Komiyama M, Bender M L, Utaka M, Takeda A
Proc Natl Acad Sci U S A. 1977 Jul;74(7):2634-8. doi: 10.1073/pnas.74.7.2634.
The effect of benzoate anion on intramolecular general base-catalyzed ester hydrolysis by the imidazolyl group in endo-5-[4'(5')-imidazolyl]bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate was examined in dioxane/H(2)O solutions. Benzoate anion exhibited a remarkable acceleration of the intramolecular general base-catalyzed hydrolysis of endo-5-[4'(5')-imidazolyl]bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate by the imidazolyl group. The rate of hydrolysis in the presence of the benzoate anion increased with the dioxane mole fraction and was proportional to the concentration of benzoate anion. On the other hand, the rate of hydrolysis of endo-5-[4'(5')-imidazolyl]bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate in the absence of benzoate anion decreased with the dioxane mole fraction. Thus, the ratio of the rate in the presence of benzoate anion to that in the absence of benzoate anion drastically increased with the dioxane mole fraction and attained a 2500-fold rate acceleration at a dioxane mole fraction of 0.42 (the highest experimentally attainable) when the concentration of benzoate anion was 0.5 M. The proposed mechanism involves proton abstraction by the benzoate anion from the imidazolyl group, followed by proton abstraction by the imidazolyl group from H(2)O, resulting in effective general base-catalysis of hydrolysis. The results of the present paper provide support for the "charge-relay" system in serine proteases.
在二氧六环/水混合溶液中，研究了苯甲酸根阴离子对内型-5-[4'(5')-咪唑基]双环[2.2.1]庚-内型-2-基反式肉桂酸酯中咪唑基催化的分子内广义碱催化酯水解反应的影响。苯甲酸根阴离子显著加速了咪唑基对内型-5-[4'(5')-咪唑基]双环[2.2.1]庚-内型-2-基反式肉桂酸酯的分子内广义碱催化水解反应。在苯甲酸根阴离子存在下，水解速率随二氧六环摩尔分数的增加而增大，且与苯甲酸根阴离子的浓度成正比。另一方面，在不存在苯甲酸根阴离子时，内型-5-[4'(5')-咪唑基]双环[2.2.1]庚-内型-2-基反式肉桂酸酯的水解速率随二氧六环摩尔分数的增加而降低。因此，在苯甲酸根阴离子存在与不存在时的速率之比随二氧六环摩尔分数急剧增大，当苯甲酸根阴离子浓度为0.5 M时，在二氧六环摩尔分数为0.42（实验可达到的最高值）时实现了2500倍的速率加速。提出的机理涉及苯甲酸根阴离子从咪唑基夺取质子，随后咪唑基从水中夺取质子，从而实现有效的水解广义碱催化。本文的结果为丝氨酸蛋白酶中的“电荷中继”系统提供了支持。
