Polgár L
Eur J Biochem. 1979 Aug 1;98(2):369-74. doi: 10.1111/j.1432-1033.1979.tb13196.x.
The experimental data presented in this paper comprise kinetic deuterium isotope effects on acylation of papain with various substrates when conducted in H2O and 2H2O. With alkyl esters of N-acylamino acids there is no or very little isotope effect, whereas with N-acylamino acid amides the ratio kappa H2O/kappa 2H2O is less than 1, i.e. there is an inverse isotope effect. Similarly, alkylation of papain with methyl bromoacetate exhibits no kinetic isotope effect, whereas for the analogous alkylation with bromoacetamide an inverse isotope effect is observed. It is concluded that (a) general base catalysis does not occur in the acylation of papain and (b) kinetic deuterium isotope effects can be affected substantially by interaction between the substrate leaving group and the enzyme, which has not been considered in previous mechanistic investigations.
本文展示的实验数据包括在H₂O和²H₂O中用各种底物对木瓜蛋白酶进行酰化反应时的动力学氘同位素效应。对于N-酰基氨基酸的烷基酯，不存在或仅有非常小的同位素效应，而对于N-酰基氨基酸酰胺，κH₂O/κ²H₂O的比值小于1，即存在逆同位素效应。同样，木瓜蛋白酶与溴乙酸甲酯的烷基化反应未表现出动力学同位素效应，而与溴乙酰胺的类似烷基化反应则观察到逆同位素效应。得出的结论是：(a) 木瓜蛋白酶酰化反应中不存在一般碱催化；(b) 动力学氘同位素效应会受到底物离去基团与酶之间相互作用的显著影响，而这在以往的机理研究中未被考虑。
