Harel M, Kleywegt G J, Ravelli R B, Silman I, Sussman J L
Department of Structural Biology, Weizmann Institute of Science, Rehovot, Israel.
Structure. 1995 Dec 15;3(12):1355-66. doi: 10.1016/s0969-2126(01)00273-8.
Fasciculin (FAS), a 61-residue polypeptide purified from mamba venom, is a three-fingered toxin which is a powerful reversible inhibitor of acetylcholinesterase (AChE). Solution of the three-dimensional structure of the AChE/FAS complex would provide the first structure of a three-fingered toxin complexed with its target.
The structure of a complex between Torpedo californica AChE and fasciculin-II (FAS-II), from the venom of the green mamba (Dendroaspis angusticeps) was solved by molecular replacement techniques, and refined at 3.0 A resolution to an R-factor of 0.231. The structure reveals a stoichiometric complex with one FAS molecule bound to each AChE subunit. The AChE and FAS conformations in the complex are very similar to those in their isolated structures. FAS is bound at the 'peripheral' anionic site of AChE, sealing the narrow gorge leading to the active site, with the dipole moments of the two molecules roughly aligned. The high affinity of FAS for AChE is due to a remarkable surface complementarity, involving a large contact area (approximately 2000 A2) and many residues either unique to FAS or rare in other three-fingered toxins. The first loop, or finger, of FAS reaches down the outer surface of the thin aspect of the gorge. The second loop inserts into the gorge, with an unusual stacking interaction between Met33 in FAS and Trp279 in AChE. The third loop points away from the gorge, but the C-terminal residue makes contact with the enzyme.
Two conserved aromatic residues in the AChE peripheral anionic site make important contacts with FAS. The absence of these residues from chicken and insect AChEs and from butyrylcholinesterase explains the very large reduction in the affinity of these enzymes for FAS. Several basic residues in FAS make important contacts with AChE. The complementarity between FAS and AChE is unusual, inasmuch as it involves a number of charged residues, but lacks any intermolecular salt linkages.
束丝毒素（FAS）是一种从曼巴蛇毒液中纯化得到的由61个氨基酸残基组成的多肽，是一种三指毒素，是乙酰胆碱酯酶（AChE）的强效可逆抑制剂。AChE/FAS复合物三维结构的解析将提供首个三指毒素与其靶标结合的结构。
通过分子置换技术解析了加州电鳐AChE与来自绿曼巴蛇（Dendroaspis angusticeps）毒液的束丝毒素-II（FAS-II）之间复合物的结构，并在3.0埃分辨率下进行精修，R因子为0.231。该结构揭示了一种化学计量复合物，每个AChE亚基结合一个FAS分子。复合物中的AChE和FAS构象与其分离结构中的构象非常相似。FAS结合在AChE的“外周”阴离子位点，封闭通向活性位点的狭窄通道，两个分子的偶极矩大致对齐。FAS对AChE的高亲和力归因于显著的表面互补性，涉及大的接触面积（约2000埃²）以及许多FAS特有的或在其他三指毒素中罕见的残基。FAS的第一个环或指向下延伸至通道细窄部分的外表面。第二个环插入通道，FAS中的Met33与AChE中的Trp279之间存在异常的堆积相互作用。第三个环指向远离通道的方向，但C末端残基与酶接触。
AChE外周阴离子位点的两个保守芳香族残基与FAS形成重要接触。鸡和昆虫的AChE以及丁酰胆碱酯酶中缺少这些残基，这解释了这些酶对FAS的亲和力大幅降低的原因。FAS中的几个碱性残基与AChE形成重要接触。FAS与AChE之间的互补性不同寻常，因为它涉及许多带电荷的残基，但缺乏任何分子间盐键。
