Sulfide quinone oxidoreductase (SQR) catalyzes the first step in sulfide clearance, coupling H₂S oxidation to coenzyme Q reduction. Recent structures of human SQR revealed a sulfur atom bridging the SQR active site cysteines in a trisulfide configuration. Here, we assessed the importance of this cofactor using kinetic, crystallographic and computational modeling approaches. Cyanolysis of SQR proceeds via formation of an intense charge transfer complex that subsequently decays to eliminate thiocyanate. Cyanolysis leads to reversible loss of SQR activity, which is restored in the presence of sulfide. We captured a crystallographic intermediate in SQR that provides clues as to how the oxidized state of the cysteines is preserved. Computational modeling and MD simulations revealed an ~10⁵-fold rate enhancement for nucleophilic addition of sulfide into the trisulfide versus a disulfide cofactor. The cysteine trisulfide in SQR is thus critical for activity and provides a significant catalytic advantage over a cysteine disulfide.

中文翻译：

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拆除和重建三硫键辅因子证明了其在人类硫化物醌醌氧化还原酶中的重要作用

硫化物醌氧化还原酶（SQR）催化硫化物清除的第一步，将H ₂ S氧化与辅酶Q还原偶联。人类SQR的最新结构显示，硫原子以三硫键的形式桥接了SQR活性位点半胱氨酸。在这里，我们使用动力学，晶体学和计算建模方法评估了该辅助因子的重要性。SQR的氰化反应通过形成强烈的电荷转移络合物而发生，该络合物随后降解以消除硫氰酸盐。氰化作用导致SQR活性的可逆损失，在硫化物存在下可以恢复。我们捕获了SQR中的晶体学中间体，该中间体提供了有关如何保留半胱氨酸氧化状态的线索。计算建模和MD仿真显示〜10 ⁵与二硫键辅因子相比，将硫醚亲核加成到三硫键中的速率提高了两倍。因此，SQR中的半胱氨酸三硫化物对于活性至关重要，并且比半胱氨酸二硫化物具有明显的催化优势。