Biology subjects

Saliba, K. J.

Publications and source records attributed to Saliba, K. J..

10 recordsLinked to original sources

bioRxiv2026

The heteromeric Plasmodium falciparum pantothenate kinase has only one active site and does not require Pf14-3-3I for activity

Coenzyme A (CoA) is an essential molecule for the intraerythrocytic stage of Plasmodium falciparum. Pantothenate kinase (PanK) catalyses the first step of the CoA biosynthesis pathway and functions as a homodimer in most organisms investigated thus far. P. falciparum possesses a novel heteromeric PanK complex composed of PfPanK1, PfPanK2 and Pf14-3-3I. Using a mutagenesis approach, we generated 10 PfPanK mutants and demonstrate that the PfPanK complex has only one functional active site, with both PfPanK1 and PfPanK2 required for activity by the complex. We also show that PfPanK2 is essential for normal intraerythrocytic parasite proliferation using a conditional knockdown system. 14-3-3 binding motifs generally contain a phosphoserine/threonine residue. Mass spectrometry analyses of phospho-peptide enriched, immunoprecipitated PfPanK samples revealed phosphorylation sites in both PfPanK1 and PfPanK2 that were additional to the previously reported sites. To investigate the role of specific sites in PfPanK1 and PfPanK2 that may be involved in Pf14-3-3I binding, five additional mutants were generated. Mutagenesis of four predicted Pf14-3-3I binding sites in PfPanK1 resulted in a significant reduction in the amount of Pf14-3-3I bound to the PfPanK complex, with S334 being the most likely binding site. Heterologous expression of the PfPanK complex in an insect cell system yielded a small amount of soluble protein that assembled in situ into a functional complex. Combined results from heterologous expression and P. falciparum mutagenesis suggest that Pf14-3-3I may not be essential for PfPanK activity but may be important for stabilising the PfPanK complex.

biochemistry↗

bioRxiv2025

Thiazole substitution of a labile amide bond - a new option towards stable pantothenamide-mimics

The emergence and spread of artemisinin-resistant, malaria-causing P. falciparum provide the impetus for the development of novel antimalarials. Pantothenamides are potent inhibitors of malaria parasite proliferation, however their clinical use is hindered by pantetheinase-mediated degradation in human serum. Here we report the synthesis and biological activity of a series of pantothenamide-mimics in which the labile amide bond is replaced by a thiazole ring with various orientations. Out of 23 novel compounds generated and tested in the presence of pantetheinase, several display sub-micromolar antiplasmodial activity in vitro. A selection of compounds was studied in more detail and CoA biosynthesis and/or utilisation pathways were confirmed to be the target. Toxicity to human cells was not observed. Kinetic studies identified the selected compounds as substrates of the HsPanK3 enzyme, but with much lower affinity compared to that of the natural substrate pantothenate. The most potent thiazole-bearing antiplasmodial compound was found to bind to PfPanK with a 120-fold higher affinity compared to HsPanK, highlighting excellent selectivity, not only against the key first enzyme in the CoA biosynthesis pathway, but also at the whole-cell level. In conclusion, thiazole substitution of the labile amide bond represents a promising avenue for the development of antimalarial pantothenamide-mimics.

pharmacology and toxicology↗

bioRxiv2024

Identification and characterization of thiamine analogues with antiplasmodial activity

Thiamine is metabolized into thiamine pyrophosphate (TPP), an essential enzyme cofactor. Previous work has shown that oxythiamine, a thiamine analogue, is metabolized by thiamine pyrophosphokinase (TPK) into oxythiamine pyrophosphate (OxPP) within the malaria parasite Plasmodium falciparum, and then inhibits TPP-dependent enzymes, killing the parasite in vitro and in vivo. To identify a more potent antiplasmodial thiamine analogue, 11 commercially available compounds were tested against P. falciparum and P. knowlesi. Five active compounds were identified, but only N3-pyridyl thiamine (N3PT), a potent transketolase inhibitor and candidate anticancer lead compound, was found to suppress P. falciparum proliferation with an IC50 value 10-fold lower than that of oxythiamine. N3PT was active against P. knowlesi and was >17 times less toxic to human fibroblast, as compared to oxythiamine. Increasing the extracellular thiamine concentration reduced the antiplasmodial activity of N3PT, consistent with N3PT competing with thiamine/TPP. A transgenic P. falciparum line overexpressing TPK was found to be hypersensitized to N3PT. Docking studies showed an almost identical binding mode in TPK between thiamine and N3PT. Furthermore, we show that [3H]thiamine accumulation, resulting from a combination of transport and metabolism, in isolated parasites is reduced by N3PT. Treatment of P. berghei-infected mice with 200 mg/kg/day N3PT reduced their parasitemia, prolonged their time to malaria symptoms, and appeared to be non-toxic to mice. Collectively, our studies are consistent with N3PT competing with thiamine for TPK binding and inhibiting parasite proliferation by reducing TPP production, as well as being converted into a TPP antimetabolite that inhibits TPP-dependent enzymes.

pharmacology and toxicology↗

bioRxiv2024

Mutation of the P. falciparum flavokinase confers resistance to roseoflavin and 8-aminoriboflavin

We previously found that two riboflavin analogues, roseoflavin and 8-aminoriboflavin, inhibit malaria parasite proliferation by targeting riboflavin utilisation. To determine the mechanism of action of roseoflavin in P. falciparum, we generated roseoflavin-resistant parasites by in vitro evolution over 27 weeks. The roseoflavin-resistant parasites were found to be four times more resistant to roseoflavin and cross-resistant to 8-aminoriboflavin. Resistant parasites were subjected to whole genome sequencing and a missense mutation (T2015A), leading to an amino acid exchange (L672H), was detected in the gene coding for a putative flavokinase (PfFK), the enzyme responsible for converting riboflavin (vitamin B2) into the cofactor flavin mononucleotide (FMN). To confirm that the L672H mutation is responsible for the observed phenotype, we generated parasites with the missense mutation incorporated into the PfFK gene via a single-crossover recombination. The IC50 values for roseoflavin (RoF) and 8-aminoriboflavin against the RoF-resistant parasites created through in vitro evolution were indistinguishable from the IC50 values for parasites in which the missense mutation was specifically introduced into the native PfFK. To investigate this mutation, we generated two parasite lines episomally-expressing GFP-tagged versions of either the wild type or mutant forms of flavokinase. We found that PfFK-GFP localises to the parasite cytosol and that immunopurified PfFK-GFP was active and phosphorylated riboflavin into flavin mononucleotide. The L672H mutation caused a reduction of the binding affinity, especially for the substrate RoF, which explains the resistance phenotype. The mutant PfFK is no longer capable of phosphorylating 8-aminoriboflavin, but its antiplasmodial activity against resistant parasites can still be antagonised by increasing the extracellular concentration of riboflavin, consistent with the compound also inhibiting parasite growth through competitive inhibition of PfFK. Our findings, therefore are consistent with roseoflavin and 8-aminoriboflavin inhibiting parasite growth by inhibiting FMN production, in addition to the generation of toxic flavin cofactor analogues.

microbiology↗

bioRxiv2022

Roseoflavin, a natural riboflavin analogue, possesses in vitro and in vivo antiplasmodial activity

The ability of the human malaria parasite Plasmodium falciparum to access and utilise vital nutrients is critical to its growth and proliferation. Molecules that interfere with these process could potentially serve as antimalarials. We found that two riboflavin analogues, roseoflavin and 8-aminoriboflavin, inhibit malaria parasite proliferation by targeting riboflavin metabolism and/or the utilisation of the riboflavin metabolites flavin mononucleotide and flavin adenine dinucleotide. An additional eight riboflavin analogues were evaluated, but none were found to be more potent than roseoflavin, nor was their activity on target. Focussing on roseoflavin, we tested its antimalarial activity in vivo against Plasmodium vinckei vinckei in mice. We found that roseoflavin decreased the parasitemia by 46-fold following a 4 day suppression test and, on average, increased the survival of mice by 4-5 days. Our data are consistent with riboflavin metabolism and/or the utilisation of riboflavin-derived cofactors being viable drug targets for the development of new antimalarials and that roseoflavin could serve as a potential starting point.

pharmacology and toxicology↗

bioRxiv2022

A screen of drug-like molecules identifies chemically diverse electron transport chain inhibitors in apicomplexan parasites

With the advent of resistance to existing treatments, new drugs are needed to combat apicomplexan parasites such as the causative agents of malaria (Plasmodium species) and toxoplasmosis (Toxoplasma gondii). To identify new inhibitors of the mitochondrial electron transport chain (ETC) in these parasites, we developed a Seahorse XFe96 flux analyzer approach to screen compounds from the Medicines for Malaria Venture Pathogen Box for ETC inhibition. We identified six chemically diverse, on-target inhibitors of the ETC of T. gondii, five of which also target the ETC of Plasmodium falciparum. Two of the identified compounds (MMV024937 and MMV688853) represent novel ETC inhibitor chemotypes. We pinpoint the molecular targets of these inhibitors, demonstrating that all target ETC Complex III, with MMV688853 additionally targeting a kinase with a key role in parasite invasion of host cells. Most of the compounds remain effective inhibitors of parasites that are resistant to the clinically used Complex III inhibitor atovaquone. In sum, we have developed a versatile screening approach to identify and characterize new inhibitors of the ETC in apicomplexan parasites.

microbiology↗

bioRxiv2021

Purification of functional Plasmodium falciparum tubulin allows for the identification of parasite-specific microtubule inhibitors

Cytoskeletal proteins are essential for parasite proliferation, growth, and transmission, and therefore represent promising drug targets. While {beta}-tubulin, the molecular building block of microtubules, is an established drug target in a variety of cancers, we still lack substantial knowledge of the biochemistry of parasite tubulins, which would allow us to exploit the structural divergence between parasite and human tubulins. Indeed, mechanistic insights have been limited by the lack of purified, functional parasite tubulin. In this study, we isolated Plasmodium falciparum tubulin that is assembly-competent and shows specific microtubule dynamics in vitro. We further present mechanistic evidence that two compounds selectively interact with parasite over host microtubules and inhibit Plasmodium microtubule polymerization at substoichiometric compound concentrations. The ability of compounds to selectively disrupt protozoan microtubule growth without affecting human microtubules provides the exciting possibility for the targeted development of novel antimalarials.

biochemistry↗

bioRxiv2021

A novel heteromeric pantothenate kinase complex in apicomplexan parasites

Coenzyme A is synthesised from pantothenate via five enzyme-mediated steps. The first step is catalysed by pantothenate kinase (PanK). All PanKs characterised to date form homodimers. Many organisms express multiple PanKs. In some cases, these PanKs are not functionally redundant, and some appear to be non-functional. Here, we investigate the PanKs in two pathogenic apicomplexan parasites, Plasmodium falciparum and Toxoplasma gondii. Each of these organisms express two PanK homologues (PanK1 and PanK2). We demonstrate that PfPanK1 and PfPanK2 associate, forming a single, functional PanK complex that includes the multi-functional protein, Pf14-3-3I. Similarly, we demonstrate that TgPanK1 and TgPanK2 form a single complex that possesses PanK activity. Both TgPanK1 and TgPanK2 are essential for T. gondii proliferation, specifically due to their PanK activity. Our study constitutes the first examples of heteromeric PanK complexes in nature and provides an explanation for the presence of multiple PanKs within certain organisms.

biochemistry↗

bioRxiv2020

Exploring heteroaromatic rings as a replacement for the labile amide of antiplasmodial pantothenamides

The Plasmodium parasites that cause malaria are adept at developing resistance to antimalarial drugs, necessitating the search for new antiplasmodials. Although several amide analogs of pantothenate (pantothenamides) show potent antiplasmodial activity, hydrolysis by pantetheinases (or vanins) present in blood rapidly inactivates them. We report herein the facile synthesis and biological activity of a small library of pantothenamide analogs in which the labile amide group is replaced with a variety of heteroaromatic rings. Several of the new analogs display antiplasmodial activity in the nanomolar range against P. falciparum and/or P. knowlesi in the presence of pantetheinase. A previously reported triazole and an isoxazole derivative presented here were further characterized and found to possess high selectivity indices, medium or high Caco-2 permeability, and medium or low microsomal clearance in vitro. Although we show here that the two compounds fail to suppress proliferation of P. berghei in vivo, pharmacokinetic and contact time data presented provide a benchmark for the compound profile required to achieve antiplasmodial activity in mice and should facilitate lead optimization.

pharmacology and toxicology↗

bioRxiv2017

Mutations in the pantothenate kinase of the malaria parasite P. falciparum confer resistance or hypersensitivity to diverse pantothenate analogues

The malaria-causing blood stage of Plasmodium falciparum requires extracellular pantothenate for proliferation. The parasite converts pantothenate into coenzyme A (CoA) via five enzymes, the first being a pantothenate kinase (PfPanK). Multiple antiplasmodial pantothenate analogues, including pantothenol and CJ-15,801, kill the parasite by targeting CoA biosynthesis/utilisation. Their mechanism of action, however, remains unknown. Here, we show that parasites pressured with pantothenol or CJ-15,801 become resistant to these analogues. Whole-genome sequencing revealed mutations in one of two putative PanK genes (Pfpank1) in each resistant line. These mutations significantly alter PfPanK activity, with two conferring a fitness cost, consistent with Pfpank1 coding for a functional PanK that is essential for normal growth. The mutants exhibit a different sensitivity profile to recently-described, potent, antiplasmodial pantothenate analogues, with one line being hypersensitive. We provide evidence consistent with different pantothenate analogue classes having different mechanisms of action: some inhibit CoA biosynthesis while others inhibit CoA-utilising enzymes.

pharmacology and toxicology↗