Disulfide bonds between cysteine residues are important post-translational modifications in proteins that have critical roles for protein structure and stability, as redox-active catalytic groups in enzymes or allosteric redox switches that govern protein function^1,2,3,4. In addition to forming disulfide bridges, cysteine residues are susceptible to oxidation by reactive oxygen species, and are thus central not only to the scavenging of these but also to cellular signalling and communication in biological as well as pathological contexts^5,6. Oxidized cysteine species are highly reactive and may form covalent conjugates with, for example, tyrosines in the active sites of some redox enzymes^7,8. However, to our knowledge, regulatory switches with covalent crosslinks other than disulfides have not previously been demonstrated. Here we report the discovery of a covalent crosslink between a cysteine and a lysine residue with a NOS bridge that serves as an allosteric redox switch in the transaldolase enzyme of Neisseria gonorrhoeae, the pathogen that causes gonorrhoea. X-ray structure analysis of the protein in the oxidized and reduced state reveals a loaded-spring mechanism that involves a structural relaxation upon redox activation, which is propagated from the allosteric redox switch at the protein surface to the active site in the protein interior. This relaxation leads to a reconfiguration of key catalytic residues and elicits an increase in enzymatic activity of several orders of magnitude. The redox switch is highly conserved in related transaldolases from other members of the Neisseriaceae; for example, it is present in the transaldolase of Neisseria meningitides (a pathogen that is the primary cause of meningitis and septicaemia in children). We surveyed the Protein Data Bank and found that the NOS bridge exists in diverse protein families across all domains of life (including Homo sapiens) and that it is often located at catalytic or regulatory hotspots. Our findings will inform strategies for the design of proteins and peptides, as well as the development of new classes of drugs and antibodies that target the lysine–cysteine redox switch^9,10.

半胱氨酸残基之间的二硫键是蛋白质中重要的翻译后修饰，对蛋白质结构和稳定性具有关键作用，如酶中的氧化还原活性催化基团或控制蛋白质功能的变构氧化还原开关^1,2,3,4。除了形成二硫键外，半胱氨酸残基还容易被活性氧物质氧化，因此不仅对这些物质的清除很重要，而且对生物和病理环境中的细胞信号传导和通讯也很重要^5,6。氧化的半胱氨酸物质具有高反应性，并可能与某些氧化还原酶活性位点中的酪氨酸等形成共价结合物^7,8. 然而，据我们所知，具有除二硫化物以外的共价交联键的调节开关以前没有被证明过。在这里，我们报告了半胱氨酸和赖氨酸残基之间的共价交联的发现，其中 NOS 桥充当淋球菌转醛缩酶中的变构氧化还原开关，导致淋病的病原体。氧化和还原状态下的蛋白质的 X 射线结构分析揭示了一种加载弹簧机制，该机制涉及氧化还原激活时的结构松弛，其从蛋白质表面的变构氧化还原开关传播到蛋白质内部的活性位点。这种松弛导致关键催化残基的重新配置，并引起酶活性增加几个数量级。氧化还原开关在来自奈瑟氏球菌科其他成员的相关转醛缩酶中高度保守；例如，它存在于脑膜炎奈瑟菌的转醛缩酶中（一种病原体，是儿童脑膜炎和败血症的主要原因）。我们调查了蛋白质数据库，发现 NOS 桥存在于所有生命领域（包括智人）的不同蛋白质家族中，并且它通常位于催化或监管热点。我们的研究结果将为蛋白质和肽的设计策略以及针对赖氨酸-半胱氨酸氧化还原开关的新型药物和抗体的开发提供信息^9,10。