Methylsulfonyl benzothiazole (MSBT): a selective protein thiol blocking reagent

Depletion of protein thiols and the accumulation of oxidized thioredoxin in Parkinsonism disclosed by a red-emitting and environment-sensitive probe

Protein sulfhydryl groups play a vital role in maintaining cellular redox homeostasis and protein functions and have attracted increasing interests for the selective detection of protein thiols over low-molecular-weight thiols (LMWTs).

SO2 Donors and Prodrugs, and Their Possible Applications: A Review

SO2 is widely recognized as an air pollutant and is a known cause of acid rain. At a sufficiently high level, it also causes respiratory diseases. A much lesser known side of SO2 is its endogenous nature and possible physiological roles. There is mounting evidence that SO2 is produced during normal cellular metabolism and may possibly function as a signaling molecule in normal physiology. The latter aspect is still at the stage of being carefully examined as to the validity of classifying SO2 as a gasotransmitter with endogenous signaling roles. One difficulty in studying the biological and pharmacological roles of SO2 is the lack of adequate tools for its controllable and precise delivery. Traditional methods of using SO2 gas or mixed sulfite salts do not meet research need for several reasons. Therefore, there has been increasing attention on the need of developing SO2 donors or prodrugs that can be used as tools for the elucidation of SO2's physiological roles, pharmacological effects, and possible mechanism(s) of action. In this review, we aim to review basic sulfur chemistry in the context of sulfur signaling and various chemical strategies used for designing SO2 donors. We will also discuss potential pharmacological applications of SO2 donors, lay out desirable features for such donors and possibly prodrugs, analyze existing problems, and give our thoughts on research needs.

H2O2 oxidation of cysteine residues in c-Jun N-terminal kinase 2 (JNK2) contributes to redox regulation in human articular chondrocytes

Reactive oxygen species (ROS), in particular H₂O₂, regulate intracellular signaling through reversible oxidation of reactive protein thiols present in a number of kinases and phosphatases. H₂O₂ has been shown to regulate mitogen-activated protein kinase (MAPK) signaling depending on the cellular context. We report here that in human articular chondrocytes, the MAPK family member c-Jun N-terminal kinase 2 (JNK2) is activated by fibronectin fragments and low physiological levels of H₂O₂ and inhibited by oxidation due to elevated levels of H₂O₂ The kinase activity of affinity-purified, phosphorylated JNK2 from cultured chondrocytes was reversibly inhibited by 5-20 μm H₂O₂ Using dimedone-based chemical probes that react specifically with sulfenylated cysteines (RSOH), we identified Cys-222 in JNK2, a residue not conserved in JNK1 or JNK3, as a redox-reactive site. MS analysis of human recombinant JNK2 also detected further oxidation at Cys-222 and other cysteines to sulfinic (RSO₂H) or sulfonic (RSO₃H) acid. H₂O₂ treatment of JNK2 resulted in detectable levels of peptides containing intramolecular disulfides between Cys-222 and either Cys-213 or Cys-177, without evidence of dimer formation. Substitution of Cys-222 to alanine rendered JNK2 insensitive to H₂O₂ inhibition, unlike C177A and C213A variants. Two other JNK2 variants, C116A and C163A, were also resistant to oxidative inhibition. Cumulatively, these findings indicate differential regulation of JNK2 signaling dependent on H₂O₂ levels and point to key cysteine residues regulating JNK2 activity. As levels of intracellular H₂O₂ rise, a switch occurs from activation to inhibition of JNK2 activity, linking JNK2 regulation to the redox status of the cell.

Covalent Modification of Biomolecules through Maleimide-Based Labeling Strategies

Since their first use in bioconjugation more than 50 years ago, maleimides have become privileged chemical partners for the site-selective modification of proteins via thio-Michael addition of biothiols and, to a lesser extent, via Diels-Alder (DA) reactions with biocompatible dienes. Prominent examples include immunotoxins and marketed maleimide-based antibody-drug conjugates (ADCs) such as Adcetris, which are used in cancer therapies. Among the key factors in the success of these groups is the availability of several maleimides that can be N-functionalized by fluorophores, affinity tags, spin labels, and pharmacophores, as well as their unique reactivities in terms of selectivity and kinetics. However, maleimide conjugate reactions have long been considered irreversible, and only recently have systematic studies regarding their reversibility and stability toward hydrolysis been reported. This review provides an overview of the diverse applications for maleimides in bioconjugation, highlighting their strengths and weaknesses, which are being overcome by recent strategies. Finally, the fluorescence quenching ability of maleimides was leveraged for the preparation of fluorogenic probes, which are mainly used for the specific detection of thiol analytes. A summary of the reported structures, their photophysical features, and their relative efficiencies is discussed in the last part of the review.

Cysteine-reactive probes and their use in chemical proteomics

Proteomic profiling using bioorthogonal chemical probes that selectively react with certain amino acids is now a widely used method in life sciences to investigate enzymatic activities, study posttranslational modifications and discover novel covalent inhibitors. Over the past two decades, researchers have developed selective probes for several different amino acids, including lysine, serine, cysteine, threonine, tyrosine, aspartate and glutamate. Among these amino acids, cysteines are particularly interesting due to their highly diverse and complex biochemical role in our cells. In this feature article, we focus on the chemical probes and methods used to study cysteines in complex proteomes.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

Cyclic Acyl Disulfides and Acyl Selenylsulfides as the Precursors for Persulfides (RSSH), Selenylsulfides (RSeSH), and Hydrogen Sulfide (H2S)

The reactions of three model compounds (1-3) for cyclic acyl disulfides and cyclic acyl selenylsulfides are studied. These compounds were found to be effective precursors for persulfides (RSSH) and selenylsulfides (RSeSH) upon reacting with nucleophilic species. They could also act as H2S donors when interacting with cellular thiols. The most interesting discovery was the generation of RSeSH from compound 3 under mild conditions. Selenylsulfides (RSeSH) are expected to be important regulating molecules involved in Sec-related redox signaling. The method of producing RSeSH should allow researchers to better understand the chemical biology of RSeSH.

Reactive Cysteine Persulphides: Occurrence, Biosynthesis, Antioxidant Activity, Methodologies, and Bacterial Persulphide Signalling

Cysteine hydropersulphide (CysSSH) is a cysteine derivative having one additional sulphur atom bound to a cysteinyl thiol group. Recent advances in the development of analytical methods for detection and quantification of persulphides and polysulphides have revealed the biological presence, in both prokaryotes and eukaryotes, of hydropersulphides in diverse forms such as CysSSH, homocysteine hydropersulphide, glutathione hydropersulphide, bacillithiol hydropersulphide, coenzyme A hydropersulphide, and protein hydropersulphides. Owing to the chemical reactivity of the persulphide moiety, biological systems utilize persulphides as important intermediates in the synthesis of various sulphur-containing biomolecules. Accumulating evidence has revealed another important feature of persulphides: their potent reducing activity, which implies that they are implicated in the regulation of redox signalling and antioxidant functions. In this chapter, we discuss the biological occurrence and possible biosynthetic mechanisms of CysSSH and related persulphides, and we include descriptions of recent advances in the analytical methods that have been used to detect and quantitate persulphide species. We also discuss the antioxidant activity of persulphide species that contributes to protecting cells from reactive oxygen species-associated damage, and we examine the signalling roles of CysSSH in bacteria.

Discovery of Heteroaromatic Sulfones As a New Class of Biologically Compatible Thiol-Selective Reagents

The selective reaction of chemical reagents with reduced protein thiols is critical to biological research. This reaction is utilized to prevent cross-linking of cysteine-containing peptides in common proteomics workflows and is applied widely in discovery and targeted redox investigations of the mechanisms underlying physiological and pathological processes. However, known and commonly used thiol blocking reagents like iodoacetamide, N-ethylmaleimide, and others were found to cross-react with oxidized protein sulfenic acids (-SOH) introducing significant errors in studies employing these reagents. We have investigated and are reporting here a new heteroaromatic alkylsulfone, 4-(5-methanesulfonyl-[1,2,3,4]tetrazol-1-yl)-phenol (MSTP), as a selective and highly reactive -SH blocking reagent compatible with biological applications.

Benzothiazole Sulfinate: A Sulfinic Acid Transfer Reagent under Oxidation-Free Conditions

Sulfinic acids are commonly encountered intermediates found in natural product synthesis and medicinal chemistry. However, because of high reactivity, instability, and harsh reaction conditions, they are difficult to synthesize. Herein we have developed an oxidation-free method to produce sulfinic acids and sulfinate salts using 2-sulfinyl benzothiazole (BTS). We have also demonstrated the synthetic usefulness by developing one-pot syntheses of sulfones and sulfonamides.

A Reactive, Rigid GdIII Labeling Tag for In-Cell EPR Distance Measurements in Proteins

The cellular environment of proteins differs considerably from in vitro conditions under which most studies of protein structures are carried out. Therefore, there is a growing interest in determining dynamics and structures of proteins in the cell. A key factor for in-cell distance measurements by the double electron-electron resonance (DEER) method in proteins is the nature of the used spin label. Here we present a newly designed Gd^III spin label, a thiol-specific DOTA-derivative (DO3MA-3BrPy), which features chemical stability and kinetic inertness, high efficiency in protein labelling, a short rigid tether, as well as favorable spectroscopic properties, all are particularly suitable for in-cell distance measurements by the DEER method carried out at W-band frequencies. The high performance of DO3MA-3BrPy-Gd^III is demonstrated on doubly labelled ubiquitin D39C/E64C, both in vitro and in HeLa cells. High-quality DEER data could be obtained in HeLa cells up to 12 h after protein delivery at in-cell protein concentrations as low as 5-10 μm.

Methylsulfonyl benzothiazoles (MSBT) derivatives: Search for new potential antimicrobial and anticancer agents

A series of novel 4 and 5-substituted methylsulfonyl benzothiazole (MSBT) compounds having amide, alkoxy, sulfonamide, nitro and amine functionality were synthesized from sequential reactions on 5-ethoxy-2-(methylsulfonyl)benzo[d]thiazole such as nitration, reduction, sulfonation, dealkylation, etc. All synthesized compounds were screened against antimicrobial and selected screened for anticancer activity. Antimicrobial activities studies reveled that among all compounds screened, out of MSBT-07, MSBT-11, MSBT-12, MSBT-14, MSBT-19, and MSBT-27 were found to have promising antimicrobial activity at MIC range of 4-50μg/ml against selected bacterial as well as fungal species. Compounds having good antimicrobial activity were screened for cervical cancer (HeLA cell lines). Of these MSBT-07 and MSBT-12 significantly reduced the cell growth. Consequently their calculated GI₅₀ values were found to be 0.1 or <0.1μM.

Iminoboronates are efficient intermediates for selective, rapid and reversible N-terminal cysteine functionalisation

We show that formyl benzeno boronic acids (2FBBA) selectively react with N-terminal cysteines to yield a boronated thiazolidine featuring a B-N bond. The reaction exhibits a very rapid constant rate (2.38 ± 0.23 × 102 M-1 s-1) under mild aqueous conditions (pH 7.4, 23 °C) and tolerates different amino acids at the position adjacent to the N-cysteine. DFT calculations highlighted the diastereoselective nature of this ligation reaction and support the involvement of the proximal boronic acid in the activation of the imine functionality and the stabilisation of the boronated thiazolidine through a chelate effect. The 2FBBA reagent allowed the effective functionalisation of model peptides (C-ovalbumin and a laminin fragment) and the boronated thiazolidine construct was shown to be stable over time, though the reaction was reversible in the presence of benzyl hydroxylamine. The reaction proved to be highly chemoselective, and 2FBBA was used to functionalize the N-terminal cysteine of calcitonin in the presence of a potentially competing in-chain thiol group. This exquisite selectivity profile enabled the dual functionalisation of calcitonin and the interactive orthogonal modification of this peptide when 2FBBA was combined with conventional maleimide chemistry. These results highlight the potential of this methodology to construct complex and well-defined bioconjugates.

Benzothiazole Sulfinate: a Water-Soluble and Slow-Releasing Sulfur Dioxide Donor

Sulfur dioxide (SO2) has long been considered a toxic environmental pollutant and byproduct of industrial processing. Recently it has become evident that SO2 may also have regulatory functions in mammalian pulmonary systems. However, the study of these effects has proven to be challenging due to the difficulty in administering SO2 in a reliable manner. In this work, we report the discovery of a new pH-dependent and water-soluble SO2 donor, benzothiazole sulfinate (BTS). We have found BTS to have slow and sustained SO2 release at physiological pH. Additionally, we have explored its vasorelaxation properties as compared to the authentic SO2 gas solutions. The slow release of BTS should make it a useful tool for the study of endogenously generated SO2.

Naphthalimide Scaffold Provides Versatile Platform for Selective Thiol Sensing and Protein Labeling

Reversible thiol modifications are fundamental of cellular redox regulation. Specific thiol detection, including thiol sensing and protein thiols labeling, is critical to study such modifications. We reported the discovery of 4-methylsulfonyl-N-n-butyl-1,8-naphthalimide (MSBN), a highly selective fluorogenic probe for thiols based on the 1,8-naphthalimide scaffold. Thiols react with MSBN nearly quantitatively via nucleophilic aromatic substitution to replace the methylsulfonyl group and restore the quenched fluorescence (>100-fold increase). MSBN was employed to selectively image thiols in live cells and specifically label protein thiols with a turn-on signal to determine diverse reversible protein thiol modifications. In addition, we introduced a bulky group into the MSBN as a mass tag to create a probe MSBN-TPP, which readily discriminates the reduced thioredoxin from the oxidized one. The specific reaction of MSBN with thiols and the easy manipulation of the naphthalimide unit enable MSBN a versatile scaffold in developing novel probes for thiol-based protein bioconjugation and studying various thiol modifications.

9-Fluorenylmethyl (Fm) Disulfides: Biomimetic Precursors for Persulfides

The development of a functional disulfide, FmSSPy-A (Fm = 9-fluorenylmethyl; Py = pyridinyl), is reported. It can effectively convert small molecule and protein thiols (−SH) to form −S-SFm adducts under mild conditions. This method allows for a H₂S-free and biomimetic protocol to generate highly reactive persulfides (in their anionic forms). The high nucleophilicity of persulfides toward a number of thiol-blocking reagents is also demonstrated. The method holds promise for further understanding the chemical biology of persulfides and S-sulfhydration.

Chemical tools for the study of hydrogen sulfide (H2S) and sulfane sulfur and their applications to biological studies

Hydrogen sulfide (H₂S) functions in many physiological processes, including relaxation of vascular smooth muscles, mediation of neurotransmission, inhibition of insulin signaling, and regulation of inflammation. On the other hand, sulfane sulfur, which is a sulfur atom with six valence electrons but no charge, has the unique ability to bind reversibly to other sulfur atoms to form hydropersulfides (R-S-SH) and polysulfides (-S-S_n-S-). H₂S and sulfane sulfur always coexist, and recent work suggests that sulfane sulfur species may be the actual signaling molecules in at least some biological phenomena. For example, one of the mechanisms of activity regulation of proteins by H₂S is the S-sulfhydration of cysteine residues (protein Cys-SSH). In this review, we summarize recent progress on chemical tools for the study of H₂S and sulfane sulfur, covering fluorescence probes utilizing various design strategies, H₂S caged compounds, inhibitors of physiological H₂S-producing enzymes (cystathionine γ-lyase, cystathionine β-synthase and 3-mercaptopyruvate sulfurtransferase), and labeling reagents. Fluorescence probes offer particular advantages as chemical tools to study physiological functions of biomolecules, including ease of use and real-time, nondestructive visualization of biological processes in live cells and tissues.

Overexpression of Cystathionine γ-Lyase Suppresses Detrimental Effects of Spinocerebellar Ataxia Type 3

Spinocerebellar ataxia type 3 (SCA3) is a polyglutamine (polyQ) disorder caused by a CAG repeat expansion in the ataxin-3 (ATXN3) gene resulting in toxic protein aggregation. Inflammation and oxidative stress are considered secondary factors contributing to the progression of this neurodegenerative disease. There is no cure that halts or reverses the progressive neurodegeneration of SCA3. Here we show that overexpression of cystathionine γ-lyase, a central enzyme in cysteine metabolism, is protective in a Drosophila model for SCA3. SCA3 flies show eye degeneration, increased oxidative stress, insoluble protein aggregates, reduced levels of protein persulfidation and increased activation of the innate immune response. Overexpression of Drosophila cystathionine γ-lyase restores protein persulfidation, decreases oxidative stress, dampens the immune response and improves SCA3-associated tissue degeneration. Levels of insoluble protein aggregates are not altered; therefore, the data implicate a modifying role of cystathionine γ-lyase in ameliorating the downstream consequence of protein aggregation leading to protection against SCA3-induced tissue degeneration. The cystathionine γ-lyase expression is decreased in affected brain tissue of SCA3 patients, suggesting that enhancers of cystathionine γ-lyase expression or activity are attractive candidates for future therapies.

Mass spectrometry in studies of protein thiol chemistry and signaling: opportunities and caveats

Mass spectrometry (MS) has become a powerful and widely utilized tool in the investigation of protein thiol chemistry, biochemistry, and biology. Very early biochemical studies of metabolic enzymes have brought to light the broad spectrum of reactivity profiles that distinguish cysteine thiols with functions in catalysis and protein stability from other cysteine residues in proteins. The development of MS methods for the analysis of proteins using electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI) coupled with the emergence of high-resolution mass analyzers has been instrumental in advancing studies of thiol modifications, both in single proteins and within the cellular context. This article reviews MS instrumentation and methods of analysis employed in investigations of thiols and their reactivity toward a range of small biomolecules. A selected number of studies are detailed to highlight the advantages brought about by the MS technologies along with the caveats associated with these analyses.

Use of the "tag-switch" method for the detection of protein S-sulfhydration

Protein S-sulfhydration (i.e., converting protein cysteines -SH to persulfides -SSH) is a redox-based posttranslational modification. This reaction plays an important role in signaling pathways mediated by hydrogen sulfide or other reactive sulfane sulfur species. Recently, our laboratories developed a "tag-switch" method which can be used to selectively label and detect protein S-sulfhydrated residues. In this chapter, we provide a comprehensive summary of this method, including the design of the method, preparation of the reagents, validation on small-molecule substrates, as well as applications in protein labeling. Experimental protocols for the use of the method are described in details.

Site-specific tagging proteins via a rigid, stable and short thiolether tether for paramagnetic spectroscopic analysis

The reaction of phenylsulfonated pyridine derivatives and protein thiols is suitable for high-resolution spectroscopic analysis by generation of a rigid, stable and short thiolether tether.

Highly selective fluorescence off-on probes for biothiols and imaging in live cells

Three sulfonyl benzothiazole-based fluorescent probes (, , and ) for the detection of biothiols (cysteine, homocysteine, and glutathione) are developed based on thiol-mediated nucleophilic aromatic substitutions. The probes exhibited good selectivity and sensitivity toward biothiols over other analytes. The probes were successfully applied for visualizing endogenous thiols in living cells.

Reactive cysteine persulfides and S-polythiolation regulate oxidative stress and redox signaling

Using methodology developed herein, it is found that reactive persulfides and polysulfides are formed endogenously from both small molecule species and proteins in high amounts in mammalian cells and tissues. These reactive sulfur species were biosynthesized by two major sulfurtransferases: cystathionine β-synthase and cystathionine γ-lyase. Quantitation of these species indicates that high concentrations of glutathione persulfide (perhydropersulfide >100 μM) and other cysteine persulfide and polysulfide derivatives in peptides/proteins were endogenously produced and maintained in the plasma, cells, and tissues of mammals (rodent and human). It is expected that persulfides are especially nucleophilic and reducing. This view was found to be the case, because they quickly react with H2O2 and a recently described biologically generated electrophile 8-nitroguanosine 3',5'-cyclic monophosphate. These results indicate that persulfides are potentially important signaling/effector species, and because H2S can be generated from persulfide degradation, much of the reported biological activity associated with H2S may actually be that of persulfides. That is, H2S may act primarily as a marker for the biologically active of persulfide species.

Detection of protein S-sulfhydration by a tag-switch technique

Protein S-sulfhydration (forming -S-SH adducts from cysteine residues) is a newly defined oxidative posttranslational modification and plays an important role in H2 S-mediated signaling pathways. In this study we report the first selective, "tag-switch" method which can directly label protein S-sulfhydrated residues by forming stable thioether conjugates. Furthermore we demonstrate that H2 S alone cannot lead to S-sulfhydration and that the two possible physiological mechanisms include reaction with protein sulfenic acids (P-SOH) or the involvement of metal centers which would facilitate the oxidation of H2 S to HS(.) .

Rapid, stable, chemoselective labeling of thiols with Julia-Kocieński-like reagents: a serum-stable alternative to maleimide-based protein conjugation

Cysteine-maleimide chemistry is widely used for peptide and protein modification. However, the formed succinimide linkage is readily hydrolyzed and is susceptible to an exchange reaction in vivo. We demonstrate that methylsulfonyl phenyloxadiazole compounds react specifically with cysteine under various buffer conditions and found that the resulting protein conjugates had superior stability to cysteine-maleimide conjugates in human plasma. This Thiol-Click chemistry promises a new approach to stable protein conjugates and pegylated proteins.

Thiol-blocking electrophiles interfere with labeling and detection of protein sulfenic acids

Cellular exposure to reactive oxygen species induces rapid oxidation of DNA, proteins, lipids and other biomolecules. At the proteome level, cysteine thiol oxidation is a prominent post-translational process that is implicated in normal physiology and numerous pathologies. Methods for investigating protein oxidation include direct labeling with selective chemical probes and indirect tag-switch techniques. Common to both approaches is chemical blocking of free thiols using reactive electrophiles to prevent post-lysis oxidation or other thiol-mediated cross-reactions. These reagents are used in large excess, and their reactivity with cysteine sulfenic acid, a critical oxoform in numerous proteins, has not been investigated. Here we report the reactivity of three thiol-blocking electrophiles, iodoacetamide, N-ethylmaleimide and methyl methanethiosulfonate, with protein sulfenic acid and dimedone, the structural core of many sulfenic acid probes. We demonstrate that covalent cysteine -SOR (product) species are partially or fully susceptible to reduction by dithiothreitol, tris(2-carboxyethyl)phosphine and ascorbate, regenerating protein thiols, or, in the case of ascorbate, more highly oxidized species. The implications of this reactivity on detection methods for protein sulfenic acids and S-nitrosothiols are discussed.