A novel covalent post-translational modification (lysine–NOS–cysteine) was discovered in proteins, initially in the enzyme transaldolase of Neisseria gonorrhoeae (NgTAL) [Nature 2021, 593, 460–464], acting as a redox switch. The identification of this novel linkage in solution was unprecedented until now. We present detection of the NOS redox switch in solution using sulfur K-edge X-ray absorption spectroscopy (XAS). The oxidized NgTAL spectrum shows a distinct shoulder on the low-energy side of the rising edge, corresponding to a dipole-allowed transition from the sulfur 1s core to the unoccupied σ* orbital of the S–O group in the NOS bridge. This feature is absent in the XAS spectrum of reduced NgTAL, where Lys-NOS-Cys is absent. Our experimental and calculated XAS data support the presence of a NOS bridge in solution, thus potentially facilitating future studies on enzyme activity regulation mediated by the NOS redox switches, drug discovery, biocatalytic applications, and protein design.

中文翻译：

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通过硫 K 边 X 射线吸收光谱在溶液中鉴定转醛酶中带有 NOS 桥的赖氨酸-半胱氨酸氧化还原开关

在蛋白质中发现了一种新的共价翻译后修饰（赖氨酸 - NOS - 半胱氨酸），最初是在淋病奈瑟菌转醛醇酶( Ng TAL) 中 [ Nature 2021 , 593 , 460–464]，充当氧化还原开关。迄今为止，在溶液中发现这种新颖的联系是前所未有的。我们使用硫 K 边 X 射线吸收光谱 (XAS) 检测溶液中的 NOS 氧化还原开关。氧化的Ng TAL 光谱在上升沿的低能侧显示出明显的肩部，对应于偶极子允许的从硫 1s 核心到 NOS 桥中 S-O 基团的未占据 σ* 轨道的跃迁。还原的Ng TAL的 XAS 谱图中不存在此特征，其中不存在 Lys-NOS-Cys。我们的实验和计算的 XAS 数据支持溶液中存在 NOS 桥，从而有可能促进未来对 NOS 氧化还原开关介导的酶活性调节、药物发现、生物催化应用和蛋白质设计的研究。