Overoxidation of 2-Cys peroxiredoxin in prokaryotes: cyanobacterial 2-Cys peroxiredoxins sensitive to oxidative stress

Isopropylthiol emission by bloom-forming Microcystis: Biochemistry, ecophysiology and semiochemistry of a volatile organosulfur compound

Microcystis species not only produce toxic cyanobacterial blooms, but can be a significant source of taste and odour. Previous studies have associated foul-smelling volatile organic sulfur compounds (VOSCs) with Microcystis blooms, but have largely attributed these compounds to bacterial bloom decomposition. However, earlier reports of the production of isopropylthio compounds by several Microcystis strains suggests that these cyanobacteria may themselves be a source of these VOSCs. Sulphur compounds have been shown to play important semiochemical roles in algal cell protection and grazer interactions in marine systems, but little is known about the production and chemical ecology of freshwater cyanobacterial VOSCs. To address this knowledge gap, we undertook the first detailed investigation of the biochemistry, ecophysiology and semiochemistry of these compounds and their production by Microcystis, and tested the hypothesis that they act as multifunctional semiochemicals in processes related to cell protection and grazer defence. Using short-term incubations and an adapted headspace-GC-MS technique, we investigated VOSC production by axenic and non-axenic strains, and verified that isopropylthio compounds are in fact produced by these cyanobacteria, identifying 5 isopropyl moiety-containing VOSCs (isopropylthiol (ISH), isopropylmethyl sulfide, isopropyl methyl disulfide, diisopropyl disulfide (ISSI) and diisopropyl trisulfide) as well as methanethiol in three strains. Further studies with the axenic strain Microcystis PCC 7806 using different light regimes, metabolic inhibitors (sodium azide, DCMU), the antioxidant enzyme catalase and stable labelled precursors (hydrogencarbonate, acetates and sulfate) demonstrated that ISH is a true exo-metabolite, synthesized via the acetate pathway. It is actively produced and continuously excreted by the cyanobacteria during growth, with minimal internal storage or post-lysis catalytic generation. The molar ratios of the redox pair ISH/ISSI are not directly involved in the photosynthetic and respiratory electron transport chains, but dependant on the redox state of the cell - likely mediated by reactive oxygen species (ROS), as shown by a marked effect of catalase. These results, along with toxicological and behavioural assays using the two aquatic invertebrates Thamnocephalus platyurus and Daphnia magna indicate that ISH plays multiple important physiological and ecological roles. It acts as an effective antioxidant against high ROS levels, as often experienced in surface blooms, it elicits avoidance-related behavioural responses in grazer communities and at high levels, it can be toxic to some invertebrates.

The atypical thioredoxin 'Alr2205', a newly identified partner of the typical 2-Cys-Peroxiredoxin, safeguards the cyanobacterium Anabaena from oxidative stress

Thioredoxins (Trxs) are ubiquitous proteins that play vital roles in several physiological processes. Alr2205, a thioredoxin-like protein from Anabaena PCC 7120, was found to be evolutionarily closer to the Trx-domain of the NADPH-Thioredoxin Reductase C than the other thioredoxins. The Alr2205 protein showed disulfide reductase activity despite the presence a non-canonical active site motif 'CPSC'. Alr2205 not only physically interacted with, but also acted as a physiological reductant of Alr4641 (the typical 2-Cys-Peroxiredoxin from Anabaena), supporting its peroxidase function. Structurally, Alr2205 was a monomeric protein that formed an intramolecular disulfide bond between the two active site cysteines (Cys-38 and Cys-41). However, the Alr2205C41S protein, wherein the resolving cysteine was mutated to serine, was capable of forming intermolecular disulfide bond and exist as a dimer when treated with H2O2. Overproduction of Alr2205 in E. coli protected cells from heavy metals, but not oxidative stress. To delve into its physiological role, Alr2205/Alr2205C41S was overexpressed in Anabaena, and the ability of the corresponding strains (An2205+ or An2205C41S+) to withstand environmental stresses was assessed. An2205+ showed higher resistance to H2O2 than An2205C41S+, indicating that the disulfide reductase function of this protein was critical to protect cells from this peroxide. Although, An2205+ did not show increased capability to withstand cadmium stress, An2205C41S+ was more susceptible to this heavy metal. This is the first study that provides a vital understanding into the function of atypical thioredoxins in countering the toxic effects of heavy metals/H2O2 in prokaryotes.

Back to the future: Transplanting the chloroplast TrxF-FBPase-SBPase redox system to cyanobacteria

Fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase) are two essential activities in the Calvin-Benson-Bassham cycle that catalyze two irreversible reactions and are key for proper regulation and functioning of the cycle. These two activities are codified by a single gene in all cyanobacteria, although some cyanobacteria contain an additional gene coding for a FBPase. Mutants lacking the gene coding for SBP/FBPase protein are not able to grow photoautotrophically and require glucose to survive. As this protein presents both activities, we have tried to elucidate which of the two are required for photoautrophic growth in Synechocystis sp PCC 6803. For this, the genes coding for plant FBPase and SBPase were introduced in a SBP/FBPase mutant strain, and the strains were tested for growth in the absence of glucose. Ectopic expression of only a plant SBPase gene did not allow growth in the absence of glucose although allowed mutation of both Synechocystis' FBPase genes. When both plant FBPase and SBPase genes were expressed, photoautrophic growth of the SBP/FBPase mutants was restored. This complementation was partial as the strain only grew in low light, but growth was impaired at higher light intensities. Redox regulation of the Calvin-Benson-Bassham cycle is essential to properly coordinate light reactions to carbon fixation in the chloroplast. Two of the best characterized proteins that are redox-regulated in the cycle are FBPase and SBPase. These two proteins are targets of the FTR-Trx redox system with Trx f being the main reductant in vivo. Introduction of the TrxF gene improves growth of the complemented strain, suggesting that the redox state of the proteins may be the cause of this phenotype. The redox state of the plant proteins was also checked in these strains, and it shows that the cyanobacterial redox system is able to reduce all of them (SBPase, FBPase, and TrxF) in a light-dependent manner. Thus, the TrxF-FBPase-SBPase plant chloroplast system is active in cyanobacteria despite that these organisms do not contain proteins related to them. Furthermore, our system opens the possibility to study specificity of the Trx system in vivo without the complication of the different isoforms present in plants.

Loss of 2-Cys-Prx affects cellular ultrastructure, disturbs redox poise and impairs photosynthesis in cyanobacteria

In a striking similarity to plant chloroplasts, the cyanobacterium Anabaena displays very low catalase activity, but expresses several peroxiredoxins (Prxs), including the typical 2-Cys-Prx (annotated as Alr4641), that detoxify H₂ O₂ . Due to the presence of multiple Prxs, the precise contribution of Alr4641 to the oxidative stress response of Anabaena is not well-defined. To unambiguously assess its in vivo function, the Alr4641 protein was knocked down using the CRISPRi approach in Anabaena PCC 7120. The knockdown strain (An-KD4641), which showed over 85% decrease in the content of Alr4641, was viable, but grew slower than the control strain (An-dCas9). An-KD4641 showed elevated levels of reactive oxygen species and the expression of several redox-responsive genes was analogous to that of An-dCas9 subjected to oxidative stress. The knockdown strain displayed reduced filament size, altered thylakoid ultrastructure, a marked drop in the ratio of phycocyanin to chlorophyll a and decreased photosynthetic parameters compared to An-dCas9. In comparison to the control strain, exposure to H₂ O₂ had a more severe effect on the photosynthetic parameters or survival of An-KD4641. Thus, in the absence of adequate catalase activity, 2-Cys-Prx appears to be the principal Prx responsible for maintaining redox homoeostasis in diverse photosynthetic systems ranging from chloroplasts to cyanobacteria.

Structural and mechanistic basis for redox sensing by the cyanobacterial transcription regulator RexT

Organisms have a myriad of strategies for sensing, responding to, and combating reactive oxygen species, which are unavoidable consequences of aerobic life. In the heterocystous cyanobacterium Nostoc sp. PCC 7120, one such strategy is the use of an ArsR-SmtB transcriptional regulator RexT that senses H₂O₂ and upregulates expression of thioredoxin to maintain cellular redox homeostasis. Different from many other members of the ArsR-SmtB family which bind metal ions, RexT has been proposed to use disulfide bond formation as a trigger to bind and release DNA. Here, we present high-resolution crystal structures of RexT in the reduced and H₂O₂-treated states. These structures reveal that RexT showcases the ArsR-SmtB winged-helix-turn-helix fold and forms a vicinal disulfide bond to orchestrate a response to H₂O₂. The importance of the disulfide-forming Cys residues was corroborated using site-directed mutagenesis, mass spectrometry, and H₂O₂-consumption assays. Furthermore, an entrance channel for H₂O₂ was identified and key residues implicated in H₂O₂ activation were pinpointed. Finally, bioinformatics analysis of the ArsR-SmtB family indicates that the vicinal disulfide “redox switch” is a unique feature of cyanobacteria in the Nostocales order, presenting an interesting case where an ArsR-SmtB protein scaffold has been evolved to showcase peroxidatic activity and facilitate redox-based regulation.

The DNA binding and H₂O₂ sensing mechanisms are revealed for RexT, a transcriptional regulator found in cyanobacteria of the Nostocales order.

Unique functional insights into the antioxidant response of the cyanobacterial Mn-catalase (KatB)

KatB, a hexameric Mn-catalase, plays a vital role in overcoming oxidative and salinity stress in the ecologically important, N₂-fixing cyanobacterium, Anabaena. The 5 N-terminal residues of KatB, which show a high degree of conservation in cyanobacteria, form an antiparallel β-strand at the subunit interface of the KatB hexamer. In this study, the contribution of these N-terminal non-active site residues, towards the maintenance of the structure, biochemical properties, and redox balance was evaluated. Each N-terminal amino acid residue from the 2nd to the 7th position of KatB was individually mutated to Ala (to express KatBF2A/KatBF3A/KatBH4A/KatBK5E/KatBK6A/KatBE7A) or this entire 6 amino acid stretch was deleted (to yield KatBTrunc). All the above-mentioned KatB variants, along with the wild-type KatB protein (KatBWT), were overproduced in E. coli and purified. In comparison to KatBWT, the KatBF2A/KatBH4A/KatBTrunc proteins were less compact, more prone to chemical/thermal denaturation, and were unexpectedly inactive. KatBF3A/KatBK5E/KatBK6A showed biophysical/biochemical properties that were in between that of KatBWT and KatBF2A/KatBH4A/KatBTrunc. Surprisingly, KatBE7A was more thermostable with higher activity than KatBWT. On exposure to H₂O₂, E. coli expressing KatBWT/KatBE7A showed considerably reduced formation of ROS and increased survival than the other KatB variants. Utilizing the KatB structure, the molecular basis responsible for the altered stability/activity of the KatB mutants was delineated. This study demonstrates the physiological importance of the N-terminal β-strand of Mn-catalases in combating H₂O₂ stress and shows that the non-active site residues can be used for rational protein engineering to develop Mn-catalases with improved characteristics.

Differential catalase activity and tolerance to hydrogen peroxide in the filamentous cyanobacteria Nostoc punctiforme ATCC 29133 and Anabaena sp. PCC 7120

Photoautotrophic cyanobacteria often confront hydrogen peroxide (H₂O₂), a reactive oxygen species potentially toxic to cells when present in sufficiently high concentrations. In this study, H₂O₂ tolerance ability of filamentous cyanobacteria Nostoc punctiforme ATCC 29133 (Nostoc 29133) and Anabaena sp. PCC 7120 (Anabaena 7120) was investigated at increasing concentrations of H₂O₂ (0-0.5 mM). In Nostoc 29133, 0.25 and 0.5 mM H₂O₂ caused a reduction in chlorophyll a content by 12 and 20%, respectively, whereas with similar treatments, a total loss of chlorophyll a was detected in Anabaena 7120. Further, Nostoc 29133 was able to maintain its photosystem II performance in the presence of H₂O₂ up to a concentration of 0.5 mM, whereas in Anabaena 7120, 0.25 mM H₂O₂ caused a complete reduction of photosystem II performance. The intracellular hydroperoxide level (indicator of oxidative status) did not increase to the same high level in Nostoc 29133, as compared to in Anabaena 7120 after H₂O₂ treatment. This might be explained by that Nostoc 29133 showed a 20-fold higher intrinsic constitutive catalase activity than Anabaena 7120, thus indicating that the superior tolerance of Nostoc 29133 to H₂O₂ stems from its higher ability to decompose H₂O₂. It is suggested that difference in H₂O₂ tolerance between closely related filamentous cyanobacteria, as revealed in this study, may be taken into account for judicious selection and effective use of strains in biotechnological applications.

Structural analysis revealed a novel conformation of the NTRC reductase domain from Chlamydomonas reinhardtii

In plant chloroplasts, thiol regulation is driven by two systems. One relies on the activity of thioredoxins through their light dependent reduction by ferredoxin via a ferredoxin-thioredoxin reductase (FTR). In the other system, a NADPH-dependent redox regulation is driven by a NADPH-thioredoxin reductase C (NTRC). While the thioredoxin system has been deeply studied, a more thorough understanding of the function of this plant specific NTRC is desirable. NTRC is a single polypeptide harbouring a thioredoxin domain (Trx) at the C-terminus of a NADPH-dependent Thioredoxin reductase (TrxR). To provide functional and structural insights, we studied the crystal structure of the TrxR domain of the NTRC from Chlamydomonas reinhardtii (CrNTRC, Cre01.g054150.t1.2) and its Cys136Ser (C136S) mutant, which is characterized by the mutation of the resolving cysteine in the active site of the TrxR domain. Furthermore, we confirmed the role of NTRC as electron donor for 2-Cys peroxiredoxin (PRX) also in C. reinhardtii. The structural data of TrxR were employed to develop a scheme of action which addresses electron transfer between TrxR and Trx of NTRC and between NTRC and its substrates.

Redox regulation of chloroplast metabolism

Regulation of enzyme activity based on thiol-disulfide exchange is a regulatory mechanism in which the protein disulfide reductase activity of thioredoxins (TRXs) plays a central role. Plant chloroplasts are equipped with a complex set of up to 20 TRXs and TRX-like proteins, the activity of which is supported by reducing power provided by photosynthetically reduced ferredoxin (FDX) with the participation of a FDX-dependent TRX reductase (FTR). Therefore, the FDX-FTR-TRXs pathway allows the regulation of redox-sensitive chloroplast enzymes in response to light. In addition, chloroplasts contain an NADPH-dependent redox system, termed NTRC, which allows the use of NADPH in the redox network of these organelles. Genetic approaches using mutants of Arabidopsis (Arabidopsis thaliana) in combination with biochemical and physiological studies have shown that both redox systems, NTRC and FDX-FTR-TRXs, participate in fine-tuning chloroplast performance in response to changes in light intensity. Moreover, these studies revealed the participation of 2-Cys peroxiredoxin (2-Cys PRX), a thiol-dependent peroxidase, in the control of the reducing activity of chloroplast TRXs as well as in the rapid oxidation of stromal enzymes upon darkness. In this review, we provide an update on recent findings regarding the redox regulatory network of plant chloroplasts, focusing on the functional relationship of 2-Cys PRXs with NTRC and the FDX-FTR-TRXs redox systems for fine-tuning chloroplast performance in response to changes in light intensity and darkness. Finally, we consider redox regulation as an additional layer of control of the signaling function of the chloroplast.

Gazing into the remarkable world of non-heme catalases through the window of the cyanobacterial Mn-catalase 'KatB'

Catalases, enzymes that decompose H₂O₂, are broadly categorized as heme catalases or non-heme catalases. The non-heme catalases are also known as Mn-catalases as they have Mn atoms in their active sites. However, unlike the well characterized heme-catalases, the study of Mn-catalases has gained importance only in the last few years. The filamentous, heterocystous, N₂-fixing cyanobacterium Anabaena PCC 7120, shows the presence of two Mn-catalases, KatA and KatB, but lacks heme catalases. Of the two Mn-catalases, KatB, which is induced by salt/desiccation, plays a major role in overcoming salinity/oxidative stress. In this mini review, we have summarized the recent advances made in the field of Mn-catalases, particularly KatB, and have interpreted these results in the larger context of stress physiology. These aspects bring to the fore the distinctive biochemical/structural properties of Mn-catalases and furthermore highlight the in vivo importance of these enzymes in adapting to oxidative stresses.

Exploring the Functional Relationship between y-Type Thioredoxins and 2-Cys Peroxiredoxins in Arabidopsis Chloroplasts

Thioredoxins (Trxs) are small, ubiquitous enzymes that catalyze disulphide–dithiol interchange in target enzymes. The large set of chloroplast Trxs, including f, m, x and y subtypes, use reducing equivalents fueled by photoreduced ferredoxin (Fdx) for fine-tuning photosynthetic performance and metabolism through the control of the activity of redox-sensitive proteins. Although biochemical analyses suggested functional diversity of chloroplast Trxs, genetic studies have established that deficiency in a particular Trx subtype has subtle phenotypic effects, leading to the proposal that the Trx isoforms are functionally redundant. In addition, chloroplasts contain an NADPH-dependent Trx reductase with a joint Trx domain, termed NTRC. Interestingly, Arabidopsis mutants combining the deficiencies of x- or f-type Trxs and NTRC display very severe growth inhibition phenotypes, which are partially rescued by decreased levels of 2-Cys peroxiredoxins (Prxs). These findings indicate that the reducing capacity of Trxs f and x is modulated by the redox balance of 2-Cys Prxs, which is controlled by NTRC. In this study, we explored whether NTRC acts as a master regulator of the pool of chloroplast Trxs by analyzing its functional relationship with Trxs y. While Trx y interacts with 2-Cys Prxs in vitro and in planta, the analysis of Arabidopsis mutants devoid of NTRC and Trxs y suggests that Trxs y have only a minor effect, if any, on the redox state of 2-Cys Prxs.

Ferredoxin-mediated reduction of 2-nitrothiophene inhibits photosynthesis: mechanism and herbicidal potential

Searching for compounds that inhibit the growth of photosynthetic organisms highlighted a prominent effect at micromolar concentrations of the nitroheteroaromatic thioether, 2-nitrothiophene, applied in the light. Since similar effects were reminiscent to those obtained also by radicals produced under excessive illumination or by herbicides, and in light of its redox potential, we suspected that 2-nitrothiophene was reduced by ferredoxin, a major reducing compound in the light. In silico examination using docking and tunneling computing algorithms of the putative interaction between 2-nitrothiophene and cyanobacterial ferredoxin has suggested a site of interaction enabling robust electron transfer from the iron-sulfur cluster of ferredoxin to the nitro group of 2-nitrothiophene. ESR and oximetry analyses of cyanobacterial cells (Anabaena PCC7120) treated with 50 μM 2-nitrothiophene under illumination revealed accumulation of oxygen radicals and peroxides. Gas chromatography mass spectrometry analysis of 2-nitrothiophene-treated cells identified cytotoxic nitroso and non-toxic amino derivatives. These products of the degradation pathway of 2-nitrohiophene, which initializes with a single electron transfer that forms a short-live anion radical, are then decomposed to nitrate and thiophene, and may be further reduced to a nitroso hydroxylamine and amino derivatives. This mechanism of toxicity is similar to that of nitroimidazoles (e.g. ornidazole and metronidazole) reduced by ferredoxin in anaerobic bacteria and protozoa, but differs from that of ornidazole in planta.

Peroxiredoxin (2-cys-prx) and catalase (katA) cyanobacterial-based bioluminescent bioreporters to detect oxidative stress in the aquatic environment

The detection of oxidative stress caused by emerging pollutants in aquatic systems is essential to carry out toxicological analysis since they can bring us information about the mechanisms of toxic action of the pollutants, which might be useful to address this contamination. To achieve this goal, two self-bioluminescent strains that respond to oxidative stress based on the filamentous cyanobacterium Nostoc sp. PCC7120, which has a high ecological relevance in aquatic continental systems, have been constructed. Nostoc sp. PCC7120 pBG2172 harbours the promoter region of the 2-cys-prx gene (P2-cys-prx), encoding a cytoplasmic peroxiredoxin, fused to luxCDABE genes of the bacterium Photorhabdus luminescens. Nostoc sp. PCC7120 pBG2173 harbours the promoter region of the KatA gene (PkatA), a cytoplasmic catalase, also fused to luxCDABE genes. Both strains have been characterized by exposing them to H₂O₂: Nostoc sp. PCC7120 pBG2172 responded while Nostoc sp. PCC7120 pBG2173 did not respond to this pollutant. In order to know their specificity, they were exposed to methyl viologen (MV), an herbicide that produces superoxide anion (O₂^-) and a bioluminescence response was observed in both strains. Besides, the utility of these strains for the detection of H₂O₂ and MV in natural water samples, both pristine and wastewater samples has been tested by spiking experiments. Finally, the possible application of these strains for the detection of the emerging pollutant triclosan has also been tested showing to be suitable bioreporters to study oxidative stress in aquatic environments.

Characterization of the peroxiredoxin 1 subfamily from Tetrahymena thermophila

Peroxiredoxins are antioxidant enzymes that use redox active Cys residues to reduce H2O2 and various organic hydroperoxides to less reactive products, and thereby protect cells against oxidative stress. In yeasts and mammals, the Prx1 proteins are sensitive to hyperoxidation and consequent loss of their peroxidase activity whereas in most bacteria they are not. In this paper we report the characterization of the Prx1 family in the non-parasitic protist Tetrahymena thermophila. In this organism, four genes potentially encoding Prx1 have been identified. In particular, we show that the mitochondrial Prx1 protein (Prx1m) from T. thermophila is relatively robust to hyperoxidation. This is surprising given that T. thermophila is a eukaryote like yeasts and mammals. In addition, the proliferation of the T. thermophila cells was relatively robust to inhibition by H2O2, cumene hydroperoxide and plant natural products that are known to promote the production of H2O2. In the presence of these agents, the abundance of the T. thermophila Prx1m protein was shown to increase. This suggested that the Prx1m protein may be protecting the cells against oxidative stress. There was no evidence for any increase in Prx1m gene expression in the stressed cells. Thus, increasing protein stability rather than increasing gene expression may explain the increasing Prx1m protein abundance we observed.

Novel molecular insights into the anti-oxidative stress response and structure-function of a salt-inducible cyanobacterial Mn-catalase

KatB, a salt-inducible Mn-catalase, protects the cyanobacterium Anabaena from salinity/oxidative stress. In this report, we provide distinctive insights into the biological-biochemical function of KatB at the molecular level. Anabaena overexpressing the wild-type KatB protein (KatBWT) detoxified H₂ O₂ efficiently, showing reduced burden of reactive oxygen species compared with the strain overproducing KatBF2V (wherein F-2 is replaced by V). Correspondingly, the KatBWT protein also displayed several folds more activity than KatBF2V. Interestingly, the KatB variants with large hydrophobic amino acids (F/W/Y) were more compact, showed enhanced activity, and were resistant to thermal/chemical denaturation than variants with smaller residues (G/A/V) at the second position. X-ray crystallography-based analysis showed that F-2 was required for appropriate interactions between two subunits. These contacts provided stability to the hexamer, making it more compact. F-2, through its interaction with F-66 and W-43, formed the proper hydrophobic pocket that held the active site together. Consequently, only residues that supported activity (i.e., F/Y/W) were selected at the second position in Mn-catalases during evolution. This study (a) demonstrates that modification of nonactive site residues can alter the response of catalases to environmental stress and (b) has expanded the scope of amino acids that can be targeted for rational protein engineering in plants.

Proteomic De-Regulation in Cyanobacteria in Response to Abiotic Stresses

Cyanobacteria are oxygenic photoautotrophs, exhibiting a cosmopolitan distribution in almost all possible environments and are significantly responsible for half of the global net primary productivity. They are well adapted to the diverse environments including harsh conditions by evolving a range of fascinating repertoires of unique biomolecules and secondary metabolites to support their growth and survival. These phototrophs are proved as excellent models for unraveling the mysteries of basic biochemical and physiological processes taking place in higher plants. Several known species of cyanobacteria have tremendous biotechnological applications in diverse fields such as biofuels, biopolymers, secondary metabolites and much more. Due to their potential biotechnological and commercial applications in various fields, there is an imperative need to engineer robust cyanobacteria in such a way that they can tolerate and acclimatize to ever-changing environmental conditions. Adaptations to stress are mainly governed by a precise gene regulation pathways resulting in the expression of novel protein/enzymes and metabolites. Despite the demand, till date few proteins/enzymes have been identified which play a potential role in improving tolerance against abiotic stresses. Therefore, it is utmost important to study environmental stress responses related to post-genomic investigations, including proteomic changes employing advanced proteomics, synthetic and structural biology workflows. In this respect, the study of stress proteomics offers exclusive advantages to scientists working on these aspects. Advancements on these fields could be helpful in dissecting, characterization and manipulation of physiological and metabolic systems of cyanobacteria to understand the stress induced proteomic responses. Till date, it remains ambiguous how cyanobacteria perceive changes in the ambient environment that lead to the stress-induced proteins thus metabolic deregulation. This review briefly describes the current major findings in the fields of proteome research on the cyanobacteria under various abiotic stresses. These findings may improve and advance the information on the role of different class of proteins associated with the mechanism(s) of stress mitigation in cyanobacteria under harsh environmental conditions.

Role of Cytosolic 2-Cys Prx1 and Prx2 in Redox Signaling

Peroxiredoxins (Prxs), a family of peroxidases, are reactive oxygen species scavengers that hydrolyze H₂O₂ through catalytic cysteine. Mammalian Prxs comprise six isoforms (typical 2-Cys Prxs; Prx1–4, atypical 2-Cys Prx; Prx5, and 1-Cys Prx; Prx6) that are distributed over various cellular compartments as they are classified according to the position and number of conserved cysteine. 2-Cys Prx1 and Prx2 are abundant proteins that are ubiquitously expressed mainly in the cytosol, and over 90% of their amino acid sequences are homologous. Prx1 and Prx2 protect cells from ROS-mediated oxidative stress through the elimination of H₂O₂ and regulate cellular signaling through redox-dependent mechanism. In addition, Prx1 and Prx2 are able to bind to a diversity of interaction partners to regulate other various cellular processes in cancer (i.e., regulation of the protein redox status, cell growth, apoptosis, and tumorigenesis). Thus, Prx1 and Prx2 can be potential therapeutic targets and it is particularly important to control their level or activity. This review focuses on cytosolic 2-Cys Prx1 and Prx2 and their role in the regulation of redox signaling based on protein-protein interaction.

Chloroplast Redox Regulatory Mechanisms in Plant Adaptation to Light and Darkness

Light is probably the most important environmental stimulus for plant development. As sessile organisms, plants have developed regulatory mechanisms that allow the rapid adaptation of their metabolism to changes in light availability. Redox regulation based on disulfide-dithiol exchange constitutes a rapid and reversible post-translational modification, which affects protein conformation and activity. This regulatory mechanism was initially discovered in chloroplasts when it was identified that enzymes of the Calvin-Benson cycle (CBC) are reduced and active during the day and become rapidly inactivated by oxidation in the dark. At present, the large number of redox-sensitive proteins identified in chloroplasts extend redox regulation far beyond the CBC. The classic pathway of redox regulation in chloroplasts establishes that ferredoxin (Fdx) reduced by the photosynthetic electron transport chain fuels reducing equivalents to the large set of thioredoxins (Trxs) of this organelle via the activity of a Fdx-dependent Trx reductase (FTR), hence linking redox regulation to light. In addition, chloroplasts harbor an NADPH-dependent Trx reductase with a joint Trx domain, termed NTRC. The presence in chloroplasts of this NADPH-dependent redox system raises the question of the functional relationship between NTRC and the Fdx-FTR-Trx pathways. Here, we update the current knowledge of these two redox systems focusing on recent evidence showing their functional interrelationship through the action of the thiol-dependent peroxidase, 2-Cys peroxiredoxin (2-Cys Prx). The relevant role of 2-Cys Prxs in chloroplast redox homeostasis suggests that hydrogen peroxide may exert a key function to control the redox state of stromal enzymes. Indeed, recent reports have shown the participation of 2-Cys Prxs in enzyme oxidation in the dark, thus providing an explanation for the long-lasting question of photosynthesis deactivation during the light-dark transition.

Expression of OsTPX Gene Improves Cellular Redox Homeostasis and Photosynthesis Efficiency in Synechococcus elongatus PCC 7942

Cyanobacterial 2-Cys peroxiredoxin (thioredoxin peroxidase, TPX) comprises a family of thiol antioxidant enzymes critically involved in cell survival under oxidative stress. In our previous study, a putative TPX was identified using a proteomics analysis of rice (Oryza sativa L. japonica, OsTPX) seedlings exposed to oxidative stress. This OsTPX gene is structurally similar to the Synechococcus elongatus TPX gene in the highly conserved redox-active disulfide bridge (Cys114, Cys236) and other highly conserved regions. In the present study, the OsTPX gene was cloned into rice plants and S. elongatus PCC 7942 strain to study hydrogen peroxide (H₂O₂) stress responses. The OsTPX gene expression was confirmed using semi-quantitative RT-PCR and western blot analysis. The OsTPX gene expression increased growth under oxidative stress by decreasing reactive oxygen species and malondialdehyde level. Additionally, the OsTPX gene expression in S. elongatus PCC 7942 (OT) strain exhibited a reduced loss of chlorophyll and enhanced photosynthesis efficiency under H₂O₂ stress, thereby increasing biomass yields twofold compared with that of the control wild type (WT) strain. Furthermore, redox balance, ion homeostasis, molecular chaperone, and photosynthetic systems showed upregulation of some genes in the OT strain than in the WT strain by RNA-Seq analysis. Thus, OsTPX gene expression enhances oxidative stress tolerance by increasing cell defense regulatory networks through the cellular redox homeostasis in the rice plants and S. elongatus PCC 7942.

2-Cys Peroxiredoxins Participate in the Oxidation of Chloroplast Enzymes in the Dark

Most redox-regulated chloroplast enzymes are reduced during the day and oxidized during the night. While the reduction mechanism of light-dependent enzymes is well known, the mechanism mediating their oxidation in the dark remains unknown. The thiol-dependent peroxidases, 2-Cys peroxiredoxins (Prxs), play a key role in light-dependent reduction of chloroplast enzymes. Prxs transfer reducing equivalents of thiols to hydrogen peroxide, suggesting the participation of these peroxidases in enzyme oxidation in the dark. Here, we have addressed this issue by analyzing the redox state of well-known redox-regulated chloroplast enzymes in response to darkness in Arabidopsis thaliana mutants deficient in chloroplast-localized Prxs (2-Cys Prxs A and B, Prx IIE, and Prx Q). Mutant plants lacking 2-Cys Prxs A and B, and plants overexpressing NADPH-dependent thioredoxin (Trx) reductase C showed delayed oxidation of chloroplast enzymes in the dark. In contrast, the deficiencies of Prx IIE or Prx Q exerted no effect. In vitro assays allowed the reconstitution of the pathway of reducing equivalents from reduced fructose 1,6-bisphosphatase to hydrogen peroxide mediated by Trxs and 2-Cys Prxs. Taken together, these results suggest that 2-Cys Prxs participate in the short-term oxidation of chloroplast enzymes in the dark.

Beyond ROS clearance: Peroxiredoxins in stress signaling and aging

Antioxidants were long predicted to have lifespan-promoting effects, but in general this prediction has not been well supported. While some antioxidants do seem to have a clear effect on longevity, this may not be primarily as a result of their role in the removal of reactive oxygen species, but rather mediated by other mechanisms such as the modulation of intracellular signaling. In this review we discuss peroxiredoxins, a class of proteinaceous antioxidants with redox signaling and chaperone functions, and their involvement in regulating longevity and stress resistance. Peroxiredoxins have a clear role in the regulation of lifespan and survival of many model organisms, including the mouse, Caenorhabditis elegans and Drosophila melanogaster. Recent research on peroxiredoxins - in these models and beyond - has revealed surprising new insights regarding the interplay between peroxiredoxins and longevity signaling, which will be discussed here in detail. As redox signaling is emerging as a potentially important player in the regulation of longevity and aging, increased knowledge of these fascinating antioxidants and their mode(s) of action is paramount.

Hyperoxidation of Peroxiredoxins: Gain or Loss of Function?

SIGNIFICANCE

In 2003, structural studies revealed that eukaryotic 2-Cys peroxiredoxins (Prx) have evolved to be sensitive to inactivation of their thioredoxin peroxidase activity by hyperoxidation (sulfinylation) of their peroxide-reacting catalytic cysteine. This was accompanied by the unexpected discovery, that the sulfinylation of this cysteine was reversible in vivo and the identification of a new enzyme, sulfiredoxin, that had apparently co-evolved specifically to reduce hyperoxidized 2-Cys Prx, restoring their peroxidase activity. Together, these findings have provided the impetus for multiple studies investigating the purpose of this reversible, Prx hyperoxidation. Recent Advances: It has been suggested that inhibition of the thioredoxin peroxidase activity by hyperoxidation can both promote and inhibit peroxide signal transduction, depending on the context. Prx hyperoxidation has also been proposed to protect cells against reactive oxygen species (ROS)-induced damage, by preserving reduced thioredoxin and/or by increasing non-peroxidase chaperone or signaling activities of Prx.

CRITICAL ISSUES

Here, we will review the evidence in support of each of these proposed functions, in view of the in vivo contexts in which Prx hyperoxidation occurs, and the role of sulfiredoxin. Thus, we will attempt to explain the basis for seemingly contradictory roles for Prx hyperoxidation in redox signaling.

FUTURE DIRECTIONS

We provide a rationale, based on modeling and experimental studies, for why Prx hyperoxidation should be considered a suitable, early biomarker for damaging levels of ROS. We discuss the implications that this has for the role of Prx in aging and the detection of hyperoxidized Prx as a conserved feature of circadian rhythms. Antioxid. Redox Signal. 28, 574-590.

An event of alternative splicing affects the expression of the NTRC gene, encoding NADPH-thioredoxin reductase C, in seed plants

The NTRC gene encodes a NADPH-dependent thioredoxin reductase with a joint thioredoxin domain, exclusive of photosynthetic organisms. An updated search shows that although most species harbor a single copy of the NTRC gene, two copies were identified in different species of the genus Solanum, Glycine max and the moss Physcomitrella patens. The phylogenetic analysis of NTRCs from different sources produced a tree with the major groups of photosynthetic organisms: cyanobacteria, algae and land plants, indicating the evolutionary success of the NTRC gene among photosynthetic eukaryotes. An event of alternative splicing affecting the expression of the NTRC gene was identified, which is conserved in seed plants but not in algae, bryophytes and lycophytes. The alternative splicing event results in a transcript with premature stop codon, which would produce a truncated form of the enzyme. The standard splicing/alternative splicing (SS/AS) transcripts ratio was higher in photosynthetic tissues from Arabidopsis, Brachypodium and tomato, in line with the higher content of the NTRC polypeptide in these tissues. Moreover, environmental stresses such as cold or high salt affected the SS/AS ratio of the NTRC gene transcripts in Brachypodium seedlings. These results suggest that the alternative splicing of the NTRC gene might be an additional mechanism for modulating the content of NTRC in photosynthetic and non-photosynthetic tissues of seed plants.

Novel molecular insights into the function and the antioxidative stress response of a Peroxiredoxin Q protein from cyanobacteria

The Peroxiredoxin Q (PrxQ) proteins are thiol-based peroxidases that are important for maintaining redox homeostasis in several organisms. Activity of PrxQs is mediated by two cysteines, peroxidatic (C_p) and resolving (C_r), in association with a reducing partner. A PrxQ, Alr3183, from the cyanobacterium, Anabaena PCC 7120, was characterized in this study. Alr3183, which required thioredoxin A (TrxA) for peroxidase activity, was an intramolecular disulfide bond-containing monomeric protein. However, Alr3183 lacking C_p (Alr3183C46S) or C_r (Alr3183C51S) formed intermolecular disulfide linkages and was dimeric. Alr3183C46S was completely inactive, while Alr3183C51S required higher concentration of TrxA for peroxidase activity. Surface plasmon resonance analysis showed that unlike Alr3183 or Alr3183C46S, Alr3183C51S bound rather poorly to TrxA. Also, compared to the oxidized protein, the DTT-treated (reduced) Alr3183 displayed decreased interaction with TrxA. In vivo, Alr3183 was found to be induced in response to γ-radiation. On exposure to H₂O₂, Anabaena strain over-expressing Alr3183 showed reduced formation of ROS, intact photosynthetic pigments and consequently better survival than the wild-type, whereas overproduction of Alr3183C46S did not provide any protection. Significantly, this study (1) reveals the importance of C_r for interaction with thioredoxins and (2) demonstrates that over-expression of PrxQs can protect cyanobacteria from oxidative stresses.

Functional Significance of NADPH-Thioredoxin Reductase C in the Antioxidant Defense System of Cyanobacterium Anabaena sp. PCC 7120

The redox regulation system is widely accepted as a crucial mechanism for controlling the activities of various metabolic enzymes. In addition to thioredoxin reductase/thioredoxin cascades, NADPH-thioredoxin reductase C (NTRC), a hybrid protein formed by an NADPH-thioredoxin reductase domain and a thioredoxin (Trx) domain, is present in chloroplasts and in most cyanobacteria species. Although several target proteins and physiological functions of NTRC in chloroplasts have been characterized, little is known about NTRC functions in cyanobacteria. Therefore, we investigated the molecular basis and physiological significance of NTRC-dependent redox regulation in the filamentous heterocyst-forming nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120). Initially, we identified six candidate NTRC targets in Anabaena 7120 using NTRC affinity chromatography. Subsequently, we compared the efficiency of reducing-equivalent transfer from NTRC and Trx-m1 to the NTRC target protein 2-Cys peroxiredoxin. Biochemical analyses revealed that compared with Trx-m1, NTRC more efficiently transfers reducing equivalents to 2-Cys peroxiredoxin. Subsequently, we constructed and analyzed an ntrC knockout strain in Anabaena 7120. The mutant showed impaired growth under oxidative stress conditions and lower concentrations of reduced 2-Cys peroxiredoxin in cells. Taken together, the present in vitro and in vivo results indicate that NTRC is a significant electron donor for 2-Cys peroxiredoxin and plays a pivotal role in antioxidant defense systems in Anabaena 7120 cells.

Role of sulfiredoxin as a peroxiredoxin-2 denitrosylase in human iPSC-derived dopaminergic neurons

Recent studies have pointed to protein S-nitrosylation as a critical regulator of cellular redox homeostasis. For example, S-nitrosylation of peroxiredoxin-2 (Prx2), a peroxidase widely expressed in mammalian neurons, inhibits both enzymatic activity and protective function against oxidative stress. Here, using in vitro and in vivo approaches, we identify a role and reaction mechanism of the reductase sulfiredoxin (Srxn1) as an enzyme that denitrosylates (thus removing -SNO) from Prx2 in an ATP-dependent manner. Accordingly, by decreasing S-nitrosylated Prx2 (SNO-Prx2), overexpression of Srxn1 protects dopaminergic neural cells and human-induced pluripotent stem cell (hiPSC)-derived neurons from NO-induced hypersensitivity to oxidative stress. The pathophysiological relevance of this observation is suggested by our finding that SNO-Prx2 is dramatically increased in murine and human Parkinson's disease (PD) brains. Our findings therefore suggest that Srxn1 may represent a therapeutic target for neurodegenerative disorders such as PD that involve nitrosative/oxidative stress.

NADPH-Thioredoxin Reductase C Mediates the Response to Oxidative Stress and Thermotolerance in the Cyanobacterium Anabaena sp. PCC7120

NADPH-thioredoxin reductase C (NTRC) is a bimodular enzyme composed of an NADPH-thioredoxin reductase and a thiioredoxin domain extension in the same protein. In plants, NTRC has been described to be involved in the protection of the chloroplast against oxidative stress damage through reduction of the 2-Cys peroxiredoxin (2-Cys Prx) as well as through other functions related to redox enzyme regulation. In cyanobacteria, the Anabaena NTRC has been characterized in vitro, however, nothing was known about its in vivo function. In order to study that, we have generated the first knockout mutant strain (ΔntrC), apart from the previously described in Arabidopsis. Detailed characterization of this strain reveals a differential sensitivity to oxidative stress treatments with respect to the wild-type Anabaena strain, including a higher level of ROS (reactive oxygen species) in normal growth conditions. In the mutant strain, different oxidative stress treatments such as hydrogen peroxide, methyl-viologen or high light irradiance provoke an increase in the expression of genes related to ROS detoxification, including AnNTRC and peroxiredoxin genes, with a concomitant increase in the amount of AnNTRC and 2-Cys Prx. Moreover, the role of AnNTRC in the antioxidant response is confirmed by the observation of a pronounced overoxidation of the 2-Cys Prx and a time-delay recovery of the reduced form of this protein upon oxidative stress treatments. Our results suggest the participation of this enzyme in the peroxide detoxification in Anabaena. In addition, we describe the role of Anabaena NTRC in thermotolerance, by the appearance of high molecular mass AnNTRC complexes, showing that the mutant strain is more sensitive to high temperature treatments.

Quantitative measures for redox signaling

Redox signaling is now recognized as an important regulatory mechanism for a number of cellular processes including the antioxidant response, phosphokinase signal transduction and redox metabolism. While there has been considerable progress in identifying the cellular machinery involved in redox signaling, quantitative measures of redox signals have been lacking, limiting efforts aimed at understanding and comparing redox signaling under normoxic and pathogenic conditions. Here we have outlined some of the accepted principles for redox signaling, including the description of hydrogen peroxide as a signaling molecule and the role of kinetics in conferring specificity to these signaling events. Based on these principles, we then develop a working definition for redox signaling and review a number of quantitative methods that have been employed to describe signaling in other systems. Using computational modeling and published data, we show how time- and concentration- dependent analyses, in particular, could be used to quantitatively describe redox signaling and therefore provide important insights into the functional organization of redox networks. Finally, we consider some of the key challenges with implementing these methods.

A Salt-Inducible Mn-Catalase (KatB) Protects Cyanobacterium from Oxidative Stress

Catalases, enzymes that detoxify H2O2, are widely distributed in all phyla, including cyanobacteria. Unlike the heme-containing catalases, the physiological roles of Mn-catalases remain inadequately characterized. In the cyanobacterium Anabaena, pretreatment of cells with NaCl resulted in unusually enhanced tolerance to oxidative stress. On exposure to H2O2, the NaCl-treated Anabaena showed reduced formation of reactive oxygen species, peroxides, and oxidized proteins than the control cells (i.e. not treated with NaCl) exposed to H2O2. This protective effect correlated well with the substantial increase in production of KatB, a Mn-catalase. Addition of NaCl did not safeguard the katB mutant from H2O2, suggesting that KatB was indeed responsible for detoxifying the externally added H2O2. Moreover, Anabaena deficient in KatB was susceptible to oxidative effects of salinity stress. The katB gene was strongly induced in response to osmotic stress or desiccation. Promoter-gfp analysis showed katB to be expressed only in the vegetative cells but not in heterocysts. Biochemically, KatB was an efficient, robust catalase that remained active in the presence of high concentrations of NaCl. Our findings unravel the role of Mn-catalase in acclimatization to salt/oxidative stress and demonstrate that the oxidative stress resistance of an organism can be enhanced by a simple compound such as NaCl.

Kinetic analysis of structural influences on the susceptibility of peroxiredoxins 2 and 3 to hyperoxidation

Mammalian 2-cysteine peroxiredoxins (Prxs) are susceptible to hyperoxidation by excess H2O2. The cytoplasmic family member Prx2 hyperoxidizes more readily than mitochondrial Prx3 due to slower dimerization of the sulfenic acid (SpOH) intermediate. Four variant amino acids near the C-terminus have been shown to contribute to this difference. We have performed kinetic analysis of the relationship between hyperoxidation and disulfide formation, using whole-protein MS and comparing wild-type (WT) Prx2 and Prx3 with tail-swap mutants in which the four amino acids were reversed. These changes make Prx3 more sensitive and Prx2 less sensitive to hyperoxidation and accounted for ∼70% of the difference between the two proteins. The tail swap mutant of Prx3 was also more susceptible when expressed in the mitochondria of HeLa cells. The hyperoxidized product at lower excesses of H2O2 was a semi-hyperoxidized dimer with one active site disulfide and the other a sulfinic acid. For Prx2, increasing the H2O2 concentration resulted in complete hyperoxidation. In contrast, only approximately half the Prx3 active sites underwent hyperoxidation and, even with high H2O2, the predominant product was the hyperoxidized dimer. Size exclusion chromatography (SEC) showed that the oligomeric forms of all redox states of Prx3 dissociated more readily into dimeric units than their Prx2 counterparts. Notably the species with one disulfide and one hyperoxidized active site was decameric for Prx2 and dimeric for Prx3. Reduction and re-oxidation of the hyperoxidized dimer of Prx3 produced hyperoxidized monomers, implying dissociation and rearrangement of the subunits of the functional homodimer.

The sulfiredoxin-peroxiredoxin (Srx-Prx) axis in cell signal transduction and cancer development

Redox signaling is a critical component of cell signaling pathways that are involved in the regulation of cell growth, metabolism, hormone signaling, immune regulation and variety of other physiological functions. Peroxiredoxin (Prx) is a family of thiol-based peroxidase that acts as a regulator of redox signaling. Members of Prx family can act as antioxidants and chaperones. Sulfiredoxin (Srx) is an antioxidant protein that exclusively reduces over-oxidized typical 2-Cys Prx. Srx has different affinities for individual Prx and it also catalyzes the deglutathionylation of variety of substrates. Individual component of the Srx-Prx system plays critical role in carcinogenesis by modulating cell signaling pathways involved in cell proliferation, migration and metastasis. Expression levels of individual component of the Srx-Prx axis have been correlated with patient survival outcome in multiple cancer types. This review will summarize the molecular basis of differences in the affinity of Srx for individual Prx and the role of individual component of the Srx-Prx system in tumor progression and metastasis. This enhanced understanding of molecular aspects of Srx-Prx interaction and its role in cell signal transduction will help define the Srx-Prx system as a future therapeutic target in human cancer.

Unraveling the mechanism responsible for the contrasting tolerance of Synechocystis and Synechococcus to Cr(VI): Enzymatic and non-enzymatic antioxidants

Two unicellular cyanobacteria, Synechocystis and Synechococcus, showed contrasting tolerance to Cr(VI); with Synechococcus being 12-fold more tolerant than Synechocystis to potassium dichromate. The mechanism responsible for this differential sensitivity to Cr(VI) was explored in this study. Total content of photosynthetic pigments as well as photosynthetic activity decreased at lower concentration of Cr(VI) in Synechocystis as compared to Synechococcus. Experiments with (51)Cr showed Cr to accumulate intracellularly in both the cyanobacteria. At lower concentrations, Cr(VI) caused excessive ROS generation in Synechocystis as compared to that observed in Synechococcus. Intrinsic levels of enzymatic antioxidants, i.e., superoxide dismutase, catalase and 2-Cys-peroxiredoxin were considerably higher in Synechococcus than Synechocystis. Content of total thiols (both protein as well as non-protein) and reduced glutathione (GSH) was also higher in Synechococcus as compared to Synechocystis. This correlated well with higher content of carbonylated proteins observed in Synechocystis than Synechococcus. Additionally, in contrast to Synechocystis, Synechococcus exhibited better tolerance to other oxidative stresses like high intensity light and H2O2. The data indicate that the disparity in the ability to detoxify ROS could be the primary mechanism responsible for the differential tolerance of these cyanobacteria to Cr(VI).

The contribution of NADPH thioredoxin reductase C (NTRC) and sulfiredoxin to 2-Cys peroxiredoxin overoxidation in Arabidopsis thaliana chloroplasts

Hydrogen peroxide is a harmful by-product of photosynthesis, which also has important signalling activity. Therefore, the level of hydrogen peroxide needs to be tightly controlled. Chloroplasts harbour different antioxidant systems including enzymes such as the 2-Cys peroxiredoxins (2-Cys Prxs). Under oxidizing conditions, 2-Cys Prxs are susceptible to inactivation by overoxidation of their peroxidatic cysteine, which is enzymatically reverted by sulfiredoxin (Srx). In chloroplasts, the redox status of 2-Cys Prxs is highly dependent on NADPH-thioredoxin reductase C (NTRC) and Srx; however, the relationship of these activities in determining the level of 2-Cys Prx overoxidation is unknown. Here we have addressed this question by a combination of genetic and biochemical approaches. An Arabidopsis thaliana double knockout mutant lacking NTRC and Srx shows a phenotype similar to the ntrc mutant, while the srx mutant resembles wild-type plants. The deficiency of NTRC causes reduced overoxidation of 2-Cys Prxs, whereas the deficiency of Srx has the opposite effect. Moreover, in vitro analyses show that the disulfide bond linking the resolving and peroxidatic cysteines protects the latter from overoxidation, thus explaining the dominant role of NTRC on the level of 2-Cys Prx overoxidation in vivo. The overoxidation of chloroplast 2-Cys Prxs shows no circadian oscillation, in agreement with the fact that neither the NTRC nor the SRX genes show circadian regulation of expression. Additionally, the low level of 2-Cys Prx overoxidation in the ntrc mutant is light dependent, suggesting that the redox status of 2-Cys Prxs in chloroplasts depends on light rather than the circadian clock.

Evidence that glutathione and the glutathione system efficiently recycle 1-cys sulfiredoxin in vivo

AIMS

Typical 2-Cys peroxiredoxins (2-Cys Prxs) are Cys peroxidases that undergo inactivation by hyperoxidation of the catalytic Cys, a modification reversed by ATP-dependent reduction by sulfiredoxin (Srx). Such an attribute is thought to provide regulation of 2-Cys Prxs functions. The initial steps of the Srx catalytic mechanism lead to a Prx/Srx thiolsulfinate intermediate that must be reduced to regenerate Srx. In Saccharomyces cerevisiae Srx, the thiolsulfinate is resolved by an extra Cys (Cys48) that is absent in mammalian, plant, and cyanobacteria Srxs (1-Cys Srxs). We have addressed the mechanism of reduction of 1-Cys Srxs using S. cerevisiae Srx mutants lacking Cys48 as a model.

RESULTS

We have tested the recycling of Srx by glutathione (GSH) by a combination of in vitro steady-state and single-turnover kinetic analyses, using enzymatic coupled assays, Prx fluorescence, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and reverse-phase chromatography coupled to mass spectrometry. We demonstrate that GSH reacts directly with the thiolsulfinate intermediate, by following saturation kinetics with an apparent dissociation constant of 34 μM, while producing S-glutathionylated Srx as a catalytic intermediate which is efficiently reduced by the glutaredoxin/glutathione reductase system. Total cellular depletion of GSH impacted the recycling of Srx, confirming in vivo that GSH is the physiologic reducer of 1-Cys Srx.

INNOVATION

Our study suggests that GSH binds to the thiolsulfinate complex, thus allowing non-rate limiting reduction. Such a structural recognition of GSH enables an efficient catalytic reduction, even at very low GSH cellular levels.

CONCLUSION

This study provides both in vitro and in vivo evidence of the role of GSH as the primary reducer of 1-Cys Srxs.

Redox-dependent chaperone/peroxidase function of 2-Cys-Prx from the cyanobacterium Anabaena PCC7120: role in oxidative stress tolerance

BACKGROUND

Cyanobacteria, progenitors of plant chloroplasts, provide a suitable model system for plants to study adaptation towards different abiotic stresses. Genome of the filamentous, heterocystous, nitrogen-fixing cyanobacterium Anabaena PCC7120 harbours a single gene (alr4641) encoding a typical 2-Cys-Peroxiredoxins (2-Cys-Prxs). 2-Cys-Prxs are thiol-based peroxidases that also function as molecular chaperones in plants and other systems. The Alr4641 protein from Anabaena PCC7120 shows high level biochemical similarities with the plant 2-Cys-Prx. The physiological role played by the Alr4641 protein in Anabaena was addressed in this study.

RESULTS

In Anabaena PCC7120, alr4641 transcript /Alr4641 protein was induced in response to abiotic stresses and its promoter was active in the vegetative cells as well as heterocysts. The wild-type Alr4641 protein or Alr4641 lacking the peroxidatic cysteine (Alr4641C56S) or the resolving cysteine (Alr4641C178S) existed as higher oligomers in their native form. The wild-type or the mutant Alr4641 proteins showed similar chaperone activity, but only the wild-type protein exhibited peroxidase activity indicating that unlike peroxidase activity, chaperone activity was not dependent on cysteines. In contrast to other 2-Cys-Prxs, chaperone/peroxidase activity of Alr4641 was dependent on its redox state and not oligomerization status. Alr4641 could protect plasmid DNA from oxidative damage and physically associate with NADPH-dependent thioredoxin reductase (NTRC). Like 2-Cys-Prxs from plants (e.g. rice), Alr4641 could detoxify various peroxides using NTRC as reductant. On exposure to H2O2, recombinant Anabaena PCC7120 strain over-expressing Alr4641 (An4641+) showed reduced content of reactive oxygen species (ROS), intact photosynthetic functions and consequently better survival than the wild-type Anabaena PCC7120, indicating that Alr4641 can protect Anabaena from oxidative stress.

CONCLUSIONS

The peroxidase/chaperone function of Alr4641, its inherent transcriptional/translational induction under different abiotic stresses and localization in both vegetative cells and heterocysts could be an adaptive strategy to battle various oxidative stresses that Anabaena encounters during its growth. Moreover, the recombinant Anabaena strain over expressing Alr4641 showed higher resistance to oxidative stress, suggesting its potential to serve as stress-tolerant biofertilizers in rice fields.

The Anabaena sp. PCC 7120 Exoproteome: Taking a Peek outside the Box

The interest in examining the subset of proteins present in the extracellular milieu, the exoproteome, has been growing due to novel insights highlighting their role on extracellular matrix organization and biofilm formation, but also on homeostasis and development. The cyanobacterial exoproteome is poorly studied, and the role of cyanobacterial exoproteins on cell wall biogenesis, morphology and even physiology is largely unknown. Here, we present a comprehensive examination of the Anabaena sp. PCC 7120 exoproteome under various growth conditions. Altogether, 139 proteins belonging to 16 different functional categories have been identified. A large fraction (48%) of the identified proteins is classified as "hypothetical", falls into the "other categories" set or presents no similarity to other proteins. The evidence presented here shows that Anabaena sp. PCC 7120 is capable of outer membrane vesicle formation and that these vesicles are likely to contribute to the exoproteome profile. Furthermore, the activity of selected exoproteins associated with oxidative stress has been assessed, suggesting their involvement in redox homeostasis mechanisms in the extracellular space. Finally, we discuss our results in light of other cyanobacterial exoproteome studies and focus on the potential of exploring cyanobacteria as cell factories to produce and secrete selected proteins.

Tuning of peroxiredoxin catalysis for various physiological roles

Peroxiredoxins (Prxs) make up an ancient family of enzymes that are the predominant peroxidases for nearly all organisms and play essential roles in reducing hydrogen peroxide, organic hydroperoxides, and peroxynitrite. Even between distantly related organisms, the core protein fold and key catalytic residues related to its cysteine-based catalytic mechanism have been retained. Given that these enzymes appeared early in biology, Prxs have experienced more than 1 billion years of optimization for specific ecological niches. Although their basic enzymatic function remains the same, Prxs have diversified and are involved in roles such as protecting DNA against mutation, defending pathogens against host immune responses, suppressing tumor formation, and--for eukaryotes--helping regulate peroxide signaling via hyperoxidation of their catalytic Cys residues. Here, we review the current understanding of the physiological roles of Prxs by analyzing knockout and knockdown studies from ∼25 different species. We also review what is known about the structural basis for the sensitivity of some eukaryotic Prxs to inactivation by hyperoxidation. In considering the physiological relevance of hyperoxidation, we explore the distribution across species of sulfiredoxin (Srx), the enzyme responsible for rescuing hyperoxidized Prxs. We unexpectedly find that among eukaryotes appearing to have a "sensitive" Prx isoform, some do not contain Srx. Also, as Prxs are suggested to be promising targets for drug design, we discuss the rationale behind recently proposed strategies for their selective inhibition.

Kinetic and structural characterization of a typical two-cysteine peroxiredoxin from Leptospira interrogans exhibiting redox sensitivity

Little is known about the mechanisms by which Leptospira interrogans, the causative agent of leptospirosis, copes with oxidative stress at the time it establishes persistent infection within its human host. We report the molecular cloning of a gene encoding a 2-Cys peroxiredoxin (LinAhpC) from this bacterium. After bioinformatic analysis we found that LinAhpC contains the characteristic GGIG and YF motifs present in peroxiredoxins that are sensitive to overoxidation (mainly eukaryotic proteins). These motifs are absent in insensitive prokaryotic enzymes. Recombinant LinAhpC showed activity as a thioredoxin peroxidase with sensitivity to overoxidation by H2O2 (Chyp 1% ~30 µM at pH 7.0 and 30°C). So far, Anabaena 2-Cys peroxiredoxin, Helicobacter pylori AhpC, and LinAhpC are the only prokaryotic enzymes studied with these characteristics. The properties determined for LinAhpC suggest that the protein could be critical for the antioxidant defense capacity in L. interrogans.

Evolutionary development of redox regulation in chloroplasts

SIGNIFICANCE

The post-translational modification of thiol groups stands out as a key strategy that cells employ for metabolic regulation and adaptation to changing environmental conditions. Nowhere is this more evident than in chloroplasts-the O2-evolving photosynthetic organelles of plant cells that are fitted with multiple redox systems, including the thioredoxin (Trx) family of oxidoreductases functional in the reversible modification of regulatory thiols of proteins in all types of cells. The best understood member of this family in chloroplasts is the ferredoxin-linked thioredoxin system (FTS) by which proteins are modified via light-dependent disulfide/dithiol (S-S/2SH) transitions.

RECENT ADVANCES

Discovered in the reductive activation of enzymes of the Calvin-Benson cycle in illuminated chloroplast preparations, recent studies have extended the role of the FTS far beyond its original boundaries to include a spectrum of cellular processes. Together with the NADP-linked thioredoxin reductase C-type (NTRC) and glutathione/glutaredoxin systems, the FTS also plays a central role in the response of chloroplasts to different types of stress.

CRITICAL ISSUES

The comparisons of redox regulatory networks functional in chloroplasts of land plants with those of cyanobacteria-prokaryotes considered to be the ancestors of chloroplasts-and different types of algae summarized in this review have provided new insight into the evolutionary development of redox regulation, starting with the simplest O2-evolving organisms.

FUTURE DIRECTIONS

The evolutionary appearance, mode of action, and specificity of the redox regulatory systems functional in chloroplasts, as well as the types of redox modification operating under diverse environmental conditions stand out as areas for future study.

Observed octameric assembly of a Plasmodium yoelii peroxiredoxin can be explained by the replacement of native "ball-and-socket" interacting residues by an affinity tag

Peroxiredoxins (Prxs) are ubiquitous and efficient antioxidant enzymes crucial for redox homeostasis in most organisms, and are of special importance for disease-causing parasites that must protect themselves against the oxidative weapons of the human immune system. Here, we describe reanalyses of crystal structures of two Prxs from malaria parasites. In addition to producing improved structures, we provide normalizing explanations for features that had been noted as unusual in the original report of these structures (Qiu et al., BMC Struct Biol 2012;12:2). Most importantly, we provide evidence that the unusual octameric assembly seen for Plasmodium yoelii Prx1a is not physiologically relevant, but arises because the structure is not of authentic P. yoelii Prx1a, but a variant we designate PyPrx1a(N*) that has seven native N-terminal residues replaced by an affinity tag. This N-terminal modification disrupts a previously unrecognized, hydrophobic "ball-and-socket" interaction conserved at the B-type dimer interface of Prx1 subfamily enzymes, and is accommodated by a fascinating two-residue "β-slip" type register shift in the β-strand association at a dimer interface. The resulting change in the geometry of the dimer provides a simple explanation for octamer formation. This study illustrates how substantive impacts can occur in protein variants in which native residues have been altered.

Structural basis of redox-dependent modulation of galectin-1 dynamics and function

Galectin-1 (Gal-1), a member of a family of multifunctional lectins, plays key roles in diverse biological processes including cell signaling, immunomodulation, neuroprotection and angiogenesis. The presence of an unusual number of six cysteine residues within Gal-1 sequence prompted a detailed analysis of the impact of the redox environment on the functional activity of this lectin. We examined the role of each cysteine residue in the structure and function of Gal-1 using both experimental and computational approaches. Our results show that: (i) only three cysteine residues present in each carbohydrate recognition domain (CRD) (Cys2, Cys16 and Cys88) were important in protein oxidation, (ii) oxidation promoted the formation of the Cys16-Cys88 disulfide bond, as well as multimers through Cys2, (iii) the oxidized protein did not bind to lactose, probably due to poor interactions with Arg48 and Glu71, (iv) in vitro oxidation by air was completely reversible and (v) oxidation by hydrogen peroxide was relatively slow (1.7 ± 0.2 M(-1) s(-1) at pH 7.4 and 25°C). Finally, an analysis of key cysteines in other human galectins is also provided in order to predict their behaviour in response to redox variations. Collectively, our data provide new insights into the structural basis of Gal-1 redox regulation with critical implications in physiology and pathology.

Dissecting the integrative antioxidant and redox systems in plant mitochondria. Effect of stress and S-nitrosylation

Mitochondrial respiration provides the energy needed to drive metabolic and transport processes in cells. Mitochondria are a significant site of reactive oxygen species (ROS) production in plant cells, and redox-system components obey fine regulation mechanisms that are essential in protecting the mitochondrial integrity. In addition to ROS, there are compelling indications that nitric oxide can be generated in this organelle by both reductive and oxidative pathways. ROS and reactive nitrogen species play a key role in signaling but they can also be deleterious via oxidation of macromolecules. The high production of ROS obligates mitochondria to be provided with a set of ROS scavenging mechanisms. The first line of mitochondrial antioxidants is composed of superoxide dismutase and the enzymes of the ascorbate-glutathione cycle, which are not only able to scavenge ROS but also to repair cell damage and possibly serve as redox sensors. The dithiol-disulfide exchanges form independent signaling nodes and act as antioxidant defense mechanisms as well as sensor proteins modulating redox signaling during development and stress adaptation. The presence of thioredoxin (Trx), peroxiredoxin (Prx) and sulfiredoxin (Srx) in the mitochondria has been recently reported. Cumulative results obtained from studies in salt stress models have demonstrated that these redox proteins play a significant role in the establishment of salt tolerance. The Trx/Prx/Srx system may be subjected to a fine regulated mechanism involving post-translational modifications, among which S-glutathionylation and S-nitrosylation seem to exhibit a critical role that is just beginning to be understood. This review summarizes our current knowledge in antioxidative systems in plant mitochondria, their interrelationships, mechanisms of compensation and some unresolved questions, with special focus on their response to abiotic stress.

Oxidative stress management in the filamentous, heterocystous, diazotrophic cyanobacterium, Anabaena PCC7120

Reactive oxygen species (ROS) are inevitably generated as by-products of respiratory/photosynthetic electron transport in oxygenic photoautotrophs. Unless effectively scavenged, these ROS can damage all cellular components. The filamentous, heterocystous, nitrogen-fixing strains of the cyanobacterium, Anabaena, serve as naturally abundant contributors of nitrogen biofertilizers in tropical rice paddy fields. Anabaena strains are known to tolerate several abiotic stresses, such as heat, UV, gamma radiation, desiccation, etc., that are known to generate ROS. ROS are detoxified by specific antioxidant enzymes like superoxide dismutases (SOD), catalases and peroxiredoxins. The genome of Anabaena PCC7120 encodes two SODs, two catalases and seven peroxiredoxins, indicating the presence of an elaborate antioxidant enzymatic machinery to defend its cellular components from ROS. This article summarizes recent findings and depicts important perspectives in oxidative stress management in Anabaena PCC7120.