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1.  Leishmania Trypanothione Synthetase-Amidase Structure Reveals a Basis for Regulation of Conflicting Synthetic and Hydrolytic Activities*S⃞ 
The Journal of Biological Chemistry  2008;283(25):17672-17680.
The bifunctional trypanothione synthetase-amidase catalyzes biosynthesis and hydrolysis of the glutathione-spermidine adduct trypanothione, the principal intracellular thiol-redox metabolite in parasitic trypanosomatids. These parasites are unique with regard to their reliance on trypanothione to determine intracellular thiol-redox balance in defense against oxidative and chemical stress and to regulate polyamine levels. Enzymes involved in trypanothione biosynthesis provide essential biological activities, and those absent from humans or for which orthologues are sufficiently distinct are attractive targets to underpin anti-parasitic drug discovery. The structure of Leishmania major trypanothione synthetase-amidase, determined in three crystal forms, reveals two catalytic domains. The N-terminal domain, a cysteine, histidine-dependent amidohydrolase/peptidase amidase, is a papain-like cysteine protease, and the C-terminal synthetase domain displays an ATP-grasp family fold common to C:N ligases. Modeling of substrates into each active site provides insight into the specificity and reactivity of this unusual enzyme, which is able to catalyze four reactions. The domain orientation is distinct from that observed in a related bacterial glutathionylspermidine synthetase. In trypanothione synthetase-amidase, the interactions formed by the C terminus, binding in and restricting access to the amidase active site, suggest that the balance of ligation and hydrolytic activity is directly influenced by the alignment of the domains with respect to each other and implicate conformational changes with amidase activity. The potential inhibitory role of the C terminus provides a mechanism to control relative levels of the critical metabolites, trypanothione, glutathionylspermidine, and spermidine in Leishmania.
PMCID: PMC2427367  PMID: 18420578
2.  Trypanothione S-transferase activity in a trypanosomatid ribosomal elongation factor 1B 
The Journal of biological chemistry  2004;279(26):27246-27256.
Trypanothione is a thiol unique to the Kinetoplastida and has been shown to be a vital component of their antioxidant defences. However, little is known as to the role of trypanothione in xenobiotic metabolism. A trypanothione S-transferase activity was detected in extracts of Leishmania major, L. infantum, L. tarentolae, Trypanosoma brucei and Crithidia fasciculata, but not Trypanosoma cruzi. No glutathione S-transferase activity was detected in any of these parasites. Trypanothione S-transferase was purified from C. fasciculata and shown to be a hexadecameric complex of three subunits with a relative molecular mass of 650,000. This enzyme complex was specific for the thiols trypanothione and glutathionylspermidine, and only used 1-chloro-2,4- dinitrobenzene from a range of glutathione S-transferases substrates. Peptide sequencing revealed that the three components were the alpha, beta and gamma subunits of ribosomal eukaryotic elongation factor 1B (eEF1B). Partial dissociation of the complex suggested that the S-transferase activity was associated with the gamma subunit. Moreover, Cibacron blue was found to be a tight-binding inhibitor and reactive blue 4 an irreversible time-dependent inhibitor that covalently modified only the gamma subunit. The rate of inactivation by reactive blue 4 was increased more than 600-fold in the presence of trypanothione and Cibacron blue protected the enzyme from inactivation by 1-chloro-2,4- dinitrobenzene, confirming that these dyes interact with the active site region. Two eEF1Bγ genes were cloned from C. fasciculata but recombinant C. fasciculata eEF1Bγ had no S-transferase activity, suggesting that eEF1Bγ is unstable in the absence of the other subunits.
PMCID: PMC3428924  PMID: 15073172
3.  A gold-containing drug against parasitic polyamine metabolism: the X-ray structure of trypanothione reductase from Leishmania infantum in complex with auranofin reveals a dual mechanism of enzyme inhibition 
Amino Acids  2011;42(2-3):803-811.
Auranofin is a gold(I)-containing drug in clinical use as an antiarthritic agent. Recent studies showed that auranofin manifests interesting antiparasitic actions very likely arising from inhibition of parasitic enzymes involved in the control of the redox metabolism. Trypanothione reductase is a key enzyme of Leishmania infantum polyamine-dependent redox metabolism, and a validated target for antileishmanial drugs. As trypanothione reductase contains a dithiol motif at its active site and gold(I) compounds are known to be highly thiophilic, we explored whether auranofin might behave as an effective enzyme inhibitor and as a potential antileishmanial agent. Notably, enzymatic assays revealed that auranofin causes indeed a pronounced enzyme inhibition. To gain a deeper insight into the molecular basis of enzyme inhibition, crystals of the auranofin-bound enzyme, in the presence of NADPH, were prepared, and the X-ray crystal structure of the auranofin–trypanothione reductase–NADPH complex was solved at 3.5 Å resolution. In spite of the rather low resolution, these data were of sufficient quality as to identify the presence of the gold center and of the thiosugar of auranofin, and to locate them within the overall protein structure. Gold binds to the two active site cysteine residues of TR, i.e. Cys52 and Cys57, while the thiosugar moiety of auranofin binds to the trypanothione binding site; thus auranofin appears to inhibit TR through a dual mechanism. Auranofin kills the promastigote stage of L. infantum at micromolar concentration; these findings will contribute to the design of new drugs against leishmaniasis.
PMCID: PMC3266496  PMID: 21833767
Gold; Auranofin; Leishmania; Trypanothione reductase
4.  Dissecting the Catalytic Mechanism of Trypanosoma brucei Trypanothione Synthetase by Kinetic Analysis and Computational Modeling* 
The Journal of Biological Chemistry  2013;288(33):23751-23764.
Background: Trypanothione synthetase catalyzes the conjugation of spermidine with two GSH molecules to form trypanothione.
Results: The kinetic parameters were measured under in vivo-like conditions. A mathematical model was developed describing the entire kinetic profile.
Conclusion: Trypanothione synthetase is affected by substrate and product inhibition.
Significance: The combined kinetic and modeling approaches provided a so far unprecedented insight in the mechanism of this parasite-specific enzyme.
In pathogenic trypanosomes, trypanothione synthetase (TryS) catalyzes the synthesis of both glutathionylspermidine (Gsp) and trypanothione (bis(glutathionyl)spermidine (T(SH)2)). Here we present a thorough kinetic analysis of Trypanosoma brucei TryS in a newly developed phosphate buffer system at pH 7.0 and 37 °C, mimicking the physiological environment of the enzyme in the cytosol of bloodstream parasites. Under these conditions, TryS displays Km values for GSH, ATP, spermidine, and Gsp of 34, 18, 687, and 32 μm, respectively, as well as Ki values for GSH and T(SH)2 of 1 mm and 360 μm, respectively. As Gsp hydrolysis has a Km value of 5.6 mm, the in vivo amidase activity is probably negligible. To obtain deeper insight in the molecular mechanism of TryS, we have formulated alternative kinetic models, with elementary reaction steps represented by linear kinetic equations. The model parameters were fitted to the extensive matrix of steady-state data obtained for different substrate/product combinations under the in vivo-like conditions. The best model describes the full kinetic profile and is able to predict time course data that were not used for fitting. This system's biology approach to enzyme kinetics led us to conclude that (i) TryS follows a ter-reactant mechanism, (ii) the intermediate Gsp dissociates from the enzyme between the two catalytic steps, and (iii) T(SH)2 inhibits the enzyme by remaining bound at its product site and, as does the inhibitory GSH, by binding to the activated enzyme complex. The newly detected concerted substrate and product inhibition suggests that TryS activity is tightly regulated.
PMCID: PMC3745322  PMID: 23814051
Enzyme Kinetics; Glutathione; Mathematical Modeling; Thiol; Trypanosoma brucei; Glutathionylspermidine
5.  Comparative structural, kinetic and inhibitor studies of Trypanosoma brucei trypanothione reductase with T. cruzi☆ 
As part of a drug discovery programme to discover new treatments for human African trypanosomiasis, recombinant trypanothione reductase from Trypanosoma brucei has been expressed, purified and characterized. The crystal structure was solved by molecular replacement to a resolution of 2.3 Å and found to be nearly identical to the T. cruzi enzyme (root mean square deviation 0.6 Å over 482 Cα atoms). Kinetically, the Km for trypanothione disulphide for the T. brucei enzyme was 4.4-fold lower than for T. cruzi measured by either direct (NADPH oxidation) or DTNB-coupled assay. The Km for NADPH for the T. brucei enzyme was found to be 0.77 μM using an NADPH-regenerating system coupled to reduction of DTNB. Both enzymes were assayed for inhibition at their respective S = Km values for trypanothione disulphide using a range of chemotypes, including CNS-active drugs such as clomipramine, trifluoperazine, thioridazine and citalopram. The relative IC50 values for the two enzymes were found to vary by no more than 3-fold. Thus trypanothione reductases from these species are highly similar in all aspects, indicating that they may be used interchangeably for structure-based inhibitor design and high-throughput screening.
PMCID: PMC2789240  PMID: 19747949
TryR, trypanothione reductase; T(S)2, trypanothione disulphide; DTNB, 5,5′-dithio-bis(2-nitrobenzoic acid); HAT, human African trypanosomiasis; Trypanothione metabolism; Trypanosome; Thiol; Enzymology; Drug discovery
6.  Antitumor Quinol PMX464 Is a Cytocidal Anti-trypanosomal Inhibitor Targeting Trypanothione Metabolism* 
The Journal of Biological Chemistry  2011;286(10):8523-8533.
Better drugs are urgently needed for the treatment of African sleeping sickness. We tested a series of promising anticancer agents belonging to the 4-substituted 4-hydroxycyclohexa-2,5-dienones class (“quinols”) and identified several with potent trypanocidal activity (EC50 < 100 nm). In mammalian cells, quinols are proposed to inhibit the thioredoxin/thioredoxin reductase system, which is absent from trypanosomes. Studies with the prototypical 4-benzothiazole-substituted quinol, PMX464, established that PMX464 is rapidly cytocidal, similar to the arsenical drug, melarsen oxide. Cell lysis by PMX464 was accelerated by addition of sublethal concentrations of glucose oxidase implicating oxidant defenses in the mechanism of action. Whole cells treated with PMX464 showed a loss of trypanothione (T(SH)2), a unique dithiol in trypanosomes, and tryparedoxin peroxidase (TryP), a 2-Cys peroxiredoxin similar to mammalian thioredoxin peroxidase. Enzyme assays revealed that T(SH)2, TryP, and a glutathione peroxidase-like tryparedoxin-dependent peroxidase were inhibited in time- and concentration-dependent manners. The inhibitory activities of various quinol analogues against these targets showed a good correlation with growth inhibition of Trypanosoma brucei. The monothiols glutathione and l-cysteine bound in a 2:1 ratio with PMX464 with Kd values of 6 and 27 μm, respectively, whereas T(SH)2 bound more tightly in a 1:1 ratio with a Kd value of 430 nm. Overexpression of trypanothione synthetase in T. brucei decreased sensitivity to PMX464 indicating that the key metabolite T(SH)2 is a target for quinols. Thus, the quinol pharmacophore represents a novel lead structure for the development of a new drug against African sleeping sickness.
PMCID: PMC3048735  PMID: 21212280
Drug Action; Metabolism; Peroxidase; Thiol; Trypanosome; Quinol; Trypanothione; Tryparedoxin Peroxidase
7.  Trypanothione Reductase: A Viable Chemotherapeutic Target for Antitrypanosomal and Antileishmanial Drug Design 
Drug Target Insights  2007;2:129-146.
Trypanosomiasis and leishmaniasis are two debilitating disease groups caused by parasites of Trypanosoma and Leishmania spp. and affecting millions of people worldwide. A brief outline of the potential targets for rational drug design against these diseases are presented, with an emphasis placed on the enzyme trypanothione reductase. Trypanothione reductase was identified as unique to parasites and proposed to be an effective target against trypanosomiasis and leishmaniasis. The biochemical basis of selecting this enzyme as a target, with reference to the simile and contrast to human analogous enzyme glutathione reductase, and the structural aspects of its active site are presented. The process of designing selective inhibitors for the enzyme trypanothione reductase has been discussed. An overview of the different chemical classes of inhibitors of trypanothione reductase with their inhibitory activities against the parasites and their prospects as future chemotherapeutic agents are briefly revealed.
PMCID: PMC3155241  PMID: 21901070
Trypanothione; glutathione; Chagas disease; sleeping sickness; rational drug design
8.  Transgenic Biosynthesis of Trypanothione Protects Escherichia coli from Radiation-Induced Toxicity 
Radiation research  2010;174(3):290-296.
Trypanothione is a unique diglutathionyl-spermidine conjugate found in abundance in trypanosomes but not in other eukaryotes. Because trypanothione is a naturally occurring polyamine thiol reminiscent of the synthetic drug amifostine, it may be a useful protector against radiation and oxidative stress. For these reasons we hypothesized that trypanothione might serve as a radioprotective agent when produced in bacteria. To accomplish this objective, the trypanothione synthetase and reductase genes from T. cruzi were introduced into E. coli and their expression was verified by qPCR and immunoblotting. Trypanothione synthesis in bacteria, detected by HPLC, resulted in decreased intracellular levels of reactive oxygen species as determined by H2DCFDA oxidation. Moreover, E. coli genomic DNA was protected from radiation-induced DNA damage by 4.6-fold in the presence of trypanothione compared to control bacteria. Concordantly, the transgenic E. coli expressing trypanothione were 4.3-fold more resistant to killing by 137Cs γ radiation compared to E. coli devoid of trypanothione expression. Thus we have shown for the first time that E. coli can be genetically engineered to express the trypanothione biosynthetic pathway and produce trypanothione, which results in their radioresistance. These results warrant further research to explore the possibility of developing trypanothione as a novel radioprotective agent.
PMCID: PMC2942016  PMID: 20726720
9.  Benznidazole Biotransformation and Multiple Targets in Trypanosoma cruzi Revealed by Metabolomics 
The first line treatment for Chagas disease, a neglected tropical disease caused by the protozoan parasite Trypanosoma cruzi, involves administration of benznidazole (Bzn). Bzn is a 2-nitroimidazole pro-drug which requires nitroreduction to become active, although its mode of action is not fully understood. In the present work we used a non-targeted MS-based metabolomics approach to study the metabolic response of T. cruzi to Bzn.
Methodology/Principal findings
Parasites treated with Bzn were minimally altered compared to untreated trypanosomes, although the redox active thiols trypanothione, homotrypanothione and cysteine were significantly diminished in abundance post-treatment. In addition, multiple Bzn-derived metabolites were detected after treatment. These metabolites included reduction products, fragments and covalent adducts of reduced Bzn linked to each of the major low molecular weight thiols: trypanothione, glutathione, γ-glutamylcysteine, glutathionylspermidine, cysteine and ovothiol A. Bzn products known to be generated in vitro by the unusual trypanosomal nitroreductase, TcNTRI, were found within the parasites, but low molecular weight adducts of glyoxal, a proposed toxic end-product of NTRI Bzn metabolism, were not detected.
Our data is indicative of a major role of the thiol binding capacity of Bzn reduction products in the mechanism of Bzn toxicity against T. cruzi.
Author Summary
The unicellular parasite Trypanosoma cruzi infects humans, leading to Chagas disease, endemic in Central and South America and responsible for 13,000 annual deaths. Only two drugs have proven effective against Chagas, nifurtimox and benznidazole (Bzn). Bzn has the best safety and efficacy profiles and is thus used as first line treatment. Bzn is a pro-drug, and possesses a nitro group which needs to be enzymatically reduced within the parasite to become active. We have investigated for the first time, by means of mass spectrometry based metabolomics, the global changes to small metabolites that occur once Bzn enters the parasite. A decrease in the levels of several thiols, including cysteine and trypanothione, and an increase in gamma-glutamyl containing dipeptides were observed after treatment. Reduced metabolites of Bzn were also detected, together with numerous covalent conjugates of the drug combined with low molecular weight thiols and some non-thiol metabolites. Overall, Bzn treatment primarily affects thiol containing molecules in T. cruzi, and this interference with thiol metabolism contributes to the drug's mode of action.
PMCID: PMC4031082  PMID: 24853684
10.  Improved Tricyclic Inhibitors of Trypanothione Reductase by Screening and Chemical Synthesis 
Chemmedchem  2009;4(8):1333-1340.
Trypanothione reductase (TryR) is a key validated enzyme in the trypanothione-based redox metabolism of pathogenic trypanosomes and leishmania parasites. This system is absent in humans, being replaced with glutathione and glutathione reductase, and as such offers a target for selective inhibition. As part of a program to discover antiparasitic drugs, the LOPAC1280 library of 1266 compounds was screened against TryR and the top hits evaluated against glutathione reductase and T. brucei parasites. The top hits included a number of known tricyclic neuroleptic drugs along with other new scaffolds for TryR. Three novel druglike hits were identified and SAR studies on one of these using information from the tricyclic neuroleptic agents led to the discovery of a competitive inhibitor (Ki=330 nm) with an improved potency against T. brucei (EC50=775 nm).
PMCID: PMC2929371  PMID: 19557801
drug discovery; inhibitors; oxidoreductases; trypanosoma brucei; trypanothione reductase
11.  Molecular Dynamics Reveal Binding Mode of Glutathionylspermidine by Trypanothione Synthetase 
PLoS ONE  2013;8(2):e56788.
The trypanothione synthetase (TryS) catalyses the two-step biosynthesis of trypanothione from spermidine and glutathione and is an attractive new drug target for the development of trypanocidal and antileishmanial drugs, especially since the structural information of TryS from Leishmania major has become available. Unfortunately, the TryS structure was solved without any of the substrates and lacks loop regions that are mechanistically important. This contribution describes docking and molecular dynamics simulations that led to further insights into trypanothione biosynthesis and, in particular, explains the binding modes of substrates for the second catalytic step. The structural model essentially confirm previously proposed binding sites for glutathione, ATP and two Mg2+ ions, which appear identical for both catalytic steps. The analysis of an unsolved loop region near the proposed spermidine binding site revealed a new pocket that was demonstrated to bind glutathionylspermidine in an inverted orientation. For the second step of trypanothione synthesis glutathionylspermidine is bound in a way that preferentially allows N1-glutathionylation of N8-glutathionylspermidine, classifying N8-glutathionylspermidine as the favoured substrate. By inhibitor docking, the binding site for N8-glutathionylspermidine was characterised as druggable.
PMCID: PMC3581523  PMID: 23451087
12.  Inhibitory Effect of Silver Nanoparticles on Trypanothione Reductase Activity and Leishmania infantum Proliferation 
ACS Medicinal Chemistry Letters  2010;2(3):230-233.
In Leishmania the glutathione/glutathione reductase eukaryotic redox sys-tem is replaced by the unique trypanothione/trypanothione reductase (TR) system. In vitro, silver is a more effective TR inhibitor than antimony, the first line drug against leishmaniasis in most endemic countries, and its mechanism of inhibition is similar to that of Sb(III). In particular, silver binds with high affinity to the catalytic triad Cys52, Cys57, and His461′, thereby inhibiting TR. Here, Ag(0) activity was tested on the promastigote and amastigote stages of Leishmania infantum using a drug-delivery system consisting in Ag(0) nanoparticles encapsulated by ferritin molecules (PfFt−AgNPs). These were able to induce an antiproliferative effect on the parasites at metal concentrations lower than those used with antimony.
PMCID: PMC4017975  PMID: 24900299
Trypanothione reductase; Leishmania; silver; nanoparticles; drug delivery
13.  Mono- and Dithiol Glutaredoxins in the Trypanothione-Based Redox Metabolism of Pathogenic Trypanosomes 
Antioxidants & Redox Signaling  2013;19(7):708-722.
Significance: Glutaredoxins are ubiquitous small thiol proteins of the thioredoxin-fold superfamily. Two major groups are distinguished based on their active sites: the dithiol (2-C-Grxs) and the monothiol (1-C-Grxs) glutaredoxins with a CXXC and a CXXS active site motif, respectively. Glutaredoxins are involved in cellular redox and/or iron sulfur metabolism. Usually their functions are closely linked to the glutathione system. Trypanosomatids, the causative agents of several tropical diseases, rely on trypanothione as principal low molecular mass thiol, and their glutaredoxins readily react with the unique bis(glutathionyl) spermidine conjugate. Recent Advances: Two 2-C-Grxs and three 1-C-Grxs have been identified in pathogenic trypanosomatids. The 2-C-Grxs catalyze the reduction of glutathione disulfide by trypanothione and display reductase activity towards protein disulfides, as well as protein-glutathione mixed disulfides. In vitro, all three 1-C-Grxs as well as the cytosolic 2-C-Grx of Trypanosoma brucei can complex an iron–sulfur cluster. Recently the structure of the 1-C-Grx1 has been solved by NMR spectroscopy. The structure is very similar to those of other 1-C-Grxs, with some differences in the loop containing the conserved cis-Pro and the surface charge distribution. Critical Issues: Although four of the five trypanosomal glutaredoxins proved to coordinate an iron–sulfur cluster in vitro, the physiological role of the mitochondrial and cytosolic proteins, respectively, has only started to be unraveled. Future Directions: The use of trypanothione by the glutaredoxins has established a novel role for this parasite-specific dithiol. Future work should reveal if these differences can be exploited for the development of novel antiparasitic drugs. Antioxid. Redox Signal. 19, 708–722.
PMCID: PMC3739957  PMID: 22978520
14.  Extrachromosomal, homologous expression of trypanothione reductase and its complementary mRNA in Trypanosoma cruzi. 
Nucleic Acids Research  1996;24(15):2942-2949.
Trypanothione reductase (TR), a flavoprotein oxidoreductase present in trypanosomatids but absent in human cells, is regarded as a potential target for the chemotherapy of several tropical parasitic diseases caused by trypanosomes and leishmanias. We investigated the possibility of modulating intracellular TR levels in Trypanosoma cruzi by generating transgenic lines that extrachromosomally overexpress either sense or antisense TR mRNA. Cells overexpressing the sense construct showed a 4-10-fold increase in levels of TR mRNA, protein and enzyme activity. In contrast, recombinant T.cruzi harbouring the antisense construct showed no significant difference in TR protein or catalytic activity when compared with control cells. Although increased levels of TR mRNA were detected in some of the antisense cells neither upregulation nor amplification of the endogenous trypanothione reductase gene (tryA) was observed. Instead, a proportion of plasmid molecules was found rearranged and, as a result, contained the tryA sequence in the sense orientation. Plasmid rescue experiments and sequence analysis of rearranged plasmids revealed that this specific gene inversion event was associated with the deletion of small regions of flanking DNA.
PMCID: PMC146039  PMID: 8760878
15.  Identification of a Small Molecule That Modifies MglA/SspA Interaction and Impairs Intramacrophage Survival of Francisella tularensis 
PLoS ONE  2013;8(1):e54498.
The transcription factors MglA and SspA of Francisella tularensis form a heterodimer complex and interact with the RNA polymerase to regulate the expression of the Francisella pathogenicity island (FPI) genes. These genes are essential for this pathogen’s virulence and survival within host cells. In this study, we used a small molecule screening to identify quinacrine as a thermal stabilizing compound for F. tularensis SCHU S4 MglA and SspA. A bacterial two-hybrid system was used to analyze the in vivo effect of quinacrine on the heterodimer complex. The results show that quinacrine affects the interaction between MglA and SspA, indicated by decreased β-galactosidase activity. Further in vitro analyses, using size exclusion chromatography, indicated that quinacrine does not disrupt the heterodimer formation, however, changes in the alpha helix content were confirmed by circular dichroism. Structure-guided site-directed mutagenesis experiments indicated that quinacrine makes contact with amino acid residues Y63 in MglA, and K97 in SspA, both located in the “cleft” of the interacting surfaces. In F. tularensis subsp. novicida, quinacrine decreased the transcription of the FPI genes, iglA, iglD, pdpD and pdpA. As a consequence, the intramacrophage survival capabilities of the bacteria were affected. These results support use of the MglA/SspA interacting surface, and quinacrine’s chemical scaffold, for the design of high affinity molecules that will function as therapeutics for the treatment of Tularemia.
PMCID: PMC3553074  PMID: 23372736
16.  ATP-dependent ligases in trypanothione biosynthesis – kinetics of catalysis and inhibition by phosphinic acid pseudopeptides 
The Febs Journal  2008;275(21):5408-5421.
Glutathionylspermidine is an intermediate formed in the biosynthesis of trypanothione, an essential metabolite in defence against chemical and oxidative stress in the Kinetoplastida. The kinetic mechanism for glutathionylspermidine synthetase (EC from Crithidia fasciculata (CfGspS) obeys a rapid equilibrium random ter-ter model with kinetic constants KGSH = 609 μm, KSpd = 157 μm and KATP = 215 μm. Phosphonate and phosphinate analogues of glutathionylspermidine, previously shown to be potent inhibitors of GspS from Escherichia coli, are equally potent against CfGspS. The tetrahedral phosphonate acts as a simple ground state analogue of glutathione (GSH) (Ki ∼ 156 μm), whereas the phosphinate behaves as a stable mimic of the postulated unstable tetrahedral intermediate. Kinetic studies showed that the phosphinate behaves as a slow-binding bisubstrate inhibitor [competitive with respect to GSH and spermidine (Spd)] with rate constants k3 (on rate) = 6.98 × 104 m−1·s−1 and k4 (off rate) = 1.3 × 10−3 s−1, providing a dissociation constant Ki = 18.6 nm. The phosphinate analogue also inhibited recombinant trypanothione synthetase (EC from C. fasciculata, Leishmania major, Trypanosoma cruzi and Trypanosoma brucei with Kiapp values 20–40-fold greater than that of CfGspS. This phosphinate analogue remains the most potent enzyme inhibitor identified to date, and represents a good starting point for drug discovery for trypanosomiasis and leishmaniasis.
PMCID: PMC2702004  PMID: 18959765
drug discovery; enzyme mechanism; glutathionylspermidine synthetase; slow-binding inhibition; trypanothione synthetase
17.  Quinacrine Impairs Enterovirus 71 RNA Replication by Preventing Binding of Polypyrimidine-Tract Binding Protein with Internal Ribosome Entry Sites 
PLoS ONE  2013;8(1):e52954.
Since the 1980s, epidemics of enterovirus 71 (EV71) and other enteroviruses have occurred in Asian countries and regions, causing a wide range of human diseases. No effective therapy is available for the treatment of these infections. Internal ribosome entry sites (IRESs) are indispensable for the initiation of translation in enteroviruses. Several cellular factors, as well as the ribosome, are recruited to the conserved IRES during this process. Quinacrine intercalates into the RNA architecture and inhibits RNA transcription and protein synthesis, and a recent study showed that quinacrine inhibited encephalomyocarditis virus and poliovirus IRES-mediated translation in vitro without disrupting internal cellular IRES. Here, we report that quinacrine was highly active against EV71, protecting cells from EV71 infection. Replication of viral RNA, expression of viral capsid protein, and production of virus were all strongly inhibited by quinacrine. Interaction of the polypyrimidine tract-binding protein (PTB) with the conserved IRES was prevented by quinacrine. Coxsackieviruses and echovirus were also inhibited by quinacrine in cultured cells. These results indicate that quinacrine may serve as a potential protective agent for use in the treatment of patients with chronic enterovirus infection.
PMCID: PMC3536785  PMID: 23301007
18.  Iron–Sulfur Cluster Binding by Mitochondrial Monothiol Glutaredoxin-1 of Trypanosoma brucei: Molecular Basis of Iron–Sulfur Cluster Coordination and Relevance for Parasite Infectivity 
Antioxidants & Redox Signaling  2013;19(7):665-682.
Aims: Monothiol glutaredoxins (1-C-Grxs) are small proteins linked to the cellular iron and redox metabolism. Trypanosoma brucei brucei, model organism for human African trypanosomiasis, expresses three 1-C-Grxs. 1-C-Grx1 is a highly abundant mitochondrial protein capable to bind an iron–sulfur cluster (ISC) in vitro using glutathione (GSH) as cofactor. We here report on the functional and structural analysis of 1-C-Grx1 in relation to its ISC-binding properties. Results: An N-terminal extension unique to 1-C-Grx1 from trypanosomatids affects the oligomeric structure and the ISC-binding capacity of the protein. The active-site Cys104 is essential for ISC binding, and the parasite-specific glutathionylspermidine and trypanothione can replace GSH as the ligands of the ISC. Interestingly, trypanothione forms stable protein-free ISC species that in vitro are incorporated into the dithiol T. brucei 2-C-Grx1, but not 1-C-Grx1. Overexpression of the C104S mutant of 1-C-Grx1 impairs disease progression in a mouse model. The structure of the Grx-domain of 1-C-Grx1 was solved by nuclear magnetic resonance spectroscopy. Despite the fact that several residues—which in other 1-C-Grxs are involved in the noncovalent binding of GSH—are conserved, different physicochemical approaches did not reveal any specific interaction between 1-C-Grx1 and free thiol ligands. Innovation: Parasite Grxs are able to coordinate an ISC formed with trypanothione, suggesting a new mechanism of ISC binding and a novel function for the parasite-specific dithiol. The first 3D structure and in vivo relevance of a 1-C-Grx from a pathogenic protozoan are reported. Conclusion: T. brucei 1-C-Grx1 is indispensable for mammalian parasitism and utilizes a new mechanism for ISC binding. Antioxid. Redox Signal. 19, 665–682.
PMCID: PMC3739951  PMID: 23259530
19.  Dissecting the essentiality of the bifunctional trypanothione synthetase-amidase in Trypanosoma brucei using chemical and genetic methods 
Molecular Microbiology  2009;74(3):529-540.
The bifunctional trypanothione synthetase-amidase (TRYS) comprises two structurally distinct catalytic domains for synthesis and hydrolysis of trypanothione (N1,N8-bis(glutathionyl)spermidine). This unique dithiol plays a pivotal role in thiol-redox homeostasis and in defence against chemical and oxidative stress in trypanosomatids. A tetracycline-dependent conditional double knockout of TRYS (cDKO) was generated in bloodstream Trypanosoma brucei. Culture of cDKO parasites without tetracycline induction resulted in loss of trypanothione and accumulation of glutathione, followed by growth inhibition and cell lysis after 6 days. In the absence of inducer, cDKO cells were unable to infect mice, confirming that this enzyme is essential for virulence in vivo as well as in vitro. To establish whether both enzymatic functions were essential, an amidase-dead mutant cDKO line was generated. In the presence of inducer, this line showed decreased growth in vitro and decreased virulence in vivo, indicating that the amidase function is not absolutely required for viability. The druggability of TRYS was assessed using a potent small molecule inhibitor developed in our laboratory. Growth inhibition correlated in rank order cDKO, single KO, wild-type and overexpressing lines and produced the predicted biochemical phenotype. The synthetase function of TRYS is thus unequivocally validated as a drug target by both chemical and genetic methods.
PMCID: PMC2784880  PMID: 19558432
20.  Stage-Dependent Expression and Up-Regulation of Trypanothione Synthetase in Amphotericin B Resistant Leishmania donovani 
PLoS ONE  2014;9(6):e97600.
Kinetoplastids differ from other organisms in their ability to conjugate glutathione and spermidine to form trypanothione which is involved in maintaining redox homeostasis and removal of toxic metabolites. It is also involved in drug resistance, antioxidant mechanism, and defense against cellular oxidants. Trypanothione synthetase (TryS) of thiol metabolic pathway is the sole enzyme responsible for the biosynthesis of trypanothione in Leishmania donovani. In this study, TryS gene of L. donovani (LdTryS) was cloned, expressed, and fusion protein purified with affinity column chromatography. The purified protein showed optimum enzymatic activity at pH 8.0–8.5. The TryS amino acids sequences alignment showed that all amino acids involved in catalytic and ligands binding of L. major are conserved in L. donovani. Subcellular localization using digitonin fractionation and immunoblot analysis showed that LdTryS is localized in the cytoplasm. Furthermore, RT-PCR coupled with immunoblot analysis showed that LdTryS is overexpressed in Amp B resistant and stationary phase promastigotes (∼2.0-folds) than in sensitive strain and logarithmic phase, respectively, which suggests its involvement in Amp B resistance. Also, H2O2 treatment upto 150 µM for 8 hrs leads to 2-fold increased expression of LdTryS probably to cope up with oxidative stress generated by H2O2. Therefore, this study demonstrates stage- and Amp B sensitivity-dependent expression of LdTryS in L. donovani and involvement of TryS during oxidative stress to help the parasites survival.
PMCID: PMC4046939  PMID: 24901644
21.  Mini review on tricyclic compounds as an inhibitor of trypanothione reductase 
Trypanosomiasis and leishmaniasis are two most ruinous parasitic infectious diseases caused by Trypanosoma and Leishmania species. The disease affects millions of people all over the world and associated with high morbidity and mortality rates. The review discuss briefly on current treatment of these parasitic diseases and trypanothione reductase (TryR) as potential targets for rational drug design. The enzyme trypanothione reductase (TryR) has been identified as unique among these parasites and has been proposed to be an effective target against for developing new drugs. The researchers have selected this enzyme as target is due to its substrate specificity in contrast to human analogous glutathione reductase and its absence from the host cell which makes this enzyme an ideal target for drug discovery. In this review we have tried to present an overview of the different tricyclic compounds which are potent inhibitors of TryR with their inhibitory activities against the parasites are briefly discussed.
PMCID: PMC4231380  PMID: 25400403
Tricyclic; trypanosomiasis and leishmaniasis; trypanothione reductase
22.  Footprinting of Inhibitor Interactions of In Silico Identified Inhibitors of Trypanothione Reductase of Leishmania Parasite 
The Scientific World Journal  2012;2012:963658.
Structure-based virtual screening of NCI Diversity set II compounds was performed to indentify novel inhibitor scaffolds of trypanothione reductase (TR) from Leishmania infantum. The top 50 ranked hits were clustered using the AuPoSOM tool. Majority of the top-ranked compounds were Tricyclic. Clustering of hits yielded four major clusters each comprising varying number of subclusters differing in their mode of binding and orientation in the active site. Moreover, for the first time, we report selected alkaloids and dibenzothiazepines as inhibitors of Leishmania infantum TR. The mode of binding observed among the clusters also potentiates the probable in vitro inhibition kinetics and aids in defining key interaction which might contribute to the inhibition of enzymatic reduction of T[S] 2. The method provides scope for automation and integration into the virtual screening process employing docking softwares, for clustering the small molecule inhibitors based upon protein-ligand interactions.
PMCID: PMC3322522  PMID: 22550471
23.  Genetic Validation of Trypanosoma brucei Glutathione Synthetase as an Essential Enzyme 
Eukaryotic Cell  2014;13(5):614-624.
Human African trypanosomiasis (HAT) is a debilitating and fatal vector-borne disease. Polyamine biosynthesis is the target of one of the key drugs (eflornithine) used for the treatment of late-stage disease, suggesting that the pathway might be exploited for the identification of additional drug targets. The polyamine spermidine is required in trypanosomatid parasites for formation of a unique redox cofactor termed trypanothione, which is formed from the conjugation of glutathione to spermidine. Here we characterize recombinant Trypanosoma brucei glutathione synthetase (TbGS) and show that depletion of TbGS in blood-form parasites using a regulated knockout strategy leads to loss of trypanothione and to cell death as quantified by fluorescence-activated cell sorter (FACS) analysis. These data suggest that >97% depletion of TbGS is required before trypanothione is depleted and cell growth arrest is observed. Exogenous glutathione was able to partially compensate for the loss of TbGS, suggesting that parasites are able to transport intact glutathione. Finally, reduced expression of TbGS leads to increased levels of upstream glutathione biosynthetic enzymes and decreased expression of polyamine biosynthetic enzymes, providing evidence that the cells cross regulate the two branches of the trypanothione biosynthetic pathway to maintain spermidine and trypanothione homeostasis.
PMCID: PMC4060477  PMID: 24610661
24.  Synthesis and Evaluation of 1-(1-(Benzo[b]thiophen-2-yl)cyclohexyl)piperidine (BTCP) Analogues as Inhibitors of Trypanothione Reductase 
Chemmedchem  2009;4(8):1341-1353.
Thirty two analogues of phencyclidine were synthesised and tested as inhibitors of trypanothione reductase (TryR), a potential drug target in trypanosome and leishmania parasites. The lead compound BTCP (1, 1-(1-benzo[b]thiophen-2-yl-cyclohexyl) piperidine) was found to be a competitive inhibitor of the enzyme (Ki=1 μm) and biologically active against bloodstream T. brucei (EC50=10 μm), but with poor selectivity against mammalian MRC5 cells (EC50=29 μm). Analogues with improved enzymatic and biological activity were obtained. The structure–activity relationships of this novel series are discussed.
PMCID: PMC2929374  PMID: 19557802
BTCP; inhibitors; medicinal chemistry; trypanosoma brucei; trypanothione reductase
25.  Dihydroquinazolines as a Novel Class of Trypanosoma brucei Trypanothione Reductase Inhibitors: Discovery, Synthesis, and Characterization of their Binding Mode by Protein Crystallography 
Journal of Medicinal Chemistry  2011;54(19):6514-6530.
Trypanothione reductase (TryR) is a genetically validated drug target in the parasite Trypanosoma brucei, the causative agent of human African trypanosomiasis. Here we report the discovery, synthesis, and development of a novel series of TryR inhibitors based on a 3,4-dihydroquinazoline scaffold. In addition, a high resolution crystal structure of TryR, alone and in complex with substrates and inhibitors from this series, is presented. This represents the first report of a high resolution complex between a noncovalent ligand and this enzyme. Structural studies revealed that upon ligand binding the enzyme undergoes a conformational change to create a new subpocket which is occupied by an aryl group on the ligand. Therefore, the inhibitor, in effect, creates its own small binding pocket within the otherwise large, solvent exposed active site. The TryR–ligand structure was subsequently used to guide the synthesis of inhibitors, including analogues that challenged the induced subpocket. This resulted in the development of inhibitors with improved potency against both TryR and T. brucei parasites in a whole cell assay.
PMCID: PMC3188286  PMID: 21851087

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