title: Mapping the elusive neonicotinoid binding site.
abstract: Two types of structurally similar nicotinic agonists have very different biological and physicochemical properties. Neonicotinoids, important insecticides including imidacloprid and thiacloprid, are nonprotonated and selective for insects and their nicotinic receptors, whereas nicotinoids such as nicotine and epibatidine are cationic and selective for mammalian systems. We discovered that a mollusk acetylcholine binding protein (AChBP), as a structural surrogate for the extracellular ligand-binding domain of the nicotinic receptor, is similarly sensitive to neonicotinoids and nicotinoids. It therefore seemed possible that the proposed very different interactions of the neonicotinoids and nicotinoids might be examined with a single AChBP by using optimized azidochloropyridinyl photoaffinity probes. Two azidoneonicotinoids with a nitro or cyano group were compared with the corresponding desnitro or descyano azidonicotinoids. The four photoactivated nitrene probes modified AChBP with up to one agonist for each subunit based on analysis of the intact derivatized protein. Identical modification sites were observed by collision-induced dissociation analysis for the neonicotinoids and nicotinoids with similar labeling frequency of Tyr-195 of loop C and Met-116 of loop E at the subunit interface. The nitro- or cyano-substituted guanidine/amidine planes of the neonicotinoids provide a unique electronic conjugation system to interact with loop C Tyr-188. The neonicotinoid nitro oxygen and cyano nitrogen contact loop C Cys-190/Ser-189, whereas the cationic head of the corresponding nicotinoids is inverted for hydrogen-bonding and cation-pi contact with Trp-147 and Tyr-93. These structural models based on AChBP directly map the elusive neonicotinoid binding site and further describe the molecular determinants of agonists on nicotinic receptors.
authors.affiliation: Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720-3112, USA.
authors.forename: Motohiro
authors.initials: M
authors.lastname: Tomizawa
authors.forename: Todd T
authors.initials: TT
authors.lastname: Talley
authors.forename: David
authors.initials: D
authors.lastname: Maltby
authors.forename: Kathleen A
authors.initials: KA
authors.lastname: Durkin
authors.forename: Katalin F
authors.initials: KF
authors.lastname: Medzihradszky
authors.forename: Alma L
authors.initials: AL
authors.lastname: Burlingame
authors.forename: Palmer
authors.initials: P
authors.lastname: Taylor
authors.forename: John E
authors.initials: JE
authors.lastname: Casida
mesh_terms: D000818:Animals; D001048:Aplysia / Q000187:drug effects / Q000378:metabolism; D001665:Binding Sites; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D006801:Humans; D013058:Mass Spectrometry; D008958:Models, Molecular; D015394:Molecular Structure; D009538:Nicotine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D019920:Photoaffinity Labels
chemical_list: D002352:Carrier Proteins; D019920:Photoaffinity Labels; D009538:Nicotine