Verification of the Interaction of a Tryparedoxin Peroxidase with Tryparedoxin by ESI-MS/MS

Looking Back at the Early Stages of Redox Biology

The beginnings of redox biology are recalled with special emphasis on formation, metabolism and function of reactive oxygen and nitrogen species in mammalian systems. The review covers the early history of heme peroxidases and the metabolism of hydrogen peroxide, the discovery of selenium as integral part of glutathione peroxidases, which expanded the scope of the field to other hydroperoxides including lipid hydroperoxides, the discovery of superoxide dismutases and superoxide radicals in biological systems and their role in host defense, tissue damage, metabolic regulation and signaling, the identification of the endothelial-derived relaxing factor as the nitrogen monoxide radical (more commonly named nitric oxide) and its physiological and pathological implications. The article highlights the perception of hydrogen peroxide and other hydroperoxides as signaling molecules, which marks the beginning of the flourishing fields of redox regulation and redox signaling. Final comments describe the development of the redox language. In the 18th and 19th century, it was highly individualized and hard to translate into modern terminology. In the 20th century, the redox language co-developed with the chemical terminology and became clearer. More recently, the introduction and inflationary use of poorly defined terms has unfortunately impaired the understanding of redox events in biological systems.

Catalysis of Peroxide Reduction by Fast Reacting Protein Thiols

Life on Earth evolved in the presence of hydrogen peroxide, and other peroxides also emerged before and with the rise of aerobic metabolism. They were considered only as toxic byproducts for many years. Nowadays, peroxides are also regarded as metabolic products that play essential physiological cellular roles. Organisms have developed efficient mechanisms to metabolize peroxides, mostly based on two kinds of redox chemistry, catalases/peroxidases that depend on the heme prosthetic group to afford peroxide reduction and thiol-based peroxidases that support their redox activities on specialized fast reacting cysteine/selenocysteine (Cys/Sec) residues. Among the last group, glutathione peroxidases (GPxs) and peroxiredoxins (Prxs) are the most widespread and abundant families, and they are the leitmotif of this review. After presenting the properties and roles of different peroxides in biology, we discuss the chemical mechanisms of peroxide reduction by low molecular weight thiols, Prxs, GPxs, and other thiol-based peroxidases. Special attention is paid to the catalytic properties of Prxs and also to the importance and comparative outlook of the properties of Sec and its role in GPxs. To finish, we describe and discuss the current views on the activities of thiol-based peroxidases in peroxide-mediated redox signaling processes.

Peroxiredoxins: guardians against oxidative stress and modulators of peroxide signaling

Peroxiredoxins (Prxs) are a ubiquitous family of cysteine-dependent peroxidase enzymes that play dominant roles in regulating peroxide levels within cells. These enzymes, often present at high levels and capable of rapidly clearing peroxides, display a remarkable array of variations in their oligomeric states and susceptibility to regulation by hyperoxidative inactivation and other post-translational modifications. Key conserved residues within the active site promote catalysis by stabilizing the transition state required for transferring the terminal oxygen of hydroperoxides to the active site (peroxidatic) cysteine residue. Extensive investigations continue to expand our understanding of the scope of their importance as well as the structures and forces at play within these critical defense and regulatory enzymes.

A primer on peroxiredoxin biochemistry

Peroxiredoxins were not recognized as a family of enzymes until the 1990s but are now known to be the dominant peroxidases in most organisms. Here, the history and fundamental properties of peroxiredoxins are briefly reviewed, with a special focus on describing how an exquisitely tunable balance between fully folded and locally unfolded conformations plays a large role in peroxiredoxin catalytic properties.

The crystal structures of the tryparedoxin-tryparedoxin peroxidase couple unveil the structural determinants of Leishmania detoxification pathway

Leishmaniasis is a neglected disease caused by Leishmania, an intracellular protozoan parasite which possesses a unique thiol metabolism based on trypanothione. Trypanothione is used as a source of electrons by the tryparedoxin/tryparedoxin peroxidase system (TXN/TXNPx) to reduce the hydroperoxides produced by macrophages during infection. This detoxification pathway is not only unique to the parasite but is also essential for its survival; therefore, it constitutes a most attractive drug target. Several forms of TXNPx, with very high sequence identity to one another, have been found in Leishmania strains, one of which has been used as a component of a potential anti-leishmanial polyprotein vaccine. The structures of cytosolic TXN and TXNPx from L. major (LmTXN and LmTXNPx) offer a unique opportunity to study peroxide reduction in Leishmania parasites at a molecular level, and may provide new tools for multienzyme inhibition-based drug discovery. Structural analyses bring out key structural features to elucidate LmTXN and LmTXNPx function. LmTXN displays an unusual N-terminal α-helix which allows the formation of a stable domain-swapped dimer. In LmTXNPx, crystallized in reducing condition, both the locally unfolded (LU) and fully folded (FF) conformations, typical of the oxidized and reduced protein respectively, are populated. The structural analysis presented here points to a high flexibility of the loop that includes the peroxidatic cysteine which facilitates Cys52 to form an inter-chain disulfide bond with the resolving cysteine (Cys173), thereby preventing over-oxidation which would inactivate the enzyme. Analysis of the electrostatic surface potentials of both LmTXN and LmTXNPx unveils the structural elements at the basis of functionally relevant interaction between the two proteins. Finally, the structural analysis of TXNPx allows us to identify the position of the epitopes that make the protein antigenic and therefore potentially suitable to be used in an anti-leishmanial polyprotein vaccine.

Leishmania spp. are protozoa responsible for Leishmaniases, neglected diseases killing up to 60,000 people every year. Current therapies rely mainly on antimonial drugs that are inadequate due to poor drug efficacy and safety, combined with increasing drug resistance. To overcome these problems, there is an urgent need to find new and more affordable drugs. Leishmania reduces the hydrogen peroxide produced by macrophages during the infection by means of the tryparedoxin/tryparedoxin peroxidase couple. The two enzymes are potentially suitable drug targets since they are both necessary for parasite survival and absent in the human host. To understand the molecular basis of peroxide reduction in the Leishmania parasites, we have solved the X-ray crystal structures of both enzymes. Structural analyses highlight oligomerization of the two proteins and allow the regions responsible for their interaction to be identified. Moreover, based on the X-ray structures and on electronic microscopy data present in literature for the homologous proteins from Trypanosoma brucei, we have generated a model of interaction between tryparedoxin and tryparedoxin peroxidase from L. major. From the X-ray structure and from this model, we have identified the epitopes of tryparedoxin peroxidase, which is part of a potential threecomponent vaccine that is presently being studied in animal models and in human.

Peroxiredoxins in parasites

SIGNIFICANCE

Parasite survival and virulence relies on effective defenses against reactive oxygen and nitrogen species produced by the host immune system. Peroxiredoxins (Prxs) are ubiquitous enzymes now thought to be central to such defenses and, as such, have potential value as drug targets and vaccine antigens.

RECENT ADVANCES

Plasmodial and kinetoplastid Prx systems are the most extensively studied, yet remain inadequately understood. For many other parasites our knowledge is even less well developed. Through parasite genome sequencing efforts, however, the key players are being discovered and characterized. Here we describe what is known about the biochemistry, regulation, and cell biology of Prxs in parasitic protozoa, helminths, and fungi. At least one Prx is found in each parasite with a sequenced genome, and a notable theme is the common patterns of expression, localization, and functionality among sequence-similar Prxs in related species.

CRITICAL ISSUES

The nomenclature of Prxs from parasites is in a state of disarray, causing confusion and making comparative inferences difficult. Here we introduce a systematic Prx naming convention that is consistent between organisms and informative about structural and evolutionary relationships.

FUTURE DIRECTIONS

The new nomenclature should stimulate the crossfertilization of ideas among parasitologists and with the broader redox research community. The diverse parasite developmental stages and host environments present complex systems in which to explore the variety of roles played by Prxs, with a view toward parlaying what is learned into novel therapies and vaccines that are urgently needed.

High throughput screening against the peroxidase cascade of African trypanosomes identifies antiparasitic compounds that inactivate tryparedoxin

In African trypanosomes, the detoxification of broad spectrum hydroperoxides relies on a unique cascade composed of trypanothione (T(SH)(2)), trypanothione reductase, tryparedoxin (Tpx), and nonselenium glutathione peroxidase-type enzymes. All three proteins are essential for Trypanosoma brucei. Here, we subjected the complete system to a high throughput screening approach with nearly 80,000 chemicals. Twelve compounds inhibited the peroxidase system. All but one carried chloroalkyl substituents. The detailed kinetic analysis showed that two compounds weakly inhibited trypanothione reductase, but none of them specifically interacted with the peroxidase. They proved to be time-dependent inhibitors of Tpx-modifying Cys-40, the first cysteine of its active site WCPPC motif. Importantly, gel shift assays verified Tpx as a target in the intact parasites. T(SH)(2), present in the in vitro assays and in the cells in high molar excess, did not interfere with Tpx inactivation. The compounds inhibited the proliferation of bloodstream T. brucei with EC(50) values down to <1 μM and exerted up to 83-fold lower toxicity toward HeLa cells. Irreversible inhibitors are traditionally regarded as unfavorable. However, a large number of antimicrobials and anticancer therapeutics acts covalently with their target protein. The compounds identified here also interacted with recombinant human thioredoxin, a distant relative of Tpx. This finding might even be exploited for thioredoxin-based anticancer drug development approaches reported recently. The fact that the T(SH)(2)/Tpx couple occupies a central position within the trypanosomal thiol metabolism and delivers electrons also for the synthesis of DNA precursors renders the parasite-specific oxidoreductase an attractive drug target molecule.

A comparison of thiol peroxidase mechanisms

Thiol peroxidases comprise glutathione peroxidases (GPx) and peroxiredoxins (Prx). The enzymes of both families reduce hydroperoxides with thiols by enzyme-substitution mechanisms. H(2)O(2) and organic hydroperoxides are reduced by all thiol peroxidases, most efficiently by SecGPxs, whereas fast peroxynitrite reduction is more common in Prxs. Reduction of lipid hydroperoxides is the domain of monomeric GPx4-type enzymes and of some Prxs. The catalysis starts with oxidation of an active-site selenocysteine (U(P)) or cysteine (C(P)). Activation of Cys (Sec) for hydroperoxide reduction in the GPx family is achieved by a typical tetrad composed of Cys (Sec), Asn, Gln, and Trp, whereas a triad of Cys Thr (or Ser) and Arg is the signature of Prx. In many of the CysGPxs and Prxs, a second Cys (C(R)) is required. In these 2-CysGPxs and 2-CysPrxs, the C(P) oxidized to a sulfenic acid forms an intra- or intermolecular disulfide (typical 2-CysPrx) with C(R), before a stepwise regeneration of ground-state enzyme by redoxin-type proteins can proceed. In SecGPxs and sporadically in Prxs, GSH is used as the reductant. Diversity combined with structural variability predestines thiol peroxidases for redox regulation via ROOH sensing and direct or indirect transduction of oxidant signals to specific protein targets.

Typical 2-Cys peroxiredoxins--modulation by covalent transformations and noncovalent interactions

2-Cys peroxiredoxins are peroxidases devoid of prosthetic groups that mediate in the defence against oxidative stress and the peroxide activation of signaling pathways. This dual capacity relies on the high reactivity of the conserved peroxidatic and resolving cysteines, whose modification embraces not only the usual thiol-disulfide exchange but also higher oxidation states of the sulfur atom. These changes are part of a complex system wherein the cooperation with other post-translational modifications - phosphorylation, acetylation - may function as major regulatory mechanisms of the quaternary structure. More importantly, modern proteomic approaches have identified the oxyacids at cysteine residues as novel protein targets for unsuspected post-translational modifications, such as phosphorylation that yields the unusual sulfi(o)nic-phosphoryl anhydride. In this article, we review the biochemical attributes of 2-Cys peroxiredoxins that, in combination with complementary studies of forward and reverse genetics, have generated stimulating molecular models to explain how this enzyme integrates into cell signaling in vivo.

Redox control in trypanosomatids, parasitic protozoa with trypanothione-based thiol metabolism

Trypanosomes and leishmania, the causative agents of several tropical diseases, possess a unique redox metabolism which is based on trypanothione. The bis(glutathionyl)spermidine is the central thiol that delivers electrons for the synthesis of DNA precursors, the detoxification of hydroperoxides and other trypanothione-dependent pathways. Many of the reactions are mediated by tryparedoxin, a distant member of the thioredoxin protein family. Trypanothione is kept reduced by the parasite-specific flavoenzyme trypanothione reductase. Since glutathione reductases and thioredoxin reductases are missing, the reaction catalyzed by trypanothione reductase represents the only connection between the NADPH- and the thiol-based redox metabolisms. Thus, cellular thiol redox homeostasis is maintained by the biosynthesis and reduction of trypanothione. Nearly all proteins of the parasite-specific trypanothione metabolism have proved to be essential.

ATP-dependent modulation and autophosphorylation of rapeseed 2-Cys peroxiredoxin

2-Cys peroxiredoxins (2-Cys Prx) are ubiquitous thiol-containing peroxidases that have been implicated in antioxidant defense and signal transduction. Although their biochemical features have been extensively studied, little is known about the mechanisms that link the redox activity and non-redox processes. Here we report that the concerted action of a nucleoside triphosphate and Mg(2+) on rapeseed 2-Cys Prx reversibly impairs the peroxidase activity and promotes the formation of high molecular mass species. Using protein intrinsic fluorescence in the analysis of site-directed mutants, we demonstrate that ATP quenches the emission intensity of Trp179, a residue close to the conserved Cys175. More importantly, we found that ATP facilitates the autophosphorylation of 2-Cys Prx when the protein is successively reduced with thiol-bearing compounds and oxidized with hydroperoxides or quinones. MS analyses reveal that 2-Cys Prx incorporates the phosphoryl group into the Cys175 residue yielding the sulfinic-phosphoryl [Prx-(Cys175)-SO(2)PO(3)(2-)] and the sulfonic-phosphoryl [Prx-(Cys175)-SO(3)PO(3)(2-)] anhydrides. Hence, the functional coupling between ATP and 2-Cys Prx gives novel insights into not only the removal of reactive oxygen species, but also mechanisms that link the energy status of the cell and the oxidation of cysteine residues.

Catalytic mechanism of the glutathione peroxidase-type tryparedoxin peroxidase of Trypanosoma brucei

Trypanosoma brucei, the causative agent of African sleeping sickness, encodes three nearly identical genes for cysteine-homologues of the selenocysteine-containing glutathione peroxidases. The enzymes, which are essential for the parasites, lack glutathione peroxidase activity but catalyse the trypanothione/Tpx (tryparedoxin)-dependent reduction of hydroperoxides. Cys47, Gln82 and Trp137 correspond to the selenocysteine, glutamine and tryptophan catalytic triad of the mammalian selenoenzymes. Site-directed mutagenesis revealed that Cys47 and Gln82 are essential. A glycine mutant of Trp137 had 13% of wild-type activity, which suggests that the aromatic residue may play a structural role but is not directly involved in catalysis. Cys95, which is conserved in related yeast and plant proteins but not in the mammalian selenoenzymes, proved to be essential as well. In contrast, replacement of the highly conserved Cys76 by a serine residue resulted in a fully active enzyme species and its role remains unknown. Thr50, proposed to stabilize the thiolate anion at Cys47, is also not essential for catalysis. Treatment of the C76S/C95S but not of the C47S/C76S double mutant with H2O2 induced formation of a sulfinic acid and covalent homodimers in accordance with Cys47 being the peroxidative active site thiol. In the wild-type peroxidase, these oxidations are prevented by formation of an intramolecular disulfide bridge between Cys47 and Cys95. As shown by MS, regeneration of the reduced enzyme by Tpx involves a transient mixed disulfide between Cys95 of the peroxidase and Cys40 of Tpx. The catalytic mechanism of the Tpx peroxidase resembles that of atypical 2-Cys-peroxiredoxins but is distinct from that of the selenoenzymes.

Cysteine reactivity and thiol-disulfide interchange pathways in AhpF and AhpC of the bacterial alkyl hydroperoxide reductase system

AhpC and AhpF from Salmonella typhimurium undergo a series of electron transfers to catalyze the pyridine nucleotide-dependent reduction of hydroperoxide substrates. AhpC, the peroxide-reducing (peroxiredoxin) component of this alkyl hydroperoxidase system, is an important scavenger of endogenous hydrogen peroxide in bacteria and acts through a reactive, peroxidatic cysteine, Cys46, and a second cysteine, Cys165, that forms an active site disulfide bond. AhpF, a separate disulfide reductase protein, regenerates AhpC every catalytic cycle via electrons from NADH which are transferred to AhpC through a tightly bound flavin and two disulfide centers, Cys345-Cys348 and Cys129-Cys132, through putative large domain movements. In order to assess cysteine reactivity and interdomain interactions in both proteins, a comprehensive set of single and double cysteine mutants (replacing cysteine with serine) of both proteins were prepared. Based on 5,5-dithiobis(2-nitrobenzoic acid) (DTNB) and AhpC reactivity with multiple mutants of AhpF, the thiolate of Cys129 in the N-terminal domain of AhpF initiates attack on Cys165 of the intersubunit disulfide bond within AhpC for electron transfer between proteins. Cys348 of AhpF has also been identified as the nucleophile attacking the Cys129 sulfur of the N-terminal disulfide bond to initiate electron transfer between these two redox centers. These findings support the modular architecture of AhpF and its need for domain rotations for function, and emphasize the importance of Cys165 in the reductive reactivation of AhpC. In addition, two new constructs have been generated, an AhpF-AhpC complex and a "twisted" form of AhpF, in which redox centers are locked together by stable disulfide bonds which mimic catalytic intermediates.

The catalytic mechanism of peroxiredoxins

Peroxiredoxins carry out the efficient reduction of a typically broad range of peroxide substrates through an absolutely conserved, activated cysteine residue within a highly conserved active site pocket structure. Though details of reductive recycling after cysteine sulfenic acid formation at the active site vary among members of different Prx classes, local unfolding around the active site cysteine is likely generally required in these proteins for disulfide bond formation with a second resolving cysteine and/or for access of the reductant to the oxidized active site. The conformational change associated with the catalytic cycle and the redox-dependent decamer formation occurring in at least some typical 2-Cys Prxs have interesting implications in the interplay between active site loop dynamics, oligomerization state, catalytic efficiency and propensity toward inactivation during turnover in these important antioxidant enzymes.

The Thioredoxin Specificity of Drosophila GPx: A Paradigm for a Peroxiredoxin-like Mechanism of many Glutathione Peroxidases

Some members of the glutathione peroxidase (GPx) family have been reported to accept thioredoxin as reducing substrate. However, the selenocysteine-containing ones oxidise thioredoxin (Trx), if at all, at extremely slow rates. In contrast, the Cys homolog of Drosophila melanogaster exhibits a clear preference for Trx, the net forward rate constant, k'(+2), for reduction by Trx being 1.5x10(6) M(-1) s(-1), but only 5.4 M(-1) s(-1) for glutathione. Like other CysGPxs with thioredoxin peroxidase activity, Drosophila melanogaster (Dm)GPx oxidized by H(2)O(2) contained an intra-molecular disulfide bridge between the active-site cysteine (C45; C(P)) and C91. Site-directed mutagenesis of C91 in DmGPx abrogated Trx peroxidase activity, but increased the rate constant for glutathione by two orders of magnitude. In contrast, a replacement of C74 by Ser or Ala only marginally affected activity and specificity of DmGPx. Furthermore, LC-MS/MS analysis of oxidized DmGPx exposed to a reduced Trx C35S mutant yielded a dead-end intermediate containing a disulfide between Trx C32 and DmGPx C91. Thus, the catalytic mechanism of DmGPx, unlike that of selenocysteine (Sec)GPxs, involves formation of an internal disulfide that is pivotal to the interaction with Trx. Hereby C91, like the analogous second cysteine in 2-cysteine peroxiredoxins, adopts the role of a "resolving" cysteine (C(R)). Molecular modeling and homology considerations based on 450 GPxs suggest peculiar features to determine Trx specificity: (i) a non-aligned second Cys within the fourth helix that acts as C(R); (ii) deletions of the subunit interfaces typical of tetrameric GPxs leading to flexibility of the C(R)-containing loop. Based of these characteristics, most of the non-mammalian CysGPxs, in functional terms, are thioredoxin peroxidases.

The trypanothione system

Trypanosomes and Leishmania, the causative agents of severe tropical diseases, employ 2-Cys-peroxiredoxins together with cysteine-homologues of glutathione peroxidases and ascorbate-dependent peroxidases for the detoxification of hydroperoxides. All three types of peroxidases gain their reducing equivalents from the parasite-specific dithiol trypanothione [bis(glutathionyl)spermidine]. Based on their primary structure and cellular localization, the trypanosomatid 2-Cys-peroxiredoxins are subdivided into two families that occur in the mitochondrion and cytosol of the parasites. In Trypanosoma brucei, the cytosolic 2-Cys-peroxiredoxin, as well as the glutathione peroxidase-type enzyme, is essential for cell viability. Despite overlapping substrate specificities and subcellular localizations, the two types of peroxidases can obviously not substitute for each other which suggests distinct cell-physiological roles.