-POSS) was prepared via acid-catalyzed hydrolysis of 3-aminopropyltriethoxysilane and polymerized on the surface of graphene oxide (GO) through an amidation reaction. Glutathione (GSH) was then modified to GO-POSS through a Michael addition reaction to obtain a GSH-functionalized GO-POSS composite (GPG). The structure and characteristics of the as-prepared GPG composite were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravity analysis, and surface charge analysis. The specific binding interactions between glutathione and GST gave GPG favorable adsorption selectivity towards GST, and other proteins did not affect GST adsorption. The adsorption behavior of GST on the GPG composite conformed to the Langmuir isotherm model, and the adsorption capacity of GST was high up to 364.94 mg g^-1 under optimal conditions. The GPG-based solid-phase adsorption process was applied to the extraction of GST from a crude enzyme solution of pig liver, and high-purity GST was obtained via SDS-PAGE identification.

Observing cellulose biosynthesis and membrane translocation in crystallo

Many biopolymers, including polysaccharides, must be translocated across at least one membrane to reach their site of biological function. Cellulose is a linear glucose polymer synthesized and secreted by a membrane-integrated cellulose synthase. Here, in crystallo enzymology with the catalytically active bacterial cellulose synthase BcsA-BcsB complex reveals structural snapshots of a complete cellulose biosynthesis cycle, from substrate binding to polymer translocation. Substrate- and product-bound structures of BcsA provide the basis for substrate recognition and demonstrate the stepwise elongation of cellulose. Furthermore, the structural snapshots show that BcsA translocates cellulose via a ratcheting mechanism involving a 'finger helix' that contacts the polymer's terminal glucose. Cooperating with BcsA's gating loop, the finger helix moves 'up' and 'down' in response to substrate binding and polymer elongation, respectively, thereby pushing the elongated polymer into BcsA's transmembrane channel. This mechanism is validated experimentally by tethering BcsA's finger helix, which inhibits polymer translocation but not elongation.

Cloning, expression and analysis of the olfactory glutathione S-transferases in coho salmon

The glutathione S-transferases (GSTs) provide cellular protection by detoxifying xenobiotics, maintaining redox status, and modulating secondary messengers, all of which are critical to maintaining olfaction in salmonids. Here, we characterized the major coho salmon olfactory GSTs (OlfGSTs), namely omega, pi, and rho subclasses. OlfGST omega contained an open reading frame of 720bp and encoded a protein of 239 amino acids. OlfGST pi and OlfGST rho contained open reading frames of 627 and 681nt, respectively, and encoded proteins of 208 and 226 amino acids. Whole-protein mass spectrometry yielded molecular weights of 29,950, 23,354, and 26,655Da, respectively, for the GST omega, pi, and rho subunits. Homology modeling using four protein-structure prediction algorithms suggest that the active sites in all three OlfGST isoforms resembled counterparts in other species. The olfactory GSTs conjugated prototypical GST substrates, but only OlfGST rho catalyzed the demethylation of the pesticide methyl parathion. OlfGST pi and rho exhibited thiol oxidoreductase activity toward 2-hydroxyethyl disulfide (2-HEDS) and conjugated 4-hydroxynonenal (HNE), a toxic aldehyde with neurodegenerative properties. The kinetic parameters for OlfGST pi conjugation of HNE were K(M)=0.16 ± 0.06mM and V(max)=0.5 ± 0.1μmolmin⁻¹mg⁻¹, whereas OlfGST rho was more efficient at catalyzing HNE conjugation (K(M)=0.022 ± 0.008 mM and V(max)=0.47 ± 0.05μmolmin⁻¹mg⁻¹). Our findings indicate that the peripheral olfactory system of coho expresses GST isoforms that detoxify certain electrophiles and pesticides and that help maintain redox status and signal transduction.

Ensemble perspective for catalytic promiscuity: calorimetric analysis of the active site conformational landscape of a detoxification enzyme

Enzymological paradigms have shifted recently to acknowledge the biological importance of catalytic promiscuity. However, catalytic promiscuity is a poorly understood property, and no thermodynamic treatment has described the conformational landscape of promiscuous versus substrate-specific enzymes. Here, two structurally similar glutathione transferase (GST, glutathione S-transferase) isoforms with high specificity or high promiscuity are compared. Differential scanning calorimetry (DSC) indicates a reversible low temperature transition for the promiscuous GSTA1-1 that is not observed with substrate-specific GSTA4-4. This transition is assigned to rearrangement of the C terminus at the active site of GSTA1-1 based on the effects of ligands and mutations. Near-UV and far-UV circular dichroism indicate that this transition is due to repacking of tertiary contacts with the remainder of the subunit, rather than "unfolding" of the C terminus per se. Analysis of the DSC data using a modified Landau theory indicates that the local conformational landscape of the active site of GSTA1-1 is smooth, with barrierless transitions between states. The partition function of the C-terminal states is a broad unimodal distribution at all temperatures within this DSC transition. In contrast, the remainder of the GSTA1-1 subunit and the GSTA4-4 protein exhibit folded and unfolded macrostates with a significant energy barrier separating them. Their partition function includes a sharp unimodal distribution of states only at temperatures that yield either folded or unfolded macrostates. At intermediate temperatures the partition function includes a bimodal distribution. The barrierless rearrangement of the GSTA1-1 active site within a local smooth energy landscape suggests a thermodynamic basis for catalytic promiscuity.

Interactions of glutathione transferases with 4-hydroxynonenal

Electrophilic products of lipid peroxidation are important contributors to the progression of several pathological states. The prototypical α,β-unsaturated aldehyde, 4-hydroxynonenal (HNE), triggers cellular events associated with oxidative stress, which can be curtailed by the glutathione-dependent elimination of HNE. The glutathione transferases (GSTs) are a major determinate of the intracellular concentration of HNE and can influence susceptibility to toxic effects, particularly when HNE and GST levels are altered in disease states. In this article, we provide a brief summary of the cellular effects of HNE, followed by a review of its GST-catalyzed detoxification, with an emphasis on the structural attributes that play an important role in the interactions with alpha-class GSTs. Some of the key determining characteristics that impart high alkenal activity reside in the unique C-terminal interactions of the GSTA4-4 enzyme. Studies encompassing both kinetic and structural analyses of related isoforms will be highlighted, with additional attention to stereochemical aspects that demonstrate the capacity of GSTA4-4 to detoxify both enantiomers of the biologically relevant racemic mixture while generating a select set of diastereomeric products with subsequent implications. A summary of the literature that examines the interplay between GSTs and HNE in model systems relevant to oxidative stress will also be discussed to demonstrate the magnitude of importance of GSTs in the overall detoxification scheme.

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.

Structure-function studies of chemokine-derived carboxy-terminal antimicrobial peptides

Recent reports which show that several chemokines can act as direct microbicidal agents have drawn renewed attention to these chemotactic signalling proteins. Here we present a structure-function analysis of peptides derived from the human chemokines macrophage inflammatory protein-3alpha (MIP-3alpha/CCL20), interleukin-8 (IL-8), neutrophil activating protein-2 (NAP-2) and thrombocidin-1 (TC-1). These peptides encompass the C-terminal alpha-helices of these chemokines, which have been suggested to be important for the direct antimicrobial activities. Far-UV CD spectroscopy showed that the peptides are unstructured in aqueous solution and that a membrane mimetic solvent is required to induce a helical secondary structure. A co-solvent mixture was used to determine solution structures of the peptides by two-dimensional (1)H-NMR spectroscopy. The highly cationic peptide, MIP-3alpha(51-70), had the most pronounced antimicrobial activity and displayed an amphipathic structure. A shorter version of this peptide, MIP-3alpha(59-70), remained antimicrobial but its structure and mechanism of action were unlike that of the former peptide. The NAP-2 and TC-1 proteins differ in their sequences only by the deletion of two C-terminal residues in TC-1, but intact TC-1 is a very potent antimicrobial while NAP-2 is inactive. The corresponding C-terminal peptides, NAP-2(50-70) and TC-1(50-68), had very limited and no bactericidal activity, respectively. This suggests that other regions of TC-1 contribute to its bactericidal activity. Altogether, this work provides a rational structural basis for the biological activities of these peptides and proteins and highlights the importance of experimental characterization of peptide fragments as distinct entities because their activities and structural properties may differ substantially from their parent proteins.

Structural analysis of a glutathione transferase A1-1 mutant tailored for high catalytic efficiency with toxic alkenals

The specificity of human glutathione transferase (GST) A1-1 is drastically altered to favor alkenal substrates in the GIMFhelix mutant designed to mimic first-sphere interactions utilized by GSTA4-4. This redesign serves as a model for improving our understanding of the structural determinants that contribute to the distinct specificities of alpha class GSTs. Herein we report the first crystal structures of GIMFhelix, both in complex with GSH and in apo form at 1.98 and 2.38 A resolution. In contrast to the preorganized hydrophobic binding pocket that accommodates alkenals in GSTA4-4, GSTA1-1 includes a dynamic alpha9 helix that undergoes a ligand-dependent localization to complete the active site. Comparisons of the GIMFhelix structures with previously reported structures show a striking similarity with the GSTA4-4 active site obtained within an essentially GSTA1-1 scaffold and reveal the alpha9 helix assumes a similar localized structure regardless of active site occupancy in a manner resembling that of GSTA4-4. However, we cannot fully account for all the structural elements important in GSTA4-4 within the mutant's active site. The contribution of Phe10 to the Tyr212-Phe10-Phe220 network prevents complete C-terminal closure and demonstrates that the presence of Phe10 within the context of a GSTA4-4-like active site may ultimately hinder Phe220, a key C-terminal residue, from effectively contributing to the active site. In total, these results illustrate the remaining structural differences presumably reflected in the previously reported catalytic efficiencies of GIMFhelix and GSTA4-4 and emphasize the F10P mutation as being necessary to completely accomplish the transformation to a highly specific GST from the more promiscuous GSTA1-1 enzyme.

Characterization of the binding of 8-anilinonaphthalene sulfonate to rat class Mu GST M1-1

Molecular docking and ANS-displacement experiments indicated that 8-anilinonaphthalene sulfonate (ANS) binds the hydrophobic site (H-site) in the active site of dimeric class Mu rGST M1-1. The naphthalene moiety provides most of the van der Waals contacts at the ANS-binding interface while the anilino group is able to sample different rotamers. The energetics of ANS binding were studied by isothermal titration calorimetry (ITC) over the temperature range of 5-30 degrees C. Binding is both enthalpically and entropically driven and displays a stoichiometry of one ANS molecule per subunit (or H-site). ANS binding is linked to the uptake of 0.5 protons at pH 6.5. Enthalpy of binding depends linearly upon temperature yielding a DeltaC(p) of -80+/-4 cal K(-1) mol(-1) indicating the burial of solvent-exposed nonpolar surface area upon ANS-protein complex formation. While ion-pair interactions between the sulfonate moiety of ANS and protein cationic groups may be significant for other ANS-binding proteins, the binding of ANS to rGST M1-1 is primarily hydrophobic in origin. The binding properties are compared with those of other GSTs and ANS-binding proteins.

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.

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.

A dynamic N-capping motif in cytochrome b5: evidence for a pH-controlled conformational switch

Apocytochrome b5 is a marginally stable protein exhibiting under native conditions a slow conformational exchange in its C-terminal region. The affected elements of secondary structure include a 3(10)-helix containing at its N-terminus a histidine Ncap and a subsequent proline. Participation of the neutral histidine side-chain in backbone amide capping lowers the imidazole pKa. To explore the nature of the conformational exchange in the protein and determine whether it is related to cis-trans isomerization of the His-Pro bond, three octapeptides encompassing the helix were synthesized and studied by NMR spectroscopy. One corresponded to the wild-type sequence, the second was the D-histidine epimer, and the third contained an alanine in place of the proline. It was found that the rates of cis-trans interconversion in the proline-containing peptides were slower than the rates of the conformational exchange in the protein. In addition, the wild-type peptide hinted at a predisposition for Ncap formation when in the trans configuration. Analysis of the pH response of the peptides and protein suggested that at pH near neutral, the conformational exchange detected in the protein involved only species with a trans His-Pro bond and could be approximated with a three-state model by which the terminal helix sampled a locally unfolded state. This state, which contained an uncapped histidine with a normal pKa, partitioned into neutral and protonated populations according to pH. The intrinsic conformational bias of the wild-type peptide and the pH-driven equilibria illustrated how a 3(10)-element could serve as a nucleation site for structural rearrangement.

The intersubunit lock-and-key motif in human glutathione transferase A1-1: role of the key residues Met51 and Phe52 in function and dimer stability

The dimeric structure of certain cytosolic GSTs (glutathione S-transferases) is stabilized by a hydrophobic lock-and-key motif at their subunit interface. In hGSTA1-1 (human class Alpha GST with two type-1 subunits), the key consists of two residues, Met51 and Phe52, that fit into a hydrophobic cavity (lock) in the adjacent subunit. SEC (size-exclusion chromatography)-HPLC, far-UV CD and tryptophan fluorescence of the M51A and M51A/F52S mutants indicated the non-disruptive nature of these mutations on the global structure. While the M51A mutant retained 80% of wild-type activity, the activity of the M51A/F52S was markedly diminished, indicating the importance of Phe52 in maintaining the correct conformation at the active site. The M51A and M51A/F52S mutations altered the binding of ANS (8-anilinonaphthalene-l-sulphonic acid) at the H-site by destabilizing helix 9 in the C-terminal region. Data from urea unfolding studies show that the dimer is destabilized by both mutations and that the dimer dissociates to aggregation-prone monomers at low urea concentrations before global unfolding. Although not essential for the assembly of the dimeric structure of hGSTA1-1, both Met51 and Phe52 in the intersubunit lock-and-key motif play important structural roles in maintaining the catalytic and ligandin functions and stability of the GST dimer.