Metz Sebastian, Wang Dongqi, Thiel Walter
Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim an der Ruhr, Germany.
J Am Chem Soc. 2009 Apr 8;131(13):4628-40. doi: 10.1021/ja805938w.
We report a combined QM/MM study on the mechanism of the reductive half-reaction of aldehyde oxidoreductase. Five possible pathways are explored concerning the binding mode of acetaldehyde and the catalytic effect of the nearby glutamic acid (Glu869), taking both possible protonation states into account. In the most favorable pathway, Glu869 participates and acts as a Lewis base to deprotonate the labile hydroxide group. This proton transfer is essential for the high activity of the enzyme toward substrate because it increases the nucleophilicity of the migrating O atom and strengthens the electrophilicity of the target C atom in the substrate. The subsequent product-forming reactions occur in two discrete steps, first nucleophilic attack and then hydride transfer, which implies that the oxidation of aldehyde is a two-electron process. A variant of this mechanism, with an additional water molecule bridging the Glu869 side chain and the substrate, has similar barriers. Judging from previous gas phase calculations and our present QM/MM data, the catalytic effect of Glu869 mainly lowers the barrier of the nucleophilic attack so that the hydride transfer becomes the rate-determining step in the reductive half-reaction.
我们报道了一项关于醛氧化还原酶还原半反应机制的量子力学/分子力学（QM/MM）联合研究。考虑到乙醛的结合模式以及附近谷氨酸（Glu869）的催化作用的两种可能质子化状态，探索了五条可能的途径。在最有利的途径中，Glu869参与并作为路易斯碱使不稳定的羟基去质子化。这种质子转移对于酶对底物的高活性至关重要，因为它增加了迁移的O原子的亲核性，并增强了底物中目标C原子的亲电性。随后的产物形成反应分两个离散步骤进行，首先是亲核攻击，然后是氢化物转移，这意味着醛的氧化是一个双电子过程。这种机制的一个变体，有一个额外的水分子桥接Glu869侧链和底物，具有相似的势垒。从先前的气相计算和我们目前的QM/MM数据判断，Glu869的催化作用主要降低了亲核攻击的势垒，使得氢化物转移成为还原半反应中的速率决定步骤。
