Molecular Mechanisms of Methoctramine Binding and Selectivity at Muscarinic Acetylcholine Receptors
作者:Jan Jakubík、Pavel Zimčík、Alena Randáková、Květoslava Fuksová、Esam E. El-Fakahany、Vladimír Doležal
DOI:10.1124/mol.114.093310
日期:2014.8
Methoctramine ( N , N '-bis[6-[[(2-methoxyphenyl)-methyl]hexyl]-1,8-octane] diamine) is an M2-selective competitive antagonist of muscarinic acetylcholine receptors and exhibits allosteric properties at high concentrations. To reveal the molecular mechanisms of methoctramine binding and selectivity we took advantage of reciprocal mutations of the M2 and M3 receptors in the second and third extracellular loops that are involved in the binding of allosteric ligands. To this end we performed measurements of kinetics of the radiolabeled antagonists N-methylscopolamine (NMS) in the presence of methoctramine and its precursors, fluorescence energy transfer between green fluorescent protein–fused receptors and an Alexa-555–conjugated precursor of methoctramine, and simulation of molecular dynamics of methoctramine association with the receptor. We confirm the hypothesis that methoctramine high-affinity binding to the M2 receptors involves simultaneous interaction with both the orthosteric binding site and the allosteric binding site located between the second and third extracellular loops. Methoctramine can bind solely with low affinity to the allosteric binding site on the extracellular domain of NMS-occupied M2 receptors by interacting primarily with glutamate 175 in the second extracellular loop. In this mode, methoctramine physically prevents dissociation of NMS from the orthosteric binding site. Our results also demonstrate that lysine 523 in the third extracellular loop of the M3 receptors forms a hydrogen bond with glutamate 219 of the second extracellular loop that hinders methoctramine binding to the allosteric site at this receptor subtype. Impaired interaction with the allosteric binding site manifests as low-affinity binding of methoctramine at the M3 receptor.
甲氧氯普胺(N , N '-双[6-[[(2-甲氧基苯基)-甲基]己基]-1,8-辛烷]二胺)是毒蕈碱乙酰胆碱受体的一种 M2 选择性竞争性拮抗剂,在高浓度时表现出异构特性。为了揭示甲氧氯普胺结合和选择性的分子机制,我们利用了 M2 和 M3 受体在第二和第三胞外环的互变,这两个环参与了异构配体的结合。为此,我们测量了放射性标记的拮抗剂 N-甲基东莨菪碱(NMS)在甲氧辛胺及其前体存在下的动力学,融合了绿色荧光蛋白的受体与 Alexa-555 结合的甲氧辛胺前体之间的荧光能量转移,以及甲氧辛胺与受体结合的分子动力学模拟。我们证实了这样的假设:甲氧辛胺与 M2 受体的高亲和力结合涉及与位于第二和第三胞外环之间的正交结合位点和异位结合位点的同时相互作用。甲氧氯普胺主要通过与第二胞外环中的谷氨酸 175 相互作用,仅能以低亲和力与 NMS 占位的 M2 受体胞外结构域上的异构结合位点结合。在这种模式下,甲氧氯普胺可物理性地阻止 NMS 从异位结合位点解离。我们的研究结果还表明,M3 受体第三胞外环中的赖氨酸 523 与第二胞外环中的谷氨酸 219 形成氢键,从而阻碍了甲氧辛胺与该受体亚型的异构位点结合。与异构结合位点的相互作用受损表现为甲氧氯普胺与 M3 受体的低亲和力结合。