A STATISTICAL-ANALYSIS OF ACETYLCHOLINE-RECEPTOR ACTIVATION IN XENOPUS MYOCYTES - STEPWISE VERSUS CONCERTED MODELS OF GATING
A STATISTICAL-ANALYSIS OF ACETYLCHOLINE-RECEPTOR ACTIVATION IN XENOPUS MYOCYTES - STEPWISE VERSUS CONCERTED MODELS OF GATING
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DOI：
10.1113/jphysiol.1993.sp019517
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发表时间：
1993-02-01
影响因子：
5.5
通讯作者：
AUERBACH, A
中科院分区：
文献类型：
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作者：
AUERBACH, A
1.用最大似然法分析了非洲爪蟾胚胎肌细胞（60 h龄）胎儿型乙酰胆碱受体单通道电流的动力学特性.在非常高的乙酰胆碱（ACh）浓度（高达5毫米）的有效开放率似乎饱和在约30000 s-1.3。动力学进行了分析，根据标准的协同方案，假定一个单一的通道开放的构象变化后，两个激动剂的绑定，和一个很少调用的逐步计划，假定半独立的构象变化在两个不同的门控域。这两种模型都假设激动剂不能从处于其激活构象的通道（或结构域）中逃逸。动力学分析表明，无论采用哪种激活方案，ACh都以约2 × 10（8）s-1 m-1的速率结合，并以约28000 s-1的速率从双配体受体上解离，激活过程是不对称的，即结合（协同模型）或门控（逐步模型）跃迁是不相等和独立的。在27个逐文件比较的18个中，逐步模型的可能性大于协调模型。在七个这样的比较中，协同模型的可能性大于逐步模型，在两个没有差异。对数似然比分布从三个文件（事件最多的文件）通过多个循环的恢复和拟合获得。这些分布的平均值显著大于零，表明逐步方案在描述受体激活方面与协调方案一样好，或更好。根据逐步的观点，两个结合位点必须被占据，两个“门”被激活才能发生传导。虽然等效结合不是逐步激活的必要方面，但结合位点可以是相同的，并且对ACh具有低亲和力（K（d）约为130 μ M）。门控结构域的异构化速率是不同的，或者它们受到其对应物的构象状态的影响，如果互补结构域处于活性构象，则活化增加约3倍，失活减少约10倍。逐步激活预测终板电流的衰减由五个速率决定。
1. The kinetic properties of single channel currents from fetal-type acetylcholine receptors in embryonic Xenopus myocytes (60 h old) have been analysed by a maximum-likelihood method.2. At very high acetylcholine (ACh) concentrations (up to 5 mm) the effective opening rate appears to saturate at approximately 30000 s-1.3. The kinetics were analysed according to the standard concerted scheme that postulates a single channel-opening conformational change after two agonists are bound, and a rarely invoked stepwise scheme that postulates semi-independent conformational changes in two distinct gating domains. Both models assume that agonist cannot escape from a channel (or domain) that is in its activated conformation.4. With either activation scheme the kinetic analyses indicate that ACh binds at a rate of approximately 2 x 10(8) s-1 m-1 and dissociates from doubly liganded receptors at a rate of approximately 28000 s-1, and that the activation process is asymmetric, i.e. the binding (concerted model) or gating (stepwise model) transitions are not equal and independent.5. In eighteen of twenty-seven file-by-file comparisons, the likelihood of the stepwise model was greater than that of the concerted model. In seven such comparisons, the likelihood of the concerted model was greater than that of the stepwise model, and in two there was no difference. Log likelihood ratio distributions were obtained from three files (those with the most events) by multiple cycles of resampling and fitting. The means of these distributions were significantly greater than zero, indicating that the stepwise scheme was as good as, or better than, the concerted scheme in describing receptor activation.6. According to the stepwise view, two binding sites must be occupied and two 'gates' activated for conduction to occur. Although equivalent binding is not an essential aspect of stepwise activation, the binding sites can be identical and have a low affinity for ACh (K(d) approximately 130 muM). Either the isomerization rates of the gating domains are different, or they are influenced by the conformational status of its counterpart, with activation increasing approximately 3-fold and deactivation decreasing approximately 10-fold if the complementary domain is in the active conformation. Stepwise activation predicts that the decay of the endplate current is determined by five rates.