Contreras M L, Wolfe B B, Molinoff P B
J Pharmacol Exp Ther. 1986 Apr;237(1):154-64.
The properties associated with ligand interactions with membrane-bound beta adrenergic receptors prepared from L6 myoblasts were examined at five temperatures between 10 degrees and 30 degrees C. The interactions of antagonists with membrane-bound receptors were insensitive to temperature, whereas the interactions of agonists were temperature-dependent. The affinity constants for the low-affinity binding states of agonists (KL) decreased slightly with decreasing assay temperature. The small temperature-dependent changes in KL values were similar to the changes in Kd values observed in studies of the binding of agonists in the presence of GTP. The high-affinity dissociation constants (KH) for binding of full agonists to membrane-bound receptors in the absence of GTP were approximately 50-fold lower at 10 degrees than at 30 degrees C. The KH values for partial agonists also decreased with decreasing temperature, but the changes were smaller in magnitude. Thermodynamically, the binding of antagonists was primarily entropy-driven, whereas the binding of agonists was enthalpy-driven. The energetics of the low-affinity component of agonist binding to membrane-bound receptors were similar to the energetics for binding of agonists to membrane-bound receptors in the presence of GTP. Under these conditions, standard enthalpy change (delta H degree) and standard entropy change (delta S degree) values for binding of agonists were more negative than the corresponding values for binding of antagonists, possibly reflecting a conformational change in the receptor or an increased ordering of the lipids surrounding the receptor. The interaction of the receptor with the guanine nucleotide-binding protein to form the high-affinity component of agonist binding was thermodynamically described by larger negative changes in enthalpy and entropy than the values for formation of the low-affinity component of agonist binding. There was a correlation between the efficacies of ligands in activating adenylate cyclase and the delta H degree and delta S degree values for high-affinity binding of agonists. Thus, the extent or nature of the interaction between the guanine nucleotide-binding protein and the receptor may determine the efficacies of ligands.
在10摄氏度至30摄氏度之间的五个温度下，研究了从L6成肌细胞制备的膜结合β肾上腺素能受体与配体相互作用的相关特性。拮抗剂与膜结合受体的相互作用对温度不敏感，而激动剂的相互作用则依赖于温度。激动剂低亲和力结合状态的亲和常数（KL）随着测定温度的降低而略有下降。KL值随温度的微小变化与在GTP存在下激动剂结合研究中观察到的Kd值变化相似。在不存在GTP的情况下，完全激动剂与膜结合受体结合的高亲和力解离常数（KH）在10摄氏度时比在30摄氏度时低约50倍。部分激动剂的KH值也随温度降低而下降，但变化幅度较小。从热力学角度来看，拮抗剂的结合主要由熵驱动，而激动剂的结合则由焓驱动。激动剂与膜结合受体低亲和力成分结合的能量学与在GTP存在下激动剂与膜结合受体结合的能量学相似。在这些条件下，激动剂结合的标准焓变（ΔH°）和标准熵变（ΔS°）值比拮抗剂结合的相应值更负，这可能反映了受体的构象变化或受体周围脂质排列的增加。受体与鸟嘌呤核苷酸结合蛋白相互作用形成激动剂结合的高亲和力成分，在热力学上表现为焓和熵的更大负变化，比激动剂结合低亲和力成分形成时的值更大。配体激活腺苷酸环化酶的效力与激动剂高亲和力结合的ΔH°和ΔS°值之间存在相关性。因此，鸟嘌呤核苷酸结合蛋白与受体之间相互作用的程度或性质可能决定配体的效力。
