Noncompetitive antagonist-binding sites of rat and housefly gamma-aminobutyric acid receptors display different enantiospecificities for tert-butyl(isopropyl)bicyclophosphorothionate

Mechanistic insights into the selectivity of bicyclophosphorothionate antagonists for housefly versus rat GABA receptors

BACKGROUND

γ-Aminobutyric acid (GABA) receptors (GABARs) are validated targets of insecticides. Bicyclophosphorothionates are a group of insecticidal compounds that act as noncompetitive antagonists of GABARs. We previously reported that the analogs exhibit various degrees of selectivity for housefly versus rat GABARs, depending on substitutions at the 3- and 4-positions. We here sought to elucidate the unsolved mechanisms of the receptor selectivity using quantitative structure-activity relationship (QSAR), molecular docking, and molecular dynamics approaches.

RESULTS

Three-dimensional (3D)-QSAR studies using Topomer comparative molecular field analysis quantitatively demonstrated how the introduction of a small alkyl group at the 3-position of bicyclophosphorothionates contributes to the housefly versus rat GABAR selectivity. To investigate the molecular mechanisms of the selective action, bicyclophosphorothionates were docked into housefly Resistance to dieldrin (RDL) GABAR and rat α1β2γ2 GABAR homology models built using the published 3D-structures of human GABARs as templates. The results of molecular docking and molecular dynamics simulations revealed that the 2'Ala and 6'Thr residues of the RDL subunit within the channel are the key amino acids for binding to the housefly GABARs, whereas the 2'Ser residue of γ2 subunit plays a crucial role in binding to rat GABARs.

CONCLUSION

We revealed the molecular mechanisms underlying the selective antagonistic action of bicyclophosphorothionates on housefly versus rat GABARs. The information presented should help design and develop novel, safe GABAR-targeting insecticides. © 2023 Society of Chemical Industry.

Non-competitive GABA antagonists: probing the mechanisms of their selectivity for insect versus mammalian receptors

A great variety of non-competitive antagonists of ionotropic gamma-aminobutyric acid (GABA) receptors have been reported. While they are structurally diverse, there are common features in their structures. Thus, it was hypothesized that they bind to an identical site in different or overlapping orientations, and this hypothesis was validated by three-dimensional structure-activity relationship (3D-QSAR) analysis using receptor-binding data. Meanwhile, although most antagonists are highly toxic to both vertebrates and invertebrates, several classes of antagonists, such as nor-diterpene lactone picrodendrins, phenyl heterocyclic compounds and disubstituted bicyclophosphorothionates, were found to exhibit selectivity for housefly versus rat GABA receptors. To probe their selectivity mechanisms, the 3D-QSAR method was applied to the three classes of antagonists. This revealed several important differences that might be related to the selectivity of antagonists between the structures of the non-competitive antagonist-binding sites of housefly and rat GABA receptors.