Zn(II) to Ag(I) Swap in Rad50 Zinc Hook Domain Leads to Interprotein Complex Disruption through the Formation of Highly Stable Ag_x(Cys)_y Cores

The widespread application of silver nanoparticles in medicinal and daily life products increases the exposure to Ag(I) of thiol-rich biological environments, which help control the cellular metallome. A displacement of native metal cofactors from their cognate protein sites is a known phenomenon for carcinogenic and otherwise toxic metal ions. Here, we examined the interaction of Ag(I) with the peptide model of the interprotein zinc hook (Hk) domain of Rad50 protein from Pyrococcus furiosus, a key player in DNA double-strand break (DSB) repair. The binding of Ag(I) to 14 and 45 amino acid long peptide models of apo-and Zn(Hk)2 was experimentally investigated by UV-vis spectroscopy, circular dichroism, isothermal titration calorimetry, and mass spectrometry. The Ag(I) binding to the Hk domain was found to disrupt its structure via the replacement of the structural Zn(II) ion by multinuclear Agx(Cys)y complexes. The ITC analysis indicated that the formed Ag(I)-Hk species are at least 5 orders of magnitude stronger than the otherwise extremely stable native Zn(Hk)2 domain. These results show that Ag(I) ions may easily disrupt the interprotein zinc binding sites as an element of silver toxicity at the cellular level.