Milstien S, Cohen L A
Proc Natl Acad Sci U S A. 1970 Nov;67(3):1143-7. doi: 10.1073/pnas.67.3.1143.
As a result of alkyl substitution in both aromatic ring and side chain, the rate constant for acid-catalyzed lactonization of hydrocoumaric acid is increased by factors as high as 10(11) and, in comparison with the bimolecular esterification of phenol and acetic acid, by almost 10(16). In the most favorable case studied, the half-life of the phenolic acid (imidazole buffer, pH 7, 30 degrees C) is 6 sec, with 90% of the total rate being due to catalysis by the buffer species. The effect is attributed to a unique interlocking of methyl groups, which produces a severe conformational restriction of the side chain and increases greatly the population of the most productive conformer. This phenomenon is presented as a model for the conformational restraint imposed by an enzyme on its substrate. The magnitude of the effect suggests that the contribution of substrate "freezing" to total rate enhancement by an enzyme can be considerably greater than previously supposed and may in fact be sufficient to complete the justification for enzyme catalysis.
由于在芳环和侧链上都发生了烷基取代，氢香豆酸的酸催化内酯化反应速率常数增加了高达10的11次方倍，与苯酚和乙酸的双分子酯化反应相比，增加了近10的16次方倍。在所研究的最有利情况下，酚酸（咪唑缓冲液，pH 7，30℃）的半衰期为6秒，总反应速率的90%是由缓冲物种催化引起的。这种效应归因于甲基的独特互锁，这对侧链产生了严重的构象限制，并大大增加了最具活性构象体的数量。这种现象被作为酶对其底物施加构象限制的一个模型提出。该效应的大小表明，底物“冻结”对酶总反应速率增强的贡献可能比以前认为的要大得多，实际上可能足以完全解释酶催化作用。
