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1.  Allosteric Activation of Trypanosomatid Deoxyhypusine Synthase by a Catalytically Dead Paralog*♦ 
The Journal of Biological Chemistry  2013;288(21):15256-15267.
Background: Deoxyhypusine synthase (DHS) catalyzes the spermidine-dependent modification of translation factor eIF5A.
Results: Trypanosomatid DHS activity is increased 3000-fold by heterotetramer formation with a catalytically dead paralog, and both gene products are essential for parasite growth.
Conclusion: Trypanosomatid DHS is a complex between catalytically impaired and inactive DHS subunits.
Significance: This activation mechanism uniquely evolved for two independent enzymes within the trypanosomatid polyamine pathway.
Polyamine biosynthesis is a key drug target in African trypanosomes. The “resurrection drug” eflornithine (difluoromethylornithine), which is used clinically to treat human African trypanosomiasis, inhibits the first step in polyamine (spermidine) biosynthesis, a highly regulated pathway in most eukaryotic cells. Previously, we showed that activity of a key trypanosomatid spermidine biosynthetic enzyme, S-adenosylmethionine decarboxylase, is regulated by heterodimer formation with a catalytically dead paralog (a prozyme). Here, we describe an expansion of this prozyme paradigm to the enzyme deoxyhypusine synthase, which is required for spermidine-dependent hypusine modification of a lysine residue in the essential translation factor eIF5A. Trypanosoma brucei encodes two deoxyhypusine synthase paralogs, one that is catalytically functional but grossly impaired, and the other is inactive. Co-expression in Escherichia coli results in heterotetramer formation with a 3000-fold increase in enzyme activity. This functional complex is also present in T. brucei, and conditional knock-out studies indicate that both DHS genes are essential for in vitro growth and infectivity in mice. The recurrent evolution of paralogous, catalytically dead enzyme-based activating mechanisms may be a consequence of the unusual gene expression in the parasites, which lack transcriptional regulation. Our results suggest that this mechanism may be more widely used by trypanosomatids to control enzyme activity and ultimately influence pathogenesis than currently appreciated.
PMCID: PMC3663545  PMID: 23525104
Parasite Metabolism; Polyamines; Protozoan; Trypanosoma brucei; Trypanosome; Deoxyhypusine; Deoxyhypusine Synthase; eIF5A; Spermidine
2.  High-Throughput Screening Affords Novel and Selective Trypanothione Reductase Inhibitors with Anti-Trypanosomal Activity 
Trypanothione reductase (TR), an enzyme that buffers oxidative stress in trypanosomatid parasites, was screened against commercial libraries containing approximately 134,500 compounds. After secondary screening, four chemotypes were identified as screening positives with selectivity for TR over human glutathione reductase. Thirteen compounds from these four chemotypes were purchased, and their in vitro activity against TR and Trypanosoma brucei are described.
PMCID: PMC3428910  PMID: 17197182
3.  Identification of a κ-opioid agonist as a potent and selective lead for drug development against human African trypanosomiasis 
Biochemical Pharmacology  2010;80(10):1478-1486.
Graphical abstract
Phenotypic screening of the LOPAC library identified several potent and selective inhibitors of African trypanosomes. The κ-opioid agonist (+)-U50,488 represents a novel lead for drug discovery against sleeping sickness.
A resazurin-based cell viability assay was developed for phenotypic screening of the LOPAC 1280 ‘library of pharmacologically active compounds’ against bloodstream forms of Trypanosoma brucei in vitro identifying 33 compounds with EC50 values <1 μM. Counter-screening vs. normal diploid human fibroblasts (MRC5 cells) was used to rank these hits for selectivity, with the most potent (<70 nM) and selective (>700-fold) compounds being suramin and pentamidine. These are well-known antitrypanosomal drugs which demonstrate the robustness of the resazurin cell viability assay. The most selective novel inhibitor was (+)-trans-(1R,2R)-U50,488 having an EC50 value of 60 nM against T. brucei and 270-fold selectivity over human fibroblasts. Interestingly, (−)-U50,488, a known CNS-active κ-opioid receptor agonist and other structurally related compounds were >70-fold less active or inactive, as were several μ- and κ-opioid antagonists. Although (+)-U50,488 was well tolerated by the oral route and displayed good pharmaceutical properties, including high brain penetration, the compound was not curative in the mouse model of infection. Nonetheless, the divergence of antinociceptive and antitrypanosomal activity represents a promising start point for further exploratory chemistry. Bioinformatic studies did not reveal any obvious candidate opioid receptors and the target of this cytostatic compound is unknown. Among the other potent, but less selective screening hits were compound classes with activity against protein kinases, topoisomerases, tubulin, as well as DNA and energy metabolism.
PMCID: PMC3025325  PMID: 20696141
Phenotypic screening; African trypanosomiasis; Target identification; Target validation; U50,488
4.  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
5.  Trypanosoma brucei UDP-Glucose:Glycoprotein Glucosyltransferase Has Unusual Substrate Specificity and Protects the Parasite from Stress▿ §  
Eukaryotic Cell  2008;8(2):230-240.
In this paper, we describe the range of N-linked glycan structures produced by wild-type and glucosidase II null mutant bloodstream form Trypanosoma brucei parasites and the creation and characterization of a bloodstream form Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase null mutant. These analyses highlight peculiarities of the Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase, including an unusually wide substrate specificity, ranging from Man5GlcNAc2 to Man9GlcNAc2 glycans, and an unusually high efficiency in vivo, quantitatively glucosylating the Asn263 N-glycan of variant surface glycoprotein (VSG) 221 and 75% of all non-VSG N glycosylation sites. We also show that although Trypanosoma brucei UDP-glucose:glycoprotein glucosyltransferase is not essential for parasite growth at 37°C, it is essential for parasite growth and survival at 40°C. The null mutant was also shown to be hypersensitive to the effects of the N glycosylation inhibitor tunicamycin. Further analysis of bloodstream form Trypanosoma brucei under normal conditions and stress conditions suggests that it does not have a classical unfolded protein response triggered by sensing unfolded proteins in the endoplasmic reticulum. Rather, judging by its uniform Grp78/BiP levels, it appears to have an unregulated and constitutively active endoplasmic reticulum protein folding system. We suggest that the latter may be particularly appropriate for this organism, which has an extremely high flux of glycoproteins through its secretory pathway.
PMCID: PMC2643610  PMID: 19114500
6.  Development of a Novel Virtual Screening Cascade Protocol to Identify Potential Trypanothione Reductase Inhibitors 
Journal of Medicinal Chemistry  2009;52(6):1670-1680.
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.
PMCID: PMC2659691  PMID: 19296695
7.  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
8.  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
9.  Synthesis and Evaluation of Indatraline-Based Inhibitors for Trypanothione Reductase 
Chemmedchem  2010;6(2):321-328.
The search for novel compounds of relevance to the treatment of diseases caused by trypanosomatid protozoan parasites continues. Screening of a large library of known bioactive compounds has led to several drug-like starting points for further optimisation. In this study, novel analogues of the monoamine uptake inhibitor indatraline were prepared and assessed both as inhibitors of trypanothione reductase (TryR) and against the parasite Trypanosoma brucei. Although it proved difficult to significantly increase the potency of the original compound as an inhibitor of TryR, some insight into the preferred substituent on the amine group and in the two aromatic rings of the parent indatraline was deduced. In addition, detailed mode of action studies indicated that two of the inhibitors exhibit a mixed mode of inhibition.
PMCID: PMC3047706  PMID: 21275055
antiprotozoal agents; drug discovery; indatraline; Nazarov reaction; trypanothione reductase
10.  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
11.  Chemical, genetic and structural assessment of pyridoxal kinase as a drug target in the African trypanosome 
Molecular Microbiology  2012;86(1):51-64.
Pyridoxal-5′-phosphate (vitamin B6) is an essential cofactor for many important enzymatic reactions such as transamination and decarboxylation. African trypanosomes are unable to synthesise vitamin B6de novo and rely on uptake of B6 vitamers such as pyridoxal and pyridoxamine from their hosts, which are subsequently phosphorylated by pyridoxal kinase (PdxK). A conditional null mutant of PdxK was generated in Trypanosoma brucei bloodstream forms showing that this enzyme is essential for growth of the parasite in vitro and for infectivity in mice. Activity of recombinant T. brucei PdxK was comparable to previously published work having a specific activity of 327 ± 13 mU mg−1 and a Kmapp with respect to pyridoxal of 29.6 ± 3.9 µM. A coupled assay was developed demonstrating that the enzyme has equivalent catalytic efficiency with pyridoxal, pyridoxamine and pyridoxine, and that ginkgotoxin is an effective pseudo substrate. A high resolution structure of PdxK in complex with ATP revealed important structural differences with the human enzyme. These findings suggest that pyridoxal kinase is an essential and druggable target that could lead to much needed alternative treatments for this devastating disease.
PMCID: PMC3470933  PMID: 22857512

Results 1-11 (11)