Oxygen-18 leaving group kinetic isotope effects on the hydrolysis of nitrophenyl glycosides. 2. Lysozyme and beta-glucosidase: acid and alkaline hydrolysis.
Oxygen-18 leaving group kinetic isotope effects on the hydrolysis of nitrophenyl glycosides. 2. Lysozyme and beta-glucosidase: acid and alkaline hydrolysis.
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Oxygen-18 离去基团动力学同位素对硝基苯糖苷水解的影响。
DOI：
10.1021/bi00514a032
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发表时间：
1981
期刊：
影响因子：
2.9
通讯作者：
J. Kirsch
中科院分区：
文献类型：
--
作者：
S. Rosenberg;J. Kirsch
以对硝基苯基β-D-糖苷为底物，测定了一系列糖基转移反应的氧-18离去基团动力学同位素效应（KIE）。酸催化水解和碱水解的KIE分别为K16/k18 = 1.0355 +/- 0.0015和1.0386 +/- 0.0032。溶菌酶和β-葡糖苷酶A在Vmax/Km（V/K）上显示KIE分别为（V/KI）16/（V/K）18 = 1.0467 +/- 0.0015和1.0377 +/-0.0061。这些KIE的大幅度要求碳-氧键断裂在这些反应的过渡态中远远提前;因此，在这些反应序列中的第一个不可逆步骤的过渡态中，对于由溶菌酶和β-葡糖苷酶A催化的反应，糖苷键的断裂必须基本上完成，这被认为分别通过SN 1和SN 2机制进行。酸催化的水解是通过一个过渡态进行的，涉及至少80%的C-O键裂解和只有部分质子转移到离开对硝基苯基氧原子。
Oxygen-18 leaving group kinetic isotope effects (KIEs) have been measured for a set of glycosyl transfer reactions with p-nitrophenyl beta-D-glycosides as substrates. Acid-catalyzed hydrolysis and alkaline hydrolysis exhibit KIEs of K16/k18 = 1.0355 +/- 0.0015 and 1.0386 +/- 0.0032, respectively. Lysozyme and beta-glucosidase A show KIEs on Vmax/Km (V/K) of (V/KI)16/(V/K)18 = 1.0467 +/- 0.0015 and 1.0377 +/0 0.0061, respectively. The large magnitude of these KIEs requires that carbon-oxygen bond scission be far advanced in the transition states for these reactions; therefore in the transition states for the first irreversible steps in these reaction sequences, scission of the glycosidic bond must be essentially complete for the reactions catalyzed by lysozyme and beta-glucosidase A, which are thought to proceed via SN1 and SN2 mechanisms, respectively. Acid-catalyzed hydrolysis is shown to proceed through a transition state involving at least 80% C-O bond cleavage and only partially proton transfer to the leaving p-nitrophenyl oxygen atom.