Glutaredoxins employ parallel monothiol-dithiol mechanisms to catalyze thiol-disulfide exchanges with protein disulfides

Identification and characterization of multidomain monothiol glutaredoxin 3 from diploblastic Hydra

Intracellular antioxidant glutaredoxin controls cell proliferation and survival. Based on the active site, structure, and conserved domain motifs, it is classified into two classes. Class I contains dithiol Grxs with two cysteines in the consensus active site sequence CXXC, while class II has monothiol Grxs with one cysteine residue in the active site. Monothiol Grxs can also have an additional N-terminal thioredoxin (Trx)-like domain. Previously, we reported the characterization of Grx1 from Hydra vulgaris (HvGrx1), which is a dithiol isoform. Here, we report the molecular cloning, expression, analysis, and characterization of another isoform of Grx, which is the multidomain monothiol glutaredoxin-3 from Hydra vulgaris (HvGrx3). It encodes a protein with 303 amino acids and is significantly larger and more divergent than HvGrx1. In-silico analysis revealed that Grx1 and Grx3 have 22.5% and 9.9% identical nucleotide and amino acid sequences, respectively. HvGrx3 has two glutaredoxin domains and a thioredoxin-like domain at its amino terminus, unlike HvGrx1, which has a single glutaredoxin domain. Like other monothiol glutaredoxins, HvGrx3 failed to reduce glutathione-hydroxyethyl disulfide. In the whole Hydra, HvGrx3 was found to be expressed all over the body column, and treatment with H2O2 led to a significant upregulation of HvGrx3. When transfected in HCT116 (human colon cancer cells) cells, HvGrx3 enhanced cell proliferation and migration, indicating that this isoform could be involved in these cellular functions. These transfected cells also tolerate oxidative stress better.

Direct C-H Sulfuration: Synthesis of Disulfides, Dithiocarbamates, Xanthates, Thiocarbamates and Thiocarbonates

In light of the important biological activities and widespread applications of organic disulfides, dithiocarbamates, xanthates, thiocarbamates and thiocarbonates, the continual persuit of efficient methods for their synthesis remains crucial. Traditionally, the preparation of such compounds heavily relied on intricate multi-step syntheses and the use of highly prefunctionalized starting materials. Over the past two decades, the direct sulfuration of C-H bonds has evolved into a straightforward, atom- and step-economical method for the preparation of organosulfur compounds. This review aims to provide an up-to-date discussion on direct C-H disulfuration, dithiocarbamation, xanthylation, thiocarbamation and thiocarbonation, with a special focus on describing scopes and mechanistic aspects. Moreover, the synthetic limitations and applications of some of these methodologies, along with the key unsolved challenges to be addressed in the future are also discussed. The majority of examples covered in this review are accomplished via metal-free, photochemical or electrochemical approaches, which are in alignment with the overraching objectives of green and sustainable chemistry. This comprehensive review aims to consolidate recent advancements, providing valuable insights into the dynamic landscape of efficient and sustainable synthetic strategies for these crucial classes of organosulfur compounds.

Computational models as catalysts for investigating redoxin systems

Thioredoxin, glutaredoxin and peroxiredoxin systems play central roles in redox regulation, signaling and metabolism in cells. In these systems, reducing equivalents from NAD(P)H are transferred by coupled thiol-disulfide exchange reactions to redoxins which then reduce a wide array of targets. However, the characterization of redoxin activity has been unclear, with redoxins regarded as enzymes in some studies and redox metabolites in others. Consequently, redoxin activities have been quantified by enzyme kinetic parameters in vitro, and redox potentials or redox ratios within cells. By analyzing all the reactions within these systems, computational models showed that many kinetic properties attributed to redoxins were due to system-level effects. Models of cellular redoxin networks have also been used to estimate intracellular hydrogen peroxide levels, analyze redox signaling and couple omic and kinetic data to understand the regulation of these networks in disease. Computational modeling has emerged as a powerful complementary tool to traditional redoxin enzyme kinetic and cellular assays that integrates data from a number of sources into a single quantitative framework to accelerate the analysis of redoxin systems.

Exploring Immune Redox Modulation in Bacterial Infections: Insights into Thioredoxin-Mediated Interactions and Implications for Understanding Host-Pathogen Dynamics

Bacterial infections trigger a multifaceted interplay between inflammatory mediators and redox regulation. Recently, accumulating evidence has shown that redox signaling plays a significant role in immune initiation and subsequent immune cell functions. This review addresses the crucial role of the thioredoxin (Trx) system in the initiation of immune reactions and regulation of inflammatory responses during bacterial infections. Downstream signaling pathways in various immune cells involve thiol-dependent redox regulation, highlighting the pivotal roles of thiol redox systems in defense mechanisms. Conversely, the survival and virulence of pathogenic bacteria are enhanced by their ability to counteract oxidative stress and immune attacks. This is achieved through the reduction of oxidized proteins and the modulation of redox-sensitive signaling pathways, which are functions of the Trx system, thereby fortifying bacterial resistance. Moreover, some selenium/sulfur-containing compounds could potentially be developed into targeted therapeutic interventions for pathogenic bacteria. Taken together, the Trx system is a key player in redox regulation during bacterial infection, and contributes to host-pathogen interactions, offering valuable insights for future research and therapeutic development.

Deciphering the mechanism of glutaredoxin-catalyzed roGFP2 redox sensing reveals a ternary complex with glutathione for protein disulfide reduction

Glutaredoxins catalyze the reduction of disulfides and are key players in redox metabolism and regulation. While important insights were gained regarding the reduction of glutathione disulfide substrates, the mechanism of non-glutathione disulfide reduction remains highly debated. Here we determined the rate constants for the individual redox reactions between PfGrx, a model glutaredoxin from Plasmodium falciparum, and redox-sensitive green fluorescent protein 2 (roGFP2), a model substrate and versatile tool for intracellular redox measurements. We show that the PfGrx-catalyzed oxidation of roGFP2 occurs via a monothiol mechanism and is up to three orders of magnitude faster when roGFP2 and PfGrx are fused. The oxidation kinetics of roGFP2-PfGrx fusion constructs reflect at physiological GSSG concentrations the glutathionylation kinetics of the glutaredoxin moiety, thus allowing intracellular structure-function analysis. Reduction of the roGFP2 disulfide occurs via a monothiol mechanism and involves a ternary complex with GSH and PfGrx. Our study provides the mechanistic basis for understanding roGFP2 redox sensing and challenges previous mechanisms for protein disulfide reduction.

Glutathione and Glutaredoxin-Key Players in Cellular Redox Homeostasis and Signaling

This Special Issue of Antioxidants on Glutathione (GSH) and Glutaredoxin (Grx) was designed to collect review articles and original research studies focused on advancing the current understanding of the roles of the GSH/Grx system in cellular homeostasis and disease processes. The tripeptide glutathione (GSH) is the most abundant non-enzymatic antioxidant/nucleophilic molecule in cells. In addition to various metabolic reactions involving GSH and its oxidized counterpart GSSG, oxidative post-translational modification (PTM) of proteins has been a focal point of keen interest in the redox field over the last few decades. In particular, the S-glutathionylation of proteins (protein-SSG formation), i.e., mixed disulfides between GSH and protein thiols, has been studied extensively. This reversible PTM can act as a regulatory switch to interconvert inactive and active forms of proteins, thereby mediating cell signaling and redox homeostasis. The unique architecture of the GSH molecule enhances its relative abundance in cells and contributes to the glutathionyl specificity of the primary catalytic activity of the glutaredoxin enzymes, which play central roles in redox homeostasis and signaling, and in iron metabolism in eukaryotes and prokaryotes under physiological and pathophysiological conditions. The class-1 glutaredoxins are characterized as cytosolic GSH-dependent oxidoreductases that catalyze reversible protein S-glutathionylation specifically, thereby contributing to the regulation of redox signal transduction and/or the protection of protein thiols from irreversible oxidation. This Special Issue includes nine other articles: three original studies and six review papers. Together, these ten articles support the central theme that GSH/Grx is a unique system for regulating thiol-redox hemostasis and redox-signal transduction, and the dysregulation of the GSH/Grx system is implicated in the onset and progression of various diseases involving oxidative stress. Within this context, it is important to appreciate the complementary functions of the GSH/Grx and thioredoxin systems not only in thiol-disulfide regulation but also in reversible S-nitrosylation. Several potential clinical applications have emerged from a thorough understanding of the GSH/Grx redox regulatory system at the molecular level, and in various cell types in vitro and in vivo, including, among others, the concept that elevating Grx content/activity could serve as an anti-fibrotic intervention; and discovering small molecules that mimic the inhibitory effects of S-glutathionylation on dimer association could identify novel anti-viral agents that impact the key protease activities of the HIV and SARS-CoV-2 viruses. Thus, this Special Issue on Glutathione and Glutaredoxin has focused attention and advanced understanding of an important aspect of redox biology, as well as spawning questions worthy of future study.

Glutaredoxin 1 protects lens epithelial cells from epithelial-mesenchymal transition by preventing casein kinase 1α S-glutathionylation during posterior capsular opacification

Oxidative stress drives protein S-glutathionylation, which regulates the structure and function of target proteins and is implicated in the pathogenesis of many diseases. Glutaredoxin 1 (Grx1), a cytoplasmic deglutathionylating enzyme, maintains a reducing environment within the cell under various conditions by reversing S-glutathionylation. Grx1 performs a wide range of antioxidant activities in the lens and prevents protein-thiol mixed disulfide accumulation, reducing protein-protein aggregation, insolubilization, and apoptosis of lens epithelial cells. Oxidative stress is related to epithelial-mesenchymal transition (EMT) during posterior capsular opacification (PCO). However, whether Grx1-regulated protein S-glutathionylation plays an essential role in PCO remains unclear. In this study, we revealed that Grx1 expression was decreased in mice following cataract surgery. Furthermore, the absence of Grx1 elevated oxidative stress and protein S-glutathionylation and aggravated EMT in both in vitro and in vivo models. Concurrently, these results could be reversed by Grx1 overexpression. Notably, liquid chromatography-tandem mass spectrometry results showed that casein kinase 1α (CK1α) was susceptible to S-glutathionylation under oxidative stress, and CK1α S-glutathionylation (CK1α-SSG) was mediated at Cys249. The absence of Grx1 upregulated CK1α-SSG, subsequently decreasing the CK1α-induced phosphorylation of β-catenin at Ser45. The consequential downregulation of degradative β-catenin and upregulation of its nuclear translocation activated the Wnt/β-catenin signaling pathway and aggravated EMT. In conclusion, the downregulated expression of Grx1 in mice following cataract surgery aggravated EMT by upregulating the extent of CK1α-SSG. To the best of our knowledge, our study is the first to report how S-glutathionylation regulates CK1α activity under oxidative stress.

De-glutathionylases: The resilient underdogs to keep neurodegeneration at bay

Proteins become S-glutathionylated as a result of the derivatization of their cysteine thiols with the thiolate anion derivative of glutathione; this process is frequently linked to diseases and protein misbehavior. Along with the other well-known oxidative modifications like S-nitrosylation, S-glutathionylation has quickly emerged as a major contributor to a number of diseases, with a focus on neurodegeneration. The immense clinical significance of S-glutathionylation in cell signaling and the genesis of diseases are progressively coming to light with advanced research, which is also creating new opportunities for prompt diagnostics that utilize this phenomenon. In-depth investigation in recent years has revealed other significant deglutathionylases in addition to glutaredoxin, necessitating the hunt for their specific substrates. The precise catalytic mechanisms of these enzymes must also be understood, along with how the intracellular environment affects their impact on protein conformation and function. These insights must then be extrapolated to the understanding of neurodegeneration and the introduction of novel and clever therapeutic approaches to clinics. Clarifying the importance of the functional overlap of glutaredoxin and other deglutathionylases and examining their complementary functions as defense systems in the face of stress are essential prerequisites for predicting and promoting cell survival under high oxidative/nitrosative stress.

Glutathione-Related Enzymes and Proteins: A Review

The tripeptide glutathione is found in all eukaryotic cells, and due to the compartmentalization of biochemical processes, its synthesis takes place exclusively in the cytosol. At the same time, its functions depend on its transport to/from organelles and interorgan transport, in which the liver plays a central role. Glutathione is determined as a marker of the redox state in many diseases, aging processes, and cell death resulting from its properties and reactivity. It also uses other enzymes and proteins, which enables it to engage and regulate various cell functions. This paper approximates the role of these systems in redox and detoxification reactions such as conjugation reactions of glutathione-S-transferases, glyoxylases, reduction of peroxides through thiol peroxidases (glutathione peroxidases, peroxiredoxins) and thiol-disulfide exchange reactions catalyzed by glutaredoxins.

Experimental Approaches for Investigating Disulfide-Based Redox Relays in Cells

Reversible oxidation of cysteine residues within proteins occurs naturally during normal cellular homeostasis and can increase during oxidative stress. Cysteine oxidation often leads to the formation of disulfide bonds, which can impact protein folding, stability, and function. Work in both prokaryotic and eukaryotic models over the past five decades has revealed several multiprotein systems that use thiol-dependent oxidoreductases to mediate disulfide bond reduction, formation, and/or rearrangement. Here, I provide an overview of how these systems operate to carry out disulfide exchange reactions in different cellular compartments, with a focus on their roles in maintaining redox homeostasis, transducing redox signals, and facilitating protein folding. Additionally, I review thiol-independent and thiol-dependent approaches for interrogating what proteins partner together in such disulfide-based redox relays. While the thiol-independent approaches rely either on predictive measures or standard procedures for monitoring protein-protein interactions, the thiol-dependent approaches include direct disulfide trapping methods as well as thiol-dependent chemical cross-linking. These strategies may prove useful in the systematic characterization of known and newly discovered disulfide relay mechanisms and redox switches involved in oxidant defense, protein folding, and cell signaling.

(NH4)2S2O8-Promoted cross-coupling of thiols/diselenides and sulfoxides for the synthesis of unsymmetrical disulfides/selenosulfides

(NH4)2S2O8-Promoted cross-coupling of thiols/diselenides and sulfoxides to construct unsymmetrical disulfides/selenosulfides is disclosed. Control experiments demonstrate that (NH4)2S2O8 acts as an acid and an oxidant while both ionic and radical routes...

Can thiol-based redox systems be utilized as parts for synthetic biology applications?

OBJECTIVES

Synthetic biology has emerged from molecular biology and engineering approaches and aims to develop novel, biologically-inspired systems for industrial and basic research applications ranging from biocomputing to drug production. Surprisingly, redoxin (thioredoxin, glutaredoxin, peroxiredoxin) and other thiol-based redox systems have not been widely utilized in many of these synthetic biology applications.

METHODS

We reviewed thiol-based redox systems and the development of synthetic biology applications that have used thiol-dependent parts.

RESULTS

The development of circuits to facilitate cytoplasmic disulfide bonding, biocomputing and the treatment of intestinal bowel disease are amongst the applications that have used thiol-based parts. We propose that genetically encoded redox sensors, thiol-based biomaterials and intracellular hydrogen peroxide generators may also be valuable components for synthetic biology applications.

DISCUSSION

Thiol-based systems play multiple roles in cellular redox metabolism, antioxidant defense and signaling and could therefore offer a vast and diverse portfolio of components, parts and devices for synthetic biology applications. However, factors limiting the adoption of redoxin systems for synthetic biology applications include the orthogonality of thiol-based components, limitations in the methods to characterize thiol-based systems and an incomplete understanding of the design principles of these systems.

Glutaredoxin: Discovery, redox defense and much more

Glutaredoxin, Grx, is a small protein containing an active site cysteine pair and was discovered in 1976 by Arne Holmgren. The Grx system, comprised of Grx, glutathione, glutathione reductase, and NADPH, was first described as an electron donor for Ribonucleotide Reductase but, from the first discovery in E.coli, the Grx family has impressively grown, particularly in the last two decades. Several isoforms have been described in different organisms (from bacteria to humans) and with different functions.

The unique characteristic of Grxs is their ability to catalyse glutathione-dependent redox regulation via glutathionylation, the conjugation of glutathione to a substrate, and its reverse reaction, deglutathionylation. Grxs have also recently been enrolled in iron sulphur cluster formation. These functions have been implied in various physiological and pathological conditions, from immune defense to neurodegeneration and cancer development thus making Grx a possible drug target.

This review aims to give an overview on Grxs, starting by a phylogenetic analysis of vertebrate Grxs, followed by an analysis of the mechanisms of action, the specific characteristics of the different human isoforms and a discussion on aspects related to human physiology and diseases.

PdCl2/DMSO-Catalyzed Thiol-Disulfide Exchange: Synthesis of Unsymmetrical Disulfide

Unsymmetrical disulfides have been effectively prepared through thiol exchange with symmetrical disulfides employing a simple PdCl₂/DMSO catalytic system. The given method features excellent functional group tolerance, a broad substrate scope, and operational simplicity. This reaction is especially useful for late-stage functionalization of bioactive scaffolds such as peptides and pharmaceuticals. Disulfide-containing organic dyes have also been prepared. This transformation could be extended to thiol-diselenide or thiol-ditelluride exchange affording RS-SeR' or RS-TeR'.

Regulacja układu zależnego od tioredoksyny jako element farmakoterapii w chorobach z zaburzeniami równowagi redoks

Działanie wielu czynników egzogennych, a także zaburzone procesy metaboliczne komórek przyczyniają się do nasilonego wytwarzania oksydantów, a to zaburza równowagę redoks, wywołując zmiany metaboliczne, w tym śmierci lub transformacji nowotworowej komórek. Jednak każda komórka zawiera antyoksydanty, które mają zapobiegać tego typu sytuacjom. Jednym z układów antyoksydacyjnych, funkcjonujących w komórkach, jest układ zależny od tioredoksyny, w skład którego wchodzą: tioredoksyna (Trx), reduktaza tioredoksyny (TrxR) oraz peroksydaza tioredoksyny (TPx), które mogą redukować utlenione składniki komórek kosztem fosforanu dinukleotydu nikotynoamidoadeninowego (NADPH). Działanie takie wynika z budowy przestrzennej Trx oraz TrxR, która umożliwia wytworzenie wewnątrzcząsteczkowego mostka disulfidowego w obrębie cząsteczki tioredoksyny oraz dwóch międzycząsteczkowych mostków selenosulfidowych w obrębie dimeru reduktazy tioredoksyny. Inną, równie istotną funkcją układu zależnego od tioredoksyny jest regulowanie ekspresji wielu białek za pośrednictwem takich czynników jak czynnik transkrypcyjnego NF-κB oraz kinaza regulująca apoptozę (ASK-1), które uruchamiają kaskady przemian metabolicznych prowadzących ostatecznie do proliferacji lub apoptozy komórek. Wzrost ekspresji/aktywności składników systemu zależnego od Trx obserwuje się w rozwoju wielu nowotworów. Dlatego też poszukiwanie selektywnych inhibitorów tioredoksyny lub reduktazy tioredoksyny jest obecnie jednym z głównych kierunków badań w farmakoterapii nowotworów. Wykazano, że wiele naturalnie występujących związków polifenolowych pochodzenia naturalnego o działaniu antyoksydacyjnym (np. kwercetyna czy kurkumina) powoduje inaktywację układu Trx-TrxR. Jednocześnie wiele syntetycznych związków, w tym związki kompleksowe, które stosowane są w terapii przeciwnowotworowej (np. cisplatyna, auranofina, moteksafina gadolinu), również hamują działanie układu zależnego od Trx.

S-glutathionylation, friend or foe in cardiovascular health and disease

Glutathione is a low molecular weight thiol that is present at high levels in the cell. The high levels of glutathione in the cell make it one of the most abundant antioxidants contributing to cellular redox homeostasis. As a general rule, throughout cardiovascular disease and progression there is an imbalance in redox homeostasis characterized by reactive oxygen species overproduction and glutathione underproduction. As research into these imbalances continues, glutathione concentrations are increasingly being observed to drive various physiological and pathological signaling responses. Interestingly in addition to acting directly as an antioxidant, glutathione is capable of post translational modifications (S-glutathionylation) of proteins through both chemical interactions and enzyme mediated events. This review will discuss both the chemical and enzyme-based S-glutathionylation of proteins involved in cardiovascular pathologies and angiogenesis.

One cysteine is enough: A monothiol Grx can functionally replace all cytosolic Trx and dithiol Grx

Glutaredoxins are small proteins of the thioredoxin superfamily that are present throughout life. Most glutaredoxins fall into two major subfamilies. Class I glutaredoxins are glutathione-dependent thiol-disulfide oxidoreductases whilst class II glutaredoxins coordinate Fe–S clusters. Class I glutaredoxins are typically dithiol enzymes with two active-site cysteine residues, however, some enzymatically active monothiol glutaredoxins are also known. Whilst both monothiol and dithiol class I glutaredoxins mediate protein deglutathionylation, it is widely claimed that only dithiol glutaredoxins are competent to reduce protein disulfide bonds. In this study, using a combination of yeast ‘viability rescue’, growth, and redox-sensitive GFP-based assays, we show that two different monothiol class I glutaredoxins can each facilitate the reduction of protein disulfide bonds in ribonucleotide reductase, methionine sulfoxide reductase and roGFP2. Our observations thus challenge the generalization of the dithiol mechanism for glutaredoxin catalysis and raise the question of why most class I glutaredoxins have two active-site cysteine residues.

The Arabidopsis Plastidial Glucose-6-Phosphate Transporter GPT1 is Dually Targeted to Peroxisomes via the Endoplasmic Reticulum

Studies on Glucose-6-phosphate (G6P)/phosphate translocator isoforms GPT1 and GPT2 reported the viability of Arabidopsis (Arabidopsis thaliana) gpt2 mutants, whereas heterozygous gpt1 mutants exhibited a variety of defects during fertilization/seed set, indicating that GPT1 is essential for this process. Among other functions, GPT1 was shown to be important for pollen and embryo-sac development. Because our previous work on the irreversible part of the oxidative pentose phosphate pathway (OPPP) revealed comparable effects, we investigated whether GPT1 may dually localize to plastids and peroxisomes. In reporter fusions, GPT2 localized to plastids, but GPT1 also localized to the endoplasmic reticulum (ER) and around peroxisomes. GPT1 contacted two oxidoreductases and also peroxins that mediate import of peroxisomal membrane proteins from the ER, hinting at dual localization. Reconstitution in yeast (Saccharomyces cerevisiae) proteoliposomes revealed that GPT1 preferentially exchanges G6P for ribulose-5-phosphate (Ru5P). Complementation analyses of heterozygous +/gpt1 plants demonstrated that GPT2 is unable to compensate for GPT1 in plastids, whereas GPT1 without the transit peptide (enforcing ER/peroxisomal localization) increased gpt1 transmission significantly. Because OPPP activity in peroxisomes is essential for fertilization, and immunoblot analyses hinted at the presence of unprocessed GPT1-specific bands, our findings suggest that GPT1 is indispensable in both plastids and peroxisomes. Together with its G6P-Ru5P exchange preference, GPT1 appears to play a role distinct from that of GPT2 due to dual targeting.

Role of rockfish (Sebastes schlegelii) glutaredoxin 1 in innate immunity, and alleviation of cellular oxidative stress: Insights into localization, molecular characteristics, transcription, and function

Glutaredoxins are a group of heat stable oxidoreductases ubiquitously found in prokaryotes and eukaryotes. They are widely known for GSH (glutathione)-dependent protein disulfide reduction and cellular redox homeostasis. This study was performed to identify and characterize rockfish (Sebastes schlegelii) glutaredoxin 1 (SsGrx1) at molecular, transcriptional, and functional levels. The coding sequence of SsGrx1 was 318 bp in length and encoded a protein containing 106 amino acids. The molecular weight and theoretical isoelectric point of the putative SsGrx1 protein were 11.6 kDa and 6.71 kDa, respectively. The amino acid sequence of SsGrx1 comprised a CPYC redox active motif surrounded by several conserved GSH binding sites. The modeled protein structure was found to consist of five α-helices and four β-sheets, similar to human Grx1. SsGrx1 showed a tissue specific expression in all the tissues tested, with the highest expression in the kidney. Immune stimulation by lipopolysaccharides (LPS), polyinosinic:polycytidylic acid (polyI:C), and Streptococcus iniae (S. iniae) could significantly modulate the SsGrx1 expression pattern in the blood and gills. Analysis of its subcellular localization disclosed that SsGrx1 was prominently localized in the cytosol. Recombinant SsGrx1 (rSsGrx1) exhibited significant activity in insulin disulfide reduction assay and HED (β-Hydroxyethyl Disulfide) assay. Furthermore, transient overexpression of SsGrx1 in FHM (fathead minnow) cells significantly enhanced cell survival upon H₂O₂-induced apoptosis. Collectively, our findings strongly suggest that SsGrx1 plays a crucial role in providing rockfish immune protection against pathogens and oxidative stress.

Catalytic site-selective substrate processing within a tubular nanoreactor

Chemists have long sought the ability to modify molecules precisely when presented with several sites of similar reactivity. We reasoned that the confinement of substrates within nanostructures might permit site-selective reactions unachievable in bulk solution, even with sophisticated reagents. In particular, the stretching and alignment of polymers within nanotubes might allow site-specific cleavage or modification. To explore this proposition, macromolecular disulfide substrates were elongated within members of a collection of tubular protein nanoreactors, which contained cysteine residues positioned at different locations along the length of each tube. For each nanoreactor, we defined the reactive location by using a set of polymer substrates (site-selectivity) and which of the two sulfur atoms was attacked (regioselectivity), and found that disulfide interchange occurs with atomic precision. Our strategy has potential for the selective processing of a wide variety of biomacromolecules, and the chemistry and substrates might be generalized yet further by using alternative nanotubes.

The role of thiols in antioxidant systems

The sulfur biochemistry of the thiol group endows cysteines with a number of highly specialized and unique features that enable them to serve a variety of different functions in the cell. Typically highly conserved in proteins, cysteines are predominantly found in functionally or structurally crucial regions, where they act as stabilizing, catalytic, metal-binding and/or redox-regulatory entities. As highly abundant low molecular weight thiols, cysteine thiols and their oxidized disulfide counterparts are carefully balanced to maintain redox homeostasis in various cellular compartments, protect organisms from oxidative and xenobiotic stressors and partake actively in redox-regulatory and signaling processes. In this review, we will discuss the role of protein thiols as scavengers of hydrogen peroxide in antioxidant enzymes, use thiol peroxidases to exemplify how protein thiols contribute to redox signaling, provide an overview over the diverse set of low molecular weight thiol-based redox systems found in biology, and illustrate how thiol-based redox systems have evolved not only to protect against but to take full advantage of a world full of molecular oxygen.

High-resolution crystal structure of the reduced Grx1 from Saccharomyces cerevisiae

Grx1, a cytosolic thiol-disulfide oxidoreductase, actively maintains cellular redox homeostasis using glutathione substrates (reduced, GSH, and oxidized, GSSG). Here, the crystallization of reduced Grx1 from the yeast Saccharomyces cerevisiae (yGrx1) in space group P212121 and its structure solution and refinement to 1.22 Å resolution are reported. To study the structure-function relationship of yeast Grx1, the crystal structure of reduced yGrx1 was compared with the existing structures of the oxidized and glutathionylated forms. These comparisons revealed structural differences in the conformations of residues neighbouring the Cys27-Cys30 active site which accompany alterations in the redox status of the protein.

Factors that determine thione(thiol)-disulfide interconversion in a bis(thiosemicarbazone) copper(ii) complex

Solvent-, acidity-, and redox-responsive thione(thiol)-disulfide interconversion were achieved by a dinuclear copper(ii) complex bearing a bis(thiosemicarbazone) (bTSC) ligand. The role of copper(ii) ion coordination was rationalized by parallel comparison with a bare bTSC ligand and zinc(ii) bTSC complexes under identical reaction conditions.

Molecular Mechanisms of Glutaredoxin Enzymes: Versatile Hubs for Thiol-Disulfide Exchange between Protein Thiols and Glutathione

The tripeptide glutathione (GSH) and its oxidized form glutathione disulfide (GSSG) constitute a key redox couple in cells. In particular, they partner protein thiols in reversible thiol-disulfide exchange reactions that act as switches in cell signaling and redox homeostasis. Disruption of these processes may impair cellular redox signal transduction and induce redox misbalances that are linked directly to aging processes and to a range of pathological conditions including cancer, cardiovascular diseases and neurological disorders. Glutaredoxins are a class of GSH-dependent oxidoreductase enzymes that specifically catalyze reversible thiol-disulfide exchange reactions between protein thiols and the abundant thiol pool GSSG/GSH. They protect protein thiols from irreversible oxidation, regulate their activities under a variety of cellular conditions and are key players in cell signaling and redox homeostasis. On the other hand, they may also function as metal-binding proteins with a possible role in the cellular homeostasis and metabolism of essential metals copper and iron. However, the molecular basis and underlying mechanisms of glutaredoxin action remain elusive in many situations. This review focuses specifically on these aspects in the context of recent developments that illuminate some of these uncertainties.

Structural and mechanistic aspects of S-S bonds in the thioredoxin-like family of proteins

Disulfide bonds play a critical role in a variety of structural and mechanistic processes associated with proteins inside the cells and in the extracellular environment. The thioredoxin family of proteins like thioredoxin (Trx), glutaredoxin (Grx) and protein disulfide isomerase, are involved in the formation, transfer or isomerization of disulfide bonds through a characteristic thiol-disulfide exchange reaction. Here, we review the structural and mechanistic determinants behind the thiol-disulfide exchange reactions for the different enzyme types within this family, rationalizing the known experimental data in light of the results from computational studies. The analysis sheds new atomic-level insight into the structural and mechanistic variations that characterize the different enzymes in the family, helping to explain the associated functional diversity. Furthermore, we review here a pattern of stabilization/destabilization of the conserved active-site cysteine residues presented beforehand, which is fully consistent with the observed roles played by the thioredoxin family of enzymes.

A glutaredoxin domain fused to the radical-generating subunit of ribonucleotide reductase (RNR) functions as an efficient RNR reductant

Class I ribonucleotide reductase (RNR) consists of a catalytic subunit (NrdA) and a radical-generating subunit (NrdB) that together catalyze reduction of ribonucleotides to their corresponding deoxyribonucleotides. NrdB from the firmicute Facklamia ignava is a unique fusion protein with N-terminal add-ons of a glutaredoxin (Grx) domain followed by an ATP-binding domain, the ATP cone. Grx, usually encoded separately from the RNR operon, is a known RNR reductant. We show that the fused Grx domain functions as an efficient reductant of the F. ignava class I RNR via the common dithiol mechanism and, interestingly, also via a monothiol mechanism, although less efficiently. To our knowledge, a Grx that uses both of these two reaction mechanisms has not previously been observed with a native substrate. The ATP cone is in most RNRs an N-terminal domain of the catalytic subunit. It is an allosteric on/off switch promoting ribonucleotide reduction in the presence of ATP and inhibiting RNR activity in the presence of dATP. We found that dATP bound to the ATP cone of F. ignava NrdB promotes formation of tetramers that cannot form active complexes with NrdA. The ATP cone bound two dATP molecules but only one ATP molecule. F. ignava NrdB contains the recently identified radical-generating cofactor MnIII/MnIV We show that NrdA from F. ignava can form a catalytically competent RNR with the MnIII/MnIV-containing NrdB from the flavobacterium Leeuwenhoekiella blandensis In conclusion, F. ignava NrdB is fused with a Grx functioning as an RNR reductant and an ATP cone serving as an on/off switch.