Huang K, Strynadka N C, Bernard V D, Peanasky R J, James M N
Department of Biochemistry, University of Alberta, Edmonton, Canada.
Structure. 1994 Jul 15;2(7):679-89. doi: 10.1016/s0969-2126(00)00068-x.
The intestinal parasitic worm, Ascaris suum, produces a variety of protein inhibitors that defend the organism against the host's proteinases. Eight different proteins from Ascaris suum have been identified as inhibitors of serine proteinases, targeting chymotrypsin, elastase and trypsin. These inhibitors share 30-40% sequence identity with one another, but have virtually no sequence identity with members of any of the other families of serine proteinase inhibitors.
The crystal structure of the complex of porcine pancreatic elastase with a chymotrypsin/elastase inhibitor from Ascaris suum (the C/E-1 inhibitor) has been solved to 2.4 A resolution by the molecular replacement method. The C/E-1 inhibitor exhibits a novel folding motif. There are only two small beta-sheets and two single-turn 3(10)-helices in this inhibitor. Unlike the majority of proteins, the C/E-1 inhibitor does not have a hydrophobic core. The presence and unique topography of the five disulfide bridges suggests that they play important roles in maintaining the tertiary structure of the inhibitor. In addition, the side chains of several charged residues from electrostatic and hydrogen-bonding cascades, which also probably compensate for the lack of extensive secondary structures and a hydrophobic core. The reactive-site loop of this inhibitor displays a conformation that is characteristic of most serine proteinase inhibitors.
The structure of the C/E-1 inhibitor confirms that inhibitors from Ascaris suum belong to a novel family of proteinase inhibitors. It also provides conclusive evidence for the correct disulfide bridge connections. The C/E-1 inhibitor probably acts by a common inhibitory mechanism proposed for other substrate-like protein inhibitors of serine proteinases. The unusual molecular scaffolding presents a challenge to current folding algorithms. Proteins like the C/E-1 inhibitor may provide a valuable model system to study how the primary sequence of a protein dictates its three-dimensional structure.
肠道寄生虫猪蛔虫会产生多种蛋白质抑制剂，以保护自身抵御宿主的蛋白酶。猪蛔虫的八种不同蛋白质已被鉴定为丝氨酸蛋白酶抑制剂，可作用于胰凝乳蛋白酶、弹性蛋白酶和胰蛋白酶。这些抑制剂彼此之间的序列同一性为30 - 40%，但与任何其他丝氨酸蛋白酶抑制剂家族的成员几乎没有序列同一性。
通过分子置换法解析了猪胰弹性蛋白酶与猪蛔虫的一种胰凝乳蛋白酶/弹性蛋白酶抑制剂（C/E - 1抑制剂）复合物的晶体结构，分辨率达到2.4埃。C/E - 1抑制剂呈现出一种新颖的折叠基序。该抑制剂中仅有两个小的β - 折叠片层和两个单圈3(10) - 螺旋。与大多数蛋白质不同，C/E - 1抑制剂没有疏水核心。五个二硫键的存在及其独特的拓扑结构表明它们在维持抑制剂的三级结构中起重要作用。此外，来自静电和氢键级联的几个带电荷残基的侧链，这也可能弥补了缺乏广泛二级结构和疏水核心的不足。该抑制剂的活性位点环呈现出大多数丝氨酸蛋白酶抑制剂所特有的构象。
C/E - 1抑制剂的结构证实了猪蛔虫的抑制剂属于一个新的蛋白酶抑制剂家族。它还为正确的二硫键连接提供了确凿证据。C/E - 1抑制剂可能通过一种针对丝氨酸蛋白酶其他类似底物的蛋白质抑制剂所提出的常见抑制机制发挥作用。这种不寻常的分子支架对当前的折叠算法提出了挑战。像C/E - 1抑制剂这样的蛋白质可能为研究蛋白质的一级序列如何决定其三维结构提供一个有价值的模型系统。
