Soft Cysteine Signaling Network: The Functional Significance of Cysteine in Protein Function and the Soft Acids/Bases Thiol Chemistry That Facilitates Cysteine Modification

Investigating sensitization activity of azobenzene disperse dyes via the Direct Peptide Reactivity Assay (DPRA)

Azobenzene disperse dyes are the fastest-growing category of commercial dyestuffs and have been found in indoor house dust and in children's polyester apparel. Azobenzene disperse dyes are implicated as potentially allergenic; however, little experimental data is available on allergenicity of these dyes. Here, we examine the binding of azobenzene disperse dyes to nucleophilic peptide residues as a proxy for their potential reactivity as electrophilic allergenic sensitizers. The Direct Peptide Reactivity Assay (DPRA) was utilized via both a spectrophotometric method and a high-performance liquid chromatography (HPLC) method. We tested dyes purified from commercial dyestuffs as well as several known transformation products. All dyes were found to react with nucleophilic peptides in a dose-dependent manner with pseudo-first order kinetics (rate constants as high as 0.04 h-1). Rates of binding reactivity were also found to correlate to electrophilic properties of dyes as measured by Hammett constants and electrophilicity indices. Reactivities of polyester shirt extracts were also tested for DPRA activity and the shirt extracts with high measured abundances of azobenzene disperse dyes were observed to induce greater peptide reactivity. Results suggest that azobenzene disperse dyes may function as immune sensitizers, and that clothing containing these dyes may pose risks for skin sensitization.

Cysteine and methionine oxidation in thrombotic disorders

Thrombosis is the leading cause of death in many diseased conditions. Oxidative stress is characteristic of these conditions. Yet, the mechanisms through which oxidants become prothrombotic are unclear. Recent evidence suggests protein cysteine and methionine oxidation as prothrombotic regulators. These oxidative post-translational modifications occur on proteins that participate in the thrombotic process, including Src family kinases, protein disulfide isomerase, β2 glycoprotein I, von Willebrand factor, and fibrinogen. New chemical tools to identify oxidized cysteine and methionine proteins in thrombosis and hemostasis, including carbon nucleophiles for cysteine sulfenylation and oxaziridines for methionine, are critical to understanding why clots occur during oxidative stress. These mechanisms will identify alternative or novel therapeutic approaches to treat thrombotic disorders in diseased conditions.

Circadian Control of Mitochondria in Reactive Oxygen Species Homeostasis

Significance: Mitochondria produce most of the cellular ATP through the process of oxidative phosphorylation. Energy metabolism in the mitochondria is associated with the production of reactive oxygen species (ROS). Excessive ROS production leads to oxidative stress and compromises cellular physiology. Energy metabolism in the mitochondria depends on nutrient flux and cellular metabolic needs, which are in turn connected with the feeding/fasting cycle. In animals, the feeding/fasting cycle is controlled by the circadian clock that generates 24-h rhythms in behavior, metabolism, and signaling. Recent Advances: Here, we discuss the role of the circadian clock and rhythms in mitochondria on ROS homeostasis. The circadian clock is involved in mitochondrial ROS production and detoxification through the control of nutrient flux and oxidation, uncoupling, antioxidant defense, and mitochondrial dynamics. Critical Issues: Little is known on the molecular mechanisms of circadian control of mitochondrial functions. The circadian clock regulates the expression and activity of mitochondrial metabolic and antioxidant enzymes. The regulation involves a direct transcriptional control by Circadian Locomotor Output Cycles Kaput/brain and muscle ARNT-like 1(CLOCK/BMAL1), nuclear factor erythroid-2-related factor 2 (NRF2) transcriptional network, and sirtuin-dependent posttranslational protein modifications. Future Perspectives: We hypothesize that the circadian clock orchestrates mitochondrial physiology to synchronize it with the feeding/fasting cycle. Circadian coordination of mitochondrial function couples energy metabolism with diets and contributes to antioxidant defense to prevent metabolic diseases and delay aging. Antioxid. Redox Signal. 37, 647-663.

From quantum-derived principles underlying cysteine reactivity to combating the COVID-19 pandemic

The COVID‐19 pandemic poses a challenge in coming up with quick and effective means to counter its cause, the SARS‐CoV‐2. Here, we show how the key factors governing cysteine reactivity in proteins derived from combined quantum mechanical/continuum calculations led to a novel multi‐targeting strategy against SARS‐CoV‐2, in contrast to developing potent drugs/vaccines against a single viral target such as the spike protein. Specifically, they led to the discovery of reactive cysteines in evolutionary conserved Zn²⁺‐sites in several SARS‐CoV‐2 proteins that are crucial for viral polypeptide proteolysis as well as viral RNA synthesis, proofreading, and modification. These conserved, reactive cysteines, both free and Zn²⁺‐bound, can be targeted using the same Zn‐ejector drug (disulfiram/ebselen), which enables the use of broad‐spectrum anti‐virals that would otherwise be removed by the virus's proofreading mechanism. Our strategy of targeting multiple, conserved viral proteins that operate at different stages of the virus life cycle using a Zn‐ejector drug combined with other broad‐spectrum anti‐viral drug(s) could enhance the barrier to drug resistance and antiviral effects, as compared to each drug alone. Since these functionally important nonstructural proteins containing reactive cysteines are highly conserved among coronaviruses, our proposed strategy has the potential to tackle future coronaviruses.

This article is categorized under:

Structure and Mechanism > Reaction Mechanisms and Catalysis

Structure and Mechanism > Computational Biochemistry and Biophysics

Electronic Structure Theory > Density Functional Theory

Protein Modifications: From Chemoselective Probes to Novel Biocatalysts

Chemical reactions can be performed to covalently modify specific residues in proteins. When applied to native enzymes, these chemical modifications can greatly expand the available set of building blocks for the development of biocatalysts. Nucleophilic canonical amino acid sidechains are the most readily accessible targets for such endeavors. A rich history of attempts to design enhanced or novel enzymes, from various protein scaffolds, has paved the way for a rapidly developing field with growing scientific, industrial, and biomedical applications. A major challenge is to devise reactions that are compatible with native proteins and can selectively modify specific residues. Cysteine, lysine, N-terminus, and carboxylate residues comprise the most widespread naturally occurring targets for enzyme modifications. In this review, chemical methods for selective modification of enzymes will be discussed, alongside with examples of reported applications. We aim to highlight the potential of such strategies to enhance enzyme function and create novel semisynthetic biocatalysts, as well as provide a perspective in a fast-evolving topic.

qPTMplants: an integrative database of quantitative post-translational modifications in plants

As a crucial molecular mechanism, post-translational modifications (PTMs) play critical roles in a wide range of biological processes in plants. Recent advances in mass spectrometry-based proteomic technologies have greatly accelerated the profiling and quantification of plant PTM events. Although several databases have been constructed to store plant PTM data, a resource including more plant species and more PTM types with quantitative dynamics still remains to be developed. In this paper, we present an integrative database of quantitative PTMs in plants named qPTMplants (http://qptmplants.omicsbio.info), which hosts 1 242 365 experimentally identified PTM events for 429 821 nonredundant sites on 123 551 proteins under 583 conditions for 23 PTM types in 43 plant species from 293 published studies, with 620 509 quantification events for 136 700 PTM sites on 55 361 proteins under 354 conditions. Moreover, the experimental details, such as conditions, samples, instruments and methods, were manually curated, while a variety of annotations, including the sequence and structural characteristics, were integrated into qPTMplants. Then, various search and browse functions were implemented to access the qPTMplants data in a user-friendly manner. Overall, we anticipate that the qPTMplants database will be a valuable resource for further research on PTMs in plants.

An immunoblot assay for cysteine oxidation by reactive oxygen species allows detection of novel thioprotective efficacy of black tea extracts

INTRODUCTION

While cysteine thiol groups help to maintain the redox status of many proteins, they can be very susceptible to damaging oxidants. Despite broad interest in their antioxidant properties, whether tea polyphenols protect against protein thiol damage of this kind is unclear. This study sought to develop a simple immunoassay for use in screening tea extracts and other antioxidants for thioprotective efficacy at protein thiol groups.

METHODS

Fresh aqueous extracts were prepared from commercially sourced green, white, black and red teas. Traut's reagent (2-iminothiolane) was used to prepare surface-thiolated bovine serum albumin for use as assay substrate in the protein oxidation assay. Oxidative damage was induced during a 15 min incubation with hydrogen peroxide (H₂O₂) in the presence of tea extracts and reference antioxidants. The substrate protein was then derivatised with dimedone before samples were loaded onto a nitrocellulose membrane housed within a Slot-Blot apparatus. After blocking nonspecific protein binding a commercially available antibody was used to detect dimedone-labelled groups.

RESULTS

While the total phenol content of tea extracts typically correlated with their activity in lipid peroxidation and galvinoxyl radical-trapping assays, the former did not fully predict their abilities to suppress H₂O₂-induced cysteine oxidation, with black tea extracts displaying greater activity than the other teas and an apparent ability to reverse pre-existing cysteine oxidation. Among the model antioxidants tested, quercetin displayed a heightened ability to suppress cysteine oxidation.

DISCUSSION

This slot-blot immunoassay is a convenient method that facilitates standardised comparisons between the thioprotective properties of structurally- and constitutively-diverse antioxidants.

iCysMod: an integrative database for protein cysteine modifications in eukaryotes

As important post-translational modifications, protein cysteine modifications (PCMs) occurring at cysteine thiol group play critical roles in the regulation of various biological processes in eukaryotes. Due to the rapid advancement of high-throughput proteomics technologies, a large number of PCM events have been identified but remain to be curated. Thus, an integrated resource of eukaryotic PCMs will be useful for the research community. In this work, we developed an integrative database for protein cysteine modifications in eukaryotes (iCysMod), which curated and hosted 108 030 PCM events for 85 747 experimentally identified sites on 31 483 proteins from 48 eukaryotes for 8 types of PCMs, including oxidation, S-nitrosylation (-SNO), S-glutathionylation (-SSG), disulfide formation (-SSR), S-sulfhydration (-SSH), S-sulfenylation (-SOH), S-sulfinylation (-SO2H) and S-palmitoylation (-S-palm). Then, browse and search options were provided for accessing the dataset, while various detailed information about the PCM events was well organized for visualization. With human dataset in iCysMod, the sequence features around the cysteine modification sites for each PCM type were analyzed, and the results indicated that various types of PCMs presented distinct sequence recognition preferences. Moreover, different PCMs can crosstalk with each other to synergistically orchestrate specific biological processes, and 37 841 PCM events involved in 119 types of PCM co-occurrences at the same cysteine residues were finally obtained. Taken together, we anticipate that the database of iCysMod would provide a useful resource for eukaryotic PCMs to facilitate related researches, while the online service is freely available at http://icysmod.omicsbio.info.

Lipid Peroxide-Derived Reactive Carbonyl Species as Mediators of Oxidative Stress and Signaling

Oxidation of membrane lipids by reactive oxygen species (ROS) or O₂/lipoxygenase leads to the formation of various bioactive compounds collectively called oxylipins. Reactive carbonyl species (RCS) are a group of oxylipins that have the α,β-unsaturated carbonyl structure, including acrolein and 4-hydroxy-(E)-2-nonenal. RCS provides a missing link between ROS stimuli and cellular responses in plants via their electrophilic modification of proteins. The physiological significance of RCS in plants has been established based on the observations that the RCS-scavenging enzymes that are overexpressed in plants or the RCS-scavenging chemicals added to plants suppress the plants' responses to ROS, i.e., photoinhibition, aluminum-induced root damage, programmed cell death (PCD), senescence, abscisic acid-induced stomata closure, and auxin-induced lateral root formation. The functions of RCS are thus a key to ROS- and redox-signaling in plants. The chemical species involved in distinct RCS signaling/damaging phenomena were recently revealed, based on comprehensive carbonyl determinations. This review presents an overview of the current status of research regarding RCS signaling functions in plants and discusses present challenges for gaining a more complete understanding of the signaling mechanisms.

How the Local Environment of Functional Sites Regulates Protein Function

Proteins form complex biological machineries whose functions in the cell are highly regulated at both the cellular and molecular levels. Cellular regulation of protein functions involves differential gene expressions, post-translation modifications, and signaling cascades. Molecular regulation, on the other hand, involves tuning an optimal local protein environment for the functional site. Precisely how a protein achieves such an optimal environment around a given functional site is not well understood. Herein, by surveying the literature, we first summarize the various reported strategies used by certain proteins to ensure their correct functioning. We then formulate three key physicochemical factors for regulating a protein's functional site, namely, (i) its immediate interactions, (ii) its solvent accessibility, and (iii) its conformational flexibility. We illustrate how these factors are applied to regulate the functions of free/metal-bound Cys and Zn sites in proteins.

Inactivation of Carbonyl-Detoxifying Enzymes by H2O2 Is a Trigger to Increase Carbonyl Load for Initiating Programmed Cell Death in Plants

H2O2-induced programmed cell death (PCD) of tobacco Bright Yellow-2 (BY-2) cells is mediated by reactive carbonyl species (RCS), degradation products of lipid peroxides, which activate caspase-3-like protease (C3LP). Here, we investigated the mechanism of RCS accumulation in the H2O2-induced PCD of BY-2 cells. The following biochemical changes were observed in 10-min response to a lethal dose (1.0 mM) of H2O2, but they did not occur in a sublethal dose (0.5 mM) of H2O2. (1) The C3LP activity was increased twofold. (2) The intracellular levels of RCS, i.e., 4-hydroxy-(E)-hexenal and 4-hydroxy-(E)-nonenal (HNE), were increased 1.2-1.5-fold. (3) The activity of a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent carbonyl reductase, scavenging HNE, and n-hexanal was decreased. Specifically, these are the earliest events leading to PCD. The proteasome inhibitor MG132 suppressed the H2O2-induced PCD, indicating that the C3LP activity of the 1 subunit of the 20S proteasome was responsible for PCD. The addition of H2O2 to cell-free protein extract inactivated the carbonyl reductase. Taken together, these results suggest a PCD-triggering mechanism in which H2O2 first inactivates a carbonyl reductase(s), allowing RCS levels to rise, and eventually leads to the activation of the C3LP activity of 20S proteasome. The carbonyl reductase thus acts as an ROS sensor for triggering PCD.

Activity-Based Sensing for Site-Specific Proteomic Analysis of Cysteine Oxidation

Oxidative post-translational modifications (OxiPTMs) of cysteine residues are the molecular foundation of thiol-based redox regulation that modulates physiological events such as cell proliferation, differentiation, and migration and, when dysregulated, can lead to biomolecule damage and cell death. Common OxiPTMs of cysteine thiols (-SH) include reversible modifications such as S-sulfenylation (-SOH), S-glutathionylation (-SSG), disulfide formation (-SSR), S-nitrosylation (-SNO), and S-sulfhydration (-SSH) as well as more biologically stable modifications like S-sulfinylation (-SO₂H) and S-sulfonylation (-SO₃H). In the past decade, our laboratory has developed first-in-class chemistry-based tools and proteomic methods to advance the field of thiol-based redox biology and oxidative stress. In this Account, we take the reader through the historical aspects of probe development and application in our laboratory, highlighting key advances in our understanding of sulfur chemistry, in the test tube and in living systems. Offering superior resolution, throughput, accuracy, and reproducibility, mass spectrometry (MS)-based proteomics coupled to chemoselective "activity-based" small-molecule probes is the most rigorous technique for global mapping of cysteine OxiPTMs. Herein, we describe the evolution of this field from indirect detection to state-of-the-art site-centric quantitative chemoproteomic approaches that enable mapping of physiological and pathological changes in cysteine oxidation. These methods enable protein and site-level identification, mechanistic studies, mapping fold-changes, and modification stoichiometry. In particular, this Account focuses on activity-based methods for profiling S-sulfenylation, S-sulfinylation, and S-sulfhydration with an eye toward new reactions and methodologies developed in our group as well as their applications that have shed new light on fundamental processes of redox biology. Among several classes of sulfenic acid probes, dimedone-based C-nucleophiles possess superior chemical selectivity and compatibility with tandem MS. Cell-permeable dimedone derivatives with a bioconjugation handle are capable of detecting of S-sulfenylation in living cells. In-depth screening of a C-nucleophile library has yielded several entities with significantly enhanced reactivity over dimedone while maintaining selectivity, and reversible linear C-nucleophiles that enable controlled target release. C-Nucleophiles have also been implemented in tag-switch methods to detect S-sulfhydration. Most recently, activity-based detection of protein S-sulfinylation with electrophilic nitrogen species (ENS), such as C-nitroso compounds and electron deficient diazines, offers significant advantages in simplicity-of-use and target specificity compared to label-free methods. When feasible, the rich information provided by site-centric quantitative proteomics should not be tainted by oxidation artifacts from cell lysis. Therefore, chemoselective probes that function in a native environment with low cytotoxicity, good cell-permeability, and competitive kinetics are desired in modern redox chemoproteomics approaches. As our understanding of sulfur chemistry and redox signaling evolves, newly discovered cysteine OxiPTMs in microorganisms, plants, cells, tissues, and disease models should innovatively promote mechanistic and therapeutic research.

Reactive oxygen species and reactive carbonyl species constitute a feed-forward loop in auxin signaling for lateral root formation

In auxin-stimulated roots, production of reactive oxygen species (ROS) via the hormone-induced activation of respiratory burst oxidase homologous NADPH oxidases facilitates lateral root (LR) formation. In this study, in order to verify that ROS can modulate auxin signaling, we examined the involvement of the lipid peroxide-derived agents known as reactive carbonyl species (RCS) in LR formation. When auxin was added to Arabidopsis thaliana roots, the levels of RCS, for example acrolein, 4-hydroxynonenal and crotonaldehyde, were increased prior to LR formation. Addition of the carbonyl scavenger carnosine suppressed auxin-induced LR formation. Addition of RCS to the roots induced the expression of the auxin-responsive DR5 promoter and the TIR1, IAA14, ARF7, LBD16 and PUCHI genes and facilitated LR formation without increasing the endogenous auxin level. DR5 and LBD16 were activated in the LR primordia. The auxin signaling-deficient mutants arf7 arf19 and slr-1 did not respond - and tir1 afb2 appeared to show a poor response - to RCS. When given to the roots RCS promoted the disappearance of the AXR3NT-GUS fusion protein, i.e. the degradation of the auxin/indole-3-acetic acid protein, as did auxin. These results indicate that the auxin-induced production of ROS and their downstream products RCS modulate the auxin signaling pathway in a feed-forward manner. RCS are key agents that connect the ROS signaling and the auxin signaling pathways.

Reactive Carbonyl Species: A Missing Link in ROS Signaling

As reactive oxygen species (ROS) play critical roles in plants to determine cell fate in various physiological situations, there is keen interest in the biochemical processes of ROS signal transmission. Reactive carbonyl species (RCS), the ,-unsaturated aldehydes and ketones produced from lipid peroxides, due to their chemical property to covalently modify protein, can mediate ROS signals to proteins. Comprehensive carbonyl analysis in plants has revealed that more than a dozen different RCS, e.g., acrolein, 4-hydroxy-(E)-2-nonenal and malondialdehyde, are produced from various membranes, and some of them increase and modify proteins in response to oxidative stimuli. At early stages of response, specific subsets of proteins are selectively modified with RCS. The involvement of RCS in ROS signaling can be judged on three criteria: (1) A stimulus to increase the ROS level in plants leads to the enhancement of RCS levels. (2) Suppression of the increase of RCS by scavenging enzymes or chemicals diminishes the ROS-induced response. (3) Addition of RCS to plants evokes responses similar to those induced by ROS. On these criteria, the RCS action as damaging/signaling agents has been demonstrated for root injury, programmed cell death, senescence of siliques, stomata response to abscisic acid, and root response to auxin. RCS thus act as damage/signal mediators downstream of ROS in a variety of physiological situations. A current picture and perspectives of RCS research are presented in this article.

Reaction of quinones with proteins: Kinetics of adduct formation, effects on enzymatic activity and protein structure, and potential reversibility of modifications

Quinones are a common motif in many biological compounds, and have been linked to tissue damage as they can undergo redox cycling to generate radicals, and/or act as Michael acceptors with nucleophiles, such as protein Cys residues, with consequent adduct formation. The kinetics and consequences of these Michael reactions are poorly characterized. In this study we hypothesized that adduction of protein Cys residues with quinones would be rapid, structure-dependent, quantitatively-significant, and result in altered protein structure and function. Multiple quinones were incubated with glyceraldehyde-3-phosphate dehydrogenase (GAPDH), creatine kinase (CK), papain, bovine (BSA) and human (HSA) serum albumins, with the kinetics of adduction and effects on protein structure and activity determined. Adduction rate constants at Cys residues, which were dependent on the quinone and protein structure, and thiol pK_a, are in the range 10²-10⁵ M^-1 s^-1. p-Benzoquinone (BQ) induced dimerization of GAPDH and CK (but not BSA, HSA, or papain) in a dose- and time-dependent manner. Incubation of purified proteins, or cell lysates, with quinones resulted in a rapid loss of GAPDH and CK activity; this loss correlated well with the rate constant for Cys adduction. Glutathione (GSH) reacts competitively with quinones, and could reverse the loss of activity and dimerization of GAPDH and CK. Mass spectrometry peptide mass mapping provided evidence for BQ adduction to GAPDH to specific Cys residues (Cys149, Cys244), whereas all Cys residues in CK were modified. These data suggested that quinones can induce biological effects by rapid and selective formation of adducts with Cys residues in proteins.

Neurodevelopmental Effects of Mercury

The toxicology of mercury (Hg) is of concern since this metal is ubiquitously distributed in the environment, and living organisms are routinely exposed to Hg at low to high levels. The toxic effects of Hg are well studied and it is known that they may differ depending on the Hg chemical species. In this chapter, we emphasize the neurotoxic effects of Hg during brain development. The immature brain is more susceptible to Hg exposure, since all the Hg chemical forms, not only the organic ones, can harm it. The possible consequences of Hg exposure during the early stages of development, the additive effects with other co-occurring neurotoxicants, and the known mechanisms of action and targets will be addressed in this chapter.

TGA2 signaling in response to reactive electrophile species is not dependent on cysteine modification of TGA2

Reactive electrophile species (RES), including prostaglandins, phytoprostanes and 12-oxo phytodienoic acid (OPDA), activate detoxification responses in plants and animals. However, the pathways leading to the activation of defense reactions related to abiotic or biotic stress as a function of RES formation, accumulation or treatment are poorly understood in plants. Here, the thiol-modification of proteins, including the RES-activated basic region/leucine zipper transcription factor TGA2, was studied. TGA2 contains a single cysteine residue (Cys186) that was covalently modified by reactive cyclopentenones but not required for induction of detoxification genes in response to OPDA or prostaglandin A1. Activation of the glutathione-S-transferase 6 (GST6) promoter was responsive to cyclopentenones but not to unreactive cyclopentanones, including jasmonic acid suggesting that thiol reactivity of RES is important to activate the TGA2-dependent signaling pathway resulting in GST6 activation We show that RES modify thiols in numerous proteins in vivo, however, thiol reactivity alone appears not to be sufficient for biological activity as demonstrated by the failure of several membrane permeable thiol reactive reagents to activate the GST6 promoter.

Pharmacogenomics of Chemically Distinct Classes of Keap1-Nrf2 Activators Identify Common and Unique Gene, Protein, and Pathway Responses In Vivo

The Kelch-like erythroid-associated protein 1 (Keap1)-NF-E2-related factor 2 (Nrf2) signaling pathway is the subject of several clinical trials evaluating the effects of Nrf2 activation on the prevention of cancer and diabetes and the treatment of chronic kidney disease and multiple sclerosis. 3H-1,2-dithiole-3-thione (D3T) and 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole (CDDO-Im) are representative members of two distinct series of Nrf2 chemical activators. Previous reports have described activator-specific effects on Nrf2-dependent gene regulation and physiologic outcomes. Here we used a robust chemical genomics approach to characterize expression profiles between D3T and CDDO-Im in livers from wild-type and Nrf2-null mice. At equally efficacious doses in wild-type mice, 406 genes show common RNA responses to both treatments. These genes enriched the Nrf2-regulated pathways of antioxidant defense and xenobiotic metabolism. In addition, 197 and 745 genes were regulated uniquely in response to either D3T or CDDO-Im, respectively. Functional analysis of the D3T-regulated set showed a significant enrichment of Nrf2-regulated enzymes involved in cholesterol biosynthesis. This result was supported by Nrf2-dependent increases in lanosterol synthase and CYP51 protein expression. CDDO-Im had no effect on cholesterol biosynthesis regardless of the dose tested. However, unlike D3T, CDDO-Im resulted in Nrf2-dependent elevation of peroxisome proliferator α and Kruppel-like factor 13, as well as the coactivator peroxisome proliferator γ coactivator 1β, together indicating regulation of β-oxidation and lipid metabolic pathways. These findings provide novel insights into the pharmacodynamic action of these two activators of Keap1-Nrf2 signaling. Although both compounds modify Keap1 to affect canonical cytoprotective gene expression, additional unique sets of Nrf2-dependent genes were regulated by each agent with enrichment of selective metabolic pathways.

NRF2 regulates core and stabilizing circadian clock loops, coupling redox and timekeeping in Mus musculus

Diurnal oscillation of intracellular redox potential is known to couple metabolism with the circadian clock, yet the responsible mechanisms are not well understood. We show here that chemical activation of NRF2 modifies circadian gene expression and rhythmicity, with phenotypes similar to genetic NRF2 activation. Loss of Nrf2 function in mouse fibroblasts, hepatocytes and liver also altered circadian rhythms, suggesting that NRF2 stoichiometry and/or timing of expression are important to timekeeping in some cells. Consistent with this concept, activation of NRF2 at a circadian time corresponding to the peak generation of endogenous oxidative signals resulted in NRF2-dependent reinforcement of circadian amplitude. In hepatocytes, activated NRF2 bound specific enhancer regions of the core clock repressor gene Cry2, increased Cry2 expression and repressed CLOCK/BMAL1-regulated E-box transcription. Together these data indicate that NRF2 and clock comprise an interlocking loop that integrates cellular redox signals into tissue-specific circadian timekeeping.

Identification of two frataxin isoforms in Zea mays: Structural and functional studies

Frataxin is a ubiquitous protein that plays a role in Fe-S cluster biosynthesis and iron and heme metabolism, although its molecular functions are not entirely clear. In non-photosynthetic eukaryotes, frataxin is encoded by a single gene, and the protein localizes to mitochondria. Here we report the presence of two functional frataxin isoforms in Zea mays, ZmFH-1 and ZmFH-2. We confirmed our previous findings regarding plant frataxins: both proteins have dual localization in mitochondria and chloroplasts. Physiological, biochemical and biophysical studies show some differences in the expression pattern, protection against oxidants and in the aggregation state of both isoforms, suggesting that the two frataxin homologs would play similar but not identical roles in plant cell metabolism. In addition, two specific features of plant frataxins were evidenced: their ability to form dimers and their tendency to undergo conformational change under oxygen exposure.

The STIM-Orai Pathway: Regulation of STIM and Orai by Thiol Modifications

Cysteines are among the least abundant amino acids found in proteins. Due to their unique nucleophilic thiol group, they are able to undergo a broad range of chemical modifications besides their known role in disulfide formation, such as S-sulfenylation (-SOH), S-sulfinylation (-SO(2)H), S-sufonylation (-SO(3)H), S-glutathionylation (-SSG), and S-sulfhydration (-SSH), among others. These posttranslational modifications can be irreversible and act as transitional modifiers or as reversible on-off switches for the function of proteins. Disturbances of the redox homeostasis, for example, in situations of increased oxidative stress, can contribute to a range of diseases. Because Ca²⁺ signaling mediated by store-operated calcium entry (SOCE) is involved in a plethora of cellular responses, the cross-talk between reactive oxygen species (ROS) and Ca²⁺ is critical for homeostatic control. Identification of calcium regulatory protein targets of thiol redox modifications is needed to understand their role in biology and disease.