A solvent‐exposed cysteine forms a peculiar Ni(II)‐binding site in the metallochaperone CooT from Rhodospirillum rubrum

In Rhodospirillum rubrum, the maturation of carbon monoxide dehydrogenase (CODH) requires three nickel chaperones, namely RrCooC, RrCooT and RrCooJ. Recently, the biophysical characterization of RrCooT homodimer and the X‐ray structure of its apo‐form revealed the existence of a solvent exposed Ni(II)‐binding site at the dimer interface, involving the strictly conserved Cys2. Here, a multifaceted approach that used NMR and X‐ray absorption spectroscopies, complemented with structural bio‐modelling methodologies, was used to characterise the binding mode of Ni(II) in RrCooT. This study suggests that Ni(II) adopts a square‐planar geometry via a N2S2 coordinating environment that comprises the two thiolate and amidate groups of both Cys2 residues at the dimer interface. The existence of a diamagnetic mononuclear Ni(II) centre with bis‐amidate/bis‐thiolate ligands, coordinated by a single cysteine motif, is unprecedented in biology and raises the question of its role in the activation of CODH, at the molecular level.