Tomizawa Motohiro, Casida John E
Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management, University of California, Berkeley, California 94720-3112, USA.
Acc Chem Res. 2009 Feb 17;42(2):260-9. doi: 10.1021/ar800131p.
Until the mid-20th century, pest insect control in agriculture relied on largely inorganic and botanical insecticides, which were inadequate. Then, the remarkable insecticidal properties of several organochlorines, organophosphates, methylcarbamates, and pyrethroids were discovered, leading to an arsenal of synthetic organics. The effectiveness of these insecticides, however, diminished over time due to the emergence of resistant insect strains with less sensitive molecular targets in their nervous systems. This created a critical need for a new type of neuroactive insecticide with a different yet highly sensitive target. Nicotine in tobacco extract was for centuries the best available agent to prevent sucking insects from damaging crops, although this alkaloid was hazardous to people and not very effective. The search for unusual structures and optimization revealed a new class of potent insecticides, known as neonicotinoids, which are similar to nicotine in their structure and action as agonists of the nicotinic acetylcholine receptor (nAChR). Fortunately, neonicotinoids are much more toxic to insects than mammals due in large part to differences in their binding site interactions at the corresponding nAChRs. This Account discusses the progress that has been made in defining the structural basis of neonicotinoid and nicotinoid potency and selectivity. The findings are based on comparisons of two acetylcholine binding proteins (AChBPs) with distinct pharmacological profiles that serve as structural surrogates for the extracellular ligand-binding domain of the nAChRs. Saltwater mollusk (Aplysia californica) AChBP has high neonicotinoid sensitivity, whereas freshwater snail (Lymnaea stagnalis) AChBP has low neonicotinoid and high nicotinoid sensitivities, pharmacologies reminiscent of insect and vertebrate nAChR subtypes, respectively. The ligand-receptor interactions for these AChBPs were established by photoaffinity labeling and X-ray crystallography. Both azidopyridinyl neonicotinoid and nicotinoid photoprobes bind in a single conformation with Aplysia AChBP; this is consistent with high-resolution crystal structures. Surprisingly, though, the electronegative nitro or cyano moiety of the neonicotinoid faced in a reversed orientation relative to the cationic nicotinoid functionality. For the Lymnaea AChBP, the azidoneonicotinoid probes modified two distinct and distant sites, while the azidonicotinoid probes, surprisingly, derivatized only one point. This meant that the neonicotinoids have two bound conformations in the vertebrate receptor model, which are completely inverted relative to each other, whereas nicotinoids appear buried in only one conserved conformation. Therefore, the unique binding conformations of nicotinic agonists in these insect and vertebrate receptor homologues define the basis for molecular recognition of neonicotinoid insecticides as the determinants of life or death.
直到20世纪中叶，农业中的害虫防治主要依赖于无机和植物性杀虫剂，但这些杀虫剂效果不佳。随后，人们发现了几种有机氯、有机磷、甲基氨基甲酸酯和拟除虫菊酯具有显著的杀虫特性，从而形成了一系列合成有机物杀虫剂。然而，随着时间的推移，由于神经系统中分子靶点敏感性较低的抗性昆虫品系的出现，这些杀虫剂的效力逐渐降低。这就迫切需要一种新型的神经活性杀虫剂，其靶点不同但高度敏感。几个世纪以来，烟草提取物中的尼古丁是防止吸食性昆虫损害作物的最佳可用药剂，尽管这种生物碱对人类有害且效果不太理想。对不同寻常结构的探索和优化揭示了一类新型的高效杀虫剂，即新烟碱类杀虫剂，它们在结构和作用上与尼古丁相似，都是烟碱型乙酰胆碱受体（nAChR）的激动剂。幸运的是，新烟碱类杀虫剂对昆虫的毒性比对哺乳动物大得多，这在很大程度上归因于它们在相应nAChR上结合位点相互作用的差异。本综述讨论了在确定新烟碱类和烟碱类杀虫剂效力及选择性的结构基础方面所取得的进展。这些发现基于对两种具有不同药理学特征的乙酰胆碱结合蛋白（AChBP）的比较，它们作为nAChR细胞外配体结合域的结构替代物。海水软体动物（加州海兔）AChBP对新烟碱类杀虫剂敏感性高，而淡水蜗牛（静水椎实螺）AChBP对新烟碱类杀虫剂敏感性低但对烟碱类杀虫剂敏感性高，这两种药理学特征分别让人联想到昆虫和脊椎动物的nAChR亚型。通过光亲和标记和X射线晶体学确定了这些AChBP的配体-受体相互作用。叠氮吡啶基新烟碱类和烟碱类光探针均以单一构象与加州海兔AChBP结合；这与高分辨率晶体结构一致。然而，令人惊讶的是，新烟碱类杀虫剂的电负性硝基或氰基部分相对于阳离子型烟碱类功能基团呈反向排列。对于静水椎实螺AChBP，叠氮新烟碱类探针修饰了两个不同且距离较远的位点，而叠氮烟碱类探针仅修饰了一个位点。这意味着在脊椎动物受体模型中，新烟碱类杀虫剂有两种相互完全倒置的结合构象，而烟碱类杀虫剂似乎仅以一种保守构象嵌入。因此，这些昆虫和脊椎动物受体同源物中烟碱型激动剂独特的结合构象，定义了新烟碱类杀虫剂作为生死决定因素的分子识别基础。
