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  • Tropical theileriosis is caused by the apicomplexan parasite


    Tropical theileriosis is caused by the apicomplexan parasite Theileria annulata which is transmitted by a tick vector from the genus Hyalomma in cattle (Echebli et al., 2014, Li et al., 2014). Theileria parasites invade the leukocytes by sporozoites secreted from the vector, schizonts mature into merozoites and infect erythrocytes subsequently; leading to high rate of morbidity and mortality (Mans et al., 2015, Razavi et al., 2011, Sharifiyazdi et al., 2012). Some recent studies reported that T. annulata has developed resistance to buparvaquone; a well-known drug used in the treatment of theileriosis (Marsolier et al., 2015, Mhadhbi et al., 2010, Sharifiyazdi et al., 2012). Therefore; T. annulata enolase could be a possible target for new drug-design studies because of emerging requirement for alternative drugs against the parasite. Homology modeling is one of the prominent step in structure-based drug design studies and provides information to estimate 3D structure and druggable candidate sites of molecular targets in the absence of experimentally solved 3D structures (Agrawal, 2013). The modeled structures ensure information about functional and evolutionary features of the target proteins (Wallner and Elofsson, 2005). Furthermore, molecular docking is used to determine optimum binding modes of ligands to a certain site of protein target in structure-based drug design (Thomsen and Christensen, 2006). Present study has been conducted to evaluate druggable potential of TaENO for new drug design studies. The 3D open and closed conformation models of TaENO were built by homology modeling approaches following the analysis of the enzyme by amino PRIMA-1MET sequence alignment. Primary and secondary structure analysis, the active site determination and docking of the substrate (2-PGA) were also performed.
    Materials and methods
    Conclusion In summary, we have predicted three-dimensional structures of open and closed conformations of Theileria annulata enolase by using the method of homology modeling. Comparative and comprehensive structural analysis indicated that preserved structural and functional characteristics found among enolase superfamily were also preserved in TaENO. In addition to the similarities, some surface characteristic such as electrostatics potential surfaces nearby the active site region and positively charged pockets could be potent regions for selective drugs and key residues in these regions could be further tested by mutagenesis studies.
    After ingestion, giardial cysts release trophozoites that colonize the small intestine where they can cause disease. However, if they are carried downstream, trophozoites must differentiate into cysts to survive in the environment , . In addition to major morphological changes, glycolytic metabolism decreases during encystation and increases during excystation . Both differentiations are essential for transmission and disease, but the roles of metabolic enzymes are not completely understood. Enolase (2-phosphoglycerate hydrolase) is a metal-ion-activated glycolytic enzyme that catalyzes the reversible elimination of water from 2-phosphoglycerate (2PGA) to form phosphoenolpyruvate. Enolase is one of the most abundantly expressed enzymes in the cytosol of a variety of organisms and its mRNA transcript is highly expressed in trophozoites . Recent evidence describes a role for enolase activity in the pathogenesis and cellular differentiation of several organisms , , , , . Moreover, despite the absence of an N-terminal secretion signal, enolase has been located on the surface of a variety of parasites , , , , . Thus, enolase is suggested to be a new surface-associated virulence factor. In , enolase is specifically secreted from trophozoites in the presence of intestinal epithelial cells . Although it is antigenic in natural human and experimental mouse infections , , it is not an effective vaccine target in mice . Enolases are highly conserved, and gEno shares a high level of identity with enolase α and γ of , and (51%), and 49% and 46% with and , respectively. The alignment also reveals that gEno has the important residues for enolase activity (). In gEno, the five active site catalytic residues correspond to H170, E222, K361, H389 and K412 (marked as stars in ). In other cells, mutation of any of these residues significantly reduces enolase activity , , , . During the 2PGA binding, enolase undergoes important conformational changes. The loops, V153-F169 and S250-G277 allow the protonation of 2PGA by H159 . In gEno, the first loop is highly conserved. In addition, gEno contains the residues that can bind divalent metal ions (i.e. Mg, Zn, and Mn). Mg is the strongest enolase activating metal , . Binding of two Mg ions is essential: the first Mg (“Mg (I)”) is required for the correct conformation of the active site and binds to E306, D257, and D336; the second Mg (“Mg (II)”) directly interacts with S41 and 2PGA .