de Azevedo W F, Ward R J, Canduri F, Soares A, Giglio J R, Arni R K
Department of Physics, IBILCE/UNESP, São José do Rio Preto - SP, Brazil.
Toxicon. 1998 Oct;36(10):1395-406. doi: 10.1016/s0041-0101(98)00017-8.
The crystal structure of Piratoxin-I (PrTX-I) a Lys49 homologue isolated from the venom of Bothrops pirajai has been determined and refined at 2.8 A to a crystallographic residual of 19.7% (Rfree = 29.7%). Amino-acid sequence differences between catalytically active phospholipases and PrTX-I in the putative Ca2+-binding loop, specifically the substitutions Tyr28 --> Asn, Gly32 --> Leu and Asp49 --> Lys, result in an altered conformation of this loop. The analysis of the position of the epsilon-amino group of Lys49 in the PrTX-I structure indicates that it fills the site normally occupied by the calcium ion in the catalytically active phospholipases. In contrast to the homologous monomeric Lys49 variant from Agkistrodon piscivorus piscivorus (App), PrTX-I is present as a dimer in the crystalline state, as observed in the structures of myotoxin II from Bothrops asper and Bothropstoxin I from Bothrops jararacussu. The two molecules in the asymmetric unit in the crystal structure of PrTX-I are related by a nearly perfect two-fold symmetry axis, yet the dimeric structure is radically different from the dimeric structure of the phospholipase from Crotalus atrox. In the C. atrox structure the dimer interface occludes the active sites, whereas in the PrTX-I structure they are exposed to solvent.
从巴西矛头蝮蛇毒液中分离出的赖氨酰49同源物——皮拉毒素-I（PrTX-I）的晶体结构已被确定，并在2.8埃分辨率下精修至晶体学残余因子为19.7%（Rfree = 29.7%）。在假定的钙离子结合环中，催化活性磷脂酶与PrTX-I之间的氨基酸序列差异，特别是酪氨酸28被天冬酰胺取代、甘氨酸32被亮氨酸取代以及天冬氨酸49被赖氨酸取代，导致该环的构象发生改变。对PrTX-I结构中赖氨酸49的ε-氨基位置的分析表明，它占据了催化活性磷脂酶中钙离子通常占据的位点。与来自食鱼蝮蛇（App）的同源单体赖氨酰49变体不同，PrTX-I在晶体状态下以二聚体形式存在，这与从墨西哥矛头蝮蛇中分离出的肌毒素II和从巴西矛头蝮蛇中分离出的巴西矛头蝮蛇毒素I的结构中观察到的情况相同。PrTX-I晶体结构不对称单元中的两个分子通过近乎完美的二次对称轴相关联，但二聚体结构与来自西部菱斑响尾蛇的磷脂酶的二聚体结构截然不同。在西部菱斑响尾蛇的结构中，二聚体界面封闭了活性位点，而在PrTX-I结构中，活性位点暴露于溶剂中。
