Investigation of the insecticidal potential of curcumin derivatives that target the Helicoverpa armigera sterol carrier protein-2

Cotton bollworm (Helicoverpa armigera) is a highly polyphagous, widely prevalent, and persistent Old World insect pest that affects numerous important crops that are directly consumed by people, including tomato, cotton, pigeon pea, chickpea, rice, sorghum, and cowpea. Insects do not synthesize steroids but obtain them from their diet. Inhibition of dietary uptake of steroids by insects is a potentially effective insecticidal mechanism that should not be toxic to humans and other mammals, who synthesize their steroids. Ten curcumin derivatives were tested against H. armigera sterol carrier protein-2 (HaSCP-2) for their potential as insecticidal agents. Curcumin derivatives were initially docked at the binding site of HaSCP-2 to determine their binding affinities and plausible binding modes. The binding modes predominantly show hydrophobic interactions of derivatives with Phe53, Phe110, and Phe89 as core interacting residues in the active site. Validation of in silico results was carried out by performing a fluorescence binding and displacement assay to determine the binding affinities of curcumin derivatives. Among a collection of curcumin derivatives tested, Cur10 showed the lowest IC50 value of 9.64 μM, while Cur07 was 19.86 μM, and Cur06 was 20.79 μM. There was a significant negative correlation between the ability of the curcumin derivatives tested to displace the fluorescent probe from the sterol binding site of HaSCP-2 and to inhibit Sf9 insect cell growth in culture, which is consistent with the curcumin derivatives acting by the novel mechanism of blocking sterol uptake. Then molecular dynamics simulation studies validated the predicted binding modes and the interactions of curcumin derivatives with HaSCP-2 protein. In conclusion, these studies support the potential use of curcumin derivatives as insecticidal agents.

Molecular cloning, gene expression analysis, and in silico characterization of UDP-N-acetylglucosamine pyrophosphorylase from Bombyx mori

The present study was aimed to explore the molecular and structural features of UDP-N-acetylglucosamine pyrophosphorylase of Bombyx mori (BmUAP), an essential enzyme for chitin synthesis in insects. The BmUAP cDNA sequence was cloned and expression profiles were monitored during the molting and feeding stages of silkworm larvae. The effect of 20-hydroxyecdysone (20E) on BmUAP expression, and on silkworm molting was studied, which revealed that 20E regulates its expression. Multiple sequence alignment of various pyrophosphorylases revealed that the residues N223, G290, N327, and K407 of human UAP (PDB ID: 1JV1) were found to be highly conserved in BmUAP and all other eukaryotic UAPs considered for the study. Phylogenetic analysis inferred that the UAPs possess discrete variations in primary structure among different insect Orders while sharing good identity between species of the Order. The structure of BmUAP was predicted and its interactions with uridine triphosphate, N-acetylglucosamine-1-phosphate, and UDP-N-acetylglucosamine were analyzed. Virtual screening with a library of natural compounds resulted in five potential hits with good binding affinities. On further analysis, these five hits were found to be mimicking substrate and product, in inducing conformational changes in the active site. This work provides crucial information on molecular interactions and structural dynamics of insect UAPs.