NMR structure of Escherichia coli glutaredoxin 3-glutathione mixed disulfide complex: implications for the enzymatic mechanism

Structural conservation in the glutathione binding in Sphingomonas sp. glutaredoxin Grx3 and variations for cold adaptation

Glutaredoxin 3 (Grx3), a redox protein with a thioredoxin-fold structure, maintains structural integrity and glutathione (GSH) binding capabilities across varying habitat temperatures. The cis-Pro loop, essential for GSH binding, relies on the Arg-Asp salt bridge (α2-α3) and Gln-His hydrogen bond (β3-β4) for its conformation. In some psychrophilic Grx3 variants, Arg in α2 is replaced with Tyr, and His in β4 is replaced with Phe. This study examines the roles of these bonds in Grx3's structure, function, and cold adaptation, using SpGrx3 from the Arctic bacterium Sphingomonas sp. Despite its cold habitat, SpGrx3 maintains the Arg51-Asp69 salt bridge and Gln56-His63 hydrogen bond. The R51Y substitution disrupts the α2-α3 salt bridge, while the H63F and H63Y substitutions hinder the salt bridge through cation-π interactions with Arg51, involving Phe63/Tyr63, thereby enhancing flexibility. Conversely, mutations that disrupt the hydrogen bond (Q56A, H63A, and H63F) reduce thermal stability. In the psychrophilic Grx3 configuration A48T/R51Y/H63F, a Thr48-Gln56 hydrogen bond stabilizes the cis-Pro loop, enhancing flexibility by disrupting both bonds. Furthermore, all mutants exhibit reduced α-helical content and catalytic efficiency. In summary, the highly conserved Arg51-Asp69 salt bridge and Gln56-His63 hydrogen bond are crucial for stabilizing the cis-Pro loop and catalytic activity in SpGrx3. His63 is favored as it avoids cation-π interactions with Arg51, unlike Phe63/Tyr63. Psychrophilic Grx3 variants have adapted to cold environments by reducing GSH binding and increasing structural flexibility. These findings deepen our understanding of the structural conservation in Grx3 for GSH binding and the critical alterations required for cold adaptation.

Thiol redox proteomics: Characterization of thiol-based post-translational modifications

Redox post-translational modifications on cysteine thiols (redox PTMs) have profound effects on protein structure and function, thus enabling regulation of various biological processes. Redox proteomics approaches aim to characterize the landscape of redox PTMs at the systems level. These approaches facilitate studies of condition-specific, dynamic processes implicating redox PTMs and have furthered our understanding of redox signaling and regulation. Mass spectrometry (MS) is a powerful tool for such analyses which has been demonstrated by significant advances in redox proteomics during the last decade. A group of well-established approaches involves the initial blocking of free thiols followed by selective reduction of oxidized PTMs and subsequent enrichment for downstream detection. Alternatively, novel chemoselective probe-based approaches have been developed for various redox PTMs. Direct detection of redox PTMs without any enrichment has also been demonstrated given the sensitivity of contemporary MS instruments. This review discusses the general principles behind different analytical strategies and covers recent advances in redox proteomics. Several applications of redox proteomics are also highlighted to illustrate how large-scale redox proteomics data can lead to novel biological insights.

Emerging chemistry and biology in protein glutathionylation

Protein S-glutathionylation serves a regulatory role in proteins and modulates distinct biological processes implicated in health and diseases. Despite challenges in analyzing the dynamic and reversible nature of S-glutathionylation, recent chemical and biological methods have significantly advanced the field of S-glutathionylation, culminating in selective identification and detection, structural motif analysis, and functional studies of S-glutathionylation. This review will highlight emerging studies of protein glutathionylation, beginning by introducing biochemical tools that enable mass spectrometric identification and live-cell imaging of S-glutathionylation. Next, it will spotlight recent examples of S-glutathionylation regulating physiology and inflammation. Lastly, we will feature two emerging lines of glutathionylation research in cryptic cysteine glutathionylation and protein C-glutathionylation.

Structural insights into the redox regulation of Oncomelania hupensis TRP14 and its potential role in the snail host response to parasite invasion

The freshwater amphibious snail Oncomelania hupensis is the unique intermediate host of Schistosoma japonicum, but little attention has been paid to the interaction between the two. In snails, the production of reactive oxygen species (ROS) by hemocytes has been shown to be vital for snail immune defense against schistosome infection. However, excessive ROS accumulation could lead to oxidative damage, requiring the antioxidant system for maintaining the cellular redox homeostasis. Previously we identified a thioredoxin-related protein of 14 kDa from O. hupensis (OhTRP14), and showed that it was involved in the scavenging of ROS in circulating hemocytes. Here, we confirmed that OhTRP14 plays a potential role in the snail host response to parasite challenge and determined the crystal structures of OhTRP14 in two different states (oxidized and transition state). The overall structure revealed a typical Trx fold and is similar to that of human TRP14 (hTRP14), but there were significant structural differences between the two states. Noticeably, there was a different pair of thiol groups from Cys30 and Cys44 in the transition state of OhTRP14, were with the similar separation of 2.9 Å as that (2.6 Å) between Cys41 and Cys44, but in a different orientation, suggesting that the Cys30 is likely to function as an important molecular switch involved in the oxidoreductase activity of OhTRP14. Comparative studies between OhTRP14 and hTRP14 by analyzing the surface characteristics, charge distribution and oxidoreductase activity toward insulin demonstrated they might have similar substrates. The results are expected to provide structural insights into the redox regulation of OhTRP14 and contribute to better understanding of TRP14 family. DATA DEPOSITION: The atomic coordinates of the structure and the structure factors were deposited in Protein Data Bank with PDB ID codes 7XQ3 and 7XPW.

Cloning, Expression, Characterization, and Antioxidant Protection of Glutaredoxin3 From Psychrophilic Bacterium Psychrobacter sp. ANT206

Glutaredoxins (Grxs) are proteins that catalyze the glutathione (GSH)-dependent reduction of protein disulfides. In this study, a Grx-related gene (264 bp), encoding a Ps-Grx3, was cloned from Psychrobacter sp. ANT206. Sequence analysis indicated the presence of the active site motif CPYC in this protein. Homology modeling showed that Ps-Grx3 had fewer hydrogen bonds and salt bridges, as well as a lower Arg/(Arg + Lys) ratio than its mesophilic homologs, indicative of an improved catalytic ability at low temperatures. Site-directed mutagenesis demonstrated that the Cys13, Pro14, and Cys16 sites were essential for the catalytic activity of Ps-Grx3, while circular dichroism (CD) spectroscopy confirmed that point mutations in these amino acid residues led to the loss or reduction of enzyme activity. Furthermore, analysis of the biochemical properties of Ps-Grx3 showed that the optimum temperature of this enzyme was 25 °C. Importantly, Ps-Grx3 was more sensitive to tBHP and CHP than to H₂O₂, and retained approximately 40% activity even when the H₂O₂ concentration was increased to 1 mm Regarding substrate specificity, Ps-Grx3 had a higher affinity for HED, L-cystine, and DHA than for S-sulfocysteine and BSA. We also investigated the DNA-protective ability of Ps-Grx3 using the pUC19 plasmid, and found that Ps-Grx3 could protect supercoiled DNA from oxidation-induced damage at 15°C for 1.5 h. This study provides new insights into the structure and catalytic activity of a cold-adapted Grx3.

S-Glutathionylation of human inducible Hsp70 reveals a regulatory mechanism involving the C-terminal α-helical lid

Heat shock protein 70 (Hsp70) proteins are a family of ancient and conserved chaperones. Cysteine modifications have been widely detected among different Hsp70 family members in vivo, but their effects on Hsp70 structure and function are unclear. Here, we treated HeLa cells with diamide, which typically induces disulfide bond formation except in the presence of excess GSH, when glutathionylated cysteines predominate. We show that in these cells, HspA1A (hHsp70) undergoes reversible cysteine modifications, including glutathionylation, potentially at all five cysteine residues. In vitro experiments revealed that modification of cysteines in the nucleotide-binding domain of hHsp70 is prevented by nucleotide binding but that Cys-574 and Cys-603, located in the C-terminal α-helical lid of the substrate-binding domain, can undergo glutathionylation in both the presence and absence of nucleotide. We found that glutathionylation of these cysteine residues results in unfolding of the α-helical lid structure. The unfolded region mimics substrate by binding to and blocking the substrate-binding site, thereby promoting intrinsic ATPase activity and competing with binding of external substrates, including heat shock transcription factor 1 (Hsf1). Thus, post-translational modification can alter the structure and regulate the function of hHsp70.

Quantitative assessment of the determinant structural differences between redox-active and inactive glutaredoxins

Class I glutaredoxins are enzymatically active, glutathione-dependent oxidoreductases, whilst class II glutaredoxins are typically enzymatically inactive, Fe-S cluster-binding proteins. Enzymatically active glutaredoxins harbor both a glutathione-scaffold site for reacting with glutathionylated disulfide substrates and a glutathione-activator site for reacting with reduced glutathione. Here, using yeast ScGrx7 as a model protein, we comprehensively identified and characterized key residues from four distinct protein regions, as well as the covalently bound glutathione moiety, and quantified their contribution to both interaction sites. Additionally, we developed a redox-sensitive GFP2-based assay, which allowed the real-time assessment of glutaredoxin structure-function relationships inside living cells. Finally, we employed this assay to rapidly screen multiple glutaredoxin mutants, ultimately enabling us to convert enzymatically active and inactive glutaredoxins into each other. In summary, we have gained a comprehensive understanding of the mechanistic underpinnings of glutaredoxin catalysis and have elucidated the determinant structural differences between the two main classes of glutaredoxins.

Glutaredoxins play a central role in numerous biological processes including cellular redox homeostasis and Fe-S cluster biogenesis. Here the authors establish the molecular basis for glutaredoxin redox catalysis through comprehensive biochemical and structural analyses.

BBE31 from the Lyme disease agent Borrelia burgdorferi, known to play an important role in successful colonization of the mammalian host, shows the ability to bind glutathione

Lyme disease is a tick-borne infection caused by Borrelia burgdorferi sensu lato complex spirochetes. The spirochete is located in the gut of the tick; as the infected tick starts the blood meal, the spirochete must travel through the hemolymph to the salivary glands, where it can spread to and infect the new host organism. In this study, we determined the crystal structures of the key outer surface protein BBE31 from B. burgdorferi and its orthologous protein BSE31 (BSPA14S_RS05060 gene product) from B. spielmanii. BBE31 is known to be important for the transfer of B. burgdorferi from the gut to the hemolymph in the tick after a tick bite. While BBE31 exerts its function by interacting with the Ixodes scapularis tick gut protein TRE31, structural and mass spectrometry data revealed that BBE31 has a glutathione (GSH) covalently attached to Cys142 suggesting that the protein may have acquired some additional functions in contrast to its orthologous protein BSE31, which lacks any interactions with GSH. In the current study, in addition to analyzing the potential reasons for GSH binding, the three-dimensional structure of BBE31 provides new insights into the molecular details of the transmission process as the protein plays an important role in the initial phase before the spirochete is physically transferred to the new host. This knowledge will be potentially used for the development of new strategies to fight against Lyme disease.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

Dithiothreitol-assisted polysulfide reduction in the interlayer of lithium-sulfur batteries: a first-principles study

Lithium-sulfur batteries are attracting more and more attention due to the high specific energy density and specific capacity density. The severe "shuttle effect" during the charge/discharge cycle causes significant performance degradation, and has become a great challenge for the practical application of rechargeable lithium-sulfur batteries. The biological reductant dithiothreitol (DTT) in the interlayer of lithium-sulfur batteries could reduce the shuttle effect by chemically cutting the polysulfide. The biocatalysts of molecular scission provide a gentle and innovative way to address the problems in lithium-sulfur batteries. Understanding the specific working principle of DTT would serve to expand the application of reducing agents in lithium-sulfur battery systems. A systematic theoretical study has been performed on the DTT-assisted polysulfide reduction. The steps for DTT to reduce the polysulfide chains, including the intermediate product of each reduction step (i.e. cleavage site of polysulfides) were clarified. The difference between the reduction of long chain and short chain polysulfides and the modification method of DTT to promote the reduction kinetics were also unraveled. It is hoped that our study could provide mechanistic insights into the DTT promoted performance of Li-S batteries and give inspiration for biological reagent expansion.

Kinetic studies reveal a key role of a redox-active glutaredoxin in the evolution of the thiol-redox metabolism of trypanosomatid parasites

Trypanosomes are flagellated protozoan parasites (kinetoplastids) that have a unique redox metabolism based on the small dithiol trypanothione (T(SH)₂). Although GSH may still play a biological role in trypanosomatid parasites beyond being a building block of T(SH)₂, most of its functions are replaced by T(SH)₂ in these organisms. Consequently, trypanosomes have several enzymes adapted to using T(SH)₂ instead of GSH, including the glutaredoxins (Grxs). However, the mechanistic basis of Grx specificity for T(SH)₂ is unknown. Here, we combined fast-kinetic and biophysical approaches, including NMR, MS, and fluorescent tagging, to study the redox function of Grx1, the only cytosolic redox-active Grx in trypanosomes. We observed that Grx1 reduces GSH-containing disulfides (including oxidized trypanothione) in very fast reactions (k > 5 × 10⁵ m^-1 s^-1). We also noted that disulfides without a GSH are much slower oxidants, suggesting a strongly selective binding of the GSH molecule. Not surprisingly, oxidized Grx1 was also reduced very fast by T(SH)₂ (4.8 × 10⁶ m^-1 s^-1); however, GSH-mediated reduction was extremely slow (39 m^-1 s^-1). This kinetic selectivity in the reduction step of the catalytic cycle suggests that Grx1 uses preferentially a dithiol mechanism, forming a disulfide on the active site during the oxidative half of the catalytic cycle and then being rapidly reduced by T(SH)₂ in the reductive half. Thus, the reduction of glutathionylated substrates avoids GSSG accumulation in an organism lacking GSH reductase. These findings suggest that Grx1 has played an important adaptive role during the rewiring of the thiol-redox metabolism of kinetoplastids.

Comparison of structures among Saccharomyces cerevisiae Grxs proteins

Glutaredoxins (Grxs) comprise a group of glutathione (GSH)-dependent oxidoreductase enzymes that respond to oxidative stress and sustain redox homeostasis. Saccharomyces cerevisiae Grx has a similar interaction patterns through its residues between the residues and the environment. The glutaredoxin domain covers 100% of the entire mature Grx1 and Grx8, while the glutaredoxin domain covers ~ 52% of the entire mature Grx6 and Grx7, which have approximately 74 additional amino acids in their N-terminal regions, whereas Grx3 and Grx4 have two functional domains: glutaredoxin and thioredoxin. We have presented the prediction of disordered regions within these protein sequences. Multiple sequence alignment combined with a phylogenetic tree enabled us to specify the key residues contributing to the differences between Saccharomyces cerevisiae Grxs and the proportion symmetry.

Neuronal Damage Induced by Perinatal Asphyxia Is Attenuated by Postinjury Glutaredoxin-2 Administration

The general disruption of redox signaling following an ischemia-reperfusion episode has been proposed as a crucial component in neuronal death and consequently brain damage. Thioredoxin (Trx) family proteins control redox reactions and ensure protein regulation via specific, oxidative posttranslational modifications as part of cellular signaling processes. Trx proteins function in the manifestation, progression, and recovery following hypoxic/ischemic damage. Here, we analyzed the neuroprotective effects of postinjury, exogenous administration of Grx2 and Trx1 in a neonatal hypoxia/ischemia model. P7 Sprague-Dawley rats were subjected to right common carotid ligation or sham surgery, followed by an exposure to nitrogen. 1 h later, animals were injected i.p. with saline solution, 10 mg/kg recombinant Grx2 or Trx1, and euthanized 72 h postinjury. Results showed that Grx2 administration, and to some extent Trx1, attenuated part of the neuronal damage associated with a perinatal hypoxic/ischemic damage, such as glutamate excitotoxicity, axonal integrity, and astrogliosis. Moreover, these treatments also prevented some of the consequences of the induced neural injury, such as the delay of neurobehavioral development. To our knowledge, this is the first study demonstrating neuroprotective effects of recombinant Trx proteins on the outcome of neonatal hypoxia/ischemia, implying clinical potential as neuroprotective agents that might counteract neonatal hypoxia/ischemia injury.

Polysulfide-Scission Reagents for the Suppression of the Shuttle Effect in Lithium-Sulfur Batteries

Lithium-sulfur batteries have become an appealing candidate for next-generation energy-storage technologies because of their low cost and high energy density. However, one of their major practical problems is the high solubility of long-chain lithium polysulfides and their infamous shuttle effect, which causes low Coulombic efficiency and sulfur loss. Here, we introduced a concept involving the dithiothreitol (DTT) assisted scission of polysulfides into lithium-sulfur system. Our designed porous carbon nanotube/S cathode coupling with a lightweight graphene/DTT interlayer (PCNTs-S@Gra/DTT) exhibited ultrahigh cycle-ability even at 5 C over 1100 cycles, with a capacity degradation rate of 0.036% per cycle. Additionally, the PCNTs-S@Gra/DTT electrode with a 3.51 mg cm^-2 sulfur mass loading delivered a high initial areal capacity of 5.29 mAh cm^-2 (1509 mAh g^-1) at current density of 0.58 mA cm^-2, and the reversible areal capacity of the cell was maintained at 3.45 mAh cm^-2 (984 mAh g^-1) over 200 cycles at a higher current density of 1.17 mA cm^-2. Employing this molecule scission principle offers a promising avenue to achieve high-performance lithium-sulfur batteries.

Formation and Reversibility of BiP Protein Cysteine Oxidation Facilitate Cell Survival during and post Oxidative Stress

Redox fluctuations within cells can be detrimental to cell function. To gain insight into how cells normally buffer against redox changes to maintain cell function, we have focused on elucidating the signaling pathways that serve to sense and respond to oxidative redox stress within the endoplasmic reticulum (ER) using yeast as a model system. Previously, we have shown that a cysteine in the molecular chaperone BiP, a Hsp70 molecular chaperone within the ER, is susceptible to oxidation by peroxide during ER-derived oxidative stress, forming a sulfenic acid (-SOH) moiety. Here, we demonstrate that this same conserved BiP cysteine is susceptible also to glutathione modification (-SSG). Glutathionylated BiP is detected both as a consequence of enhanced levels of cellular peroxide and also as a by-product of increased levels of oxidized glutathione (GSSG). Similar to sulfenylation, we observe glutathionylation decouples BiP ATPase and peptide binding activities, turning BiP from an ATP-dependent foldase into an ATP-independent holdase. We show glutathionylation enhances cell proliferation during oxidative stress, which we suggest relates to modified BiP's increased ability to limit polypeptide aggregation. We propose the susceptibility of BiP to modification with glutathione may serve also to prevent irreversible oxidation of BiP by peroxide.

Structural and Biochemical Characterizations of Methanoredoxin from Methanosarcina acetivorans, a Glutaredoxin-Like Enzyme with Coenzyme M-Dependent Protein Disulfide Reductase Activity

Glutaredoxins (GRXs) are thiol-disulfide oxidoreductases abundant in prokaryotes, although little is understood of these enzymes from the domain Archaea. The numerous characterized GRXs from the domain Bacteria utilize a diversity of low-molecular-weight thiols in addition to glutathione as reductants. We report here the biochemical and structural properties of a GRX-like protein named methanoredoxin (MRX) from Methanosarcina acetivorans of the domain Archaea. MRX utilizes coenzyme M (CoMSH) as reductant for insulin disulfide reductase activity, which adds to the diversity of thiol protectants in prokaryotes. Cell-free extracts of M. acetivorans displayed CoMS-SCoM reductase activity that complements the CoMSH-dependent activity of MRX. The crystal structure exhibits a classic thioredoxin-glutaredoxin fold comprising three α-helices surrounding four antiparallel β-sheets. A pocket on the surface contains a CVWC motif, identifying the active site with architecture similar to GRXs. Although it is a monomer in solution, the crystal lattice has four monomers in a dimer of dimers arrangement. A cadmium ion is found within the active site of each monomer. Two such ions stabilize the N-terminal tails and dimer interfaces. Our modeling studies indicate that CoMSH and glutathione (GSH) bind to the active site of MRX similar to the binding of GSH in GRXs, although there are differences in the amino acid composition of the binding motifs. The results, combined with our bioinformatic analyses, show that MRX represents a class of GRX-like enzymes present in a diversity of methane-producing Archaea.

The specificity of thioredoxins and glutaredoxins is determined by electrostatic and geometric complementarity

Thiol-disulfide oxidoreductases from the thioredoxin (Trx) family of proteins have a broad range of well documented functions and possess distinct substrate specificities. The mechanisms and characteristics that control these specificities are key to the understanding of both the reduction of catalytic disulfides as well as allosteric disulfides (thiol switches). Here, we have used the catalytic disulfide of E. coli 3'-phosphoadenosine 5'-phosphosulfate (PAPS) reductase (PR) that forms between the single active site thiols of two monomers during the reaction cycle as a model system to investigate the mechanisms of Trx and Grx protein specificity. Enzyme kinetics, ΔE'0 determination, and structural analysis of various Trx and Grx family members suggested that the redox potential does not determine specificity nor efficiency of the redoxins as reductant for PR. Instead, the efficiency of PR with various redoxins correlated strongly to the extent of a negative electric field of the redoxins reaching into the solvent outside the active site, and electrostatic and geometric complementary contact surfaces. These data suggest that, in contrast to common assumption, the composition of the active site motif is less important for substrate specificity than other amino acids in or even outside the immediate contact area.

Cysteines under ROS attack in plants: a proteomics view

Plants generate reactive oxygen species (ROS) as part of their metabolism and in response to various external stress factors, potentially causing significant damage to biomolecules and cell structures. During the course of evolution, plants have adapted to ROS toxicity, and use ROS as signalling messengers that activate defence responses. Cysteine (Cys) residues in proteins are one of the most sensitive targets for ROS-mediated post-translational modifications, and they have become key residues for ROS signalling studies. The reactivity of Cys residues towards ROS, and their ability to react to different oxidation states, allow them to appear at the crossroads of highly dynamic oxidative events. As such, a redox-active cysteine can be present as S-glutathionylated (-SSG), disulfide bonded (S-S), sulfenylated (-SOH), sulfinylated (-SO2H), and sulfonylated (-SO3H). The sulfenic acid (-SOH) form has been considered as part of ROS-sensing pathways, as it leads to further modifications which affect protein structure and function. Redox proteomic studies are required to understand how and why cysteines undergo oxidative post-translational modifications and to identify the ROS-sensor proteins. Here, we update current knowledge of cysteine reactivity with ROS. Further, we give an overview of proteomic techniques that have been applied to identify different redox-modified cysteines in plants. There is a particular focus on the identification of sulfenylated proteins, which have the potential to be involved in plant signal transduction.

The GSH- and GSSG-bound structures of glutaredoxin from Clostridium oremlandii

Glutaredoxin (Grx) is a major redox enzyme that reduces disulfide bonds using glutathione (GSH) as an electron donor. The anaerobic bacterium Clostridium oremlandii possesses a selenocysteine-containing Grx (cGrx1) and a cysteine-containing homolog (cGrx2). Here, the crystal structure of the GSSG-bound form of cGrx2 was determined for the first time at a resolution of 1.95Å. In addition, its monothiol variant cGrx2/C15S in complex with GSH was also determined at a resolution of 1.58Å. cGrx2 is a monomeric protein with an overall structure that consists of the typical thioredoxin fold composed of four α-helices and four β-strands. Two ligands, GSH and GSSG, share a conserved binding site consisting of CPYC, TVP, and CDD motifs. The cysteinyl and γ-glutamyl moieties show similar binding interactions in the two structures, whereas the glycine moiety shows different interactions. Interestingly, the structures revealed that only one GSH moiety of GSSG is sufficient for its binding to the protein. The GSSG-bound structure of cGrx2 was obtained as an oxidized form with a disulfide bond at the CPYC motif. Comparison of the GSH-binding mode in cGrx2 to other known Grxs revealed similarities as well as some diversity.

Enzyme structure captures four cysteines aligned for disulfide relay

Thioredoxin superfamily proteins introduce disulfide bonds into substrates, catalyze the removal of disulfides, and operate in electron relays. These functions rely on one or more dithiol/disulfide exchange reactions. The flavoenzyme quiescin sulfhydryl oxidase (QSOX), a catalyst of disulfide bond formation with an interdomain electron transfer step in its catalytic cycle, provides a unique opportunity for exploring the structural environment of enzymatic dithiol/disulfide exchange. Wild-type Rattus norvegicus QSOX1 (RnQSOX1) was crystallized in a conformation that juxtaposes the two redox-active di-cysteine motifs in the enzyme, presenting the entire electron-transfer pathway and proton-transfer participants in their native configurations. As such a state cannot generally be enriched and stabilized for analysis, RnQSOX1 gives unprecedented insight into the functional group environments of the four cysteines involved in dithiol/disulfide exchange and provides the framework for analysis of the energetics of electron transfer in the presence of the bound flavin adenine dinucleotide cofactor. Hybrid quantum mechanics/molecular mechanics (QM/MM) free energy simulations based on the X-ray crystal structure suggest that formation of the interdomain disulfide intermediate is highly favorable and secures the flexible enzyme in a state from which further electron transfer via the flavin can occur.

Structure of Escherichia coli Grx2 in complex with glutathione: a dual-function hybrid of glutaredoxin and glutathione S-transferase

The structure of glutaredoxin 2 (Grx2) from Escherichia coli co-crystallized with glutathione (GSH) was solved at 1.60 Å resolution. The structure of a mutant with the active-site residues Cys9 and Cys12 changed to serine crystallized in the absence of glutathione was solved to 2.4 Å resolution. Grx2 has an N-terminal domain characteristic of glutaredoxins, and the overall structure is congruent with the structure of glutathione S-transferases (GSTs). Purified Grx2 exhibited GST activity. Grx2, which is the physiological electron donor for arsenate reduction by E. coli ArsC, was docked with ArsC. The docked structure could be fitted with GSH bridging the active sites of the two proteins. It is proposed that Grx2 is a novel Grx/GST hybrid that functions in two steps of the ArsC catalytic cycle: as a GST it catalyzes glutathionylation of the ArsC-As(V) intermediate and as a glutaredoxin it catalyzes deglutathionylation of the ArsC-As(III)-SG intermediate.

Proteomic identification and quantification of S-glutathionylation in mouse macrophages using resin-assisted enrichment and isobaric labeling

S-Glutathionylation (SSG) is an important regulatory posttranslational modification on protein cysteine (Cys) thiols, yet the role of specific cysteine residues as targets of modification is poorly understood. We report a novel quantitative mass spectrometry (MS)-based proteomic method for site-specific identification and quantification of S-glutathionylation across different conditions. Briefly, this approach consists of initial blocking of free thiols by alkylation, selective reduction of glutathionylated thiols, and covalent capture of reduced thiols using thiol affinity resins, followed by on-resin tryptic digestion and isobaric labeling with iTRAQ (isobaric tags for relative and absolute quantitation) for MS-based identification and quantification. The overall approach was initially validated by application to RAW 264.7 mouse macrophages treated with different doses of diamide to induce glutathionylation. A total of 1071 Cys sites from 690 proteins were identified in response to diamide treatment, with ~90% of the sites displaying >2-fold increases in SSG modification compared to controls. This approach was extended to identify potential SSG-modified Cys sites in response to H2O2, an endogenous oxidant produced by activated macrophages and many pathophysiological stimuli. The results revealed 364 Cys sites from 265 proteins that were sensitive to S-glutathionylation in response to H2O2 treatment, thus providing a database of proteins and Cys sites susceptible to this modification under oxidative stress. Functional analysis revealed that the most significantly enriched molecular function categories for proteins sensitive to SSG modifications were free radical scavenging and cell death/survival. Overall the results demonstrate that our approach is effective for site-specific identification and quantification of SSG-modified proteins. The analytical strategy also provides a unique approach to determining the major pathways and cellular processes most susceptible to S-glutathionylation under stress conditions.

Novel highly active recombinant glutaredoxin from Chlorella sorokiniana T-89

Glutaredoxin (Grx) is a thiol/disulfide oxidoreductase that maintains the cellular thiol/disulfide ratio. A 321 bp cDNA fragment encoding a putative Grx (named CsT-89Grx) was cloned from heat-tolerant Chlorella sorokiniana T-89 and expressed in an Escherichia coli system. The sequence analysis of CsT-89Grx and site-directed mutations showed that the putative active site within the CPYC motif belonged to the dithiol superfamily. The biochemical property analyses showed that the optimal pH and temperature of CsT-89Grx are pH 8.5 and 50 °C, respectively. The activity of CsT-89Grx showed high thermal stability (retained 70% activity at 80 °C for 30 min) and broad pH stability (retained over 70% activity for 1 h) ranging from pH 3 to 11. The kinetic parameter kcat/Km was 20,982 min(-1) mM(-1), which suggested that CsT-89Grx exhibited the highest catalytic efficiency in reducing the disulfide bond among all the Grx reported in the related literature and is therefore potentially useful for industrial applications.

Electrostatics of cysteine residues in proteins: parameterization and validation of a simple model

One of the most popular and simple models for the calculation of pK(a) s from a protein structure is the semi-macroscopic electrostatic model MEAD. This model requires empirical parameters for each residue to calculate pK(a) s. Analysis of current, widely used empirical parameters for cysteine residues showed that they did not reproduce expected cysteine pK(a) s; thus, we set out to identify parameters consistent with the CHARMM27 force field that capture both the behavior of typical cysteines in proteins and the behavior of cysteines which have perturbed pK(a) s. The new parameters were validated in three ways: (1) calculation across a large set of typical cysteines in proteins (where the calculations are expected to reproduce expected ensemble behavior); (2) calculation across a set of perturbed cysteines in proteins (where the calculations are expected to reproduce the shifted ensemble behavior); and (3) comparison to experimentally determined pK(a) values (where the calculation should reproduce the pK(a) within experimental error). Both the general behavior of cysteines in proteins and the perturbed pK(a) in some proteins can be predicted reasonably well using the newly determined empirical parameters within the MEAD model for protein electrostatics. This study provides the first general analysis of the electrostatics of cysteines in proteins, with specific attention paid to capturing both the behavior of typical cysteines in a protein and the behavior of cysteines whose pK(a) should be shifted, and validation of force field parameters for cysteine residues.

Chaperone-like properties of tobacco plastid thioredoxins f and m

Thioredoxins (Trxs) are ubiquitous disulphide reductases that play important roles in the redox regulation of many cellular processes. However, some redox-independent functions, such as chaperone activity, have also been attributed to Trxs in recent years. The focus of our study is on the putative chaperone function of the well-described plastid Trxs f and m. To that end, the cDNA of both Trxs, designated as NtTrxf and NtTrxm, was isolated from Nicotiana tabacum plants. It was found that bacterially expressed tobacco Trx f and Trx m, in addition to their disulphide reductase activity, possessed chaperone-like properties. In vitro, Trx f and Trx m could both facilitate the reactivation of the cysteine-free form of chemically denatured glucose-6 phosphate dehydrogenase (foldase chaperone activity) and prevent heat-induced malate dehydrogenase aggregation (holdase chaperone activity). Our results led us to infer that the disulphide reductase and foldase chaperone functions prevail when the proteins occur as monomers and the well-conserved non-active cysteine present in Trx f is critical for both functions. By contrast, the holdase chaperone activity of both Trxs depended on their oligomeric status: the proteins were functional only when they were associated with high molecular mass protein complexes. Because the oligomeric status of both Trxs was induced by salt and temperature, our data suggest that plastid Trxs could operate as molecular holdase chaperones upon oxidative stress, acting as a type of small stress protein.

Thioredoxin 1-mediated post-translational modifications: reduction, transnitrosylation, denitrosylation, and related proteomics methodologies

Despite the significance of redox post-translational modifications (PTMs) in regulating diverse signal transduction pathways, the enzymatic systems that catalyze reversible and specific oxidative or reductive modifications have yet to be firmly established. Thioredoxin 1 (Trx1) is a conserved antioxidant protein that is well known for its disulfide reductase activity. Interestingly, Trx1 is also able to transnitrosylate or denitrosylate (defined as processes to transfer or remove a nitric oxide entity to/from substrates) specific proteins. An intricate redox regulatory mechanism has recently been uncovered that accounts for the ability of Trx1 to catalyze these different redox PTMs. In this review, we will summarize the available evidence in support of Trx1 as a specific disulfide reductase, and denitrosylation and transnitrosylation agent, as well as the biological significance of the diverse array of Trx1-regulated pathways and processes under different physiological contexts. The dramatic progress in redox proteomics techniques has enabled the identification of an increasing number of proteins, including peroxiredoxin 1, whose disulfide bond formation and nitrosylation status are regulated by Trx1. This review will also summarize the advancements of redox proteomics techniques for the identification of the protein targets of Trx1-mediated PTMs. Collectively, these studies have shed light on the mechanisms that regulate Trx1-mediated reduction, transnitrosylation, and denitrosylation of specific target proteins, solidifying the role of Trx1 as a master regulator of redox signal transduction.

Redox, mutagenic and structural studies of the glutaredoxin/arsenate reductase couple from the cyanobacterium Synechocystis sp. PCC 6803

The arsenate reductase from the cyanobacterium Synechocystis sp. PCC 6803 has been characterized in terms of the redox properties of its cysteine residues and their role in the reaction catalyzed by the enzyme. Of the five cysteines present in the enzyme, two (Cys13 and Cys35) have been shown not to be required for catalysis, while Cys8, Cys80 and Cys82 have been shown to be essential. The as-isolated enzyme contains a single disulfide, formed between Cys80 and Cys82, with an oxidation-reduction midpoint potential (E(m)) value of -165mV at pH 7.0. It has been shown that Cys15 is the only one of the four cysteines present in Synechocystis sp. PCC 6803 glutaredoxin A required for its ability to serve as an electron donor to arsenate reductase, while the other three cysteines (Cys18, Cys36 and Cys70) play no role. Glutaredoxin A has been shown to contain a single redox-active disulfide/dithiol couple, with a two-electron, E(m) value of -220mV at pH 7.0. One cysteine in this disulfide/dithiol couple has been shown to undergo glutathionylation. An X-ray crystal structure, at 1.8Å resolution, has been obtained for glutaredoxin A. The probable orientations of arsenate reductase disulfide bonds present in the resting enzyme and in a likely reaction intermediate of the enzyme have been examined by in silico modeling, as has the surface environment of arsenate reductase in the vicinity of Cys8, the likely site for the initial reaction between arsenate and the enzyme.

S-Glutathionyl quantification in the attomole range using glutaredoxin-3-catalyzed cysteine derivatization and capillary gel electrophoresis with laser-induced fluorescence detection

S-glutathionylation (Pr-SSG) is a specific post-translational modification of cysteine residues by the addition of glutathione. S-Glutathionylated proteins induced by oxidative or nitrosative stress play an essential role in understanding the pathogenesis of the aging and age-related disorder, such as Alzheimer's disease (AD). The purpose of this research is to develop a novel and ultrasensitive method to accurately and rapidly quantify the Pr-SSG by using capillary gel electrophoresis with laser-induced fluorescence detection (CGE-LIF). The derivatization method is based on the specific reduction of protein-bound S-glutathionylation with glutaredoxin (Grx) and labeling with thiol-reactive fluorescent dye (Dylight 488 maleimide). The experiments were performed by coupling the derivatization method with CGE-LIF to study electrophoretic profiling in in vitro oxidative stress model-S-glutathionylated bovine serum albumin (BSA-SSG), oxidant-induced human colon adenocarcinoma (HT-29) cells, brain tissues, and whole blood samples from an AD transgenic (Tg) mouse model. The results showed almost an eightfold increase in S-glutathionyl abundance when subjecting HT-29 cells in an oxidant environment, resulting in Pr-SSG at 232 ± 10.64 (average ±SD; n=3) nmol/mg. In the AD-Tg mouse model, an initial quantitative measurement demonstrated the extent of protein S-glutathionylation in three brain regions (hippocampus, cerebellum, and cerebrum), ranging from 1 to 10 nmol/mg. Additionally, we described our developed method to potentially serve as a highly desirable diagnostic tool for monitoring S-glutathionylated protein profile in minuscule amount of whole blood. The whole blood samples for S-glutathionyl expression of 5-month-old AD-Tg mice are quantified as 16.3 μmol/L (=7.2 nmol/mg protein). Altogether, this is a fast, easy, and accurate method, reaching the lowest limit of Pr-SSG detection at 1.8 attomole (amol) level, reported to date.

Redox pioneer: Professor Arne Holmgren

Dr. Arne Holmgren (Ph.D., 1968) is recognized here as a redox pioneer, because he has published at least one article on redox biology that has been cited over 1000 times and has published at least 10 articles, each cited over 100 times. He is widely known for his seminal discoveries and in-depth studies of thioredoxins, thioredoxin reductases, and glutaredoxins. Dr. Holmgren, active throughout his career at Karolinska Institutet, Sweden, has led the field of research about these classes of proteins for more than 45 years, continuously building upon his sequence determination of Escherichia coli thioredoxin in the late 1960s and discovery of the thioredoxin fold in the 1970s. He discovered and named glutaredoxin and he determined the structure and function of several members of these glutathione-dependent disulfide oxidoreductases. He still continues to broaden the frontiers of knowledge of thioredoxin and glutaredoxin systems. The thioredoxin fold is today recognized as one of the most common protein folds and the intriguing complexity of redox systems, redox signaling, and redox control of cellular function is constantly increasing. The legacy of Dr. Holmgren's research is therefore highly relevant and important also in the context of present science. In a tribute to his work, questions need to be addressed toward the physiological importance of redox signaling and the impact of glutaredoxin and thioredoxin systems on health and disease. Dr. Holmgren helped lay the foundation for the redox biology field and opened new vistas in the process. He is truly a redox pioneer.

Arabidopsis chloroplastic glutaredoxin C5 as a model to explore molecular determinants for iron-sulfur cluster binding into glutaredoxins

Unlike thioredoxins, glutaredoxins are involved in iron-sulfur cluster assembly and in reduction of specific disulfides (i.e. protein-glutathione adducts), and thus they are also important redox regulators of chloroplast metabolism. Using GFP fusion, AtGrxC5 isoform, present exclusively in Brassicaceae, was shown to be localized in chloroplasts. A comparison of the biochemical, structural, and spectroscopic properties of Arabidopsis GrxC5 (WCSYC active site) with poplar GrxS12 (WCSYS active site), a chloroplastic paralog, indicated that, contrary to the solely apomonomeric GrxS12 isoform, AtGrxC5 exists as two forms when expressed in Escherichia coli. The monomeric apoprotein possesses deglutathionylation activity mediating the recycling of plastidial methionine sulfoxide reductase B1 and peroxiredoxin IIE, whereas the dimeric holoprotein incorporates a [2Fe-2S] cluster. Site-directed mutagenesis experiments and resolution of the x-ray crystal structure of AtGrxC5 in its holoform revealed that, although not involved in its ligation, the presence of the second active site cysteine (Cys(32)) is required for cluster formation. In addition, thiol titrations, fluorescence measurements, and mass spectrometry analyses showed that, despite the presence of a dithiol active site, AtGrxC5 does not form any inter- or intramolecular disulfide bond and that its activity exclusively relies on a monothiol mechanism.

The dithiol glutaredoxins of african trypanosomes have distinct roles and are closely linked to the unique trypanothione metabolism

Trypanosoma brucei, the causative agent of African sleeping sickness, possesses two dithiol glutaredoxins (Grx1 and Grx2). Grx1 occurs in the cytosol and catalyzes protein deglutathionylations with k(cat)/K(m)-values of up to 2 × 10(5) M(-1) S(-1). It accelerates the reduction of ribonucleotide reductase by trypanothione although less efficiently than the parasite tryparedoxin and has low insulin disulfide reductase activity. Despite its classical CPYC active site, Grx1 forms dimeric iron-sulfur complexes with GSH, glutathionylspermidine, or trypanothione as non-protein ligands. Thus, contrary to the generally accepted assumption, replacement of the Pro is not a prerequisite for cluster formation. T. brucei Grx2 shows an unusual CQFC active site, and orthologues occur exclusively in trypanosomatids. Grx2 is enriched in mitoplasts, and fractionated digitonin lysis resulted in a co-elution with cytochrome c, suggesting localization in the mitochondrial intermembrane space. Grx2 catalyzes the reduction of insulin disulfide but not of ribonucleotide reductase and exerts deglutathionylation activity 10-fold lower than that of Grx1. RNA interference against Grx2 caused a growth retardation of procyclic cells consistent with an essential role. Grx1 and Grx2 are constitutively expressed with cellular concentrations of about 2 μM and 200 nM, respectively, in both the mammalian bloodstream and insect procyclic forms. Trypanothione reduces the disulfide form of both proteins with apparent rate constants that are 3 orders of magnitude higher than those with glutathione. Grx1 and, less efficiently, also Grx2 catalyze the reduction of GSSG by trypanothione. Thus, the Grxs play exclusive roles in the trypanothione-based thiol redox metabolism of African trypanosomes.

Determinants of activity in glutaredoxins: an in vitro evolved Grx1-like variant of Escherichia coli Grx3

The Escherichia coli glutaredoxins 1 and 3 (Grx1 and Grx3) are structurally similar (37% sequence identity), yet have different activities in vivo. Unlike Grx3, Grx1 efficiently reduces protein disulfides in proteins such as RR (ribonucleotide reductase), whereas it is poor at reducing S-glutathionylated proteins. An E. coli strain lacking genes encoding thioredoxins 1 and 2 and Grx1 is not viable on either rich or minimal medium; however, a M43V mutation in Grx3 restores growth under these conditions and results in a Grx1-like protein [Ortenberg, Gon, Porat and Beckwith (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 7439–7944]. To uncover the structural basis of this change in activity, we have compared wild-type and mutant Grx3 using CD and NMR spectroscopy. Ligand-induced stability measurements demonstrate that the Grx3(M43V/C65Y) mutant has acquired affinity for RR. Far-UV CD spectra reveal no significant differences, but differences are observed in the near-UV region indicative of tertiary structural changes. NMR 1H-15N HSQC (heteronuclear single quantum coherence) spectra show that approximately half of the 82 residues experience significant (Δδ>0.03 p.p.m.) chemical shift deviations in the mutant, including nine residues experiencing extensive (Δδ≥0.15 p.p.m.) deviations. To test whether the M43V mutation alters dynamic properties of Grx3, H/D (hydrogen/deuterium) exchange experiments were performed demonstrating that the rate at which backbone amides exchange protons with the solvent is dramatically enhanced in the mutant, particularly in the core of the protein. These data suggest that the Grx1-like activity of the Grx3(M43V/C65Y) mutant may be explained by enhanced intrinsic motion allowing for increased specificity towards larger substrates such as RR.

Thiol-disulfide exchange between glutaredoxin and glutathione

Glutaredoxins are ubiquitous thiol-disulfide oxidoreductases which catalyze the reduction of glutathione-protein mixed disulfides. Belonging to the thioredoxin family, they contain a conserved active site CXXC motif. The N-proximal active site cysteine can form a mixed disulfide with glutathione or an intramolecular disulfide with the C-proximal cysteine. The C-proximal cysteine is not known to be involved in the catalytic mechanism. The stability of the mixed disulfide with glutathione has been investigated in detail using a mutant variant of yeast glutaredoxin 1, in which the C-proximal active site cysteine has been replaced with serine. The exchange reaction between the reduced protein and oxidized glutathione leading to formation of the mixed disulfide could readily be monitored by isothermal titration calorimetry (ITC) due to the enthalpic contributions from the noncovalent interactions and the protonation of glutathione thiolate. An algorithm for the analysis of this type of reaction by ITC was developed and showed that the interaction is enthalpy driven with a large entropy penalty. The applicability of the method was verified by a mass spectrometry-based approach, which gave a standard reduction potential of -295 mV for the mixed disulfide. In another set of experiments, the pK(a) value of the active site cysteine was determined. In line with what has been observed for other glutaredoxins, this cysteine was found to have a very low pK(a) value. The glutathionylation of glutaredoxin was shown to have a substantial effect on the thermal stability of the protein as revealed by differential scanning calorimetry.

A residue outside the active site CXXC motif regulates the catalytic efficiency of Glutaredoxin 3

The glutaredoxin (Grx) family of oxidoreductases has a conserved residue at position 8 that varies between Arginine in Grx1 and Lysine in Grx3. It has been proposed that this Arg/Lys change is the main cause for the 35 mV difference in redox potential between the two enzymes. To gain insights into the catalytic machinery of Grx3 and directly evaluate the role of residue 8 in the catalysis of thiol-disulfide exchange by this enzyme, we synthesized the "wild type" enzyme (sGrx3), and four analogues substituting the lysine at position 8 with arginine, ornithine (Orn), citrulline (Cit) and norvaline (Nva). The redox potential and equilibration kinetics with thioredoxin (Trx1) were determined for each enzyme by fluorescence intensity. While minor effects on redox potential were observed, we found that residue 8 had a more marked effect on the catalytic efficiency of this enzyme. Surprisingly, truncation of the functional group resulted in a more efficient enzyme, Lys8Nva, exhibiting rate constants that are an order of magnitude higher than sGrx3 for both forward and reverse reactions. These observations pose the question why would a residue that reduces the rate of enzyme turnover be evolutionarily conserved? The significant changes in the kinetic parameters suggest that this position plays an important role in the thiol-disulfide exchange reaction by affecting the nucleophilic thiolate through electrostatic or hydrogen bonding interactions. Since the reduced Grx has an exposed thiol that could easily be alkylated, either Arg or Lys could act as a gatekeeper that deters unwanted electrophiles from attacking the active site thiolate.

The C-terminal active site cysteine of Escherichia coli glutaredoxin 1 determines the glutathione specificity of the second step of peptide deglutathionylation

Glutaredoxins are oxidoreductases specialized in reducing glutathione-protein mixed disulfides. In the first step of deglutathionylation, glutaredoxins form a mixed disulfide with glutathione, releasing reduced peptide. The specificity of this reaction is based on the unusual amide linkage formed between the gamma-carboxylate of the N-terminal glutamic acid and the alpha-amino group of the cysteine present in glutathione. In the second step of deglutathionylation, glutathione reduces the glutaredoxin-glutathione mixed disulfide. Here we show that the specificity of this second reaction for Escherichia coli Grx1, but not for human or yeast Grx1, also is based on the unusual gamma-linkage present in glutathione. Mutating Tyr13, Thr58, and/or Asp74 to alanine in E. coli Grx1 results in the glutaredoxin-peptide mixed disulfide being thermodynamically favored over the glutaredoxin-glutathione mixed disulfide in the first step of the reaction. An increased propensity to form glutaredoxin-protein mixed disulfides was observed in vivo for these same mutants. Furthermore, we demonstrate that all mutations studied in Cys14, the C-terminal active site cysteine, abolish the specificity of E. coli Grx1 for glutathione over the corresponding tripeptide Glu-Cys-Gly, which has a normal peptide bond linking Glu-Cys instead of the gamma-linkage present in glutathione, in the second step of deglutathionylation.

The structure of the bacterial oxidoreductase enzyme DsbA in complex with a peptide reveals a basis for substrate specificity in the catalytic cycle of DsbA enzymes

Oxidative protein folding in Gram-negative bacteria results in the formation of disulfide bonds between pairs of cysteine residues. This is a multistep process in which the dithiol-disulfide oxidoreductase enzyme, DsbA, plays a central role. The structure of DsbA comprises an all helical domain of unknown function and a thioredoxin domain, where active site cysteines shuttle between an oxidized, substrate-bound, reduced form and a DsbB-bound form, where DsbB is a membrane protein that reoxidizes DsbA. Most DsbA enzymes interact with a wide variety of reduced substrates and show little specificity. However, a number of DsbA enzymes have now been identified that have narrow substrate repertoires and appear to interact specifically with a smaller number of substrates. The transient nature of the DsbA-substrate complex has hampered our understanding of the factors that govern the interaction of DsbA enzymes with their substrates. Here we report the crystal structure of a complex between Escherichia coli DsbA and a peptide with a sequence derived from a substrate. The binding site identified in the DsbA-peptide complex was distinct from that observed for DsbB in the DsbA-DsbB complex. The structure revealed details of the DsbA-peptide interaction and suggested a mechanism by which DsbA can simultaneously show broad specificity for substrates yet exhibit specificity for DsbB. This mode of binding was supported by solution nuclear magnetic resonance data as well as functional data, which demonstrated that the substrate specificity of DsbA could be modified via changes at the binding interface identified in the structure of the complex.

Mechanistic and kinetic details of catalysis of thiol-disulfide exchange by glutaredoxins and potential mechanisms of regulation

Glutaredoxins are small, heat-stable proteins that exhibit a characteristic thioredoxin fold and a CXXC/S active-site motif. A variety of glutathione (GSH)-dependent catalytic activities have been attributed to the glutaredoxins, including reduction of ribonucleotide reductase, arsenate, and dehydroascorbate; assembly of iron sulfur cluster complexes; and protein glutathionylation and deglutathionylation. Catalysis of reversible protein glutathionylation by glutaredoxins has been implicated in regulation of redox signal transduction and sulfhydryl homeostasis in numerous contexts in health and disease. This forum review is presented in two parts. Part I is focused primarily on the mechanism of the deglutathionylation reaction catalyzed by prototypical dithiol glutaredoxins, especially human Grx1 and Grx2. Grx-catalyzed protein deglutathionylation proceeds by a nucleophilic, double-displacement mechanism in which rate enhancement is attributed to special reactivity of the low pK(a) cysteine at its active site, and to increased nucleophilicity of the second substrate, GSH. Glutaredoxins (and Grx domains) have been identified in most organisms, and many exhibit deglutathionylation or other activities or both. Further characterization according to glutathionyl selectivity, physiological substrates, and intracellular roles may lead to subclassification of this family of enzymes. Part II presents potential mechanisms for in vivo regulation of Grx activity, providing avenues for future studies.

Structure-function relationship of the chloroplastic glutaredoxin S12 with an atypical WCSYS active site

Glutaredoxins (Grxs) are efficient catalysts for the reduction of mixed disulfides in glutathionylated proteins, using glutathione or thioredoxin reductases for their regeneration. Using GFP fusion, we have shown that poplar GrxS12, which possesses a monothiol (28)WCSYS(32) active site, is localized in chloroplasts. In the presence of reduced glutathione, the recombinant protein is able to reduce in vitro substrates, such as hydroxyethyldisulfide and dehydroascorbate, and to regenerate the glutathionylated glyceraldehyde-3-phosphate dehydrogenase. Although the protein possesses two conserved cysteines, it is functioning through a monothiol mechanism, the conserved C terminus cysteine (Cys(87)) being dispensable, since the C87S variant is fully active in all activity assays. Biochemical and crystallographic studies revealed that Cys(87) exhibits a certain reactivity, since its pK(a) is around 5.6. Coupled with thiol titration, fluorescence, and mass spectrometry analyses, the resolution of poplar GrxS12 x-ray crystal structure shows that the only oxidation state is a glutathionylated derivative of the active site cysteine (Cys(29)) and that the enzyme does not form inter- or intramolecular disulfides. Contrary to some plant Grxs, GrxS12 does not incorporate an iron-sulfur cluster in its wild-type form, but when the active site is mutated into YCSYS, it binds a [2Fe-2S] cluster, indicating that the single Trp residue prevents this incorporation.

Methods for analysis of protein glutathionylation and their application to photosynthetic organisms

Protein S-glutathionylation, the reversible formation of a mixed-disulfide between glutathione and protein thiols, is involved in protection of protein cysteines from irreversible oxidation, but also in protein redox regulation. Recent studies have implicated S-glutathionylation as a cellular response to oxidative/nitrosative stress, likely playing an important role in signaling. Considering the potential importance of glutathionylation, a number of methods have been developed for identifying proteins undergoing glutathionylation. These methods, ranging from analysis of purified proteins in vitro to large-scale proteomic analyses in vivo, allowed identification of nearly 200 targets in mammals. By contrast, the number of known glutathionylated proteins is more limited in photosynthetic organisms, although they are severely exposed to oxidative stress. The aim of this review is to detail the methods available for identification and analysis of glutathionylated proteins in vivo and in vitro. The advantages and drawbacks of each technique will be discussed as well as their application to photosynthetic organisms. Furthermore, an overview of known glutathionylated proteins in photosynthetic organisms is provided and the physiological importance of this post-translational modification is discussed.

Enzymes or redox couples? The kinetics of thioredoxin and glutaredoxin reactions in a systems biology context

Systems biology approaches, such as kinetic modelling, could provide valuable insights into how thioredoxins, glutaredoxins and peroxiredoxins (here collectively called redoxins), and the systems that reduce these molecules are regulated. However, it is not clear whether redoxins should be described as redox couples (with redox potentials) or as enzymes (with Michaelis-Menten parameters) in such approaches. We show that in complete redoxin systems, redoxin substrate saturation and other purported enzymatic behaviours result from limitations in the redoxin redox cycles in these systems. Michaelis-Menten parameters are therefore inappropriate descriptors of redoxin activity; data from redoxin kinetic experiments should rather be interpreted in terms of the complete system of reactions under study. These findings were confirmed by fitting kinetic models of the thioredoxin and glutaredoxin systems to in vitro datasets. This systems approach clarifies the inconsistencies with the descriptions of redoxins and emphasizes the roles of redoxin systems in redox regulation.

Glutathione reductase and thioredoxin reductase at the crossroad: the structure of Schistosoma mansoni thioredoxin glutathione reductase

Thioredoxin glutathione reductase (TGR) is a key flavoenzyme expressed by schistosomes that bridges two detoxification pathways crucial for the parasite survival in the host's organism. In this article we report the crystal structure (at 2.2 A resolution) of TGR from Schistosoma mansoni (SmTGR), deleted in the last two residues. The structure reveals the peculiar architecture of this chimeric enzyme: the small Glutaredoxin (Grx) domain at the N-terminus is joined to the large thioredoxin reductase (TR) one via an extended complementary surface, involving residues not conserved in the Grx superfamily; the TR domain interacts with an identical partner via its C-terminal domain, forming a dimer with a twisted "W" shape. Although lacking the penultimate Selenocysteine residue (Sec), the enzyme is still able to reduce oxidized glutathione. These data update the interpretation of the interdomain communication in TGR enzymes. The possible function of this enzyme in pathogenic parasites is discussed.

The multiple functions of the thiol-based electron flow pathways of Escherichia coli: Eternal concepts revisited

Electron flow via thiols is a theme with many variations in all kingdoms of life. The favourable physichochemical properties of the redox active couple of two cysteines placed in the optimised environment of the thioredoxin fold allow for two electron transfers in between top biological reductants and ultimate oxidants. The reduction of ribonucleotide reductases by thioredoxin and thioredoxin reductase of Escherichia coli (E. coli) was one of the first pathways to be elucidated. Diverse functions such as protein folding in the periplasm, maturation of respiratory enzymes, detoxification of hydrogen peroxide and prevention of oxidative damage may be based on two electron transfers via thiols. A growing field is the relation of thiol reducing pathways and the interaction of E. coli with different organisms. This concept combined with the sequencing of the genomes of different bacteria may allow for the identification of fine differences in the systems employing thiols for electron flow between pathogens and their corresponding mammalian hosts. The emerging possibility is the development of novel antibiotics.