Department of Chemistry, University of Toronto, UTM, 3359 Mississauga Road North, Mississauga, Ontario, Canada L5L 1C6.
J Am Chem Soc. 2013 Jun 26;135(25):9465-74. doi: 10.1021/ja404305k. Epub 2013 Jun 14.
G protein-coupled receptors exhibit a wide variety of signaling behaviors in response to different ligands. When a small label was incorporated on the cytosolic interface of transmembrane helix 6 (Cys-265), (19)F NMR spectra of the β2 adrenergic receptor (β2AR) reconstituted in maltose/neopentyl glycol detergent micelles revealed two distinct inactive states, an activation intermediate state en route to activation, and, in the presence of a G protein mimic, a predominant active state. Analysis of the spectra as a function of temperature revealed that for all ligands, the activation intermediate is entropically favored and enthalpically disfavored. β2AR enthalpy changes toward activation are notably lower than those observed with rhodopsin, a likely consequence of basal activity and the fact that the ionic lock and other interactions stabilizing the inactive state of β2AR are weaker. Positive entropy changes toward activation likely reflect greater mobility (configurational entropy) in the cytoplasmic domain, as confirmed through an order parameter analysis. Ligands greatly influence the overall changes in enthalpy and entropy of the system and the corresponding changes in population and amplitude of motion of given states, suggesting a complex landscape of states and substates.
G 蛋白偶联受体在响应不同配体时表现出多种信号转导行为。当在跨膜螺旋 6（Cys-265）的胞质界面上掺入一个小标签时，（19）F NMR 光谱研究表明，β2 肾上腺素能受体（β2AR）在麦芽糖/新戊二醇去污剂胶束中再构成时存在两种不同的非活性状态，一种是朝向激活的激活中间状态，另一种是在 G 蛋白模拟物存在下的主要活性状态。对光谱随温度的变化进行分析表明，对于所有配体，激活中间态在热力学上是有利的，而在热力学上是不利的。β2AR 向激活的焓变明显低于视紫红质，这可能是由于基础活性和离子锁以及其他稳定β2AR 非活性状态的相互作用较弱所致。朝向激活的正熵变化可能反映了细胞溶质结构域更大的流动性（构象熵），这通过顺序参数分析得到了证实。配体极大地影响了系统焓和熵的整体变化，以及给定状态的运动的种群和幅度的相应变化，这表明状态和亚状态的复杂景观。
