Zhorov B S, Ananthanarayanan V S
Department of Biochemistry, McMaster University, Hamilton, Ontario, L8N 3Z5, Canada.
Arch Biochem Biophys. 2000 Mar 1;375(1):31-49. doi: 10.1006/abbi.1999.1529.
Metal ions affect ligand binding to G-protein-coupled receptors by as yet unknown mechanisms. In particular, Na(+) increases the affinity for antagonists but decreases it for agonists. We had modeled the mu-opioid receptor (muR) based on the low-resolution structure of rhodopsin by G. F. X. Schertler, C. Villa, and R. Henderson (1993, Nature 362, 770-772) and proposed that metal ions may be directly involved in the binding of ligands and receptor activation (B. S. Zhorov and V. S. Ananthanarayanan, 1998, J. Biomol. Struct. Dyn. 15, 631-637). Developing this concept further, we present here homology models of muR using as templates the structure of rhodopsin elaborated by I. D. Pogozheva, A. L. Lomize, and H. I. Mosberg (1997, Biophys. J. 70, 1963-1985) and J. M. Baldwin, G. F. X. Schertler, and V. M. Unger (1997, J. Mol. Biol., 272, 144-164). Using the Monte Carlo minimization (MCM) method, we docked the Na(+)-bound forms of muR ligands: naloxone, bremazocine, and carfentanyl. The resultant low-energy complexes showed that the two positive charges in the protonated metal-bound ligands interact with the two negative charges at Asp(3.32) and Asp(2.50) (for notations, see J. A. Ballesteros and H. Weinstein, 1995, Methods Neurosci. 25, 366-426). MCM computation on morphine docked inside the model of muR by I. D. Pogozheva, A. L. Lomize, and H. I. Mosberg (1998, Biophys. J. 75, 612-634) yielded two binding modes with the ligand's ammonium group salt-bridged either to Asp(3.32) (generally regarded as the ligand recognition site) or to Asp(2.50). The latter is the presumed site for Na(+) ion, which is known to modulate ligand binding. Assuming that in the low-dielectric transmembrane region of muR, organic and inorganic cations would compete for Asp(3.32) and Asp(2.50), we propose that ligand binding, as visualized in the above models, would first displace Na(+) from Asp(3.32). A subsequent progress of the ligand toward Asp(2.50) would result in either the retention of Na(+) at Asp(2.50) in the case of antagonists or the displacement of Na(+) from Asp(2.50) in the case of agonists. The displaced Na(+) would move toward the salt-bridged Asp(3.49)-Arg(3.50) and disengage the salt bridge. This, in turn, would result in conformational changes at the cytoplasmic face of the receptor that facilitate the interaction with the G-protein.
金属离子通过尚不明确的机制影响配体与G蛋白偶联受体的结合。具体而言，Na(+)增加了拮抗剂的亲和力，但降低了激动剂的亲和力。我们基于G. F. X. Schertler、C. Villa和R. Henderson（1993年，《自然》362卷，770 - 772页）测定的视紫红质低分辨率结构，构建了μ-阿片受体（μR）模型，并提出金属离子可能直接参与配体结合和受体激活过程（B. S. Zhorov和V. S. Ananthanarayanan，1998年，《生物分子结构与动力学杂志》15卷，631 - 637页）。在此基础上进一步拓展这一概念，我们以I. D. Pogozheva、A. L. Lomize和H. I. Mosberg（1997年，《生物物理学杂志》70卷，1963 - 1985页）以及J. M. Baldwin、G. F. X. Schertler和V. M. Unger（1997年，《分子生物学杂志》272卷，144 - 164页）阐述的视紫红质结构为模板，呈现了μR的同源模型。利用蒙特卡罗最小化（MCM）方法，我们对接了与Na(+)结合的μR配体形式：纳洛酮、布瑞马佐辛和卡芬太尼。所得的低能复合物显示，质子化的金属结合配体中的两个正电荷与Asp(3.32)和Asp(2.50)处的两个负电荷相互作用（关于符号表示，见J. A. Ballesteros和H. Weinstein，1995年，《神经科学方法》25卷，366 - 426页）。I. D. Pogozheva、A. L. Lomize和H. I. Mosberg（1998年，《生物物理学杂志》75卷，612 - 634页）在μR模型中对接吗啡的MCM计算产生了两种结合模式，配体的铵基团通过盐桥分别连接到Asp(3.32)（通常被视为配体识别位点）或Asp(2.50)。后者是推测的Na(+)离子位点，并已知其可调节配体结合。假设在μR的低介电跨膜区域，有机和无机阳离子会竞争Asp(3.32)和Asp(2.50)，我们提出，如上述模型所示，配体结合首先会将Na(+)从Asp(3.32)上取代。随后配体向Asp(2.50)移动，对于拮抗剂而言，会导致Na(+)保留在Asp(2.50)处；对于激动剂而言，则会使Na(+)从Asp(2.50)上被取代。被取代的Na(+)会向盐桥连接的Asp(3.49)-Arg(3.50)移动并使盐桥断开。反过来，这会导致受体胞质面发生构象变化，从而促进与G蛋白的相互作用。
