The O2-scavenging flavodiiron protein in the human parasite Giardia intestinalis

Antigiardial activity of novel triazolyl-quinolone-based chalcone derivatives: when oxygen makes the difference

Giardiasis is a common diarrheal disease worldwide caused by the protozoan parasite Giardia intestinalis. It is urgent to develop novel drugs to treat giardiasis, due to increasing clinical resistance to the gold standard drug metronidazole (MTZ). New potential antiparasitic compounds are usually tested for their killing efficacy against G. intestinalis under anaerobic conditions, in which MTZ is maximally effective. On the other hand, though commonly regarded as an ‘anaerobic pathogen,’ G. intestinalis is exposed to relatively high O₂ levels in vivo, living attached to the mucosa of the proximal small intestine. It is thus important to test the effect of O₂ when searching for novel potential antigiardial agents, as outlined in a previous study [Bahadur et al. (2014) Antimicrob. Agents Chemother. 58, 543]. Here, 45 novel chalcone derivatives with triazolyl-quinolone scaffold were synthesized, purified, and characterized by high resolution mass spectrometry, ¹H and ¹³C nuclear magnetic resonance and infrared spectroscopy. Efficacy of the compounds against G. intestinalis trophozoites was tested under both anaerobic and microaerobic conditions, and selectivity was assessed in a counter-screen on human epithelial colorectal adenocarcinoma cells. MTZ was used as a positive control in the assays. All the tested compounds proved to be more effective against the parasite in the presence of O₂, with the exception of MTZ that was less effective. Under anaerobiosis eighteen compounds were found to be as effective as MTZ or more (up to three to fourfold); the same compounds proved to be up to >100-fold more effective than MTZ under microaerobic conditions. Four of them represent potential candidates for the design of novel antigiardial drugs, being highly selective against Giardia trophozoites. This study further underlines the importance of taking O₂ into account when testing novel potential antigiardial compounds.

Dioxygen and nitric oxide scavenging by Treponema denticola flavodiiron protein: a mechanistic paradigm for catalysis

Flavodiiron proteins (FDPs) contain a unique active site consisting of a non-heme diiron carboxylate site proximal to a flavin mononucleotide (FMN). FDPs serve as the terminal components for reductive scavenging of dioxygen (to water) or nitric oxide (to nitrous oxide), which combats oxidative or nitrosative stress in many bacteria. Characterizations of FDPs from spirochetes or from any oral microbes have not been previously reported. Here, we report characterization of an FDP from the anaerobic spirochete, Treponema (T.) denticola, which is associated with chronic periodontitis. The isolated T. denticola FDP exhibited efficient four-electron dioxygen reductase activity and lower but significant anaerobic nitric oxide reductase activity. A mutant T. denticola strain containing the inactivated FDP-encoding gene was significantly more air-sensitive than the wild-type strain. Single turnover reactions of the four-electron-reduced FDP (FMNH2-Fe(II)Fe(II)) (FDPred) with O2 monitored on the milliseconds to seconds time scale indicated initial rapid formation of a spectral feature consistent with a cis-μ-1,2-peroxo-diferric intermediate, which triggered two-electron oxidation of FMNH2. Reaction of FDPred with NO showed apparent cooperativity between binding of the first and second NO to the diferrous site. The resulting diferrous dinitrosyl complex triggered two-electron oxidation of the FMNH2. Our cumulative results on this and other FDPs indicate that smooth two-electron FMNH2 oxidation triggered by the FDPred/substrate complex and overall four-electron oxidation of FDPred to FDPox constitutes a mechanistic paradigm for both dioxygen and nitric oxide reductase activities of FDPs. Four-electron reductive O2 scavenging by FDPs could contribute to oxidative stress protection in many other oral bacteria.

Cyanobacterial Oxygenic Photosynthesis is Protected by Flavodiiron Proteins

Flavodiiron proteins (FDPs, also called flavoproteins, Flvs) are modular enzymes widely present in Bacteria and Archaea. The evolution of cyanobacteria and oxygenic photosynthesis occurred in concert with the modulation of typical bacterial FDPs. Present cyanobacterial FDPs are composed of three domains, the β-lactamase-like, flavodoxin-like and flavin-reductase like domains. Cyanobacterial FDPs function as hetero- and homodimers and are involved in the regulation of photosynthetic electron transport. Whilst Flv2 and Flv4 proteins are limited to specific cyanobacterial species (β-cyanobacteria) and function in photoprotection of Photosystem II, Flv1 and Flv3 proteins, functioning in the "Mehler-like" reaction and safeguarding Photosystem I under fluctuating light conditions, occur in nearly all cyanobacteria and additionally in green algae, mosses and lycophytes. Filamentous cyanobacteria have additional FDPs in heterocyst cells, ensuring a microaerobic environment for the function of the nitrogenase enzyme under the light. Here, the evolution, occurrence and functional mechanisms of various FDPs in oxygenic photosynthetic organisms are discussed.

A diferrous-dinitrosyl intermediate in the N2O-generating pathway of a deflavinated flavo-diiron protein

Flavo-diiron proteins (FDPs) function as anaerobic nitric oxide scavengers in some microorganisms, catalyzing reduction of nitric to nitrous oxide. The FDP from Thermotoga maritima can be prepared in a deflavinated form with an intact diferric site (deflavo-FDP). Hayashi et al. [(2010) Biochemistry 49, 7040–7049] reported that reaction of NO with reduced deflavo-FDP produced substoichiometric N₂O. Here we report a multispectroscopic approach to identify the iron species in the reactions of deflavo-FDP with NO. Mössbauer spectroscopy identified two distinct ferrous species after reduction of the antiferromagnetically coupled diferric site. Approximately 60% of the total ferrous iron was assigned to a diferrous species associated with the N₂O-generating pathway. This pathway proceeds through successive diferrous-mononitrosyl (S = ¹/₂ Fe^II{FeNO}⁷) and diferrous-dinitrosyl (S = 0 [{FeNO}⁷]₂) species that form within ∼100 ms of mixing of the reduced protein with NO. The diferrous-dinitrosyl intermediate converted to an antiferromagnetically coupled diferric species that was spectroscopically indistinguishable from that in the starting deflavinated protein. These diiron species closely resembled those reported for the flavinated FDP [Caranto et al. (2014) J. Am. Chem. Soc. 136, 7981–7992], and the time scales of their formation and decay were consistent with the steady state turnover of the flavinated protein. The remaining ∼40% of ferrous iron was inactive in N₂O generation but reversibly bound NO to give an S = ³/₂ {FeNO}⁷ species. The results demonstrate that N₂O formation in FDPs can occur via conversion of S = 0 [{FeNO}⁷]₂ to a diferric form without participation of the flavin cofactor.

Flavodiiron oxygen reductase from Entamoeba histolytica: modulation of substrate preference by tyrosine 271 and lysine 53

Flavodiiron proteins (FDPs) are a family of enzymes endowed with bona fide oxygen- and/or nitric-oxide reductase activity, although their substrate specificity determinants remain elusive. After a comprehensive comparison of available three-dimensional structures, particularly of FDPs with a clear preference toward either O2 or NO, two main differences were identified near the diiron active site, which led to the construction of site-directed mutants of Tyr(271) and Lys(53) in the oxygen reducing Entamoeba histolytica EhFdp1. The biochemical and biophysical properties of these mutants were studied by UV-visible and electron paramagnetic resonance (EPR) spectroscopies coupled to potentiometry. Their reactivity with O2 and NO was analyzed by stopped-flow absorption spectroscopy and amperometric methods. These mutations, whereas keeping the overall properties of the redox cofactors, resulted in increased NO reductase activity and faster inactivation of the enzyme in the reaction with O2, pointing to a role of the mutated residues in substrate selectivity.

The nitric oxide reductase mechanism of a flavo-diiron protein: identification of active-site intermediates and products

The unique active site of flavo-diiron proteins (FDPs) consists of a nonheme diiron-carboxylate site proximal to a flavin mononucleotide (FMN) cofactor. FDPs serve as the terminal components for reductive scavenging of dioxygen or nitric oxide to combat oxidative or nitrosative stress in bacteria, archaea, and some protozoan parasites. Nitric oxide is reduced to nitrous oxide by the four-electron reduced (FMNH2-Fe(II)Fe(II)) active site. In order to clarify the nitric oxide reductase mechanism, we undertook a multispectroscopic presteady-state investigation, including the first Mössbauer spectroscopic characterization of diiron redox intermediates in FDPs. A new transient intermediate was detected and determined to be an antiferromagnetically coupled diferrous-dinitrosyl (S = 0, [{FeNO}(7)]2) species. This species has an exchange energy, J ≥ 40 cm(-1) (JS1 ° S2), which is consistent with a hydroxo or oxo bridge between the two irons. The results show that the nitric oxide reductase reaction proceeds through successive formation of diferrous-mononitrosyl (S = ½, Fe(II){FeNO}(7)) and the S = 0 diferrous-dinitrosyl species. In the rate-determining process, the diferrous-dinitrosyl converts to diferric (Fe(III)Fe(III)) and by inference N2O. The proximal FMNH2 then rapidly rereduces the diferric site to diferrous (Fe(II)Fe(II)), which can undergo a second 2NO → N2O turnover. This pathway is consistent with previous results on the same deflavinated and flavinated FDP, which detected N2O as a product (Hayashi Biochemistry 2010, 49, 7040). Our results do not support other proposed mechanisms, which proceed either via "super-reduction" of [{FeNO}(7)]2 by FMNH2 or through Fe(II){FeNO}(7) directly to a diferric-hyponitrite intermediate. The results indicate that an S = 0 [{FeNO}(7)}]2 complex is a proximal precursor to N-N bond formation and N-O bond cleavage to give N2O and that this conversion can occur without redox participation of the FMN cofactor.

O2-dependent large electron flow functioned as an electron sink, replacing the steady-state electron flux in photosynthesis in the cyanobacterium Synechocystis sp. PCC 6803, but not in the cyanobacterium Synechococcus sp. PCC 7942

To determine whether alternative electron flow (AEF) can replace the photosynthetic electron flow in cyanobacteria, we used an open O2-electrode system to monitor O2-exchange over a long period. In air-grown Synechocystis sp. PCC 6803 (S. 6803(WT)), the quantum yield of PSII, Y(II), held even after photosynthesis was suppressed by CO2 shortage. The S. 6803 mutant, deficient in flavodiiron (FLV) proteins 1 and 3, showed the same phenotype as S. 6803(WT). In contrast, Y(II) decreased in Synechococcus sp. PCC 7942 (S. 7942). These results suggest that AEF functioned as the Y(II) in S. 6803 and replaced the photosynthetic electron flux. In contrast, the activity of AEF in S. 7942 was lower. The affinity of AEF for O2 in S. 6803 did not correspond to those of FLVs in bacteria or terminal oxidases in respiration. AEF might be driven by photorespiration.

Peroxynitrite and peroxiredoxin in the pathogenesis of experimental amebic liver abscess

The molecular mechanisms by which Entamoeba histolytica causes amebic liver abscess (ALA) are still not fully understood. Amebic mechanisms of adherence and cytotoxic activity are pivotal for amebic survival but apparently do not directly cause liver abscess. Abundant evidence indicates that chronic inflammation (resulting from an inadequate immune response) is probably the main cause of ALA. Reports referring to inflammatory mechanisms of liver damage mention a repertoire of toxic molecules by the immune response (especially nitric oxide and reactive oxygen intermediates) and cytotoxic substances released by neutrophils and macrophages after being lysed by amoebas (e.g., defensins, complement, and proteases). Nevertheless, recent evidence downplays these mechanisms in abscess formation and emphasizes the importance of peroxynitrite (ONOO⁻). It seems that the defense mechanism of amoebas against ONOO⁻, namely, the amebic thioredoxin system (including peroxiredoxin), is superior to that of mammals. The aim of the present text is to define the importance of ONOO⁻ as the main agent of liver abscess formation during amebic invasion, and to explain the superior capacity of amoebas to defend themselves against this toxic agent through the peroxiredoxin and thioredoxin system.

Novel functions of an iron-sulfur flavoprotein from Trichomonas vaginalis hydrogenosomes

Iron-sulfur flavoproteins (Isf) are flavin mononucleotide (FMN)- and FeS cluster-containing proteins commonly encountered in anaerobic prokaryotes. However, with the exception of Isf from Methanosarcina thermophila, which participates in oxidative stress management by removing oxygen and hydrogen peroxide, none of these proteins has been characterized in terms of function. Trichomonas vaginalis, a sexually transmitted eukaryotic parasite of humans, was found to express several iron-sulfur flavoprotein (TvIsf) homologs in its hydrogenosomes. We show here that in addition to having oxygen-reducing activity, the recombinant TvIsf also functions as a detoxifying reductase of metronidazole and chloramphenicol, both of which are antibiotics effective against a variety of anaerobic microbes. TvIsf can utilize both NADH and reduced ferredoxin as electron donors. Given the prevalence of Isf in anaerobic prokaryotes, we propose that these proteins are central to a novel defense mechanism against xenobiotics.

Functional characterization of peroxiredoxins from the human protozoan parasite Giardia intestinalis

The microaerophilic protozoan parasite Giardia intestinalis, causative of one of the most common human intestinal diseases worldwide, infects the mucosa of the proximal small intestine, where it has to cope with O₂ and nitric oxide (NO). Elucidating the antioxidant defense system of this pathogen lacking catalase and other conventional antioxidant enzymes is thus important to unveil novel potential drug targets. Enzymes metabolizing O₂, NO and superoxide anion (O₂^−•) have been recently reported for Giardia, but it is yet unknown how the parasite copes with H₂O₂ and peroxynitrite (ONOO⁻). Giardia encodes two yet uncharacterized 2-cys peroxiredoxins (Prxs), GiPrx1a and GiPrx1b. Peroxiredoxins are peroxidases implicated in virulence and drug resistance in several parasitic protozoa, able to protect from nitroxidative stress and repair oxidatively damaged molecules. GiPrx1a and a truncated form of GiPrx1b (deltaGiPrx1b) were expressed in Escherichia coli, purified and functionally characterized. Both Prxs effectively metabolize H₂O₂ and alkyl-hydroperoxides (cumyl- and tert-butyl-hydroperoxide) in the presence of NADPH and E. coli thioredoxin reductase/thioredoxin as the reducing system. Stopped-flow experiments show that both proteins in the reduced state react with ONOO⁻ rapidly (k = 4×10⁵ M⁻¹ s⁻¹ and 2×10⁵ M⁻¹ s⁻¹ at 4°C, for GiPrx1a and deltaGiPrx1b, respectively). Consistent with a protective role against oxidative stress, expression of GiPrx1a (but not deltaGiPrx1b) is induced in parasitic cells exposed to air O₂ for 24 h. Based on these results, GiPrx1a and deltaGiPrx1b are suggested to play an important role in the antioxidant defense of Giardia, possibly contributing to pathogenesis.

Giardia intestinalis causes one of the most common human intestinal diseases worldwide, called giardiasis. This microorganism infects the small intestine where it has to cope with O₂, nitric oxide (NO) and related reactive species that are toxic for Giardia as it lacks most of the conventional antioxidant enzymes. Understanding how this pathogen survives oxidative stress is thus important because it may help to identify novel drug targets to combat giardiasis. Some enzymes playing a role in the antioxidant defense of Giardia have been recently reported, but it is yet unknown how the parasite copes with two well-known oxidants, hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO⁻). In this study, the Authors show that Giardia expresses two enzymes (called peroxiredoxins), yet uncharacterized, that are able not only to degrade both H₂O₂ and ONOO⁻, but also to repair damaged molecules (called hydroperoxides) that accumulate in the cell under oxidative stress conditions. These results are totally unprecedented because no enzymes with these types of functions have been reported for Giardia to date. If these two enzymes will prove to be essential for Giardia virulence in future studies, a new way will be paved towards the discovery of novel drugs to treat giardiasis.

O(2)-dependent efficacy of novel piperidine- and piperazine-based chalcones against the human parasite Giardia intestinalis

Giardia intestinalis is the most frequent protozoan agent of intestinal diseases worldwide. Though commonly regarded as an anaerobic pathogen, it preferentially colonizes the fairly oxygen-rich mucosa of the proximal small intestine. Therefore, when testing new potential antigiardial drugs, O2 should be taken into account, since it also reduces the efficacy of metronidazole, the gold standard drug against giardiasis. In this study, 46 novel chalcones were synthesized by microwave-assisted Claisen-Schmidt condensation, purified, characterized by high-resolution mass spectrometry, (1)H and (13)C nuclear magnetic resonance, and infrared spectroscopy, and tested for their toxicity against G. intestinalis under standard anaerobic conditions. As a novel approach, compounds showing antigiardial activity under anaerobiosis were also assayed under microaerobic conditions, and their selectivity against parasitic cells was assessed in a counterscreen on human epithelial colorectal adenocarcinoma cells. Among the tested compounds, three [30(a), 31(e), and 33] were more effective in the presence of O2 than under anaerobic conditions and killed the parasite 2 to 4 times more efficiently than metronidazole under anaerobiosis. Two of them [30(a) and 31(e)] proved to be selective against parasitic cells, thus representing potential candidates for the design of novel antigiardial drugs. This study highlights the importance of testing new potential antigiardial agents not only under anaerobic conditions but also at low, more physiological O2 concentrations.

Mitochondria-derived organelles in the diplomonad fish parasite Spironucleus vortens

In some eukaryotes, mitochondria have become modified during evolution to yield derived organelles (MDOs) of a similar size (hydrogenosomes), or extremely reduced to produce tiny cellular vesicles (mitosomes). The current study provides evidence for the presence of MDOs in the highly infectious fish pathogen Spironucleus vortens, an organism that produces H₂ and is shown here to have no detectable cytochromes. Transmission electron microscopy (TEM) reveals that S. vortens trophozoites contain electron-dense, membranous structures sometimes with an electron-dense core (200 nm-1 μm), resembling the hydrogenosomes previously described in other protists from habitats deficient in O₂. Confocal microscopy establishes that these organelles exhibit autofluorescence emission spectra similar to flavoprotein constituents previously described for mitochondria and also present in hydrogenosomes. These organelles possess a membrane potential and are labelled by a fluorescently labeled antibody against Fe-hydrogenase from Blastocystis hominis. Heterologous antibodies raised to mitochondrial proteins frataxin and Isu1, also exhibit a discrete punctate pattern of localization in S. vortens; however these labelled structures are distinctly smaller (90-150 nm) than hydrogenosomes as observed previously in other organisms. TEM confirms the presence of double-membrane bounded organelles of this smaller size. In addition, strong background immunostaining occurs in the cytosol for frataxin and Isu1, and labelling by anti-ferredoxin antibody is generally distributed and not specifically localized except for at the anterior polar region. This suggests that some of the functions traditionally attributed to such MDOs may also occur elsewhere. The specialized parasitic life-style of S. vortens may necessitate more complex intracellular compartmentation of redox reactions than previously recognized. Control of infection requires biochemical characterization of redox-related organelles.

Histidine ligand variants of a flavo-diiron protein: effects on structure and activities

Flavo-diiron proteins (FDPs) contain non-heme diiron and proximal flavin mononucleotide (FMN) active sites and function as terminal components of a nitric oxide reductase (NOR) and/or a four-electron dioxygen reductase (O(2)R). While most FDPs show similar structural, spectroscopic, and redox properties, O(2)R and NOR activities vary significantly among FDPs. A potential source of this variability is the iron ligation status of a conserved His residue that provides an iron ligand in all known FDP structures but one, where this His residue is rotated away from iron and replaced by a solvent ligand. In order to test the effect of this His ligation status, we changed this ligating His residue (H90) in Thermotoga maritima (Tm) FDP to either Asn or Ala. The wild-type Tm FDP shows significantly higher O(2)R than NOR activity. Single crystal X-ray crystallography revealed a remarkably conserved diiron site structure in the H90N and -A variants, differing mainly by either Asn or solvent coordination, respectively, in place of H90. The steady-state activities were minimally affected by the H90 substitutions, remaining significantly higher for O(2)R versus NOR. The pre-steady-state kinetics of the fully reduced FDP with O(2) were also minimally affected by the H90 substitutions. The results indicate that the coordination status of this His ligand does not significantly modulate the O(2)R or NOR activities, and that FDPs can retain these activities when the individual iron centers are differentiated by His ligand substitution. This differentiation may have implications for the O(2)R and NOR mechanisms of FDPs.

Low-molecular-mass antioxidants in parasites

SIGNIFICANCE

Parasitic infections continue to be a major problem for global human health. Vaccines are practically not available and chemotherapy is highly unsatisfactory. One approach toward a novel antiparasitic drug development is to unravel pathways that may be suited as future targets. Parasitic organisms show a remarkable diversity with respect to the nature and functions of their main low-molecular-mass antioxidants and many of them developed pathways that do not have a counterpart in their mammalian hosts.

RECENT ADVANCES

Work of the last years disclosed the individual antioxidants employed by parasites and their distinct pathways. Entamoeba, Trichomonas, and Giardia directly use cysteine as main low-molecular-mass thiol but have divergent cysteine metabolisms. Malarial parasites rely exclusively on cysteine uptake and generate glutathione (GSH) as main free thiol as do metazoan parasites. Trypanosomes and Leishmania have a unique trypanothione-based thiol metabolism but employ individual mechanisms for their cysteine supply. In addition, some trypanosomatids synthesize ovothiol A and/or ascorbate. Various essential parasite enzymes such as trypanothione synthetase and trypanothione reductase in Trypanosomatids and the Schistosoma thioredoxin GSH reductase are currently intensively explored as drug target molecules.

CRITICAL ISSUES

Essentiality is a prerequisite but not a sufficient property of an enzyme to become a suited drug target. The availability of an appropriate in vivo screening system and many other factors are equally important.

FUTURE DIRECTIONS

The current organism-wide RNA-interference and proteome analyses are supposed to reveal many more interesting candidates for future drug development approaches directed against the parasite antioxidant defense systems.

A detoxifying oxygen reductase in the anaerobic protozoan Entamoeba histolytica

We report the characterization of a bacterial-type oxygen reductase abundant in the cytoplasm of the anaerobic protozoan parasite Entamoeba histolytica. Upon host infection, E. histolytica is confronted with various oxygen tensions in the host intestine, as well as increased reactive oxygen and nitrogen species at the site of local tissue inflammation. Resistance to oxygen-derived stress thus plays an important role in the pathogenic potential of E. histolytica. The genome of E. histolytica has four genes that encode flavodiiron proteins, which are bacterial-type oxygen or nitric oxide reductases and were likely acquired by lateral gene transfer from prokaryotes. The EhFdp1 gene has higher expression in virulent than in nonvirulent Entamoeba strains and species, hinting that the response to oxidative stress may be one correlate of virulence potential. We demonstrate that EhFdp1 is abundantly expressed in the cytoplasm of E. histolytica and that the protein levels are markedly increased (up to ~5-fold) upon oxygen exposure. Additionally, we produced fully functional recombinant EhFdp1 and demonstrated that this enzyme is a specific and robust oxygen reductase but has poor nitric oxide reductase activity. This observation represents a new mechanism of oxygen resistance in the anaerobic protozoan pathogen E. histolytica.

Biochemistry and evolution of anaerobic energy metabolism in eukaryotes

Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.

Vibrational analysis of mononitrosyl complexes in hemerythrin and flavodiiron proteins: relevance to detoxifying NO reductase

Flavodiiron proteins (FDPs) play important roles in the microbial nitrosative stress response in low-oxygen environments by reductively scavenging nitric oxide (NO). Recently, we showed that FMN-free diferrous FDP from Thermotoga maritima exposed to 1 equiv NO forms a stable diiron-mononitrosyl complex (deflavo-FDP(NO)) that can react further with NO to form N(2)O [Hayashi, T.; Caranto, J. D.; Wampler, D. A; Kurtz, D. M., Jr.; Moënne-Loccoz, P. Biochemistry 2010, 49, 7040-7049]. Here we report resonance Raman and low-temperature photolysis FTIR data that better define the structure of this diiron-mononitrosyl complex. We first validate this approach using the stable diiron-mononitrosyl complex of hemerythrin, Hr(NO), for which we observe a ν(NO) at 1658 cm(-1), the lowest ν(NO) ever reported for a nonheme {FeNO}(7) species. Both deflavo-FDP(NO) and the mononitrosyl adduct of the flavinated FPD (FDP(NO)) show ν(NO) at 1681 cm(-1), which is also unusually low. These results indicate that, in Hr(NO) and FDP(NO), the coordinated NO is exceptionally electron rich, more closely approaching the Fe(III)(NO(-)) resonance structure. In the case of Hr(NO), this polarization may be promoted by steric enforcement of an unusually small FeNO angle, while in FDP(NO), the Fe(III)(NO(-)) structure may be due to a semibridging electrostatic interaction with the second Fe(II) ion. In Hr(NO), accessibility and steric constraints prevent further reaction of the diiron-mononitrosyl complex with NO, whereas in FDP(NO) the increased nucleophilicity of the nitrosyl group may promote attack by a second NO to produce N(2)O. This latter scenario is supported by theoretical modeling [Blomberg, L. M.; Blomberg, M. R.; Siegbahn, P. E. J. Biol. Inorg. Chem. 2007, 12, 79-89]. Published vibrational data on bioengineered models of denitrifying heme-nonheme NO reductases [Hayashi, T.; Miner, K. D.; Yeung, N.; Lin, Y.-W.; Lu, Y.; Moënne-Loccoz, P. Biochemistry 2011, 50, 5939-5947 ] support a similar mode of activation of a heme {FeNO}(7) species by the nearby nonheme Fe(II).

The superoxide reductase from the early diverging eukaryote Giardia intestinalis

Unlike superoxide dismutases (SODs), superoxide reductases (SORs) eliminate superoxide anion (O(2)(•-)) not through its dismutation, but via reduction to hydrogen peroxide (H(2)O(2)) in the presence of an electron donor. The microaerobic protist Giardia intestinalis, responsible for a common intestinal disease in humans, though lacking SOD and other canonical reactive oxygen species-detoxifying systems, is among the very few eukaryotes encoding a SOR yet identified. In this study, the recombinant SOR from Giardia (SOR(Gi)) was purified and characterized by pulse radiolysis and stopped-flow spectrophotometry. The protein, isolated in the reduced state, after oxidation by superoxide or hexachloroiridate(IV), yields a resting species (T(final)) with Fe(3+) ligated to glutamate or hydroxide depending on pH (apparent pK(a)=8.7). Although showing negligible SOD activity, reduced SOR(Gi) reacts with O(2)(•-) with a pH-independent second-order rate constant k(1)=1.0×10(9) M(-1) s(-1) and yields the ferric-(hydro)peroxo intermediate T(1); this in turn rapidly decays to the T(final) state with pH-dependent rates, without populating other detectable intermediates. Immunoblotting assays show that SOR(Gi) is expressed in the disease-causing trophozoite of Giardia. We propose that the superoxide-scavenging activity of SOR in Giardia may promote the survival of this air-sensitive parasite in the fairly aerobic proximal human small intestine during infection.

Pyruvate:ferredoxin oxidoreductase and thioredoxin reductase are involved in 5-nitroimidazole activation while flavin metabolism is linked to 5-nitroimidazole resistance in Giardia lamblia

OBJECTIVES

The mechanism of action of, and resistance to, metronidazole in the anaerobic (or micro-aerotolerant) protozoan parasite Giardia lamblia has long been associated with the reduction of ferredoxin (Fd) by the enzyme pyruvate:ferredoxin oxidoreductase (PFOR) and the subsequent activation of metronidazole by Fd to toxic radical species. Resistance to metronidazole has been associated with down-regulation of PFOR and Fd. The aim of this study was to determine whether the PFOR/Fd couple is the only pathway involved in metronidazole activation in Giardia.

METHODS

PFOR and Fd activities were measured in extracts of highly metronidazole-resistant (MTR(r)) lines and activities of recombinant G. lamblia thioredoxin reductase (GlTrxR) and NADPH oxidase were assessed for their involvement in metronidazole activation and resistance.

RESULTS

We demonstrated that several lines of highly MTR(r) G. lamblia have fully functional PFOR and Fd indicating that PFOR/Fd-independent mechanisms are involved in metronidazole activation and resistance in these cells. Flavin-dependent GlTrxR, like TrxR of other anaerobic protozoa, reduces 5-nitroimidazole compounds including metronidazole, although expression of TrxR is not decreased in MTR(r) Giardia. However, reduction of flavins is suppressed in highly MTR(r) cells, as evidenced by as much as an 80% decrease in NADPH oxidase flavin mononucleotide reduction activity. This suppression is consistent with generalized impaired flavin metabolism in highly MTR(r) Trichomonas vaginalis.

CONCLUSIONS

These data add to the mounting evidence against the dogma that PFOR/Fd is the only couple with a low enough redox potential to reduce metronidazole in anaerobes and point to the multi-factorial nature of metronidazole resistance.

Interplay between flavodiiron proteins and photorespiration in Synechocystis sp. PCC 6803

Flavodiiron (Flv) proteins are involved in detoxification of O(2) and NO in anaerobic bacteria and archaea. Cyanobacterial Flv proteins, on the contrary, function in oxygenic environment and possess an extra NAD(P)H:flavin oxidoreductase module. Synechocystis sp. PCC 6803 has four genes (sll1521, sll0219, sll0550, and sll0217) encoding Flv proteins (Flv1, Flv2, Flv3, and Flv4). Previous in vitro studies with recombinant Flv3 protein from Synechocystis provided evidence that it functions as a NAD(P)H:oxygen oxidoreductase, and subsequent in vivo studies with Synechocystis confirmed the role of Flv1 and Flv3 proteins in the Mehler reaction (photoreduction of O(2) to H(2)O). Interestingly, homologous proteins to Flv1 and Flv3 can be found also in green algae, mosses, and Selaginella. Here, we addressed the function of Flv1 and Flv3 in Synechocystis using the Δflv1, Δflv3, and Δflv1/Δflv3 mutants and applying inorganic carbon (C(i))-deprivation conditions. We propose that only the Flv1/Flv3 heterodimer form is functional in the Mehler reaction in vivo. (18)O(2) labeling was used to discriminate between O(2) evolution in photosynthetic water splitting and O(2) consumption. In wild type, ∼20% of electrons originated from water was targeted to O(2) under air level CO(2) conditions but increased up to 60% in severe limitation of C(i). Gas exchange experiments with Δflv1, Δflv3, and Δflv1/Δflv3 mutants demonstrated that a considerable amount of electrons in these mutants is directed to photorespiration under C(i) deprivation. This assumption is in line with increased transcript abundance of photorespiratory genes and accumulation of photorespiratory intermediates in the WT and to a higher extent in mutant cells under C(i) deprivation.

Giardia intestinalis escapes oxidative stress by colonizing the small intestine: A molecular hypothesis

Giardia intestinalis is the microaerophilic protozoon causing giardiasis, a common infectious intestinal disease. Giardia possesses an O(2) -scavenging activity likely essential for survival in the host. We report that Giardia trophozoites express the O(2) -detoxifying flavodiiron protein (FDP), detected by immunoblotting, and are able to reduce O(2) to H(2) O rapidly (∼3 μM O(2) × min × 10(6) cells at 37 °C) and with high affinity (C(50) = 3.4 ± 0.7 μM O(2)). Following a short-term (minutes) exposure to H(2) O(2) ≥ 100 μM, the O(2) consumption by the parasites is irreversibly impaired, and the FDP undergoes a degradation, prevented by the proteasome-inhibitor MG132. Instead, H(2) O(2) does not cause degradation or inactivation of the isolated FDP. On the basis of the elevated susceptibility of Giardia to oxidative stress, we hypothesize that the parasite preferentially colonizes the small intestine since, compared with colon, it is characterized by a greater capacity for redox buffering and a lower propensity to oxidative stress.

Bioinspired heme, heme/nonheme diiron, heme/copper, and inorganic NOx chemistry: *NO((g)) oxidation, peroxynitrite-metal chemistry, and *NO((g)) reductive coupling

The focus of this Forum Article highlights work from our own laboratories and those of others in the area of biochemical and biologically inspired inorganic chemistry dealing with nitric oxide [nitrogen monoxide, *NO((g))] and its biological roles and reactions. The latter focus is on (i) oxidation of *NO((g)) to nitrate by nitric oxide dioxygenases (NODs) and (ii) reductive coupling of two molecules of *NO((g)) to give N(2)O(g). In the former case, NODs are described, and the highlighting of possible peroxynitrite/heme intermediates and the consequences of this are given by a discussion of recent works with myoglobin and a synthetic heme model system for NOD action. Summaries of recent copper complex chemistries with *NO((g)) and O(2)(g), leading to peroxynitrite species, are given. The coverage of biological reductive coupling of *NO((g)) deals with bacterial nitric oxide reductases (NORs) with heme/nonheme diiron active sites and on heme/copper oxidases such as cytochrome c oxidase, which can mediate the same chemistry. Recently designed protein and synthetic model compounds (heme/nonheme/diiron or heme/copper) as functional mimics are discussed in some detail. We also highlight examples from the chemical literature, not necessarily involving biologically relevant metal ions, that describe the oxidation of *NO((g)) to nitrate (or nitrite) and possible peroxynitrite intermediates or reductive coupling of *NO((g)) to give nitrous oxide.

Insights into the nitric oxide reductase mechanism of flavodiiron proteins from a flavin-free enzyme

Flavodiiron proteins (FDPs) catalyze reductive scavenging of dioxygen and nitric oxide in air-sensitive microorganisms. FDPs contain a distinctive non-heme diiron/flavin mononucleotide (FMN) active site. Alternative mechanisms for the nitric oxide reductase (NOR) activity consisting of either protonation of a diiron-bridging hyponitrite or "super-reduction" of a diferrous-dinitrosyl by the proximal FMNH(2) in the rate-determining step have been proposed. To test these alternative mechanisms, we examined a deflavinated FDP (deflavo-FDP) from Thermotoga maritima. The deflavo-FDP retains an intact diiron site but does not exhibit multiturnover NOR or O(2) reductase (O(2)R) activity. Reactions of the reduced (diferrous) deflavo-FDP with nitric oxide were examined by UV-vis absorption, EPR, resonance Raman, and FTIR spectroscopies. Anaerobic addition of nitric oxide up to one NO per diferrous deflavo-FDP results in formation of a diiron-mononitrosyl complex characterized by a broad S = (1)/(2 )EPR signal arising from antiferromagnetic coupling of an S = (3)/(2) {FeNO}(7) with an S = 2 Fe(II). Further addition of NO results in two reaction pathways, one of which produces N(2)O and the diferric site and the other of which produces a stable diiron-dinitrosyl complex. Both NO-treated and as-isolated deflavo-FDPs regain full NOR and O(2)R activities upon simple addition of FMN. The production of N(2)O upon addition of NO to the mononitrosyl deflavo-FDP supports the hyponitrite mechanism, but the concomitant formation of a stable diiron-dinitrosyl complex in the deflavo-FDP is consistent with a super-reduction pathway in the flavinated enzyme. We conclude that a diiron-mononitrosyl complex is an intermediate in the NOR catalytic cycle of FDPs.

Flavohemoglobin and nitric oxide detoxification in the human protozoan parasite Giardia intestinalis

Flavohemoglobins (flavoHbs), commonly found in bacteria and fungi, afford protection from nitrosative stress by degrading nitric oxide (NO) to nitrate. Giardia intestinalis, a microaerophilic parasite causing one of the most common intestinal human infectious diseases worldwide, is the only pathogenic protozoon as yet identified coding for a flavoHb. By NO amperometry we show that, in the presence of NADH, the recombinant Giardia flavoHb metabolizes NO with high efficacy under aerobic conditions (TN=116+/-10s(-1) at 1microM NO, T=37 degrees C). The activity is [O(2)]-dependent and characterized by an apparent K(M,O2)=22+/-7microM. Immunoblotting analysis shows that the protein is expressed at low levels in the vegetative trophozoites of Giardia; accordingly, these cells aerobically metabolize NO with low efficacy. Interestingly, in response to nitrosative stress (24-h incubation with 5mM nitrite) flavoHb expression is enhanced and the trophozoites thereby become able to metabolize NO efficiently, the activity being sensitive to both cyanide and carbon monoxide. The NO-donors S-nitrosoglutathione (GSNO) and DETA-NONOate mimicked the effect of nitrite on flavoHb expression. We propose that physiologically flavoHb contributes to NO detoxification in G. intestinalis.

Diversity in mitochondrial metabolic pathways in parasitic protists Plasmodium and Cryptosporidium

Apicomplexans are obligate intracellular parasites and occupy diverse niches. They have remodeled mitochondrial carbon and energy metabolism through reductive evolution. Plasmodium lacks mitochondrial pyruvate dehydrogenase and H(+)-translocating NADH dehydrogenase (Complex I, NDH1). The mitochondorion contains a minimal mtDNA ( approximately 6kb) and carries out oxidative phosphorylation in the insect vector stages, by using 2-oxoglutarate as an alternative means of entry into the TCA cycle and a single-subunit flavoprotein as an alternative NADH dehydrogenase (NDH2). In the blood stages of mammalian hosts, mitochondrial enzymes are down-regulated and parasite energy metabolism relies mainly on glycolysis. Mitosomes of Cryptosporidium parvum and Cryptosporidium hominis (human intestine parasites) lack mtDNA, pyruvate dehydrogenase, TCA cycle enzymes except malate-quinone oxidoreductase (MQO), and ATP synthase subunits except alpha and beta. In contrast, mitosomes of Cryptosporidium muris (a rodent gastric parasite) retain all TCA cycle enzymes and functional ATP synthase and carry out oxidative phosphorylation with pyruvate-NADP(+) oxidoreductase (PNO) and a simple and unique respiratory chain consisting of NDH2 and alternative oxidase (AOX). Cryptosporidium and Perkinsus are early branching groups of chromoalveolates (apicomplexa and dinoflagellates, respectively), and both Cryptosporidium mitosome and Perkinsus mitochondrion use PNO, MQO, and AOX. All apicomplexan parasites and dinoflagellates share MQO, which has been acquired from epsilon-proteobacteria via lateral gene transfer. By genome data mining on Plasmodium, Cryptosporidium and Perkinsus, here we summarized their mitochondrial metabolic pathways, which are varied largely from those of mammalian hosts. We hope that our findings will help in understanding the apicomplexan metabolism and development of new chemotherapeutics with novel targets.

Thermosynechococcus elongatus DpsA binds Zn(II) at a unique three histidine-containing ferroxidase center and utilizes O2 as iron oxidant with very high efficiency, unlike the typical Dps proteins

The cyanobacterium Thermosynechococcus elongatus is one the few bacteria to possess two Dps proteins, DpsA-Te and Dps-Te. The present characterization of DpsA-Te reveals unusual structural and functional features that differentiate it from Dps-Te and the other known Dps proteins. Notably, two Zn(II) are bound at the ferroxidase center, owing to the unique substitution of a metal ligand at the A-site (His78 in place of the canonical aspartate) and to the presence of a histidine (His164) in place of a hydrophobic residue at a metal-coordinating distance in the B-site. Only the latter Zn(II) is displaced by incoming iron, such that Zn(II)-Fe(III) complexes are formed upon oxidation, as indicated by absorbance and atomic emission spectroscopy data. In contrast to the typical behavior of Dps proteins, where Fe(II) oxidation by H(2)O(2) is about 100-fold faster than by O(2), in DpsA-Te the ferroxidation efficiency of O(2) is very high and resembles that of H(2)O(2). Oxygraphic experiments show that two Fe(II) are required to reduce O(2), and that H(2)O(2) is not released into solution at the end of the reaction. On this basis, a reaction mechanism is proposed that also takes into account the formation of Zn(II)-Fe(III) complexes. The physiological significance of the DpsA-Te behavior is discussed in the framework of a possible localization of the protein at the thylakoid membranes, where photosynthesis takes place, with the consequent increased formation of reactive oxygen species.

Redox properties of the oxygen-detoxifying flavodiiron protein from the human parasite Giardia intestinalis

Flavodiiron proteins (FDPs) are enzymes identified in prokaryotes and a few pathogenic protozoa, which protect microorganisms by reducing O(2) to H(2)O and/or NO to N(2)O. Unlike most prokaryotic FDPs, the protozoan enzymes from the human pathogens Giardia intestinalis and Trichomonas vaginalis are selective towards O(2). UV/vis and EPR spectroscopy showed that, differently from the NO-consuming bacterial FDPs, the Giardia FDP contains an FMN with reduction potentials for the formation of the single and the two-electron reduced forms very close to each other (E(1)=-66+/-15mV and E(2)=-83+/-15mV), a condition favoring destabilization of the semiquinone radical. Giardia FDP contains also a non-heme diiron site with significantly up-shifted reduction potentials (E(1)=+163+/-20mV and E(2)=+2+/-20mV). These properties are common to the Trichomonas hydrogenosomal FDP, and likely reflect yet undetermined subtle structural differences in the protozoan FDPs, accounting for their marked O(2) specificity.

Dioxygen and nitric oxide pathways and affinity to the catalytic site of rubredoxin:oxygen oxidoreductase from Desulfovibrio gigas

Rubredoxin:oxygen oxidoreductase (ROO) is the terminal oxidase of a soluble electron transfer chain found in Desulfovibrio gigas. This protein belongs to the flavodiiron family and was initially described as an oxygen reductase, converting this substrate to water and acting as an oxygen-detoxifying system. However, more recent studies evidenced also the ability for this protein to act as a nitric oxide reductase, suggesting an alternative physiological role. To clarify the apparent bifunctional nature of this protein, we performed molecular dynamics simulations of the protein, in different redox states, together with O(2) and NO molecules in aqueous solution. The two small molecules were parameterized using free-energy calculations of the hydration process. With these simulations we were able to identify specific protein paths that allow the diffusion of both these molecules through the protein towards the catalytic centers. Also, we have tried to characterize the preference of ROO towards the presence of O(2) and/or NO at the active site. By using free-energy simulations, we did not find any significant preference for ROO to accommodate both O(2) and NO. Also, from our molecular dynamics simulations we were able to identify similar diffusion profiles for both O(2) and NO molecules. These two conclusions are in good agreement with previous experimental works stating that ROO is able to catalyze both O(2) and NO.

Reductive dioxygen scavenging by flavo-diiron proteins of Clostridium acetobutylicum

Two flavo-diiron proteins (FDPs), FprA1 and FprA2, are up-regulated when the strictly anaerobic solvent producer, Clostridium acetobutylicum, is exposed to dioxygen. These two FDPs were purified following heterologous overexpression in Escherichia coli as N-terminal Strep-tag fusion proteins. The recombinant FprA1 and FprA2 were found to be homodimeric and homotetrameric, respectively, and both FDPs functioned as terminal components of NADH oxidases (NADH:O(2) oxidoreductases) when using C. acetobutylicum NADH:rubredoxin oxidoreductase (NROR) and rubredoxin (Rd) as electron transport intermediaries. Both FDPs catalyzed the four-electron reduction of molecular oxygen to water with similar specific activities. The results are consistent with these FDPs functioning as efficient scavengers of intracellular dioxygen under aerobic or microoxic growth conditions.

Quaternary structure of flavorubredoxin as revealed by synchrotron radiation small-angle X-ray scattering

Flavodiiron proteins (FDP) are modular enzymes which function as NO and/or O(2) reductases. Although the majority is composed of two structural domains, the homolog found in Escherichia coli, flavorubredoxin, possesses an extra C-terminal module consisting of a linker and a rubredoxin (Rd) domain necessary for interprotein redox processes. In order to investigate the location of the Rd domain with respect to the flavodiiron structural core, small-angle X-ray scattering was used to construct low-resolution structural models of flavorubredoxin. Scattering patterns from the Rd domain, the FDP core, and full-length flavorubredoxin were collected. The latter two species were found to be tetrameric in solution. Ab initio shape reconstruction and rigid-body modeling indicate a peripheral location for the Rd domains, which appear to have weak contacts with the FDP core. This finding suggests that Rd behaves as an independent domain and is freely available to participate in redox reactions with protein partners.

Flavodiiron protein from Trichomonas vaginalis hydrogenosomes: the terminal oxygen reductase

Trichomonas vaginalis is one of a few eukaryotes that have been found to encode several homologues of flavodiiron proteins (FDPs). Widespread among anaerobic prokaryotes, these proteins are believed to function as oxygen and/or nitric oxide reductases to provide protection against oxidative/nitrosative stresses and host immune responses. One of the T. vaginalis FDP homologues is equipped with a hydrogenosomal targeting sequence and is expressed in the hydrogenosomes, oxygen-sensitive organelles that participate in carbohydrate metabolism and assemble iron-sulfur clusters. The bacterial homologues characterized thus far have been dimers or tetramers; the trichomonad protein is a dimer of identical 45-kDa subunits, each noncovalently binding one flavin mononucleotide. The protein reduces dioxygen to water but is unable to utilize nitric oxide as a substrate, similarly to its closest homologue from another human parasite Giardia intestinalis and related archaebacterial proteins. T. vaginalis FDP is able to accept electrons derived from pyruvate or NADH via ferredoxin and is proposed to play a role in the protection of hydrogenosomes against oxygen.

Entamoeba histolytica modulates a complex repertoire of novel genes in response to oxidative and nitrosative stresses: implications for amebic pathogenesis

Upon host infection, the protozoan parasite Entamoeba histolytica is confronted with reactive oxygen and nitrogen species and must survive these stresses in order to cause invasive disease. We analysed the parasite's response to oxidative and nitrosative stresses, probing the transcriptional changes of trophozoites of a pathogenic strain after a 60 min exposure to H2O2 (1 mM) or a NO donor (dipropylenetriamine-NONOate, 200 microM), using whole-genome DNA microarrays. Genes encoding reactive oxygen and nitrogen species detoxification enzymes had high transcriptional levels under basal conditions and upon exposure to both stresses. On a whole-genome level, there was significant modulation of gene expression by H2O2 (286 genes regulated) and dipropylenetriamine-NONOate (1036 genes regulated) with a significant overlap of genes modulated under both conditions (164 genes). A number of transcriptionally regulated genes were in signalling/regulatory and repair/metabolic pathways. However, the majority of genes with altered transcription encode unknown proteins, suggesting as yet unraveled response pathways in E. histolytica. Trophozoites of a non-pathogenic E. histolytica strain had a significantly muted transcriptional response to H2O2 compared with the pathogenic strain, hinting that differential response to oxidative stress may be one factor that contributes to the pathogenic potential of E. histolytica.