Department of Chemistry, Purdue University, West Lafayette, IN 47907.
Department of Biological Chemistry, Institute of Advanced Chemistry of Catalonia, IQAC-CSIC, E-08034 Barcelona, Spain.
Proc Natl Acad Sci U S A. 2020 Aug 4;117(31):18216-18223. doi: 10.1073/pnas.2006639117. Epub 2020 Jul 17.
Cysteine disulfides, which constitute an important component in biological redox buffer systems, are highly reactive toward the hydroxyl radical (OH). The mechanistic details of this reaction, however, remain unclear, largely due to the difficulty in characterizing unstable reaction products. Herein, we have developed a combined approach involving mass spectrometry (MS) and theoretical calculations to investigate reactions of OH with cysteine disulfides (Cys-S-S-R) in the gas phase. Four types of first-generation products were identified: protonated ions of the cysteine thiyl radical (Cys-S), cysteine (Cys-SH), cysteine sulfinyl radical (Cys-SO), and cysteine sulfenic acid (Cys-SOH). The relative reaction rates and product branching ratios responded sensitively to the electronic property of the R group, providing key evidence to deriving a two-step reaction mechanism. The first step involved OH conducting a back-side attack on one of the sulfur atoms, forming sulfenic acid (-SOH) and thiyl radical (-S) product pairs. A subsequent H transfer step within the product complex was favored for protonated systems, generating sulfinyl radical (-SO) and thiol (-SH) products. Because sulfenic acid is a potent scavenger of peroxyl radicals, our results implied that cysteine disulfide can form two lines of defense against reactive oxygen species, one using the cysteine disulfide itself and the other using the sulfenic acid product of the conversion of cysteine disulfide. This aspect suggested that, in a nonpolar environment, cysteine disulfides might play a more active role in the antioxidant network than previously appreciated.
二硫键是生物氧化还原缓冲体系的重要组成部分，它们高度反应性的与羟基自由基（OH）反应。然而，该反应的机制细节仍不清楚，这主要是因为难以对不稳定的反应产物进行表征。在此，我们开发了一种组合方法，涉及质谱（MS）和理论计算，以研究气相中 OH 与半胱氨酸二硫键（Cys-S-S-R）的反应。鉴定了四种第一代产物：半胱氨酸硫基自由基（Cys-S）、半胱氨酸（Cys-SH）、半胱氨酸亚磺酰基自由基（Cys-SO）和半胱氨酸亚磺酸（Cys-SOH）的质子化离子。相对反应速率和产物分支比对 R 基团的电子性质敏感，提供了推导两步反应机制的关键证据。第一步涉及 OH 对其中一个硫原子进行背侧攻击，形成亚磺酸（-SOH）和硫基自由基（-S）产物对。在质子化体系中，产物络合物内的 H 转移步骤是有利的，生成亚磺酰基自由基（-SO）和硫醇（-SH）产物。因为亚磺酸是过氧自由基的有效清除剂，我们的结果表明，半胱氨酸二硫键可以形成两条防御线来对抗活性氧，一条使用半胱氨酸二硫键本身，另一条使用半胱氨酸二硫键转化的亚磺酸产物。这方面表明，在非极性环境中，半胱氨酸二硫键在抗氧化网络中可能发挥比以前认为的更活跃的作用。
