Plasmodium falciparum 2-Cys peroxiredoxin reacts with plasmoredoxin and peroxynitrite

Neospora caninum peroxiredoxin 1 is an essential virulence effector with antioxidant function

Neospora caninum, an obligate intracellular parasitic protozoan discovered by Dubey in 1988, is the pathogen of neosporosis, which causes neurological symptoms in dogs and abortions in cows. Since there is no effective drug or vaccine against N. caninum, a deeper understanding of the molecules critical to parasite survival inside host cells is necessary. This study aimed to determine the role of N. caninum peroxiredoxin 1 (NcPrx1) in maintaining redox homeostasis and virulence of N. caninum. By determining the localization of NcPrx1 protein and establishing NcPrx1 gene knockout strain (ΔNcPrx1), the roles of NcPrx1 in N. caninum for invasion, replication, growth, oxidative stress, as well as pathogenicity were investigated. Our results showed that a predicted Alkyl Hydroperoxide1 (AHP1) domain was found in the amino acid sequence of NcPrx1, which displayed a high degree of similarity to homologs of several protozoa. Immunofluorescence assay (IFA) indicated that NcPrx1 was a cytoplasmic protein in N. caninum tachyzoites. Compared to wild type (WT) strain, ΔNcPrx1 strain showed reduced plaque area, invasion and egress rates. Reactive oxygen species (ROS) and malondialdehyde (MDA) were accumulated, and total antioxidant capacity (T-AOC) was attenuated in ΔNcPrx1 tachyzoites, which indicated that ΔNcPrx1 strain was more sensitive to oxidative stress. Furthermore, ΔNcPrx1 strain-infected C57BL/6 mice showed improved survival rate, reduced parasite burden, alleviated pathological changes in tissues, and decreased secretions of IL-6, IL-12, TNF-α, and IFN-γ in serum compared to the WT strain group. These findings suggested that NcPrx1 was a virulence factor of N. caninum which played an important role in maintaining the redox homeostasis of the parasite.

Crystal structure of plasmoredoxin, a redox-active protein unique for malaria parasites

Plasmoredoxin is a 22 ​kDa thiol–disulfide oxidoreductase involved in cellular redox regulatory processes and antioxidant defense. The 1.6 ​Å structure of the protein, solved via X-ray crystallography, adopts a modified thioredoxin fold. The structure reveals that plasmoredoxin, unique for malarial parasites, forms a new subgroup of thioredoxin-like proteins together with tryparedoxin, unique for kinetoplastids. Unlike most members of this superfamily, Plrx does not have a proline residue within the CxxC redox motif. In addition, the Plrx structure has a distinct C-terminal domain. Similar to human thioredoxin, plasmoredoxin forms monomers and dimers, which are also structurally similar to the human thioredoxin dimer, and, as in humans, plasmoredoxin is inactive as a dimer. Monomer–dimer equilibrium depends on the surrounding redox conditions, which could support the parasite in reacting to oxidative challenges. Based on structural considerations, the residues of the dimer interface are likely to interact with target proteins. In contrast to human and Plasmodium falciparum thioredoxin, however, there is a cluster of positively charged residues at the dimer interface of plasmoredoxin. These intersubunit (lysine) residues might allow binding of the protein to cellular membranes or to plasminogen. Malaria parasites lack catalase and glutathione peroxidase and therefore depend on their other glutathione and thioredoxin-dependent redox relays. Plasmoredoxin could be part of a so far unknown electron transfer system that only occurs in these parasites. Since the surface charge of plasmoredoxin differs significantly from other members of the thioredoxin superfamily, its three-dimensional structure can provide a model for designing selective redox-modulatory inhibitors.

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Two high resolution X-ray structures – confirmed that Plrx belongs to the thioredoxin superfamily.

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Structure and surface charge differ from the other members of the thioredoxin superfamily.

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The highest relationship in terms from sequence and structural fold is found with tryparedoxins.

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Similar to human thioredoxin, plasmoredoxin forms monomers and dimers.

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Potential as drug target.

Redox interactome in malaria parasite Plasmodium falciparum

The malaria-causing parasite Plasmodium falciparum is a severe threat to human health across the globe. This parasite alone causes the highest morbidity and mortality than any other species of Plasmodium. The parasites dynamically multiply in the erythrocytes of the vertebrate hosts, a large number of reactive oxygen species that damage biological macromolecules are produced in the cell during parasite growth. To relieve this intense oxidative stress, the parasite employs an NADPH-dependent thioredoxin and glutathione system that acts as an antioxidant and maintains redox status in the parasite. The mutual interaction of both redox proteins is involved in various biological functions and the survival of the erythrocytic stage of the parasite. Since the Plasmodium species is deficient in catalase and classical glutathione peroxidase, so their redox balance relies on a complex set of five peroxiredoxins, differentially positioned in the cytosol, mitochondria, apicoplast, and nucleus with partly overlapping substrate preferences. Moreover, Plasmodium falciparum possesses a set of members belonging to the thioredoxin superfamily, such as three thioredoxins, two thioredoxin-like proteins, one dithiol, three monocysteine glutaredoxins, and one redox-active plasmoredoxin with largely redundant functions. This review paper aims to discuss and encapsulate the biological function and current knowledge of the functional redox network of Plasmodium falciparum.

A Novel Thioredoxin-Dependent Peroxiredoxin (TPx-Q) Plays an Important Role in Defense Against Oxidative Stress and Is a Possible Drug Target in Babesia microti

Thioredoxin peroxidases (TPxs) are ubiquitous cysteine-based peroxidases that reduce peroxides as part of antioxidant defenses and redox signaling and are essential for Babesia microti protection against adverse environment agents like reactive oxygen species (ROS) and reactive nitrogen species (RNS). To better systematically understand TPxs, we identified a novel 2-Cys peroxiredoxin-Q (BmTPx-Q) of B. microti. The full-length BmTPx-Q gene is 653 bp that consists of an intact open reading frame of 594 bp that encodes a 197-amino acid protein. The predicted protein has a molecular weight of 22.3 kDa and an isoelectric point of 9.18. Moreover, BmTPx-Q showed low identity at the amino acid level to other peroxiredoxins (Prxs) among the currently known subfamilies. The recombinant BmTPx-Q protein (rBmTPx-Q) was expressed in Escherichia coli and purified with beads. The native protein BmTPx-Q was detected using mouse anti-BmTPx-Q polyclonal serum with western blotting and indirect immunofluorescence assay (IFA). In addition, enzyme activity was observed using nicotinamide adenine dinucleotide phosphate (NADPH) as substrate and triggered the NADPH-dependent reduction of the Trx/TrxR system. It was also discovered that BmTPx-Q mainly exists as a monomer whether under its native or functional states. In addition, when incubated with Chloroquine diphosphate salt for 24 h in vitro, the expression of BmTPx-Q showed a marked downward trend with the increase of drug concentration. These results suggest that B. microti uses BmTPx-Q to reduce and detoxify hydrogen peroxides to survive and proliferate inside the host. Furthermore, BmTPx-Q showed the lowest identity with host enzymes and could be a potential drug target for the development of novel strategies to control B. microti infection.

The interactome of 2-Cys peroxiredoxins in Plasmodium falciparum

Peroxiredoxins (Prxs) are crucially involved in maintaining intracellular H2O2homeostasis via their peroxidase activity. However, more recently, this class of proteins was found to also transmit oxidizing equivalents to selected downstream proteins, which suggests an important function of Prxs in the regulation of cellular protein redox relays. Using a pull-down assay based on mixed disulfide fishing, we characterized the thiol-dependent interactome of cytosolic Prx1a and mitochondrial Prx1m from the apicomplexan malaria parasitePlasmodium falciparum(Pf). Here, 127 cytosolic and 20 mitochondrial proteins that are components of essential cellular processes were found to interact withPfPrx1a andPfPrx1m, respectively. Notably, our data obtained with active-site mutants suggests that reducing equivalents might also be transferred from Prxs to target proteins. Initial functional analyses indicated that the interaction with Prx can strongly impact the activity of target proteins. The results provide initial insights into the interactome of Prxs at the level of a eukaryotic whole cell proteome. Furthermore, they contribute to our understanding of redox regulatory principles and thiol-dependent redox relays of Prxs in subcellular compartments.

Genome Editing of Babesia bovis Using the CRISPR/Cas9 System

Babesia bovis, the most virulent causative agent of bovine babesiosis, is prevalent in tropical and subtropical regions of the world. Although the whole-genome sequence was released more than a decade ago, functional analysis of the genomics of this parasite is hampered by the limited breadth of genetic engineering tools. In this study, we implemented the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system for B. bovis and demonstrated its potential for genome editing. Cas9 and human dihydrofolate reductase (hDHFR) were simultaneously expressed by the B. bovis elongation factor-1α bidirectional promoter, and a single guide RNA was expressed via the B. bovis U6 spliceosomal RNA promoter. Using a single plasmid construct, we were able to add an epitope tag to spherical body protein 3 (SBP3), introduce a point mutation into thioredoxin peroxidase 1 (tpx-1) to impair the function of the product, and replace the tpx-1 open reading frame with the other protein. Epitope tagging of SBP3 was efficient using this system, with a negligible number of remaining wild-type parasites and a pure transgenic population produced by allelic replacement of tpx-1 This advancement in genetic engineering tools for B. bovis will aid functional analysis of the genome and underpin characterization of candidate drug and vaccine targets.IMPORTANCE Babesia bovis is the most virulent cause of bovine babesiosis worldwide. The disease consequences are death, abortion, and economical loss due to reduced milk and meat production. Available vaccines are not effective, treatment options are limited, and emergence of drug and acaricide resistance has been reported from different regions. There is an urgent need to identify new drug and vaccine targets. Greater than half of the genes in B. bovis genome, including several expanded gene families which are unique for Babesia spp., have no predicted function. The available genetic engineering tools are based on conventional homologous recombination, which is time-consuming and inefficient. In this study, we adapted the CRISPR/Cas9 system as a robust genetic engineering tool for B. bovis This advancement will aid future functional studies of uncharacterized genes.

The Architecture of Thiol Antioxidant Systems among Invertebrate Parasites

The use of oxygen as the final electron acceptor in aerobic organisms results in an improvement in the energy metabolism. However, as a byproduct of the aerobic metabolism, reactive oxygen species are produced, leaving to the potential risk of an oxidative stress. To contend with such harmful compounds, living organisms have evolved antioxidant strategies. In this sense, the thiol-dependent antioxidant defense systems play a central role. In all cases, cysteine constitutes the major building block on which such systems are constructed, being present in redox substrates such as glutathione, thioredoxin, and trypanothione, as well as at the catalytic site of a variety of reductases and peroxidases. In some cases, the related selenocysteine was incorporated at selected proteins. In invertebrate parasites, antioxidant systems have evolved in a diversity of both substrates and enzymes, representing a potential area in the design of anti-parasite strategies. The present review focus on the organization of the thiol-based antioxidant systems in invertebrate parasites. Differences between these taxa and its final mammal host is stressed. An understanding of the antioxidant defense mechanisms in this kind of parasites, as well as their interactions with the specific host is crucial in the design of drugs targeting these organisms.

Identification and functional study of a novel 2-cys peroxiredoxin (BmTPx-1) of Babesia microti

Babesia microti is an emerging human pathogen and the primary causative agent of human babesiosis in many regions of the world. Although the peroxiredoxins (Prxs) or thioredoxin peroxidases (TPx) enzymes of this parasite have been sequenced and annotated, their biological properties remain largely unknown. Prxs are a family of antioxidant enzymes that protect biological molecules against metabolically produced reactive oxygen species (ROS) and reduce hydrogen peroxide (H₂O₂) to water in both eukaryotes and prokaryotes. In this study, TPx-1 cDNA was cloned from B. microti (designated BmTPx-1). Recombinant BmTPx-1 (rBmTPx-1) was expressed in Escherichia coli as a histidine fusion protein and purified using Ni-NTA His bind resin. To test the defense capacity of enzymatic antioxidants against the effect of ROS, a mixed-function oxidation system was utilized with the recombinant BmTPx-1 protein. A decreased ability of rBmTPx-1 to donate electrons to the thioredoxin (Trx)/TrxR reductase system was clarified by reaction with H₂O₂. These results suggest that BmTPx-1 has a great impact on protecting parasites from oxidative stress in the erythrocytic stage.

Typical 2-Cys peroxiredoxins in human parasites: Several physiological roles for a potential chemotherapy target

Peroxiredoxins (Prxs) are ubiquitary proteins able to play multiple physiological roles, that include thiol-dependent peroxidase, chaperone holdase, sensor of H2O2, regulator of H2O2-dependent signal cascades, and modulator of the immune response. Prxs have been found in a great number of human pathogens, both eukaryotes and prokaryotes. Gene knock-out studies demonstrated that Prxs are essential for the survival and virulence of at least some of the pathogens tested, making these proteins potential drug targets. However, the multiplicity of roles played by Prxs constitutes an unexpected obstacle to drug development. Indeed, selective inhibitors of some of the functions of Prxs are known (namely of the peroxidase and holdase functions) and are here reported. However, it is often unclear which function is the most relevant in each pathogen, hence which one is most desirable to inhibit. Indeed there are evidences that the main physiological role of Prxs may not be the same in different parasites. We here review which functions of Prxs have been demonstrated to be relevant in different human parasites, finding that the peroxidase and chaperone activities figure prominently, whereas other known functions of Prxs have rarely, if ever, been observed in parasites, or have largely escaped detection thus far.

Therapeutic potential of peroxynitrite decomposition catalysts: a patent review

INTRODUCTION

Peroxynitrite is a cytotoxic oxidant species implicated in a host of pathologies, including inflammatory and neurodegenerative diseases, cancer, radiation injury and chronic pain. With the recognition of the role of peroxynitrite in disease, numerous experimental and therapeutic tools have arisen to probe peroxyntirite's pathophysiological contribution and attenuate its oxidative damage. Peroxynitrite decomposition catalysts (PNDCs) are redox-active compounds that detoxify peroxynitrite by catalyzing its isomerization or reduction to nitrate or nitrite.

AREAS COVERED

This review discusses recent research articles and patents published 1995 - 2014 on the development and therapeutic use of PNDCs. Iron and manganese metalloporphyrin PNDCs attenuate the toxic effects of peroxynitrite and are currently being developed for clinical applications. Additionally, some Mn porphyrin-based PNDCs have optimized pharmaceutical properties such that they exhibit greater peroxynitrite selectivity. Other classes of PNDC agents, including bis(hydroxyphenyl)dipyrromethenes and metallocorroles, have demonstrated preclinical efficacy, oral availability and reduced toxicity risk.

EXPERT OPINION

Interest in the drug-like properties of peroxynitrite-neutralizing agents has grown with the realization that PNDCs will be powerful tools in the treatment of disease. The design of compounds with enhanced oral availability and peroxynitrite selectivity is a critical step toward the availability of safe, effective and selective redox modulators for the treatment of peroxynitrite-associated pathologies.

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.

Selection of genes for gene-environment interaction studies: a candidate pathway-based strategy using asthma as an example

BACKGROUND

The identification of gene by environment (GxE) interactions has emerged as a challenging but essential task to fully understand the complex mechanism underlying multifactorial diseases. Until now, GxE interactions have been investigated by candidate approaches examining a small number of genes, or agnostically at the genome wide level.

PRESENTATION OF THE HYPOTHESIS

In this paper, we propose a gene selection strategy for investigation of gene-environment interactions. This strategy integrates the information on biological processes shared by genes, the canonical pathways to which they belong and the biological knowledge related to the environment in the gene selection process. It relies on both bioinformatics resources and biological expertise.

TESTING THE HYPOTHESIS

We illustrate our strategy by considering asthma, tobacco smoke as the environmental exposure, and genes sharing the same biological function of "response to oxidative stress". Our filtering strategy leads to a list of 28 pathways involving 182 genes for further GxE investigation.

IMPLICATIONS OF THE HYPOTHESIS

By integrating the environment into the gene selection process, we expect that our strategy will improve the ability to identify the joint effects and interactions of environmental and genetic factors in disease.

2-Cys peroxiredoxin of Plasmodium falciparum is involved in resistance to heat stress of the parasite

In the cytoplasm of Plasmodium falciparum, two peroxiredoxins: PfTPx-1 and Pf1-Cys-Prx, are expressed at different time-points of the parasite cell cycle during the intraerythrocytic stage. In the present study, to gain insight into the functions of Prxs in the cytoplasm of P. falciparum, we investigated the heat stress sensitivity of the previously established PfTPx-1 KO line and found that PfTPx-1 disruption renders the parasite hypersensitive to heat stress. In addition, we established Pf1-Cys-Prx knockout (KO) parasite lines. The phenotypes of Pf1-Cys-Prx KO lines were different to those of the PfTPx-1 KO line and did not show hypersensitivity to reactive oxygen species, reactive nitrogen species, chloroquine or heat stress. These results suggest that the function of Pf1-Cys-Prx in the parasite cytoplasm is independent from that of PfTPx-1. The hyperthermal protective function of the PfTPx-1 is obviously important for the parasite physiology in the human patient body, in which it must survive repeated incidences of fever.

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.

Different genes interact with particulate matter and tobacco smoke exposure in affecting lung function decline in the general population

BACKGROUND

Oxidative stress related genes modify the effects of ambient air pollution or tobacco smoking on lung function decline. The impact of interactions might be substantial, but previous studies mostly focused on main effects of single genes.

OBJECTIVES

We studied the interaction of both exposures with a broad set of oxidative-stress related candidate genes and pathways on lung function decline and contrasted interactions between exposures.

METHODS

For 12679 single nucleotide polymorphisms (SNPs), change in forced expiratory volume in one second (FEV(1)), FEV(1) over forced vital capacity (FEV(1)/FVC), and mean forced expiratory flow between 25 and 75% of the FVC (FEF(25-75)) was regressed on interval exposure to particulate matter <10 µm in diameter (PM10) or packyears smoked (a), additive SNP effects (b), and interaction terms between (a) and (b) in 669 adults with GWAS data. Interaction p-values for 152 genes and 14 pathways were calculated by the adaptive rank truncation product (ARTP) method, and compared between exposures. Interaction effect sizes were contrasted for the strongest SNPs of nominally significant genes (p(interaction)<0.05). Replication was attempted for SNPs with MAF>10% in 3320 SAPALDIA participants without GWAS.

RESULTS

On the SNP-level, rs2035268 in gene SNCA accelerated FEV(1)/FVC decline by 3.8% (p(interaction) = 2.5×10(-6)), and rs12190800 in PARK2 attenuated FEV1 decline by 95.1 ml p(interaction) = 9.7×10(-8)) over 11 years, while interacting with PM10. Genes and pathways nominally interacting with PM10 and packyears exposure differed substantially. Gene CRISP2 presented a significant interaction with PM10 (p(interaction) = 3.0×10(-4)) on FEV(1)/FVC decline. Pathway interactions were weak. Replications for the strongest SNPs in PARK2 and CRISP2 were not successful.

CONCLUSIONS

Consistent with a stratified response to increasing oxidative stress, different genes and pathways potentially mediate PM10 and tobacco smoke effects on lung function decline. Ignoring environmental exposures would miss these patterns, but achieving sufficient sample size and comparability across study samples is challenging.

Factors affecting protein thiol reactivity and specificity in peroxide reduction

Protein thiol reactivity generally involves the nucleophilic attack of the thiolate on an electrophile. A low pK(a) means higher availability of the thiolate at neutral pH but often a lower nucleophilicity. Protein structural factors contribute to increasing the reactivity of the thiol in very specific reactions, but these factors do not provide an indiscriminate augmentation in general reactivity. Notably, reduction of hydroperoxides by the catalytic cysteine of peroxiredoxins can achieve extraordinary reaction rates relative to free cysteine. The discussion of this catalytic efficiency has centered in the stabilization of the thiolate as a way to increase nucleophilicity. Such stabilization originates from electrostatic and polar interactions of the catalytic cysteine with the protein environment. We propose that the set of interactions is better described as a means of stabilizing the anionic transition state of the reaction. The enhanced acidity of the critical cysteine is concurrent but not the cause of catalytic efficiency. Protein stabilization of the transition state is achieved by (a) a relatively static charge distribution around the cysteine that includes a conserved arginine and the N-terminus of an α-helix providing a cationic environment that stabilizes the reacting thiolate, the transition state, and also the anionic leaving group; (b) a dynamic set of polar interactions that stabilize the thiolate in the resting enzyme and contribute to restraining its reactivity in the absence of substrate; but upon peroxide binding these active/binding site groups switch interactions from thiolate to peroxide oxygens, simultaneously increasing the nucleophilicity of the attacking sulfur and facilitating the correct positioning of the substrate. The switching of polar interaction provides further acceleration and, importantly, confers specificity to the thiol reactivity. The extraordinary thiol reactivity and specificity toward H(2)O(2) combined with their ubiquity and abundance place peroxiredoxins, along with glutathione peroxidases, as obligate hydroperoxide cellular sensors.

Identification of proteins targeted by the thioredoxin superfamily in Plasmodium falciparum

The malarial parasite Plasmodium falciparum possesses a functional thioredoxin and glutathione system comprising the dithiol-containing redox proteins thioredoxin (Trx) and glutaredoxin (Grx), as well as plasmoredoxin (Plrx), which is exclusively found in Plasmodium species. All three proteins belong to the thioredoxin superfamily and share a conserved Cys-X-X-Cys motif at the active site. Only a few of their target proteins, which are likely to be involved in redox reactions, are currently known. The aim of the present study was to extend our knowledge of the Trx-, Grx-, and Plrx-interactome in Plasmodium. Based on the reaction mechanism, we generated active site mutants of Trx and Grx lacking the resolving cysteine residue. These mutants were bound to affinity columns to trap target proteins from P. falciparum cell extracts after formation of intermolecular disulfide bonds. Covalently linked proteins were eluted with dithiothreitol and analyzed by mass spectrometry. For Trx and Grx, we were able to isolate 17 putatively redox-regulated proteins each. Furthermore, the approach was successfully established for Plrx, leading to the identification of 21 potential target proteins. In addition to confirming known interaction partners, we captured potential target proteins involved in various processes including protein biosynthesis, energy metabolism, and signal transduction. The identification of three enzymes involved in S-adenosylmethionine (SAM) metabolism furthermore suggests that redox control is required to balance the metabolic fluxes of SAM between methyl-group transfer reactions and polyamine synthesis. To substantiate our data, the binding of the redoxins to S-adenosyl-L-homocysteine hydrolase and ornithine aminotransferase (OAT) were verified using BIAcore surface plasmon resonance. In enzymatic assays, Trx was furthermore shown to enhance the activity of OAT. Our approach led to the discovery of several putatively redox-regulated proteins, thereby contributing to our understanding of the redox interactome in malarial parasites.

Protection from oxidative stress and efficient redox regulation are essential for malarial parasites which have to grow and multiply rapidly in various environments. As shown by glucose-6 phosphate dehydrogenase deficiency, a genetic variation protecting from malaria, the parasite–host cell unit is very susceptible to disturbances in redox equilibrium. This is the major reason why redox active proteins of Plasmodium currently belong to the most attractive antimalarial drug targets. The dithiol-containing redox proteins thioredoxin (Trx) and glutaredoxin (Grx), as well as plasmoredoxin (Plrx), which is exclusively found in Plasmodium species, represent central players in the redox network of malarial parasites. To extend our knowledge of interacting partners and the functions of these proteins, we carried out pull-down assays with immobilized active site mutants of Trx, Grx, and Plrx and whole cell parasite lysate. After elution of bound proteins and mass spectrometric identification, about 20 interacting partners were identified for each of the redox proteins. Data was supported using BIAcore surface plasmon resonance. The identified interacting proteins, which are likely to be redox-regulated, are involved in important cellular processes including protein biosynthesis, energy metabolism, polyamine synthesis, and signal transduction. Our results contribute to our understanding of the redox interactome in malarial parasites.

The peroxidase and peroxynitrite reductase activity of human erythrocyte peroxiredoxin 2

Peroxiredoxin 2 (Prx2) is a 2-Cys peroxiredoxin extremely abundant in the erythrocyte. The peroxidase activity was studied in a steady-state approach yielding an apparent K(M) of 2.4 microM for human thioredoxin and a very low K(M) for H2O2 (0.7 microM). Rate constants for the reaction of peroxidatic cysteine with the peroxide substrate, H2O2 or peroxynitrite, were determined by competition kinetics, k(2) = 1.0 x 10(8) and 1.4 x 10(7) M(-1) s(-1) at 25 degrees C and pH 7.4, respectively. Excess of both oxidants inactivated the enzyme by overoxidation and also tyrosine nitration and dityrosine were observed with peroxynitrite treatment. Prx2 associates into decamers (5 homodimers) and we estimated a dissociation constant K(d) < 10(-23) M(4) which confirms the enzyme exists as a decamer in vivo. Our kinetic results indicate Prx2 is a key antioxidant enzyme for the erythrocyte and reveal red blood cells as active oxidant scrubbers in the bloodstream.

Peroxynitrite detoxification and its biologic implications

Peroxynitrite is a cytotoxic oxidant formed in vivo from the diffusional-controlled reaction between nitric oxide and superoxide radicals. Increased peroxynitrite formation has been related to the pathogenesis of multiple diseases, thus underlining the importance of understanding the mechanisms of its detoxification. In nature, different enzymatic routes for peroxynitrite decomposition have evolved. Among them, peroxiredoxins catalytically reduce peroxynitrite in vitro; modulation of their expression affects peroxynitrite-mediated cytotoxicity, and their content changes in pathologic conditions associated with increased peroxynitrite formation in vivo, thus indicating a physiologic role of these enzymes in peroxynitrite reduction. Selenium-containing glutathione peroxidase also catalyzes peroxynitrite reduction, but its role in vivo is still a matter of debate. In selected cellular systems, heme proteins also play a role in peroxynitrite detoxification, such as its isomerization by oxyhemoglobin in red blood cells. Moreover, different pharmacologic approaches have been used to decrease the toxicity related to peroxynitrite formation. Manganese or iron porphyrins catalyze peroxynitrite decomposition, and their protective role in vivo has been confirmed in biologic systems. Glutathione peroxidase mimetics also rapidly reduce peroxynitrite, but their biologic role is less well established. Flavonoids, nitroxides, and tyrosine-containing peptides decreased peroxynitrite-mediated toxicity under different conditions, but their mechanism of action is indirect.

Depletion of Plasmodium berghei plasmoredoxin reveals a non-essential role for life cycle progression of the malaria parasite

Proliferation of the pathogenic Plasmodium asexual blood stages in host erythrocytes requires an exquisite capacity to protect the malaria parasite against oxidative stress. This function is achieved by a complex antioxidant defence system composed of redox-active proteins and low MW antioxidants. Here, we disrupted the P. berghei plasmoredoxin gene that encodes a parasite-specific 22 kDa member of the thioredoxin superfamily. The successful generation of plasmoredoxin knockout mutants in the rodent model malaria parasite and phenotypic analysis during life cycle progression revealed a non-vital role in vivo. Our findings suggest that plasmoredoxin fulfils a specialized and dispensable role for Plasmodium and highlights the need for target validation to inform drug development strategies.

Pre-steady state kinetic characterization of human peroxiredoxin 5: Taking advantage of Trp84 fluorescence increase upon oxidation

Human peroxiredoxin 5 (PRDX5) catalyzes different peroxides reduction by enzymatic substitution mechanisms. Enzyme oxidation caused an increase in Trp84 fluorescence, allowing performing pre-steady state kinetic measurements. The technique was validated by comparing with data available from the literature or obtained herein by alternative approaches. PRDX5 reacted with organic hydroperoxides with rate constants in the 10(6)-10(7)M(-1)s(-1) range, similar to peroxynitrite-mediated PRDX5 oxidation, whereas its reaction with hydrogen peroxide was slower (10(5)M(-1)s(-1)). The method allowed determining the kinetics of intramolecular disulfide formation as well as thioredoxin 2-mediated reduction. The reactivities of PRDXs with peroxides were surprisingly high considering thiol pK(a), indicating that other protein determinants are involved in PRDXs specialization. The order of reactivities between PRDX5 towards oxidizing substrates differ from other PRDXs studied, pointing to a selective action of PRDXs with respect to peroxide detoxification, helping to rationalize the multiple enzyme isoforms present even in the same cellular compartment.

Peroxiredoxins in malaria parasites: parasitologic aspects

Malaria is one of the most debilitating and life threatening diseases in tropical regions of the world. Over 500 million clinical cases occur, and 2-3 million people die of the disease each year. Because Plasmodium lacks genuine glutathione peroxidase and catalase, the two major antioxidant enzymes in the eukaryotic cell, malaria parasites are likely to utilize members of the peroxiredoxin (Prx) family as the principal enzymes to reduce peroxides, which increase in the parasite cell due to metabolism and parasitism during parasite development. In addition to its function of protecting macromolecules from H(2)O(2), Prx has also been reported to regulate H(2)O(2) as second messenger in transmission of redox signals, which mediate cell proliferation, differentiation, and apoptosis. In the malaria parasite, several lines of experimental data have suggested that the parasite uses Prxs as multifunctional molecules to adapt themselves to asexual and sexual development. In this review, we summarize the accumulated knowledge on the Prx family with respect to their functions in mammalian cells and their possible function(s) in malaria parasites.

Peroxiredoxin systems of protozoal parasites

Cellular redox metabolism is considered to be involved in the pathophysiology of diseases caused by protozoal parasites such as Toxoplasma, Trypanosoma, Leishmania, and Plasmodia. Redox reactions furthermore are thought to play a major role in the action of and the resistance to some clinically used antiparasitic drugs. Interestingly, in malarial parasites, the antioxidant enzymes catalase and glutathione peroxidase are absent which indicates a crucial role of the thioredoxin system in redox control. Besides a glutathione peroxidase-like thioredoxin peroxidase and a glutathione S-transferase with slight peroxidase activity, Plasmodium falciparum (the causative agent of tropical malaria) possesses four classical peroxiredoxins: Two peroxiredoxins of the typical 2-Cys Prx class, one 1-Cys peroxiredoxin with homology to the atypical 2-Cys Prx class, and a peroxiredoxin of the 1-Cys Prx class have been identified and partially characterized In our article we give an introduction to redox-based drug development strategies against protozoal parasites and summarize the present knowledge on peroxiredoxin systems in Plasmodium.

Kinetics of peroxiredoxins and their role in the decomposition of peroxynitrite

Methodologies and results of studies on the kinetics of peroxiredoxins (Prx) are reviewed. Peroxiredoxins are broad-spectrum peroxidases that catalyze the reduction of H2O2, organic hydroperoxides and peroxynitrite by thiols. Their catalytic cycle starts with the oxidation of a particularly reactive cysteine residue (C(P)) to a sulfenic acid derivative by the peroxide substrate, the sulfenic acid then reacts with a thiol to form a disulfide, and the cycle is completed by thiol/disulfide exchange reactions that regenerate the ground-state enzyme. Depending on the subtype of peroxiredoxin, the thiol reacting with the primary oxidation product (E-SOH) may be a cysteine residue of a second subunit (typical 2-Cys Prx), a cysteine residue of the same subunit (atypical 2-Cys Prx) or reducing substrate (1-Cys Prx and at least one example of an atypical 2-Cys Prx). In a typical 2-Cys Prx the intra-subunit disulfide formation with the second "resolving" cysteine (C(R)) is mandatory for the reduction by the specific substrate, which is a protein characterized by a CXXC motif such as thioredoxin, tryparedoxin or AhpF. These consecutive redox reactions define the catalysis as an enzyme substitution mechanism, which is corroborated by a ping-pong pattern that is commonly observed in steady-state analyses, chemical identification of catalytic intermediates and stopped-flow analyses of partial reactions. More complex kinetic patterns are discussed in terms of cooperativity between the subunits of the oligomeric enzymes, generation of different oxidized intermediates or partial over-oxidation of C(P) to a sulfinic acid. Saturation kinetics is often not observed indicating that a typical complex between reduced enzyme and hydroperoxide is not formed and that, in these cases, formation of the complex between the oxidized enzyme and its reducing substrate is slower than the reaction within this complex. Working with sulphur catalysis, Prxs are usually less efficient than the heme- or selenium-containing peroxidases, but in some cases the k(+1) values (bimolecular rate constant for oxidation of reduced E by ROOH) are comparable, the overall range being 2 x 10(3)-4 x 10(7) M(-1)s(-1) depending on the hydroperoxide and the individual Prx. For the reduction of peroxynitrite k(+1) values of 1 x 10(6) up to 7 x 10(7) M(-1)s(-1) have been measured. The net forward rate constants k'(+2) for the reductive part of the cycle range between 2 x 10(4)-1 x 10(7) M(-1)s(-1). These kinetic characteristics qualify the peroxiredoxins as moderately efficient devices to detoxify hydroperoxides, which is pivotal to organisms devoid of more efficient peroxidases, and as most relevant to the detoxification of peroxynitrite. In higher organisms, their specific role is seen in the regulation of signalling cascades that are modulated by H2O2, lipid hydroperoxides or peroxynitrite.

The thiol-based redox networks of pathogens: unexploited targets in the search for new drugs

Hydroperoxide metabolism in diverse pathogens is reviewed under consideration of involved enzymes as potential drug targets. The common denominator of the peroxidase systems of Trypanosoma, Leishmania, Plasmodium, and Mycobacterium species is the use of NAD(P)H to reduce hydroperoxides including peroxynitrite via a flavin-containing disulfide reductase, a thioredoxin (Trx)-related protein and a peroxidase that operates with thiol catalysis. In Plasmodium falciparum, thioredoxin- and glutathione dependent systems appear to be linked via glutaredoxin and plasmoredoxin to terminal thioredoxin peroxidases belonging to both, the peroxiredoxin (Prx) and glutathione peroxidase (GPx) family. In Mycobacterium tuberculosis, a catalase-type peroxidase is complemented by the typical 2-C-Prx AhpC that, in contrast to most bacteria, is reduced by TrxC, and an atypical 2-C-Prx reduced by TrxB or C. A most complex variation of the scheme is found in trypanosomatids, where the unique redox metabolite trypanothione reduces the thioredoxin-related tryparedoxin, which fuels Prx- and GPx-type peroxidases as well as ribonucleotide reductase. In Trypanosoma brucei and Leishmania donovani the system has been shown to be essential for viability and virulence by inversed genetics. It is concluded that optimum efficacy can be expected from inhibitors of the most upstream components of the redox cascades. For trypanosomatids attractive validated drug targets are trypanothione reductase and trypanothione synthetase; for mycobacteria thioredoxin reductase appears most appealing, while in Plasmodium simultaneous inhibition of both the thioredoxin and the glutathione pathway appears advisable to avoid mutual substitution in co-substrate supply to the peroxidases. Financial and organisational needs to translate the scientific progress into applicable drugs are discussed under consideration of the socio-economic impact of the addressed diseases.