Matsui H, Blanchard J S, Brewer C F, Hehre E J
Department of Microbiology, Albert Einstein College of Medicine, Bronx, New York 10461.
J Biol Chem. 1989 May 25;264(15):8714-6.
alpha-Secondary tritium kinetic isotope effects ranging from 1.17 to 1.26 were measured for the hydrolysis of alpha-D-glucopyranosyl fluoride (forming beta-D-glucose) catalyzed by several glucoamylases and a glucodextranase. These results indicate that cleavage of the C-F bond is slow and that the enzymic transition state has significant oxo-carbonium ion character. Strong support for this conclusion is provided by the agreement found in the case of Rhizopus niveus glucoamylase (alpha-TV/K 1.26; Km 26 mM) between measured values of the alpha-secondary deuterium kinetic isotope effects (alpha-DV/K 1.16; alpha-DV 1.20) and those calculated from the tritium isotope effect. The data are consistent with the promotion of an intramolecular elimination of fluoride by the present exo-alpha-glucanases based on their ability to stabilize, perhaps with a counter ion, the development of a carbonium ion-like transition state. Although the oxo-carbonium ion is formally denoted as an intermediate it could represent a transition state along a reaction pathway to a covalent glucosyl intermediate.
在几种糖化酶和一种葡聚糖酶催化α-D-吡喃葡萄糖基氟水解（生成β-D-葡萄糖）的过程中，测得α-二级氚动力学同位素效应在1.17至1.26之间。这些结果表明，C-F键的断裂较慢，并且酶促过渡态具有显著的氧鎓离子特征。米根霉糖化酶（α-TV/K 1.26；Km 26 mM）的α-二级氘动力学同位素效应（α-DV/K 1.16；α-DV 1.20）的测量值与由氚同位素效应计算得到的值之间的一致性，为这一结论提供了有力支持。这些数据与目前的外切α-葡聚糖酶促进分子内氟消除的作用相符，这是基于它们能够（可能借助抗衡离子）稳定类碳正离子过渡态的形成。尽管氧鎓离子在形式上被表示为一种中间体，但它可能代表了通向共价葡萄糖基中间体的反应途径中的一个过渡态。
