Abstract
蛇毒蛋白(snake venom)是從毒蛇唾液中所分離的多成份混合物。依其作 用機制，蛇毒蛋白可以區分為神經毒素(neurotoxin)與心臟毒素 (cardiotoxin)。而神經毒素﹐依其毒性作用的機制來看又可以分為節前 神經毒蛋白(presynaptic neurotoxin)與節後神經毒蛋白(postsynaptic neurotoxin)。節後神經毒蛋白能與乙醯膽素受體結合而阻止了乙醯膽素 與 AChR(acetylcholine receptor)的結合而使得肌肉無法收縮﹐進而麻 痺。節前神經毒蛋白則是作用於節前神經終極板上(presynaptic motor nerve terminal)﹐而阻止了肌肉神經的傳導。節後神經毒蛋白一般又可 以分為兩類﹕短鏈神經毒蛋白具有四個雙硫鍵及60~62個胺基酸﹔長的神 經毒蛋白則具有五個雙硫鍵及66~74個胺基酸。神經毒蛋白研究的目的﹐ 除了在於毒蛇咬傷時的救治之外,同時也是利用其對AChR具高親和性( affinity)和高特異性(specificity)的結合來了解神經毒性之分子機制。 a-cobratoxin (Walkinshaw et al., 1980; Betzel et al ., 1991; La Goas et al ., 1992) 與a-bungarotoxin(Love & stroud, 1986; Bause et al., 1988)是 技術解出三維結構的神經毒蛋白﹔最近另一個長鏈神經 毒蛋白LSIII(Connolly et al ., 及三維的NMR﹐解出了其在 C這些蛇毒 蛋白整體結構大致相似﹔即具有三股及二股反平 c形成三個手指狀的迴路 自活性中心向外延伸。在本篇論文我們以眼鏡蛇王(Ophiophagus hannah) 的長的神經毒蛋白Toxin b為對象﹐以二維核磁共振光譜加上動態模擬焠 熄(dynamic simulated annealing)技術來研究其在水溶液中的三維結構 ﹔獲致了與上述結構相類似的結果。 The solution structure of Toxin b, a long neurotoxin ( 73 no acids and five disulfides), from the venom of Ophiophagus hannah( King Cobra) has been determined using 1H NMR and modeling. The structures were calculated using 415 distance constraints and 52 dihedral angle restraints. The average atoms RMSD between the twelve refined structures and the mean structure is 0.7 □for the backbone heavy atoms, and 3.1for all the heavy atoms. The protein consists of a core region from which three finger like loops extend outwards. The secondary structure includes a short double and a triple antiparallel b sheets. Comparison with the solution structures of other long neurotoxins indicates that the structure of toxin b is quite similar to previously reported long neurotoxin structures but clear local structural differences are observed in regions thought to be for binding of neurotoxins to the acetylcholine receptor (AChR). Loop II, which is important for the binding of the toxin to the acetylcholine receptor is shorter in toxin b as compared to the other long neurotoxins and the tip of Loop II has a well defined local structure.