Blow D M, Smith J M
Philos Trans R Soc Lond B Biol Sci. 1975 Nov 6;272(915):87-97. doi: 10.1098/rstb.1975.0072.
An enzyme is designed to bind most tightly to a substrate when it is in the transition state of the reaction which the enzyme catalyses. The consequent reduction of the activation energy of the reaction constitutes the catalytic mechanism. The energetic contributions of different features of the interaction can only be crudely assessed, but they are dominated by entropically driven effects. The binding site of trypsin orients the substrate so that the reacting groups are correctly placed for reaction to occur. Apart from two side chains which take part in chemical steps of the reaction, the enzyme behaves almost as a rigid body. The full binding interactions are only developed when the substrate is in an intermediate stage of the reaction. The tightly bound complexes of trypsin with protein trypsin inhibitors have proved amenable to structural analysis. Enzyme inhibitor interactions, which account for almost 80 kJ mol-1 of interaction energy, are known fairly accurately. The similarity of the two known trypsin inhibitor structures, close to the primary binding site, indicates a high specificity, even for this simple interaction. In cases where no large conformational changes occur the specificity of an enzyme should be predictable from accurate knowledge of its tertiary structure.
酶被设计成在其催化的反应处于过渡态时与底物结合得最紧密。反应活化能的降低构成了催化机制。相互作用不同特征的能量贡献只能粗略评估，但它们主要由熵驱动效应主导。胰蛋白酶的结合位点使底物定向，以便反应基团能正确定位以发生反应。除了参与反应化学步骤的两条侧链外，酶几乎表现为刚体。只有当底物处于反应的中间阶段时，才会形成完整的结合相互作用。已证明胰蛋白酶与蛋白质胰蛋白酶抑制剂的紧密结合复合物适合进行结构分析。酶与抑制剂的相互作用能约为80 kJ/mol，已相当准确地得知。两种已知的胰蛋白酶抑制剂结构在靠近主要结合位点处的相似性表明，即使对于这种简单的相互作用也具有高度特异性。在没有大的构象变化的情况下，从对酶三级结构的准确了解应该可以预测其特异性。
