Itoh S, Kawakami H, Fukuzumi S
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
Biochemistry. 1998 May 5;37(18):6562-71. doi: 10.1021/bi9800092.
Mechanistic studies on the action of calcium-containing quinoprotein alcohol dehydrogenases have been performed by using a series of PQQ model compounds in anhydrous organic media. The PQQ model compounds are shown to form 1:1 complexes with a series of alkaline earth metal ions by spectroscopic methods and theoretical calculations. The site of coordination of the PQQ molecule to the metal ions in solution is indicated to be the same as in the case of enzymatic systems. It has also been found that Ca2+ binds to the quinone most strongly among the alkaline earth metal ions. Formation of the C-5 hemiacetal derivatives with methanol, ethanol, and 2-propanol is also investigated spectrophotometrically to show that the alcohol addition to the quinone is enhanced in the presence of the metal ions. In this case as well, Ca2+ shows the highest efficiency for the enhancement of the C-5 hemiacetal formation. Addition of a strong base such as DBU into an MeCN solution containing the Ca2+ complex of the PQQ model compounds and the alcohols leads to the redox reactions to afford reduced PQQ derivatives and the corresponding aldehydes. On the basis of detailed kinetic studies on the redox reactions, including structural effects of PQQ analogues and metal ion effects, we propose the addition-oxidative elimination mechanism through the C-5 hemiacetal intermediate.
通过在无水有机介质中使用一系列吡咯喹啉醌（PQQ）模型化合物，对含钙醌蛋白醇脱氢酶的作用进行了机理研究。通过光谱方法和理论计算表明，PQQ模型化合物与一系列碱土金属离子形成1:1配合物。溶液中PQQ分子与金属离子的配位位点与酶系统中的情况相同。还发现，在碱土金属离子中，Ca2+与醌的结合最强。还通过分光光度法研究了与甲醇、乙醇和2-丙醇形成C-5半缩醛衍生物的情况，结果表明在金属离子存在下，醌与醇的加成反应增强。在这种情况下，Ca2+在增强C-5半缩醛形成方面也显示出最高的效率。向含有PQQ模型化合物的Ca2+配合物和醇的MeCN溶液中加入强碱（如DBU）会导致氧化还原反应，生成还原型PQQ衍生物和相应的醛。基于对氧化还原反应的详细动力学研究，包括PQQ类似物的结构效应和金属离子效应，我们提出了通过C-5半缩醛中间体的加成-氧化消除机理。
