The structure of Trypanosoma cruzi trypanothione reductase in the oxidized and NADPH reduced state

Antiparasitic Activity of Coumarin-Chalcone (3-Cinnamoyl-2H-Chromen-2-Ones) Hybrids

Coumarin-chalcone hybrids are promising compounds that could be used as lead structures in the fight against parasitic diseases. In this work, 16 hybrids of coumarin-chalcone (3-cinnamoyl-2H-chromen-2-ones) were synthesized, and their in vitro biological activity was evaluated against intracellular amastigotes of Leishmania braziliensis and Trypanosoma cruzi, as well as their cytotoxicity in the U-937 cell line. Compounds (E)-3-(3-(3-ethoxy-4-hydroxyphenyl)acryloyl)-7-methoxy-2H-chromen-2-one (H25) and (E)-7-(diethylamino)-3-(4-(methoxyphenyl)acryloyl)-2H-chromen-2-one (H12) showed the highest antileishmanial activity with EC50 values of 18.6 ± 3.5 and 25.6 ± 0.4 µM, respectively. In general, all 16 compounds exhibited moderate-to-high antitrypanosomal activity. The H25 hybrid also demonstrated the greatest antitrypanosomal activity, with an EC50 value of 13.2 ± 0.4 µM. Notably, the H25 hybrid displayed activity similar to that of benznidazole, which is known for its antiparasitic effects against T. cruzi. The results indicated that all compounds met the drug-like properties criteria. Taking into account the high antiparasitic activity of H25, a molecular docking study with the enzyme trypanothione reductase was performed. The substituent at C7 in the coumarinyl system is an important structural requirement for the antileishmanial and antitrypanosomal activities.

In Silico Exploration of the Trypanothione Reductase (TryR) of L. mexicana

Human leishmaniasis is a neglected tropical disease which affects nearly 1.5 million people every year, with Mexico being an important endemic region. One of the major defense mechanisms of these parasites is based in the polyamine metabolic pathway, as it provides the necessary compounds for its survival. Among the enzymes in this route, trypanothione reductase (TryR), an oxidoreductase enzyme, is crucial for the Leishmania genus' survival against oxidative stress. Thus, it poses as an attractive drug target, yet due to the size and features of its catalytic pocket, modeling techniques such as molecular docking focusing on that region is not convenient. Herein, we present a computational study using several structure-based approaches to assess the druggability of TryR from L. mexicana, the predominant Leishmania species in Mexico, beyond its catalytic site. Using this consensus methodology, three relevant pockets were found, of which the one we call σ-site promises to be the most favorable one. These findings may help the design of new drugs of trypanothione-related diseases.

Mitochondrial Ca2+ and Reactive Oxygen Species in Trypanosomatids

Significance: Millions of people are infected with trypanosomatids and new therapeutic approaches are needed. Trypanosomatids possess one mitochondrion per cell and its study has led to discoveries of general biological interest. These mitochondria, as in their animal counterparts, generate reactive oxygen species (ROS) and have evolved enzymatic and nonenzymatic defenses against them. Mitochondrial calcium ion (Ca2+) overload leads to generation of ROS and its study could lead to relevant information on the biology of trypanosomatids and to novel drug targets. Recent Advances: Mitochondrial Ca2+ is normally involved in maintaining the bioenergetics of trypanosomes, but when Ca2+ overload occurs, it is associated with cell death. Trypanosomes lack key players in the mechanism of cell death described in mammalian cells, although mitochondrial Ca2+ overload results in collapse of their membrane potential, production of ROS, and cytochrome c release. They are also very resistant to mitochondrial permeability transition, and cell death after mitochondrial Ca2+ overload depends on generation of ROS. Critical Issues: In this review, we consider the mechanisms of mitochondrial oxidant generation and removal and the involvement of Ca2+ in trypanosome cell death. Future Directions: More studies are required to determine the reactions involved in generation of ROS by the mitochondria of trypanosomatids, their enzymatic and nonenzymatic defenses against ROS, and the occurrence and composition of a mitochondrial permeability transition pore. Antioxid. Redox Signal. 36, 969-983.

Trypanothione Metabolism as Drug Target for Trypanosomatids

Chagas Disease, African sleeping sickness, and leishmaniasis are neglected diseases caused by pathogenic trypanosomatid parasites, which have a considerable impact on morbidity and mortality in poor countries. The available drugs used as treatment have high toxicity, limited access, and can cause parasite drug resistance. Long-term treatments, added to their high toxicity, result in patients that give up therapy. Trypanosomatids presents a unique trypanothione based redox system, which is responsible for maintaining the redox balance. Therefore, inhibition of these essential and exclusive parasite's metabolic pathways, absent from the mammalian host, could lead to the development of more efficient and safe drugs. The system contains different redox cascades, where trypanothione and tryparedoxins play together a central role in transferring reduced power to different enzymes, such as 2-Cys peroxiredoxins, non-selenium glutathione peroxidases, ascorbate peroxidases, glutaredoxins and methionine sulfoxide reductases, through NADPH as a source of electrons. There is sufficient evidence that this complex system is essential for parasite survival and infection. In this review, we explore what is known in terms of essentiality, kinetic and structural data, and the development of inhibitors of enzymes from this trypanothione-based redox system. The recent advances and limitations in the development of lead inhibitory compounds targeting these enzymes have been discussed. The combination of molecular biology, bioinformatics, genomics, and structural biology is fundamental since the knowledge of unique features of the trypanothione-dependent system will provide tools for rational drug design in order to develop better treatments for these diseases.

Synthesis of new N,S-acetal analogs derived from juglone with cytotoxic activity against Trypanossoma cruzi

A series of 11 new N,S-acetal juglone derivatives were synthesized and evaluated against T. cruzi epimastigote forms. These compounds were obtained in good to moderate yields using a microwave irradiation protocol. Among all compounds, two N,S-acetal analogs, showed significant trypanocidal activity. Notably, one compound 11g exhibited selectivity index 10-fold higher than the reference drug benznidazole for epimastigote. The compound 11h was more effective for amastigote forms. Both prototypes exhibited S.I. higher than the benznidazole description. Thus, both compounds proving to be useful candidate molecules to further studies in infected animals.

Advances in preclinical approaches to Chagas disease drug discovery

Introduction: Chagas disease affects 8-10 million people worldwide, mainly in Latin America. The current therapy for Chagas disease is limited to nifurtimox and benznidazole, which are effective in treating only the acute phase of the disease but with severe side effects. Therefore, there is an unmet need for new drugs and for the exploration of innovative approaches which may lead to the discovery of new effective and safe drugs for its treatment. Areas covered: The authors report and discuss recent approaches including structure-based design that have led to the discovery of new promising small molecule candidates for Chagas disease which affect prime targets that intervene in the sterol pathway of T. cruzi. Other trypanosome targets, phenotypic screening, the use of artificial intelligence and the challenges with Chagas disease drug discovery are also discussed. Expert opinion: The application of recent scientific innovations to the field of Chagas disease have led to the discovery of new promising drug candidates for Chagas disease. Phenotypic screening brought new hits and opportunities for drug discovery. Artificial intelligence also has the potential to accelerate drug discovery in Chagas disease and further research into this is warranted.

The Potential of Secondary Metabolites from Plants as Drugs or Leads against Protozoan Neglected Diseases-Part III: In-Silico Molecular Docking Investigations

Malaria, leishmaniasis, Chagas disease, and human African trypanosomiasis continue to cause considerable suffering and death in developing countries. Current treatment options for these parasitic protozoal diseases generally have severe side effects, may be ineffective or unavailable, and resistance is emerging. There is a constant need to discover new chemotherapeutic agents for these parasitic infections, and natural products continue to serve as a potential source. This review presents molecular docking studies of potential phytochemicals that target key protein targets in Leishmania spp., Trypanosoma spp., and Plasmodium spp.

Structure and catalytic mechanism of monodehydroascorbate reductase, MDHAR, from Oryza sativa L. japonica

Ascorbic acid (AsA) maintains redox homeostasis by scavenging reactive oxygen species from prokaryotes to eukaryotes, especially plants. The enzyme monodehydroascorbate reductase (MDHAR) regenerates AsA by catalysing the reduction of monodehydroascorbate, using NADH or NADPH as an electron donor. The detailed recycling mechanism of MDHAR remains unclear due to lack of structural information. Here, we present the crystal structures of MDHAR in the presence of cofactors, nicotinamide adenine dinucleotide (NAD⁺) and nicotinamide adenine dinucleotide phosphate (NADP⁺), and complexed with AsA as well as its analogue, isoascorbic acid (ISD). The overall structure of MDHAR is similar to other iron-sulphur protein reductases, except for a unique long loop of 63–80 residues, which seems to be essential in forming the active site pocket. From the structural analysis and structure-guided point mutations, we found that the Arg320 residue plays a major substrate binding role, and the Tyr349 residue mediates electron transfer from NAD(P)H to bound substrate via FAD. The enzymatic activity of MDHAR favours NADH as an electron donor over NADPH. Our results show, for the first time, structural insights into this preference. The MDHAR-ISD complex structure revealed an alternative binding conformation of ISD, compared with the MDHAR-AsA complex. This implies a broad substrate (antioxidant) specificity and resulting greater protective ability of MDHAR.

Trypanothione reductase inhibitors: Overview of the action of thioridazine in different stages of Chagas disease

Thioridazine (TDZ) is a phenothiazine that has been shown to be one of the most potent phenothiazines to inhibit trypanothione reductase irreversibly. Trypanothione reductase is an essential enzyme for the survival of Trypanosoma cruzi in the host. Here, we reviewed the use of this drug for the treatment of T. cruzi experimental infection. In our laboratory, we have studied the effect of TDZ for the treatment of mice infected with different strains of T. cruzi and treated in the acute or in the chronic phases of the experimental infection, using two different schedules: TDZ at a dose of 80 mg/kg/day, for 3 days starting 1h after infection (acute phase), or TDZ 80 mg/kg/day for 12 days starting 180 days post infection (d.p.i.) (chronic phase). In our experience, the treatment of infected mice, in the acute or in the chronic phases of the infection, with TDZ led to a large reduction in the mortality rates and in the cardiac histological and electrocardiographical abnormalities, and modified the natural evolution of the experimental infection. These analyses reinforce the importance of treatment in the chronic phase to decrease, retard or stop the evolution to chagasic myocardiopathy. Other evidence leading to the use of this drug as a potential chemotherapeutic agent for Chagas disease treatment is also revised.

Conformational behavior of flavin adenine dinucleotide: conserved stereochemistry in bound and free states

Metabolic enzymes utilize the cofactor flavin adenine dinucleotide (FAD) to catalyze essential biochemical reactions. Because these enzymes have been implicated in disease pathways, it will be necessary to target them via FAD-based structural analogues that can either activate/inhibit the enzymatic activity. To achieve this, it is important to explore the conformational space of FAD in the enzyme-bound and free states. Herein, we analyze X-ray crystallographic data of the enzyme-bound FAD conformations and sample conformations of the molecule in explicit water by molecular dynamics (MD) simulations. Enzyme-bound FAD conformations segregate into five distinct groups based on dihedral angle principal component analysis (PCA). A notable feature in the bound FADs is that the adenine base and isoalloxazine ring are oppositely oriented relative to the pyrophosphate axis characterized by near trans hypothetical dihedral angle "δV" values. Not surprisingly, MD simulations in water show final compact but not perfectly stacked ring structures in FAD. Simulation data did not reveal noticeable changes in overall conformational dynamics of the dinucleotide in reduced and oxidized forms and in the presence and/or absence of ions. During unfolding-folding dynamics, the riboflavin moiety is more flexible than the adenosine monophosphate group in the molecule. Conversely, the isoalloxazine ring is more stable than the variable adenine base. The pyrophosphate group depicts an unusually highly organized fluctuation illustrated by its dihedral angle distribution. Conformations sampled from enzymes and MD are quantified. The extent to which the protein shifts the distribution from the unbound state is discussed in terms of prevalent FAD shapes and dihedral angle population.

Inhibition of Leishmania infantum trypanothione reductase by azole-based compounds: a comparative analysis with its physiological substrate by X-ray crystallography

Herein we report a study aimed at discovering a new class of compounds that are able to inhibit Leishmania donovani cell growth. Evaluation of an in-house library of compounds in a whole-cell screening assay highlighted 4-((1-(4-ethylphenyl)-2-methyl-5-(4-(methylthio)phenyl)-1H-pyrrol-3-yl)methyl)thiomorpholine (compound 1) as the most active. Enzymatic assays on Leishmania infantum trypanothione reductase (LiTR, belonging to the Leishmania donovani complex) shed light on both the interaction with, and the nature of inhibition by, compound 1. A molecular modeling approach based on docking studies and on the estimation of the binding free energy aided our rationalization of the biological data. Moreover, X-ray crystal structure determination of LiTR in complex with compound 1 confirmed all our results: compound 1 binds to the T(SH)2 binding site, lined by hydrophobic residues such as Trp21 and Met113, as well as residues Glu18 and Tyr110. Analysis of the structure of LiTR in complex with trypanothione shows that Glu18 and Tyr110 are also involved in substrate binding, according to a competitive inhibition mechanism.

The orphan protein bis-γ-glutamylcystine reductase joins the pyridine nucleotide disulfide reductase family

Facile DNA sequencing became possible decades after many enzymes had been purified and characterized. Consequently, there are still "orphan" enyzmes for which activities are known but for which encoding genes have not been identified. Identification of the genes encoding orphan enzymes is important because it allows correct annotation of genes of unknown function or with misassigned function. Bis-γ-glutamylcystine reductase (GCR) is an orphan protein that was purified in 1988. This enzyme catalyzes the reduction of bis-γ-glutamylcystine. γ-Glutamylcysteine is the major low-molecular weight thiol in halobacteria. We purified GCR from Halobacterium sp. NRC-1 and identified the sequence of 23 tryptic peptides by nano-liquid chromatography electrospray ionization tandem mass spectrometry. These peptides cover 62% of the protein predicted to be encoded by a gene in Halobacterium sp. NRC-1 that is annotated as mercuric reductase. GCR and mercuric reductase activities were assayed using enzyme that was expressed in Escherichia coli and refolded from inclusion bodies. The enzyme had robust GCR activity but no mercuric reductase activity. The genomes of most, but not all, halobacteria for which whole genome sequences are available have close homologues of GCR, suggesting that there is more to be learned about the low-molecular weight thiols used in halobacteria.

The receptor-dependent LQTA-QSAR: application to a set of trypanothione reductase inhibitors

A new Receptor-Dependent LQTA-QSAR approach, RD-LQTA-QSAR, is proposed as a new 4D-QSAR method. It is an evolution of receptor independent LQTA-QSAR. This approach uses the free GROMACS package to carry out molecular dynamics simulations and generates a conformational ensemble profile for each compound. Such an ensemble is used to build molecular interaction field-based QSAR models, as in CoMFA. To show the potential of this methodology, a set of 38 phenothiazine derivatives that are specific competitive T. cruzi trypanothione reductase inhibitors, was chosen. Using a combination of molecular docking and molecular dynamics simulations, the binding mode of the phenotiazine derivatives was evaluated in a simulated induced fit approach. The ligands alignments were performed using both ligand and binding site atoms, enabling unbiased alignment. The models obtained were extensively validated by leave-N-out cross-validation and y-randomization techniques to test for their robustness and absence of chance correlation. The final model presented Q(2) LOO of 0.87 and R² of 0.92 and a suitable external prediction of [Formula: see text]= 0.78. The adapted binding site obtained is useful to perform virtual screening and ligand structure-based design and the descriptors in the final model can aid in the design new inhibitors.

Trypanocidal drugs: mechanisms, resistance and new targets

The protozoan parasitesTrypanosoma bruceiandTrypanosoma cruziare the causative agents of African trypanosomiasis and Chagas disease, respectively. These are debilitating infections that exert a considerable health burden on some of the poorest people on the planet. Treatment of trypanosome infections is dependent on a small number of drugs that have limited efficacy and can cause severe side effects. Here, we review the properties of these drugs and describe new findings on their modes of action and the mechanisms by which resistance can arise. We further outline how a greater understanding of parasite biology is being exploited in the search for novel chemotherapeutic agents. This effort is being facilitated by new research networks that involve academic and biotechnology/pharmaceutical organisations, supported by public–private partnerships, and are bringing a new dynamism and purpose to the search for trypanocidal agents.

Molecular basis of antimony treatment in leishmaniasis

Leishmaniasis is a disease that affects 2 million people and kills 70000 persons every year. It is caused by Leishmania species, which are human protozoan parasites of the trypanosomatidae family. Trypanosomatidae differ from the other eukaryotes in their specific redox metabolism because the glutathione/glutathione reductase system is replaced by the unique trypanothione/trypanothione reductase system. The current treatment of leishmaniasis relies mainly on antimonial drugs. The crystal structures of oxidized trypanothione reductase (TR) from Leishmania infantum and of the complex of reduced TR with NADPH and Sb(III), reported in this paper, disclose for the first time the molecular mechanism of action of antimonial drugs against the parasite. Sb(III), which is coordinated by the two redox-active catalytic cysteine residues (Cys52 and Cys57), one threonine residue (Thr335), and His461' of the 2-fold symmetry related subunit in the dimer, strongly inhibits TR activity. Because TR is essential for the parasite survival and virulence and it is absent in mammalian cells, these findings provide insights toward the design of new more affordable and less toxic drugs against Leishmaniasis.

Development of a novel virtual screening cascade protocol to identify potential trypanothione reductase inhibitors

The implementation of a novel sequential computational approach that can be used effectively for virtual screening and identification of prospective ligands that bind to trypanothione reductase (TryR) is reported. The multistep strategy combines a ligand-based virtual screening for building an enriched library of small molecules with a docking protocol (AutoDock, X-Score) for screening against the TryR target. Compounds were ranked by an exhaustive conformational consensus scoring approach that employs a rank-by-rank strategy by combining both scoring functions. Analysis of the predicted ligand−protein interactions highlights the role of bulky quaternary amine moieties for binding affinity. The scaffold hopping (SHOP) process derived from this computational approach allowed the identification of several chemotypes, not previously reported as antiprotozoal agents, which includes dibenzothiepine, dibenzooxathiepine, dibenzodithiepine, and polycyclic cationic structures like thiaazatetracyclo-nonadeca-hexaen-3-ium. Assays measuring the inhibiting effect of these compounds on T. cruzi and T. brucei TryR confirm their potential for further rational optimization.

Structures of the multicomponent Rieske non-heme iron toluene 2,3-dioxygenase enzyme system

Bacterial Rieske non-heme iron oxygenases catalyze the initial hydroxylation of aromatic hydrocarbon substrates. The structures of all three components of one such system, the toluene 2,3-dioxygenase system, have now been determined. This system consists of a reductase, a ferredoxin and a terminal dioxygenase. The dioxygenase, which was cocrystallized with toluene, is a heterohexamer containing a catalytic and a structural subunit. The catalytic subunit contains a Rieske [2Fe-2S] cluster and mononuclear iron at the active site. This iron is not strongly bound and is easily removed during enzyme purification. The structures of the enzyme with and without mononuclear iron demonstrate that part of the structure is flexible in the absence of iron. The orientation of the toluene substrate in the active site is consistent with the regiospecificity of oxygen incorporation seen in the product formed. The ferredoxin is Rieske type and contains a [2Fe-2S] cluster close to the protein surface. The reductase belongs to the glutathione reductase family of flavoenzymes and consists of three domains: an FAD-binding domain, an NADH-binding domain and a C-terminal domain. A model for electron transfer from NADH via FAD in the reductase and the ferredoxin to the terminal active-site mononuclear iron of the dioxygenase is proposed.

Mechanism and structure-activity relationships of norspermidine-based peptidic inhibitors of trypanothione reductase

A library of polyamine-peptide conjugates based around some previously identified inhibitors of trypanothione reductase was synthesised by parallel solid-phase chemistry and screened. Kinetic analysis of library members established that subtle structural changes altered their mechanism of action, switching between competitive and non-competitive inhibition. The mode of action of the non-competitive inhibitors was investigated in detail by a variety of techniques including enzyme kinetic analysis (looking at both NADPH and trypanothione disulfide substrates), gel filtration chromatography and analytical ultracentrifugation, leading to the identification of an allosteric mode of inhibition.

Inhibitors of Trypanosoma cruzi trypanothione reductase revealed by virtual screening and parallel synthesis

In an approach to discover new inhibitors of trypanothione reductase from Trypanosoma cruzi, the causative agent of Chagas' disease, a virtual high-throughput screening was performed. Two structurally new types of inhibitors emerged, the antimicrobial chlorhexidine {1,1'-hexamethylenebis[5-(4-chlorophenyl)biguanide]}, a linear competitive inhibitor (K(i) = 2 +/- 1 microM), and a piperidine derivative acting as mixed inhibitor (K(i) = 6.2 +/- 2 microM and K(i)' = 8.5 +/- 2 microM). Neither compound interferes with human glutathione reductase. Based on chlorhexidine, different series of compounds were synthesized and studied as inhibitors of T. cruzi trypanothione reductase. Most efficient derivatives were three bis(amidines) showing mixed type inhibition with K(i,slope) and K(i,int) values of 2-5 microM and 16-47 microM, respectively. Although these compounds did not exert an improved inhibitory potency compared to chlorhexidine, the change from competitive to mixed-type inhibition is advantageous, since substrate accumulation does not overcome inhibition. Remarkably, all three derivatives carried two copies of an identical 2-methoxy-4-methyl-1-(phenylmethoxy)benzene substituent.

Dithiol Proteins as Guardians of the Intracellular Redox Milieu in Parasites: Old and New Drug Targets in Trypanosomes and Malaria-Causing Plasmodia

Parasitic diseases such as sleeping sickness, Chagas' heart disease, and malaria are major health problems in poverty-stricken areas. Antiparasitic drugs that are not only active but also affordable and readily available are urgently required. One approach to finding new drugs and rediscovering old ones is based on enzyme inhibitors that paralyze antioxidant systems in the pathogens. These antioxidant ensembles are essential to the parasites as they are attacked in the human host by strong oxidants such as peroxynitrite, hypochlorite, and H2O2. The pathogen-protecting system consists of some 20 thiol and dithiol proteins, which buffer the intraparasitic redox milieu at a potential of -250 mV. In trypanosomes and leishmania the network is centered around the unique dithiol trypanothione (N1,N8-bis(glutathionyl)spermidine). In contrast, malaria parasites have a more conservative dual antioxidative system based on glutathione and thioredoxin. Inhibitors of antioxidant enzymes such as trypanothione reductase are, indeed, parasiticidal but they can also delay or prevent resistance against a number of other antiparasitic drugs.

Quinoxaline N , N ′-dioxide derivatives and related compounds as growth inhibitors of Trypanosoma cruzi . Structure–activity relationships

Quinoxaline derivatives presented good inhibitor activity of growth of Trypanosoma cruzi in in vitro assays. The 50% inhibitory doses were of the same order of that of Nifurtimox. Derivative 13, a quinoxaline N,N'-dioxide derivative, and the reduced derivatives 19 and 20 were the most cytotoxic compounds against the protozoan. Structural requirements for optimal activity were studied by computational methods. From statistical analysis we could establish a multiple correlation between activity and lipophilic properties and LUMO energy.

Flavoprotein Disulfide Reductases: Advances in Chemistry and Function

The flavoprotein disulfide reductases represent a family of enzymes that show high sequence and structural homology. They catalyze the pyridine-nucleotide-dependent reduction of a variety of substrates, including disulfide-bonded substrates (lipoamide dehydrogenase, glutathione reductase and functional homologues, thioredoxin reductase, and alkylhydroperoxide reductase), mercuric ion (mercuric ion reductase), hydrogen peroxide (NADH peroxidase), molecular oxygen (NADH oxidase), and the reductive cleavage of a carbonyl-activated carbon-sulfur bond followed by carboxylation (2-ketopropyl-coenzyme-M carboxylase?oxidoreductase). They use at least one nonflavin redox center to transfer electrons from reduced pyridine nucleotide to their substrate through flavin adenine dinucleotide. The nature of the nonflavin redox center located adjacent to the flavin varies and three types have been identified: an enzymic disulfide (most commonly), an enzymic cysteine sulfenic acid (NADH peroxidase and NADH oxidase), and a mixed Cys-S-S-CoA disulfide (coenzyme A disulfide reductase). Selection of the particular nonflavin redox center and utilization of a second, or even a third, nonflavin redox center in some cases presumably represents the most efficient strategy for reduction of the individual substrate.

The parasite-specific trypanothione metabolism of trypanosoma and leishmania

The bis(glutathionyl)spermidine trypanothione exclusively occurs in parasitic protozoa of the order Kinetoplastida, such as trypanosomes and leishmania, some of which are the causative agents of several tropical diseases. The dithiol is kept reduced by the flavoenzyme trypanothione reductase and the trypanothione system replaces in these parasites the nearly ubiquitous glutathione/glutathione reductase couple. Trypanothione is a reductant of thioredoxin and tryparedoxin, small dithiol proteins, which in turn deliver reducing equivalents for the synthesis of deoxyribonucleotides as well as for the detoxification of hydroperoxides by different peroxidases. Depending on the individual organism and the developmental state, the parasites also contain significant amounts of glutathione, mono-glutathionylspermidine and ovothiol, whereby all four low molecular mass thiols are directly (trypanothione and mono-glutathionylspermidine) or indirectly (glutathione and ovothiol) maintained in the reduced state by trypanothione reductase. Thus the trypanothione system is central for any thiol regeneration and trypanothione reductase has been shown to be an essential enzyme in these parasites. The absence of this pathway from the mammalian host and the sensitivity of trypanosomatids toward oxidative stress render the enzymes of the trypanothione metabolism attractive target molecules for the rational development of new drugs against African sleeping sickness, Chagas' disease and the different forms of leishmaniasis.

The trypanothione-thiol system in Trypanosoma cruzi as a key antioxidant mechanism against peroxynitrite-mediated cytotoxicity

Peroxynitrite, the reaction product between superoxide (O(*2)) and nitric oxide (*NO), is a powerful oxidizing species that contributes to macrophage competence against pathogens. In this context, peroxynitrite appears to play an important role in controlling infection by Trypanosoma cruzi, the unicellular parasite responsible for Chagas disease. T. cruzi contains various enzyme systems for the decomposition of hydroperoxides, all of which involve the participation of the low-molecular-weight dithiol trypanothione (N(1),N(8)-bis(glutathionyl)spermidine) as a critical redox partner. A large fraction of the trypanothione-dependent antioxidant capacity of T. cruzi is linked to the tryparedoxin-tryparedoxin peroxidase system which has critical protein thiol groups. In this report we demonstrate that dihydrotrypanothione is readily consumed during peroxynitrite challenge to cells to yield the corresponding trypanothione disulfide. On the other hand, glutathione, which is present in T. cruzi at lower concentrations than trypanothione, is consumed to a much lesser extent and mainly evolves to glutathione-protein mixed disulfides. The inhibition of glutathione biosynthesis by buthionine sulfoximine, which decreases glutathione concentration to 10% of control after 20 h, neither affects the concentration of dihydrotrypanothione nor sensitizes T. cruzi to peroxynitrite-mediated cytotoxicity. On the other hand, pretreatment of T. cruzi with diamide, which leads to a significant depletion (>70%) of dihydrotrypanothione, largely increases the extent of cellular nitration and inhibition of cell growth caused by peroxynitrite. Altogether, our findings support a key protective role for dihydrotrypanothione and the trypanothione-dependent antioxidant system in T. cruzi against peroxynitrite, which may facilitate the survival of trypanosomes within the oxidative environment of activated macrophages.

Coupling of a competitive and an irreversible ligand generates mixed type inhibitors of Trypanosoma cruzi trypanothione reductase

9-Aminoacridines and (terpyridine)platinum(II) complexes are competitive and irreversible inhibitors, respectively, of trypanothione reductase from Trypanosoma cruzi, the causative agent of Chagas' disease. Four chimeric compounds in which 2-methoxy-6-chloro-9-aminoacridine was covalently linked to the (2-hydroxyethanethiolate)(2,2':6',2' '-terpyridine)platinum(II) complex were synthesized and studied as inhibitors of the parasite enzyme. The derivatives differed by the nature and/or the length of the spacer connecting the two aromatic systems. All four compounds were effective mixed type inhibitors of trypanothione reductase with K(i) and K(i)' values of 0.3-4 and 2-11 microM, respectively. The most potent inhibitor had an ethylthioether linkage between the two aromatic ring systems, and the other compounds contained an alkyl ether group with 4-6 methylene groups. In contrast to the parasite enzyme, human glutathione reductase, the closest related host enzyme was not inhibited by these compounds. The finding that the conjugation of a competitive and an irreversible inhibitor can give rise to reversible mixed type inhibitors underlines the difficulties associated with inhibitor design based on the three-dimensional structure of trypanothione reductase.

(2,2':6',2"-Terpyridine)platinum(II) complexes are irreversible inhibitors of Trypanosoma cruzi trypanothione reductase but not of human glutathione reductase

(2,2':6',2"-terpyridine)platinum(II) complexes possess pronounced cytostatic activities against trypanosomes and leishmania. As shown here, the complexes are irreversible inhibitors of trypanothione reductase (TR) from Trypanosoma cruzi, the causative agent of Chagas' disease. The most effective derivatives are the (4'-chloro-2, 2':6',2"-terpyridine)platinum(II) ammine and the (4-picoline)(4'-p-bromophenyl-2,2':6',2" -terpyridine)platinum(II) complexes which in the presence of NADPH inhibit TR with second-order rate constants of about 1.3 x 10(4) M(-1) s(-1). The modified enzyme species possess increased oxidase activities. The inhibition is not reversed upon dialysis or treatment with low-molecular-mass thiols. Kinetic and spectroscopic data suggest that Cys52 in the active site has been specifically altered. Inhibition of this key enzyme of parasite thiol metabolism probably contributes to the antitrypanosomal activity of the compounds. In contrast to the parasite enzyme, most (terpyridine)platinum complexes interact only reversibly with human glutathione reductase and an initial inhibition is completely abolished during the course of the assay.

Crystal structure of NADH-dependent ferredoxin reductase component in biphenyl dioxygenase

Oxidative biodegradation of aromatic compounds by bacteria usually begins with hydroxylation of the aromatic ring by multi-component dioxygenases like benzene dioxygenase, biphenyl dioxygenase, and others. These enzymes are composed of ferredoxin reductase, ferredoxin, and terminal oxygenase. Reducing equivalents that originate from NADH are transferred from ferredoxin reductase to ferredoxin and, in turn, to the terminal oxygenase, thus resulting in the activation of a dioxygen. BphA4 is the ferredoxin reductase component of biphenyl dioxygenase from Pseudomonas sp. strain KKS102. The amino acid sequence of BphA4 exhibits significant homology with the putidaredoxin reductase of the cytochrome P450cam system in Pseudomonas putida, as well as with various other oxygenase-coupled NADH-dependent ferredoxin reductases (ONFRs) of bacteria. To date, no structural information has been provided for the ferredoxin reductase component of the dioxygenase systems. In order to provide a structural basis for discussing the mechanism of electron transport between ferredoxin reductase and ferredoxin, crystal structures of BphA4 and its NADH complex were solved. The three-dimensional structure of BphA4 is different from those of ferredoxin reductases whose structures have already been determined, but adopts essentially the same fold as the enzymes of the glutathione reductase (GR) family. Also the three-dimensional structure of the first two domains of BphA4 adopts a fold similar to that of adrenodoxin reductase (AdR) in the mitochondrial cytochrome P450 system. Comparing the amino acid sequence with what is known of the three-dimensional structure of BphA4 strongly suggests that the other ONFRs have secondary structural features that are similar to that of BphA4. This analysis of the crystal structures of BphA4 suggests that Lys53 and Glu159 seem to be involved in the hydride transfer from NADH to FAD. Since the amino acid residues around the active site, some of which seem to be important to electron transport, are highly conserved among ONFRs, it is likely that the mechanism of electron transport of BphA4 is quite applicable to other ONFRs.

Synthesis and antitrypanosomal evaluation of E-isomers of 5-nitro-2-furaldehyde and 5-nitrothiophene-2-carboxaldehyde semicarbazone derivatives. structure-activity relationships

Several novel semicarbazone derivatives were prepared from 5-nitro-2-furaldehyde or 5-nitrothiophene-2-carboxaldehyde and semicarbazides bearing a spermidine-mimetic moiety. All derivatives presented the E-configuration, as determined by NMR-NOE experiments. These compounds were tested in vitro as potential antitrypanosomal agents, and some of them, together with the parent compounds, 5-nitro-2-furaldehyde and 5-nitrothiophene-2-carboxaldehyde semicarbazone derivatives, were also evaluated in vivo using infected mice. Structure-activity relationship studies were carried out using voltammetric response and lipophilic-hydrophilic balance as parameters. Two of the compounds (1 and 3) displayed the highest in vivo activity. A correlation was found between lipophilic-hydrophilic properties and trypanocidal activity, high R(M) values being associated with low in vivo effects.

Inhibition of Trypanosoma cruzi trypanothione reductase by acridines: kinetic studies and structure-activity relationships

Series of 9-amino and 9-thioacridines have been synthesized and studied as inhibitors of trypanothione reductase (TR) from Trypanosoma cruzi, the causative agent of Chagas' disease. The compounds are structural analogues of the acridine drug mepacrine (quinacrine), which is a competitive inhibitor of the parasite enzyme, but not of human glutathione reductase, the closest related host enzyme. The 9-aminoacridines yielded apparent K(i) values for competitive inhibition between 5 and 43 microM. The most effective inhibitors were those with the methoxy and chlorine substituents of mepacrine and NH(2) or NHCH(CH(3))(CH(2))(4)N(Et)(2) at C9. Detailed kinetic analyses revealed that in the case of 9-aminoacridines more than one inhibitor molecule can bind to the enzyme. In contrast, the 9-thioacridine derivatives inhibit TR with mixed-type kinetics. The kinetic data are discussed in light of the three-dimensional structure of the TR-mepacrine complex. The conclusion that structurally very similar acridine compounds can give rise to completely different inhibition patterns renders modelling studies and quantitative structure-activity relationships difficult.

1,2,5-Oxadiazole N-oxide derivatives and related compounds as potential antitrypanosomal drugs: structure-activity relationships

The syntheses of a new series of derivatives of 1,2,5-oxadiazole N-oxide, benzo[1,2-c]1,2,5-oxadiazole N-oxide, and quinoxaline di-N-oxide are described. In vitro antitrypanosomal activity of these compounds was tested against epimastigote forms of Trypanosoma cruzi. For the most effective drugs, derivatives IIIe and IIIf, the 50% inhibitory dose (ID50) was determined as well as their cytotoxicity against mammalian fibroblasts. Electrochemical studies and ESR spectroscopy show that the highest activities observed are associated with the facile monoelectronation of the N-oxide moiety. Lipophilic-hydrophilic balance of the compounds could also play an important role in their effectiveness as antichagasic drugs.

Coenzyme A disulfide reductase, the primary low molecular weight disulfide reductase from Staphylococcus aureus. Purification and characterization of the native enzyme

The human pathogen Staphylococcus aureus does not utilize the glutathione thiol/disulfide redox system employed by eukaryotes and many bacteria. Instead, this organism produces CoA as its major low molecular weight thiol. We report the identification and purification of the disulfide reductase component of this thiol/disulfide redox system. Coenzyme A disulfide reductase (CoADR) catalyzes the specific reduction of CoA disulfide by NADPH. CoADR has a pH optimum of 7.5-8.0 and is a dimer of identical subunits of Mr 49,000 each. The visible absorbance spectrum is indicative of a flavoprotein with a lambdamax = 452 nm. The liberated flavin from thermally denatured enzyme was identified as flavin adenine dinucleotide. Steady-state kinetic analysis revealed that CoADR catalyzes the reduction of CoA disulfide by NADPH at pH 7.8 with a Km for NADPH of 2 muM and for CoA disulfide of 11 muM. In addition to CoA disulfide CoADR reduces 4,4'-diphosphopantethine but has no measurable ability to reduce oxidized glutathione, cystine, pantethine, or H2O2. CoADR demonstrates a sequential kinetic mechanism and employs a single active site cysteine residue that forms a stable mixed disulfide with CoA during catalysis. These data suggest that S. aureus employs a thiol/disulfide redox system based on CoA/CoA-disulfide and CoADR, an unorthodox new member of the pyridine nucleotide-disulfide reductase superfamily.

Phenothiazine inhibitors of trypanothione reductase as potential antitrypanosomal and antileishmanial drugs

Given the role of trypanothione in the redox defenses of pathogenic trypanosomal and leishmanial parasites, in contrast to glutathione for their mammalian hosts, selective inhibitors of trypanothione reductase are potential drug leads against trypanosomiasis and leishmaniasis. In the present study, the rational drug design approach was used to discover tricyclic neuroleptic molecular frameworks as lead structures for the development of inhibitors, selective for trypanothione reductase over host glutathione reductase. From a homology-modeled structure for trypanothione reductase, replaced in the later stages of the study by the X-ray coordinates for the enzyme from Crithidia fasciculata, a series of inhibitors based on phenothiazine was designed. These were shown to be reversible inhibitors of trypanothione reductase from Trypanosoma cruzi, linearly competitive with trypanothione as substrate and noncompetitive with NADPH, consistent with ping-pong bi bi kinetics. Analogues, synthesized to define structure-activity relationships for the active site, included N-acylpromazines, 2-substituted phenothiazines, and trisubstituted promazines. Analysis of Ki and I50 data, on the basis of calculated log P and molar refractivity values, provided evidence of a specially favored fit of small 2-substituents (especially 2-chloro and 2-trifluoromethyl), with a remote hydrophobic patch on the enzyme accessible for larger, hydrophobic 2-substituents. There was also evidence of an additional hydrophobic enzymic region available to suitable N-substituents of the promazine nucleus. Ki data also indicated that the phenothiazine nucleus can adopt more than one inhibitory orientation in its binding site. Selected compounds were tested for in vitro activity against Trypanosoma brucei, T. cruzi, and Leishmania donovani, with selective activities in the micromolar range being determined for a number of them.

Rational design of selective ligands for trypanothione reductase from Trypanosoma cruzi. Structural effects on the inhibition by dibenzazepines based on imipramine

Trypanothione reductase, the enzyme which in trypanosomal and leishmanial parasites catalyses the reduction of trypanothione disulphide to the redox-protective dithiol and has been identified as a potential target for rational antiparasite drug design, has been found to be strongly inhibited by tricyclic compounds containing the saturated dibenzazepine (imipramine) nucleus, with Ki values in the low micromolar range. This drug lead structure was designed by molecular graphics analysis of a three-dimensional homology model, focussing on the active-site. Inhibition studies were carried out to determine the effect of inhibitor structure on the inhibitory strength towards recombinant trypanothione reductase from Trypanosoma cruzi. Hansch analysis showed that inhibitory strength depended on terms in pi, pi 2 and sigma m indicating dependence on both lipophilicity and inductive effect for ring-substituted analogues of imipramine. The side-chain omega-aminoalkyl chain had to be longer than 2-carbon units for inhibition. The effect on inhibition strength of the substituent at the omega-amino position on the side-chain of the central ring nitrogen atom depended markedly on the detailed substitution pattern of the rest of the molecule. This provides kinetic evidence studies of multiple binding modes within a single, blanket binding site for the inhibitor with the tricyclic ring system in the general region of the hydrophobic pocket lined by Trp21, Tyr110, Met113 and Phe114. This aspect of the structural sensitivity of the precise active-site triangulation adopted by the inhibitor is probably a function of the use of hydrophobic interactions of low directional specificity in this pocket combined with an electrostatic anchoring by the omega-N+ HMe2 function of the inhibitor, presumably with a glutamate side-chain, such as Glu-18, Glu-466' and/or Glu-467'.

Inhibiting effects of spermidine derivatives on Trypanosoma cruzi trypanothione reductase

Trypanothione reductase is a vital component of the antioxidant defenses of trypanosomes. This enzyme reduces trypanothione, a spermidine-glutathione conjugate. The inhibitory effects of several spermidine derivatives on the reduction of trypanothione by Trypanosoma cruzi trypanothione reductase were assessed. Spermidine derivatives containing hydrophobic aromatic substituents were found to be competitive inhibitors of trypanothione reductase. N4-acylated spermidine derivatives were less effective inhibitors than the corresponding N4-alkylated derivatives. The most effective compounds studied were N1, N8-bis(2-naphthylmethyl)spermidine and N4-(2-naphthylmethyl)spermidine, with Ki values of 9.5 and 108 microM, respectively.

Covalent structure of the flavoprotein subunit of the flavocytochrome c: sulfide dehydrogenase from the purple phototrophic bacterium Chromatium vinosum

The amino acid sequence of the flavoprotein subunit of Chromatium vinosum flavocytochrome c-sulfide dehydrogenase (FCSD) was determined by automated Edman degradation and mass spectrometry in conjunction with the three-dimensional structure determination (Chen Z et al., 1994, Science 266:430-432). The sequence of the diheme cytochrome c subunit was determined previously. The flavoprotein contains 401 residues and has a calculated protein mass, including FAD, of 43,568 Da, compared with a mass of 43,652 +/- 44 Da measured by LDMS. There are six cysteine residues, among which Cys 42 provides the site of covalent attachment of the FAD. Cys 161 and Cys 337 form a disulfide bond adjacent to the FAD. The flavoprotein subunit of FCSD is most closely related to glutathione reductase (GR) in three-dimensional structure and, like that protein, contains three domains. However, approximately 20 insertions and deletions are necessary for alignment and the overall identity in sequence is not significantly greater than for random comparisons. The first domain binds FAD in both proteins. Domain 2 of GR is the site of NADP binding, but has an unknown role in FCSD. We postulate that it is the binding site for a cofactor involved in oxidation of reduced sulfur compounds. Domains 1 and 2 of FCSD, as of GR, are homologous to one another and represent an ancient gene doubling. The third domain provides the dimerization interface for GR, but is the site of binding of the cytochrome subunit in FCSD. The four functional entities, predicted to be near the FAD from earlier studies of the kinetics of sulfite adduct formation and decay, have now been identified from the three-dimensional structure and the sequence as Cys 161/Cys 337 disulfide, Trp 391, Glu 167, and the positive end of a helix dipole.

Charge is the major discriminating factor for glutathione reductase versus trypanothione reductase inhibitors

Benson et al. (Biochem. J. 1992, 286, 9) reported three novel competitive inhibitors of trypanothione reductase (TR), which were selected to complement a hydrophobic region identified on the TR structure which was not present on human glutathione reductase (hGR). Benson et al. also noted that chlorpromazine, a tricyclic antidepressant known to have trypanocidal activity, was an inhibitor of TR. Here we show that chlorpromazine is a competitive inhibitor of TRs from Crithidia fasciculata (Ki = 14 microM) and Trypanosoma cruzi (Ki = 10 microM), but the drug binds > 50-fold more weakly (Ki = 762 microM) to hGR. Analogues of chlorpromazine differing in the length of the side chain carrying the positively charged R-group are also selective TR inhibitors whereas, a tricyclic structure carrying a negatively charged side chain is a competitive inhibitor with selectivity for hGR (K(hGR)i = 165 microM vs. K(TR)i = 1400 microM). This finding suggests that simple charge characteristics, rather than differences in hydrophobicity, may account for a significant portion of the selectivity of this series of inhibitors for these two enzymes. Electrostatic analysis of the structures of TR and hGR thus provides a rationale for these results, and offers a new principle for inhibitor design. The principle gains further support from the observation that all known tricyclic competitive inhibitors of TR are positively charged. In order to investigate the in vivo relevance of our findings we have examined the effect of chlorpromazine and its negatively charged analogue on the growth of C. fasciculata parasites. Consistent with our kinetics, chlorpromazine (50 microM) inhibited the growth of parasites by 50%, while no measurable decrease in parasite growth rate was noted in the presence of the negatively charged inhibitor (400 microM). Furthermore, the highly similar inhibitory profiles of C. fasciculata TR and T. cruzi TR suggest that drug-design studies using the structurally better-studied C. fasciculata TR are also relevant to the human pathogen T. cruzi.

Crystal structure of the ternary complex of mouse lung carbonyl reductase at 1.8 A resolution: the structural origin of coenzyme specificity in the short-chain dehydrogenase/reductase family

BACKGROUND

Mouse lung carbonyl reductase (MLCR) is a member of the short-chain dehydrogenase/reductase (SDR) family. Although it uses both NADPH and NADH as coenzymes, the structural basis of its strong preference for NADPH is unknown.

RESULTS

The crystal structure of the ternary complex of MLCR (with NADPH and 2-propanol) has been determined at 1.8 A resolution. This is the first three-dimensional structure of a carbonyl reductase, and MLCR is the first member of the SDR family to be solved in complex with NADPH (rather than NADH). Comparison of the MLCR ternary complex with three structures reported previously for enzymes of the SDR family (all crystallized as complexes with NADH) reveals a pair of basic residues (Lys17 and Arg39) making strong electrostatic interactions with the 2'-phosphate group of NADPH. This pair of residues is well conserved among the NADPH-preferring enzymes of the SDR family, but not among the NADH-preferring enzymes. In the latter, an aspartate side chain occupies the position of the two basic side chains. The aspartate residue, which would come into unacceptably close contact with the 2'-phosphate group of the adenosine moiety of NADPH, is replaced by a threonine or alanine in the primary sequences of NADPH-preferring enzymes of the SDR family.

CONCLUSIONS

The cofactor preferences exhibited by the enzymes of the SDR family are mainly determined by the electrostatic environment surrounding the 2'-hydroxyl (or phosphate) group of the adenosine ribose moiety of NADH (or NADPH). Thus, positively charged and negatively charged environments correlate with preference for NADPH and NADH respectively.