半胱氨酸双加氧酶(CDO)是一种非血红素单核铁酶,催化半胱氨酸氧化为半胱氨酸亚磺酸。哺乳动物CDO的晶体结构研究表明,在其活性位点存在交联的半胱氨酸-酪氨酸(Cys-Tyr)辅因子。此外,Cys-Tyr辅因子的形成需要金属辅因子(Fe2+)和O2,并且其先前被认为显著提高CDO的催化效率和半衰期。最近,在厌氧条件下通过定点诱变方法使不包括Cys-Tyr辅因子的纯人CDO(F2-CDO)结晶。在这项工作中,为了深入了解Cys-Tyr辅因子的形成机制,以及它是否能真正促进CDO的催化活性,已经构建了一系列计算模型,并进行了量子力学/分子力学(QM/MM)计算。我们的计算结果表明,WT-CDO和F2-CDO遵循不同的Cys-Tyr辅因子形成机制。在F2-CDO中,辅因子形成包含H-抽象,C-S键的形成,分子内F迁移,和残留物F2Y157的芳构化,其中Fe-配位的双氧可以在形成辅因子后回收;然而,在WT-CDO中,辅因子形成显示出一些差异。在反应过程中,产生过氧化氢,最后的芳构化需要一个水分子的帮助。此外,无论辅因子的存在与否,对于WT-CDO和F2-CDO,辅因子形成的总体障碍总是高于l-半胱氨酸氧化。因此,我们可以从理论上证实Cys-Tyr辅因子对于CDO的氧化活性不是必需的,和辅因子的形成只是伴随反应,但不是氧化反应的先决条件。这些结果可能为理解CDO的催化作用提供有用的信息。
Cysteine dioxygenase (CDO) is a nonheme mononuclear iron enzyme, which catalyzes the oxidation of cysteine to cysteine sulfinic acid. Crystal structure studies of mammalian CDO showed that there is a cross-linked cysteine-tyrosine (Cys-Tyr) cofactor in its active site. Moreover, the formation of the Cys-Tyr cofactor requires the metal cofactor (Fe2+) and O2, and it was previously considered to substantially enhance the catalytic efficiency and half-life of CDO. Recently, a pure human CDO (F2-CDO) without including the Cys-Tyr cofactor was crystalized by the site-directed mutagenesis approach in the anaerobic condition. In this work, to gain insights into the formation mechanism of the Cys-Tyr cofactor and whether it can really promote the catalytic reactivity of CDO, a series of computational models have been constructed, and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that WT-CDO and F2-CDO follow different mechanisms for the formation of the Cys-Tyr cofactor. In F2-CDO, the cofactor formation contains the H-abstraction, C-S bond formation, intramolecular F migration, and aromatization of the residue F2Y157, in which the Fe-coordinate dioxygen can be recovered after the formation cofactor; however, in the WT-CDO, the cofactor formation shows some differences. During the reaction, hydrogen peroxide is generated, and the final aromatization requires the assistance of one water molecule. Furthermore, the overall barriers of cofactor formation are always higher than l-cysteine oxidation for both WT-CDO and F2-CDO irrespective of the absence or presence of the cofactor. Thus, we can theoretically confirm that the Cys-Tyr cofactor is not essential for the oxidation activity of CDO, and cofactor formation is just an accompanying reaction but not a prerequisite for the oxidation reaction. These results may provide useful information for understanding the catalysis of CDO.