Ethylbenzene dehydrogenase (EbDH; enzyme commission (EC) number: 1.17.99.2) is a unique biocatalyst that hydroxylates alkylaromatic and alkylheterocyclic compounds to (S)-secondary alcohols under anaerobic conditions. The enzyme exhibits a high promiscuity catalyzing oxidation of over 30 substrates, inter alia, para-substituted alkylphenols and alkylanilines. Secondary alcohols with OH and NH2 substituents in the aromatic ring are highly valuable synthons for many biologically active compounds in the fine chemical industry. EbDH hydroxylates most of the studied compounds highly enantioselectively, except for five substrates that harbour OH and NH2 groups in the para position, which exhibit a significant decrease in the percent enantiomeric excess (% ee). This phenomenon is inconsistent with the previously suggested enzyme mechanism, but it may be linked to a stabilization of the carbocation intermediate by deprotonation of the OH or NH2 substituent in the active site that yields a transient quinone (imine) ethide species. This would initiate an alternative reaction pathway involving the addition of a water molecule to a C=C double bond. This hypothesis was cross-validated by density functional theory (DFT) cluster modelling of the alternative reaction pathway with 4-ethylphenol, as well as by experimental assessment of the pH dependency of enantiomeric excesses. The results reported herein suggest that the alternative reaction pathway may significantly contribute to the overall reaction if the carbocation intermediates are stabilized by deprotonation.
乙苯脱氢酶(EbDH;酶学委员会(EC)编号:1.17.99.2)是一种独特的生物催化剂,在厌氧条件下将烷基芳香化合物和烷基杂环化合物羟基化为(S)-次生醇。该酶表现出很高的催化广谱性,催化氧化超过30种底物,包括对位取代的烷基酚和烷基苯胺。具有芳香环中OH和NH2取代基的次生醇对于精细化工行业中许多生物活性化合物是非常有价值的合成物。EbDH在高对映选择性地羟基化大多数研究过的化合物,除了五种在对位位置带有OH和NH2基团的底物,其对映异构体过量百分比(% ee)显著降低。这种现象与先前提出的酶机制不一致,但可能与在活性位点上通过去质子化OH或NH2取代基稳定卡宾中间体有关,从而产生瞬时的醌(亚胺)乙烯基物种。这将启动涉及将水分子加到C=C双键的另一反应途径。通过密度泛函理论(DFT)对4-乙基酚的替代反应途径进行了簇模拟,以及通过对对映异构体过量的pH依赖性的实验评估,交叉验证了这一假设。本文报道的结果表明,如果通过去质子化稳定卡宾中间体,则替代反应途径可能会显著影响整体反应。