Targeting prolyl-tRNA synthetase via a series of ATP-mimetics to accelerate drug discovery against toxoplasmosis.

The prolyl-tRNA synthetase (PRS) is a validated drug target for febrifugine and its synthetic analog halofuginone (HFG) against multiple apicomplexan parasites including Plasmodium falciparum and Toxoplasma gondii. Here, a novel ATP-mimetic centered on 1-(pyridin-4-yl) pyrrolidin-2-one (PPL) scaffold has been validated to bind to Toxoplasma gondii PRS and kill toxoplasma parasites. PPL series exhibited potent inhibition at the cellular (T. gondii parasites) and enzymatic (TgPRS) levels compared to the human counterparts. Cell-based chemical mutagenesis was employed to determine the mechanism of action via a forward genetic screen. Tg-resistant parasites were analyzed with wild-type strain by RNA-seq to identify mutations in the coding sequence conferring drug resistance by computational analysis of variants. DNA sequencing established two mutations, T477A and T592S, proximal to terminals of the PPL scaffold and not directly in the ATP, tRNA, or L-pro sites, as supported by the structural data from high-resolution crystal structures of drug-bound enzyme complexes. These data provide an avenue for structure-based activity enhancement of this chemical series as anti-infectives.

Structural characterization of glutamyl-tRNA synthetase (GluRS) from Plasmodium falciparum.

Aminoacyl-tRNA synthetases (aaRSs) are essential enzymes in protein translation machinery that provide the charged tRNAs needed for protein synthesis. Over the past decades, aaRSs have been studied as anti-parasitic, anti-bacterial, and anti-fungal drug targets. This study focused on the cytoplasmic glutamyl-tRNA synthetase (GluRS) from Plasmodium falciparum, which belongs to class Ib in aaRSs. GluRS unlike most other aaRSs requires tRNA to activate its cognate amino acid substrate L-Glutamate (L-Glu), and fails to form an intermediate adenylate complex in the absence of tRNA. The crystal structures of the Apo, ATP, and ADP-bound forms of Plasmodium falciparum glutamyl-tRNA synthetase (PfGluRS) were solved at 2.1 Å, 2.2 Å, and 2.8 Å respectively. The structural comparison of the Apo- and ATP-bound holo-forms of PfGluRS showed considerable conformational changes in the loop regions around the ATP-binding pocket of the enzyme. Biophysical characterization of the PfGluRS showed binding of the enzyme substrates L-Gluand ATP.. The sequence and structural conservation were evident across GluRS compared to other species. The structural dissection of the PfGluRS gives insight into the critical residues involved in the binding of ATP substrate, which can be harvested to develop new antimalarial drugs.

A single amino acid substitution alters activity and specificity in Plasmodium falciparum aspartyl & asparaginyl-tRNA synthetases.

The specificity of each aminoacyl-tRNA synthetase (aaRS) for its cognate amino acid ensures correct tRNA esterification and allows fidelity in protein synthesis. The aaRSs discriminate based on the chemical properties of their amino acid substrates and structural features of the binding pockets. In this study, we characterized aspartyl-(DRS) and asparaginyl-tRNA synthetase (NRS) from Plasmodium falciparum to determine the basis of their specificity towards L-asp and L-asn respectively. The negatively charged L-asp and its analogue L-asn differ only in their side-chain groups i.e., -OH and -NH2. Further, the amino acid binding sites are highly conserved within these two enzymes. Analysis of the substrate (L-asp/L-asn) binding sites across species revealed two highly conserved residues in PfDRS (D408 and K372) and PfNRS (E395 and L360) that are involved in recognition of the Oδ2/Nδ2 of L-asp/L-asn respectively. These residues were mutated and swapped between the D408→E in PfDRS and the corresponding E395→D in PfNRS. A similar approach was employed for residue number K372→L in PfDRS and L360→K in PfNRS. The mutated PfDRSD408E retained its enzymatic activity during step 1 of aminoacylation reaction towards L-asp and L-asn and esterified tRNAAsp with L-asp like wild type enzyme, while the PfDRSK372L was rendered enzymatically inactive. The correspondingly mutated PfNRSE395D was enzymatically inactive. The mutated PfNRSL360K had an altered specificity and esterified tRNAAsn with non-cognate amino acid L-asp and not L-asn. These data suggest that the residue K372 is crucial for the enzymatic activity of PfDRS while the residue L360 in PfNRS imparts specificity towards L-asn.

Structural analyses of the malaria parasite aminoacyl-tRNA synthetases provide new avenues for antimalarial drug discovery.

Malaria is a parasitic illness caused by the genus Plasmodium from the apicomplexan phylum. Five plasmodial species of P. falciparum (Pf), P. knowlesi, P. malariae, P. ovale, and P. vivax (Pv) are responsible for causing malaria in humans. According to the World Malaria Report 2020, there were 229 million cases and ~ 0.04 million deaths of which 67% were in children below 5 years of age. While more than 3 billion people are at risk of malaria infection globally, antimalarial drugs are their only option for treatment. Antimalarial drug resistance keeps arising periodically and thus threatens the main line of malaria treatment, emphasizing the need to find new alternatives. The availability of whole genomes of P. falciparum and P. vivax has allowed targeting their unexplored plasmodial enzymes for inhibitor development with a focus on multistage targets that are crucial for parasite viability in both the blood and liver stages. Over the past decades, aminoacyl-tRNA synthetases (aaRSs) have been explored as anti-bacterial and anti-fungal drug targets, and more recently (since 2009) aaRSs are also the focus of antimalarial drug targeting. Here, we dissect the structure-based knowledge of the most advanced three aaRSs-lysyl- (KRS), prolyl- (PRS), and phenylalanyl- (FRS) synthetases in terms of development of antimalarial drugs. These examples showcase the promising potential of this family of enzymes to provide druggable targets that stall protein synthesis upon inhibition and thereby kill malaria parasites selectively.

Structural basis of malaria parasite phenylalanine tRNA-synthetase inhibition by bicyclic azetidines.

The inhibition of Plasmodium cytosolic phenylalanine tRNA-synthetase (cFRS) by a novel series of bicyclic azetidines has shown the potential to prevent malaria transmission, provide prophylaxis, and offer single-dose cure in animal models of malaria. To date, however, the molecular basis of Plasmodium cFRS inhibition by bicyclic azetidines has remained unknown. Here, we present structural and biochemical evidence that bicyclic azetidines are competitive inhibitors of L-Phe, one of three substrates required for the cFRS-catalyzed aminoacylation reaction that underpins protein synthesis in the parasite. Critically, our co-crystal structure of a PvcFRS-BRD1389 complex shows that the bicyclic azetidine ligand binds to two distinct sub-sites within the PvcFRS catalytic site. The ligand occupies the L-Phe site along with an auxiliary cavity and traverses past the ATP binding site. Given that BRD1389 recognition residues are conserved amongst apicomplexan FRSs, this work lays a structural framework for the development of drugs against both Plasmodium and related apicomplexans.

Crystal structures of the two domains that constitute the Plasmodium vivax p43 protein.

Scaffold modules known as aminoacyl-tRNA synthetase (aaRS)-interacting multifunctional proteins (AIMPs), such as AIMP1/p43, AIMP2/p38 and AIMP3/p18, are important in driving the assembly of multi-aaRS (MARS) complexes in eukaryotes. Often, AIMPs contain an N-terminal glutathione S-transferase (GST)-like domain and a C-terminal OB-fold tRNA-binding domain. Recently, the apicomplexan-specific Plasmodium falciparum p43 protein (Pfp43) has been annotated as an AIMP and its tRNA binding, tRNA import and membrane association have been characterized. The crystal structures of both the N- and C-terminal domains of the Plasmodium vivax p43 protein (Pvp43), which is an ortholog of Pfp43, have been resolved. Analyses reveal the overall oligomeric structure of Pvp43 and highlight several notable features that show Pvp43 to be a soluble, cytosolic protein. The dimeric assembly of the N-terminal GST-like domain of Pvp43 differs significantly from canonical GST dimers, and it is tied to the C-terminal tRNA-binding domain via a linker region. This work therefore establishes a framework for dissecting the additional roles of p43 orthologs in eukaryotic multi-protein MARS complexes.

Conformational heterogeneity in apo and drug-bound structures of Toxoplasma gondii prolyl-tRNA synthetase.

Prolyl-tRNA synthetase (PRS) is a member of the aminoacyl-tRNA synthetase family that drives protein translation in cells. The apicomplexan PRSs are validated targets of febrifugine (FF) and its halogenated derivative halofuginone (HF). PRSs are of great interest for drug development against Plasmodium falciparum and Toxoplasma gondii. In this study, structures of apo and FF-bound T. gondii (TgPRS) are revealed and the dynamic nature of the conformational changes that occur upon FF binding is unraveled. In addition, this study highlights significant conformational plasticity within two different crystal structures of apo PRSs but not within drug-bound PRSs. The apo PRSs exist in multi-conformational states and manifest pseudo-dimeric structures. In contrast, when FF is bound the PRS dimer adopts a highly symmetrical architecture. It is shown that TgPRS does not display extant fold switching, in contrast to P. falciparum PRS, despite having over 65% sequence identity. Finally, structure-comparison analyses suggest the utility of r.m.s.d. per residue (r.m.s.d./res) as a robust tool to detect structural alterations even when the r.m.s.d. is low. Apo TgPRS reveals FF/HF-induced rigidity and this work has implications for drug-design studies that rely on the apo structures of target proteins.

Engagement Rules That Underpin DBL-DARC Interactions for Ingress of Plasmodium knowlesi and Plasmodium vivax Into Human Erythrocytes.

Malaria parasite erythrocytic stages comprise of repeated bursts of parasites via cyclical invasion of host erythrocytes using dedicated receptor-ligand interactions. A family of erythrocyte-binding proteins from Plasmodium knowlesi (Pk) and Plasmodium vivax (Pv) attach to human Duffy antigen receptor for chemokines (DARC) via their Duffy binding-like domains (DBLs) for invasion. Here we provide a novel, testable and overarching interaction model that rationalizes even contradictory pieces of evidence that have so far existed in the literature on Pk/Pv-DBL/DARC binding determinants. We further address the conundrum of how parasite-encoded Pk/Pv-DBLs recognize human DARC and collate evidence for two distinct DARC integration sites on Pk/Pv-DBLs.

Structural and functional analysis of Glutaminyl-tRNA synthetase (TtGlnRS) from Thermus thermophilus HB8 and its complexes.

Aminoacyl-tRNA synthetases (AaRSs) are vital enzymes for translation of proteins in cells. AaRSs catalyse the esterification of a specific amino acid to corresponding tRNAs to form an aminoacyl-tRNA that is used in ribosome-based protein synthesis. We focused on Glutaminyl tRNA synthetase (GlnRS) enzyme from the extreme thermophile Thermus thermophilus for structural studies. Our thermal shift assays show binding of enzyme substrates L-Gln and ATP as well as of various metals including cesium. We resolved crystal structures of apo-GlnRS as well as those in complex with AMP and ATP at 2.8 Å, 2.4 Šand 2.6 Šrespectively. The bound cesium was found at the site of magnesium that typically binds to GlnRS. High structural conservation was evident in the Thermus thermophilus GlnRS when compared to those from Escherichia coli GlnRS.

Structure of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase-dihydropteroate synthase from Plasmodium vivax sheds light on drug resistance.

The genomes of the malaria-causing Plasmodium parasites encode a protein fused of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) and dihydropteroate synthase (DHPS) domains that catalyze sequential reactions in the folate biosynthetic pathway. Whereas higher organisms derive folate from their diet and lack the enzymes for its synthesis, most eubacteria and a number of lower eukaryotes including malaria parasites synthesize tetrahydrofolate via DHPS. Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) HPPK-DHPSs are currently targets of drugs like sulfadoxine (SDX). The SDX effectiveness as an antimalarial drug is increasingly diminished by the rise and spread of drug-resistant mutations. Here, we present the crystal structure of PvHPPK-DHPS in complex with four substrates/analogs, revealing the bifunctional PvHPPK-DHPS architecture in an unprecedented state of enzymatic activation. SDX's effect on HPPK-DHPS is due to 4-amino benzoic acid (pABA) mimicry, and the PvHPPK-DHPS structure sheds light on the SDX-binding cavity, as well as on mutations that effect SDX potency. We mapped five dominant drug resistance mutations in PvHPPK-DHPS: S382A, A383G, K512E/D, A553G, and V585A, most of which occur individually or in clusters proximal to the pABA-binding site. We found that these resistance mutations subtly alter the intricate enzyme/pABA/SDX interactions such that DHPS affinity for pABA is diminished only moderately, but its affinity for SDX is changed substantially. In conclusion, the PvHPPK-DHPS structure rationalizes and unravels the structural bases for SDX resistance mutations and highlights architectural features in HPPK-DHPSs from malaria parasites that can form the basis for developing next-generation anti-folate agents to combat malaria parasites.

Specific Stereoisomeric Conformations Determine the Drug Potency of Cladosporin Scaffold against Malarial Parasite.

The dependence of drug potency on diastereomeric configurations is a key facet. Using a novel general divergent synthetic route for a three-chiral center antimalarial natural product cladosporin, we built its complete library of stereoisomers (cladologs) and assessed their inhibitory potential using parasite-, enzyme-, and structure-based assays. We show that potency is manifest via tetrahyropyran ring conformations that are housed in the ribose binding pocket of parasite lysyl tRNA synthetase (KRS). Strikingly, drug potency between top and worst enantiomers varied 500-fold, and structures of KRS-cladolog complexes reveal that alterations at C3 and C10 are detrimental to drug potency whereas changes at C3 are sensed by rotameric flipping of glutamate 332. Given that scores of antimalarial and anti-infective drugs contain chiral centers, this work provides a new foundation for focusing on inhibitor stereochemistry as a facet of antimicrobial drug development.

Targeting Prolyl-tRNA Synthetase to Accelerate Drug Discovery against Malaria, Leishmaniasis, Toxoplasmosis, Cryptosporidiosis, and Coccidiosis.

Developing anti-parasitic lead compounds that act on key vulnerabilities are necessary for new anti-infectives. Malaria, leishmaniasis, toxoplasmosis, cryptosporidiosis and coccidiosis together kill >500,000 humans annually. Their causative parasites Plasmodium, Leishmania, Toxoplasma, Cryptosporidium and Eimeria display high conservation in many housekeeping genes, suggesting that these parasites can be attacked by targeting invariant essential proteins. Here, we describe selective and potent inhibition of prolyl-tRNA synthetases (PRSs) from the above parasites using a series of quinazolinone-scaffold compounds. Our PRS-drug co-crystal structures reveal remarkable active site plasticity that accommodates diversely substituted compounds, an enzymatic feature that can be leveraged for refining drug-like properties of quinazolinones on a per parasite basis. A compound we termed In-5 exhibited a unique double conformation, enhanced drug-like properties, and cleared malaria in mice. It thus represents a new lead for optimization. Collectively, our data offer insights into the structure-guided optimization of quinazolinone-based compounds for drug development against multiple human eukaryotic pathogens.

Structural and Biochemical Characterization of Apicomplexan Inorganic Pyrophosphatases.

Inorganic pyrophosphatases (PPase) participate in energy cycling and they are essential for growth and survival of organisms. Here we report extensive structural and functional characterization of soluble PPases from the human parasites Plasmodium falciparum (PfPPase) and Toxoplasma gondii (TgPPase). Our results show that PfPPase is a cytosolic enzyme whose gene expression is upregulated during parasite asexual stages. Cambialistic PfPPase actively hydrolyzes linear short chain polyphosphates like PPi, polyP3 and ATP in the presence of Zn2+. A remarkable new feature of PfPPase is the low complexity asparagine-rich N-terminal region that mediates its dimerization. Deletion of N-region has an unexpected and substantial effect on the stability of PfPPase domain, resulting in aggregation and significant loss of enzyme activity. Significantly, the crystal structures of PfPPase and TgPPase reveal unusual and unprecedented dimeric organizations and provide new fundamental insights into the variety of oligomeric assemblies possible in eukaryotic inorganic PPases.

Structure-Based Targeting of Orthologous Pathogen Proteins Accelerates Antiparasitic Drug Discovery.

Parasitic diseases caused by eukaryotic pathogens impose significant health and economic burden worldwide. The level of research funding available for many parasitic diseases is insufficient in relation to their adverse social and economic impact. In this article, we discuss that extant 3D structural data on protein-inhibitor complexes can be harnessed to accelerate drug discovery against many related pathogens. Assessment of sequence conservation within drug/inhibitor-binding residues in enzyme-inhibitor complexes can be leveraged to predict and validate both new lead compounds and their molecular targets in multiple parasitic diseases. Hence, structure-based targeting of orthologous pathogen proteins accelerates the discovery of new antiparasitic drugs. This approach offers significant benefits for jumpstarting the discovery of new lead compounds and their molecular targets in diverse human, livestock, and plant pathogens.

Protein Translation Enzyme lysyl-tRNA Synthetase Presents a New Target for Drug Development against Causative Agents of Loiasis and Schistosomiasis.

Helminth parasites are an assemblage of two major phyla of nematodes (also known as roundworms) and platyhelminths (also called flatworms). These parasites are a major human health burden, and infections caused by helminths are considered under neglected tropical diseases (NTDs). These infections are typified by limited clinical treatment options and threat of drug resistance. Aminoacyl-tRNA synthetases (aaRSs) are vital enzymes that decode genetic information and enable protein translation. The specific inhibition of pathogen aaRSs bores well for development of next generation anti-parasitics. Here, we have identified and annotated aaRSs and accessory proteins from Loa loa (nematode) and Schistosoma mansoni (flatworm) to provide a glimpse of these protein translation enzymes within these parasites. Using purified parasitic lysyl-tRNA synthetases (KRSs), we developed series of assays that address KRS enzymatic activity, oligomeric states, crystal structure and inhibition profiles. We show that L. loa and S. mansoni KRSs are potently inhibited by the fungal metabolite cladosporin. Our co-crystal structure of Loa loa KRS-cladosporin complex reveals key interacting residues and provides a platform for structure-based drug development. This work hence provides a new direction for both novel target discovery and inhibitor development against eukaryotic pathogens that include L. loa and S. mansoni.

Dimerization of Arginyl-tRNA Synthetase by Free Heme Drives Its Inactivation in Plasmodium falciparum.

Excess cellular heme is toxic, and malaria parasites regulate its levels during hemoglobin digestion. Aminoacyl-tRNA synthetases are ubiquitous enzymes, and of these, arginyl-tRNA synthetase (RRS) is unique as its enzymatic product of charged tRNA is required for protein synthesis and degradation. We show that Plasmodium falciparum arginyl-tRNA synthetase (PfRRS) is an active, cytosolic, and monomeric enzyme. Its high-resolution crystal structure highlights critical structural differences with the human enzyme. We further show that hemin binds to and inhibits the aminoacylation activity of PfRRS. Hemin induces a dimeric form of PfRRS that is thus rendered enzymatically dead as it is unable to recognize its cognate tRNA(arg). Excessive hemin in chloroquine-treated malaria parasites results in significantly reduced charged tRNA(arg) levels, thus suggesting deceleration of protein synthesis. These data together suggest that the inhibition of Plasmodium falciparum arginyl-tRNA synthetase can now be synergized with existing antimalarials for more potent drug cocktails against malaria parasites.

Structural and functional attributes of malaria parasite diadenosine tetraphosphate hydrolase.

Malaria symptoms are driven by periodic multiplication cycles of Plasmodium parasites in human red blood corpuscles (RBCs). Malaria infection still accounts for ~600,000 annual deaths, and hence discovery of both new drug targets and drugs remains vital. In the present study, we have investigated the malaria parasite enzyme diadenosine tetraphosphate (Ap4A) hydrolase that regulates levels of signalling molecules like Ap4A by hydrolyzing them to ATP and AMP. We have tracked the spatial distribution of parasitic Ap4A hydrolase in infected RBCs, and reveal its unusual localization on the infected RBC membrane in subpopulation of infected cells. Interestingly, enzyme activity assays reveal an interaction between Ap4A hydrolase and the parasite growth inhibitor suramin. We also present a high resolution crystal structure of Ap4A hydrolase in apo- and sulphate- bound state, where the sulphate resides in the enzyme active site by mimicking the phosphate of substrates like Ap4A. The unexpected infected erythrocyte localization of the parasitic Ap4A hydrolase hints at a possible role of this enzyme in purinerigic signaling. In addition, atomic structure of Ap4A hydrolase provides insights for selective drug targeting.

Structural and Functional Highlights of Vacuolar Soluble Protein 1 from Pathogen Trypanosoma brucei brucei.

Trypanosoma brucei (T. brucei) is responsible for the fatal human disease called African trypanosomiasis, or sleeping sickness. The causative parasite, Trypanosoma, encodes soluble versions of inorganic pyrophosphatases (PPase), also called vacuolar soluble proteins (VSPs), which are localized to its acidocalcisomes. The latter are acidic membrane-enclosed organelles rich in polyphosphate chains and divalent cations whose significance in these parasites remains unclear. We here report the crystal structure of T. brucei brucei acidocalcisomal PPases in a ternary complex with Mg(2+) and imidodiphosphate. The crystal structure reveals a novel structural architecture distinct from known class I PPases in its tetrameric oligomeric state in which a fused EF hand domain arranges around the catalytic PPase domain. This unprecedented assembly evident from TbbVSP1 crystal structure is further confirmed by SAXS and TEM data. SAXS data suggest structural flexibility in EF hand domains indicative of conformational plasticity within TbbVSP1.

Structure of Prolyl-tRNA Synthetase-Halofuginone Complex Provides Basis for Development of Drugs against Malaria and Toxoplasmosis.

The Chinese herb Dichroa febrifuga has traditionally treated malaria-associated fever. Its active component febrifugine (FF) and derivatives such as halofuginone (HF) are potent anti-malarials. Here, we show that FF-based derivatives arrest parasite growth by direct interaction with and inhibition of the protein translation enzyme prolyl-tRNA synthetase (PRS). Dual administration of inhibitors that target different tRNA synthetases suggests high utility of these drug targets. We reveal the ternary complex structure of PRS-HF and adenosine 5'-(ß,γ-imido)triphosphate where the latter facilitates HF integration into the PRS active site. Structural analyses also highlight spaces within the PRS architecture for HF derivatization of its quinazolinone, but not piperidine, moiety. We also show a remarkable ability of HF to kill the related human parasite Toxoplasma gondii, suggesting wider HF efficacy against parasitic PRSs. Hence, our cell-, enzyme-, and structure-based data on FF-based inhibitors strengthen the case for their inclusion in anti-malarial and anti-toxoplasmosis drug development efforts.

Inhibition of protein synthesis and malaria parasite development by drug targeting of methionyl-tRNA synthetases.

Aminoacyl-tRNA synthetases (aaRSs) are housekeeping enzymes that couple cognate tRNAs with amino acids to transmit genomic information for protein translation. The Plasmodium falciparum nuclear genome encodes two P. falciparum methionyl-tRNA synthetases (PfMRS), termed PfMRS(cyt) and PfMRS(api). Phylogenetic analyses revealed that the two proteins are of primitive origin and are related to heterokonts (PfMRS(cyt)) or proteobacteria/primitive bacteria (PfMRS(api)). We show that PfMRS(cyt) localizes in parasite cytoplasm, while PfMRS(api) localizes to apicoplasts in asexual stages of malaria parasites. Two known bacterial MRS inhibitors, REP3123 and REP8839, hampered Plasmodium growth very effectively in the early and late stages of parasite development. Small-molecule drug-like libraries were screened against modeled PfMRS structures, and several "hit" compounds showed significant effects on parasite growth. We then tested the effects of the hit compounds on protein translation by labeling nascent proteins with (35)S-labeled cysteine and methionine. Three of the tested compounds reduced protein synthesis and also blocked parasite growth progression from the ring stage to the trophozoite stage. Drug docking studies suggested distinct modes of binding for the three compounds, compared with the enzyme product methionyl adenylate. Therefore, this study provides new targets (PfMRSs) and hit compounds that can be explored for development as antimalarial drugs.