Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, New York, USA.
Biophys J. 2013 Feb 5;104(3):565-74. doi: 10.1016/j.bpj.2012.11.3833.
Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.
激动剂，包括神经递质乙酰胆碱（ACh），在神经肌肉 ACh 受体通道（AChR）的两个位点结合，以促进蛋白质构象的可逆、全局变化，从而调节离子穿过肌细胞膜的流动。在突触间隙中，ACh 被水解为乙酸和胆碱。将递质酯基乙酰基替换为羟基（ACh→胆碱），会导致每个激动剂分子从低亲和力到高亲和力的递质结合位点的门控能量降低 1.8 千卡/摩尔（ΔG（B））。为了理解结构相关激动剂分子的不同作用，我们测量了 10 种相关胆碱衍生物的ΔG（B）。用不同的取代基（如氢、氯、甲基或胺）替换胆碱的羟基，会增加门控能量（即，相对于胆碱，使ΔG（B）更负）。将胆碱的乙基羟基延伸至丙基和丁基羟基也会增加这种能量。我们的发现揭示了不同结构相关激动剂提供的 AChR 构象变化的可用能量。我们推测，胆碱羟基和αW149 骨干羰基之间的氢键将此激动剂的季铵基团定位，从而减少该部分与结合位点的芳族基团之间的阳离子-π相互作用。
