Crucial residues in falcipains that mediate hemoglobin hydrolysis

Understanding the complex formation of falstatin; an endogenous macromolecular inhibitor of falcipains

Proteolytic activity constitutes a fundamental process essential for the survival of the malaria parasite and is thus highly regulated. Falstatin, a protease inhibitor of Plasmodium falciparum, tightly regulates the activity of cysteine hemoglobinases, falcipain-2 and 3 (FP2, FP3), by inhibiting FP2 through a single surface exposed loop. However, the multimeric nature of falstatin and its interaction with FP2 remained unexplored. Here we report that the N-terminal falstatin region is highly disordered, and needs chaperone activity (heat-shock protein 70, HSP70) for its folding. Protein-protein interaction assays showed a significant interaction between falstatin and HSP70. Further, characterization of the falstatin multimer through a series of biophysical techniques identified the formation of a falstatin decamer, which was extremely thermostable. Computational analysis of the falstatin decamer showed the presence of five falstatin dimers, with each dimer aligned in a head-to-tail orientation. Further, the falstatin C-terminal region was revealed to be primarily involved in the oligomerization process. Stoichiometric analysis of the FP2-falstatin multimer showed the formation of a heterooligomeric complex in a 1:1 ratio, with the participation of ten subunits of each protein. Taken together, our results report a novel protease-inhibitor complex and strengthens our understanding of the regulatory mechanisms of major plasmodium hemoglobinases.

Screening of potential antiplasmodial agents targeting cysteine protease-Falcipain 2: a computational pipeline

The spread of antimalarial drug resistance is a substantial challenge in achieving global malaria elimination. Consequently, the identification of novel therapeutic candidates is a global health priority. Malaria parasite necessitates hemoglobin degradation for its survival, which is mediated by Falcipain 2 (FP2), a promising antimalarial target. In particular, FP2 is a key enzyme in the erythrocytic stage of the parasite's life cycle. Here, we report the screening of approved drugs listed in DrugBank using a computational pipeline that includes drug-likeness, toxicity assessments, oral toxicity evaluation, oral bioavailability, docking analysis, maximum common substructure (MCS) and molecular dynamics (MD) Simulations analysis to identify capable FP2 inhibitors, which are hence potential antiplasmodial agents. A total of 45 drugs were identified, which have positive drug-likeness, no toxic features and good bioavailability. Among these, six drugs showed good binding affinity towards FP2 compared to E64, an epoxide known to inhibit FP2. Notably, two of them, Cefalotin and Cefoxitin, shared the highest MCS with E64, which suggests that they possess similar biological activity as E64. In an investigation using MD for 100 ns, Cefalotin and Cefoxitin showed adequate protein compactness as well as satisfactory complex stability. Overall, these computational approach findings can be applied for designing and developing specific inhibitors or new antimalarial agents for the treatment of malaria infections.Communicated by Ramaswamy H. Sarma.

Hemoglobin Precipitation: An Index of In Vitro Vasoconstrictive Activities of Methanol Leaf Extracts of Croton megalocarpus Hutch and Lantana camara Linn

The function of innate hemostasis aids the body in bleeding control, preventing the loss of excessive amounts of blood following low-degree injuries. However, injuries of a higher degree may require extrinsic intervention to stop life-threatening blood loss. Astringent agents' actions result in mechanical constriction of small blood vessels and shrinkage of body tissues, thereby stopping blood loss. This enhances the primary phase of hemostasis, where vasoconstriction is the main mechanism at play during the initial response to injury. The effects of plant extracts on protein precipitation have been linked to blood vessel vasoconstriction. Traditionally, the leaves of Croton megalocarpus Hutch and Lantana camara Linn plants are used by communities living in Makueni County, Kenya, for peripheral bleeding control. However, the effects of extracts of both plants on hemoglobin precipitation have not been evaluated scientifically. In the current study, the activities of methanol extracts of C. megalocarpus (H.) and L. camara (L.) on blood protein precipitation were investigated. The leaves were harvested, cleaned, air-dried, milled, and extracted in absolute methanol before being concentrated into dry powders. A qualitative phytochemical screen revealed the presence of terpenoids, steroids, tannins, phenols, flavonoids, reducing sugars, cardiac glycosides, and carbohydrates in the methanol extract of C. megalocarpus (H.). The methanol extracts of L. camara (L.) contained cardiac glycosides, saponins, tannins, phenols, terpenoids, reducing sugars, and carbohydrates. The hemoglobin precipitation ability of various concentrations of extracts using mice samples was presented as relative astringency following the tannic acid external standard method. Methanol extracts C. megalocarpus (H.) and L. camara (L.) had significantly higher relative astringency compared with the normal control, indicating a protein precipitating activity. The relative astringency observed in both plant extracts is linked to the activity of tannins, phenols, flavonoids, and saponins detected during preliminary phytochemical screening.

Plasmodium falciparum metacaspase-2 capture its natural substrate in a non-canonical way

Programmed cell death (PCD) is a multi-step process initiated by a set of proteases, which interacts and cleaves diverse proteins, thus modulating their biochemical and cellular functions. In metazoans, PCD is mediated by proteolytic enzymes called caspases, which triggered cell death by proteolysis of human Tudor staphylococcus nuclease (TSN). Non-metazoans lack a close homologue of caspases but possess an ancestral family of cysteine proteases termed ‘metacaspases’. Studies supported that metacaspases are involved in PCD, but their natural substrates remain unknown. In this study, we performed the Plasmodium falciparum TSN (PfTSN) cleavage assay using wild and selected mutants of P. falciparum metacaspases-2 (PfMCA-2) in vitro and in vivo. Interestingly, PfMCA-2, cleaved a phylogenetically conserved protein, PfTSN at multiple sites. Deletion or substitution mutation in key interacting residues at the active site, Cys157 and His205 of PfMCA-2, impaired its enzymatic activity with the artificial substrate, z-GRR-AMC. However, the mutant Tyr224A did not affect the activity with z-GRR-AMC but abolished the cleavage of PfTSN. These results indicated that the catalytic dyad, Cys157 and His205 of PfMCA-2 was essential for its enzymatic activity with an artificial substrate, whereas Tyr224 and Cys157 residues were responsible for its interaction with the natural substrate and subsequent degradation of PfTSN. Our results suggested that MCA-2 interacts with TSN substrate in a non-canonical way using non-conserved or conformationally available residues for its binding and cleavage. In future, it would be interesting to explore how this interaction leads to the execution of PCD in the Plasmodium.

Targeting Plasmodium with constrained peptides and peptidomimetics

Malaria remains a worldwide health concern with an estimated quarter of a billion people infected and nearly half a million deaths annually. Malaria is caused by a parasite infection from Plasmodium strains which are transmitted from mosquitoes into the human host. Although several small molecule inhibitors have been found to target the early stages of transmission and prevent parasite proliferation, multiple drug resistant parasite strains have emerged and drug resistance remains a major hurdle. As an alternative to small molecule inhibition, several peptide-based therapeutics have been explored for their potential as antimalarial compounds. Chemically constrained peptides or peptidomimetics were developed to target large binding interfaces of parasite-based proteins that have historically been difficult to selectively inhibit using small molecules. Here, we review ongoing research aimed at developing constrained peptides targeting protein-protein interactions pertinent to malaria pathogenesis. These targets include Falcipain-2, the J domain of CDPK1, myosin A tail domain interacting protein, the PKA signaling pathway, and an unclear signaling pathway involving angiotensin-derived peptides. Diverse synthetic methods were also used for each target. Merging parasite biology with synthetic strategies may provide new opportunities to develop alternative methods for uncovering novel antimalarials and may offer an alternate source for targeting drug-resistant parasite strains.