Identification of active <it>Plasmodium falciparum</it> calpain to establish screening system for <it>Pf</it>-calpain-based drug development

<p>Abstract</p> <p>Background</p> <p>With the increasing resistance of malaria parasites to available drugs, there is an urgent demand to develop new anti-malarial drugs. Calpain inhibitor, ALLN, is proposed to inhibit parasite proliferation by suppressing haemoglobin degradation. This provides <it>Plasmodium</it> calpain as a potential target for drug development. <it>Pf</it>-calpain, a cysteine protease of <it>Plasmodium falciparum</it>, belongs to calpain-7 family, which is an atypical calpain not harboring Ca²⁺-binding regulatory motifs. In this present study, in order to establish the screening system for <it>Pf</it>-calpain specific inhibitors, the active form of <it>Pf</it>-calpain was first identified.</p> <p>Methods</p> <p>Recombinant <it>Pf</it>-calpain including catalytic subdomain IIa (r<it>Pf</it>cal-IIa) was heterologously expressed and purified. Enzymatic activity was determined by both fluorogenic substrate assay and gelatin zymography. Molecular homology modeling was carried out to address the activation mode of <it>Pf</it>-calpain in the aspect of structural moiety.</p> <p>Results</p> <p>Based on the measurement of enzymatic activity and protease inhibitor assay, it was found that the active form of <it>Pf</it>-calpain only contains the catalytic subdomain IIa, suggesting that <it>Pf</it>-calpain may function as a monomeric form. The sequence prediction indicates that the catalytic subdomain IIa contains all amino acid residues necessary for catalytic triad (Cys-His-Asn) formation. Molecular modeling suggests that the <it>Pf</it>-calpain subdomain IIa makes an active site, holding the catalytic triad residues in their appropriate orientation for catalysis. The mutation analysis further supports that those amino acid residues are functional and have enzymatic activity.</p> <p>Conclusion</p> <p>The identified active form of <it>Pf</it>-calpain could be utilized to establish high-throughput screening system for <it>Pf</it>-calpain inhibitors. Due to its unique monomeric structural property, <it>Pf</it>-calpain could be served as a novel anti-malarial drug target, which has a high specificity for malaria parasite. In addition, the monomeric form of enzyme may contribute to relatively simple synthesis of selective inhibitors.</p