Detsikas E, Tsikaris V, Sakarellos-Daitsiotis M, Sakarellos C, Cung M T, Marraud M, Vatzaki E, Tzartos S J
Department of Chemistry, University of Ioannina, Greece.
Pept Res. 1993 Jan-Feb;6(1):17-23.
The majority of autoantibodies against the nicotinic acetylcholine receptor (AChR) bind to an extracellular region of the AChR's alpha-subunit, named main immunogenic region (MIR), with the sequence W67-N-P-A-DY-G-G-I-K76 for the Torpedo californica electric organ. We report on the synthesis and the biological and 1H-NMR studies of two cyclic MIR compounds--namely, [D71,K76]-MIR-NH2 and Ac-[Orn68,D71,A76]-MIR-NH2. The relatively small chemical shift differences between [D71,K76]-MIR-NH2 and the biologically active [A76]-analogue suggest that both MIR derivatives possess similar conformations. Thus, the observed limited anti-MIR MAb binding capacity of [D71,K76]-MIR-NH2 is attributed to the D71,K76 side-chain blockage, through lactam. Formation of the Orn68,D71 cycle in the Ac-[Orn68,D71,A76]-MIR-NH2 preserves, unchanged, the low antigenicity of the linear Ac-[Orn68,A76]-MIR-NH2, thus confirming the key role of position 68. The low temperature coefficient value of A70-NH and the observed NOE effect between P69-C delta H2 and A70-NH in Ac-[Orn68,D71,A76]-MIR-NH2 argue in favor of a type I beta-turn in the Trp67-Orn-P-A70 sequence. However, the N-terminus beta-folding and the Orn68,D71 cycle appear ineffective for optimal antibody molecular recognition.
大多数针对烟碱型乙酰胆碱受体（AChR）的自身抗体与AChRα亚基的一个细胞外区域结合，该区域被称为主要免疫原性区域（MIR），加州电鳐电器官的MIR序列为W67-N-P-A-DY-G-G-I-K76。我们报道了两种环状MIR化合物——即[D71,K76]-MIR-NH2和Ac-[Orn68,D71,A76]-MIR-NH2的合成、生物学及1H-NMR研究。[D71,K76]-MIR-NH2与具有生物活性的[A76]类似物之间相对较小的化学位移差异表明，这两种MIR衍生物具有相似的构象。因此，观察到的[D71,K76]-MIR-NH2有限的抗MIR单克隆抗体结合能力归因于通过内酰胺形成的D71,K76侧链阻断。在Ac-[Orn68,D71,A76]-MIR-NH2中形成的Orn68,D71环保持了线性Ac-[Orn68,A76]-MIR-NH2的低抗原性不变，从而证实了68位的关键作用。A70-NH的低温系数值以及在Ac-[Orn68,D71,A76]-MIR-NH2中观察到的P69-CδH2与A70-NH之间的核Overhauser效应（NOE）支持Trp67-Orn-P-A70序列中存在I型β-转角。然而，N端β-折叠和Orn68,D71环对于最佳抗体分子识别似乎无效。
