De Meester P, Brick P, Lloyd L F, Blow D M, Onesti S
Blackett Laboratory, Imperial College, London SW7 2BZ, England.
Acta Crystallogr D Biol Crystallogr. 1998 Jul 1;54(Pt 4):589-97. doi: 10.1107/s0907444997015849.
The Kunitz-type soybean trypsin inhibitor (STI) has played a key role in the early study of proteinases, having been used as the main substrate in the biochemical and kinetic work that led to the definition of the standard mechanism of action of proteinase inhibitors. A partial structure of STI complexed with porcine trypsin has previously been reported, in which the first 93 residues of the inhibitor, including the region of contact with trypsin, were relatively well defined, whereas for the remaining part of the peptide chain only some Calpha atoms were located. The structure of the inhibitor in its free form has now been determined by molecular replacement to 2.5 A, using the coordinates of the homologous Erythrina trypsin inhibitor as a search model. When the refined atomic coordinates of STI are compared with the partial model previously available, the conformation of the reactive-site loop and its position with respect to the main body of the molecule does not change when the inhibitor interacts with trypsin. There are instead, despite the high similarity in the overall tertiary structure, significant differences between STI and Erythrina trypsin inhibitor (ETI) in the region which is in contact with the enzyme in the STI:trypsin crystal structure. Some of these differences can explain the unique specificity of ETI and its ability to inhibit the fibrinolytic enzyme tissue-type plasminogen activator.
库尼茨型大豆胰蛋白酶抑制剂（STI）在蛋白酶的早期研究中发挥了关键作用，它被用作生化和动力学研究中的主要底物，这些研究促成了蛋白酶抑制剂标准作用机制的定义。此前已报道了与猪胰蛋白酶复合的STI的部分结构，其中抑制剂的前93个残基，包括与胰蛋白酶接触的区域，定义相对明确，而对于肽链的其余部分，仅定位了一些Cα原子。现在已通过分子置换法将游离形式的抑制剂结构确定为2.5埃，使用同源的刺桐胰蛋白酶抑制剂的坐标作为搜索模型。当将STI的精制原子坐标与先前可用的部分模型进行比较时，当抑制剂与胰蛋白酶相互作用时，活性位点环的构象及其相对于分子主体的位置不会改变。相反，尽管整体三级结构高度相似，但在STI：胰蛋白酶晶体结构中与酶接触的区域，STI和刺桐胰蛋白酶抑制剂（ETI）之间存在显著差异。其中一些差异可以解释ETI的独特特异性及其抑制纤维蛋白溶解酶组织型纤溶酶原激活剂的能力。
