PYRIDINE-NUCLEOTIDE CHEMISTRY - A NEW MECHANISM FOR THE HYDROXIDE-CATALYZED HYDROLYSIS OF THE NICOTINAMIDE GLYCOSYL BOND
PYRIDINE-NUCLEOTIDE CHEMISTRY - A NEW MECHANISM FOR THE HYDROXIDE-CATALYZED HYDROLYSIS OF THE NICOTINAMIDE GLYCOSYL BOND
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DOI：
10.1021/ja00215a041
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发表时间：
1988-03-30
影响因子：
15
通讯作者：
OPPENHEIMER, NJ
中科院分区：
文献类型：
--
作者：
JOHNSON, RW;MARSCHNER, TM;OPPENHEIMER, NJ
氢氧化物催化- β糖基键水解的机理。-NAD+已被重新研究。-的水解速率与pH值的关系。-NAD+及相关化合物在pH 4.3至13.5范围内测定。在pH 8.5和11之间，速率常数的对数与pH呈线性关系，斜率为1。低于pH 6.5和高于pH 12.5的反应与pH无关。在整个pH范围内反应的产物是烟酰胺。对数据进行非线性最小二乘拟合，得到了烟酰胺核糖二醇电离的pKeq，该pKeq由13C核磁共振独立测定。-的甲醇分解。-NAD+与。beta的比值为3.7:1。和.alpha。1''- o -甲基- adp核糖的异构体。水解是一种甲醇/水混合物，根据攻击物质的亲核性，没有攻击选择性。用β的异丙基衍生物考察了核糖二醇在水解反应中的重要性。烟酰胺核苷。氢氧化物催化分解。-2',3' - o -异丙基烟酰胺核苷在pH 7以下与pH无关，在pH 10以上与氢氧化物浓度线性相关。与.beta的结果相反。-NAD+，在高pH下没有观察到与pH无关的区域，pH依赖反应的产物是2-羟基-3-吡啶甲醛；即，没有可检测到的氢氧化物催化的烟酰胺释放被观察到。基于这些数据，以及溶剂同位素效应和以前的研究数据，我们提出了一种新的机制，其中烟酰胺核糖二醇阴离子通过氧碳正离子中间体的非共价稳定促进了烟酰胺-糖基键的解离裂解。
The mechanism of hydroxide-catalyzed hydrolysis of the glycosyl bond of .beta.-NAD+ has been reinvestigated. The pH dependence of the rate of hydrolysis of .beta.-NAD+ and related compounds has been determined over a range from pH 4.3 to 13.5. Between pH 8.5 and 11 the log of the rate constant is linearly dependent on pH with a slope of unity. Below pH 6.5 and above pH 12.5 the reaction becomes pH independent. The product of the reaction over the entire pH range is nicotinamide. A nonlinear least-squares fit of the data yields a pKeq corresponding to the pKa of ionization of the nicotinamide ribose diol, which has been determined independently by 13C NMR. Methanolysis of .beta.-NAD+ yields a ratio of 3.7:1 for the .beta. and .alpha. anomers of 1''-O-methyl-ADP ribose. Hydrolysis is a methanol/water mixture shows no selectivity for attack on the basis of the nucleophilicity of the attacking species. The importance of the ribose diol in the hydrolysis reaction was investigated with the isopropylidene derivative of .beta.-nicotinamide riboside. Hydroxide-catalyzed decomposition of .beta.-2'',3''-O-isopropylidene nicotinamide riboside is pH independent below pH 7 and linearly dependent on hydroxide concentration above pH 10. In contrast to the results for .beta.-NAD+, no pH-independent region is observed at high pH and the product of the pH-dependent reaction is 2-hydroxy-3-pyridinecarboxaldehyde; i.e., no detectable hydroxide-catalyzed release of nicotinamide is observed. On the basis of these data, as well as solvent isotope effects and data from previous investigations, we propose a new mechanism in which dissociative cleavage of the nicotinamide-glycosyl bond is facilitated by the nicotinamide ribose diol anion through noncovalent stabilization of an oxo carbocation intermediate.