The drugs salmeterol, formoterol, and salbutamol constitute the frontline treatment of asthma and other chronic pulmonary diseases. These drugs activate the β2-adrenergic receptors (β2-AR), a class A G protein–coupled receptor (GPCR), and differ significantly in their clinical onset and duration of actions. According to the microkinetic model, the long duration of action of salmeterol and formoterol compared with salbutamol were attributed, at least in part, to their high lipophilicity and increased local concentrations in the membrane near the receptor. However, the structural and molecular bases of how the lipophilic drugs reach the binding site of the receptor from the surrounding membrane remain unknown. Using a variety of classic and enhanced molecular dynamics simulation techniques, we investigated the membrane partitioning characteristics, binding, and unbinding mechanisms of the ligands. The obtained results offer remarkable insight into the functional role of membrane lipids in the ligand association process. Strikingly, salmeterol entered the binding site from the bilayer through transmembrane helices 1 and 7. The entry was preceded by membrane-facilitated rearrangement and presentation of its phenyl-alkoxy-alkyl tail as a passkey to an access route gated by F193, a residue known to be critical for salmeterol’s affinity. Formoterol’s access is through the aqueous path shared by other β2-AR agents. We observed a novel secondary path for salbutamol that is distinct from its primary route. Our study offers a mechanistic description for the membrane-facilitated access and binding of ligands to a membrane protein and establishes a groundwork for recognizing membrane lipids as an integral component in the molecular recognition process.

中文翻译：

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长效β2-肾上腺素受体激动剂的膜促进受体进入和结合机制

药物沙美特罗、福莫特罗和沙丁胺醇构成了哮喘和其他慢性肺部疾病的一线治疗药物。这些药物激活β2-肾上腺素能受体（β2-AR)，一种 AG 蛋白偶联受体 (GPCR)，在临床起效和作用持续时间方面存在显着差异。根据微动力学模型，与沙丁胺醇相比，沙美特罗和福莫特罗的长作用时间至少部分归因于它们的高亲脂性和受体附近膜中局部浓度的增加。然而，亲脂性药物如何从周围膜到达受体结合位点的结构和分子基础仍然未知。使用各种经典和增强的分子动力学模拟技术，我们研究了配体的膜分配特性、结合和解结合机制。获得的结果为膜脂在配体结合过程中的功能作用提供了显着的洞察力。引人注目的是，沙美特罗通过跨膜螺旋 1 和 7 从双层进入结合位点。进入之前是膜促进重排，并呈现其苯基-烷氧基-烷基尾巴作为通向由 F193 控制的通路的密钥，这是一种已知的残基对沙美特罗的亲和力至关重要。福莫特罗的访问是通过其他共享的水路径β 2-AR 剂。我们观察到一种不同于其主要途径的沙丁胺醇的新次要途径。我们的研究为膜促进配体与膜蛋白的接近和结合提供了机制描述，并为将膜脂识别为分子识别过程中的一个组成部分奠定了基础。