Lysine-cysteine "Nitrogen-Oxygen-Sulfur" (NOS) bonds () were first reported in 2021 in transaldolases[1]. The sidechains of a lysine and a cysteine, joined by an NOS bond, make a covalent linkage between polypeptide chains. For comparison, the disulfide bond is a far more common type of covalent linkage between polypeptide chains.
near the N-terminus of the 352 amino acid Neisseria gonorrhoeae transaldolase chain, between Lys8 and Cys38, near the surface.
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Oxidation breaks the NOS bond. In transaldolase, breaking the NOS bond causes subtle allosteric shifts in the catalytic site, decreasing enzymatic activity by several orders of magnitude[1]. Thus, the NOS bond is described as an allosteric redox switch[1].
A survey of the data in the Protein Data Bank revealed that the NOS bond likely exists "in diverse protein families across all domains of life (including Homo sapiens) and that it is often located at catalytic or regulatory hotspots."[1] Because the NOS bond was unknown before 2021, it could easily have been overlooked in earlier interpretations of electron density maps.[1]
