Flexibility of the CueR Metal Site Probed by Instantaneous Change of Element and Oxidation State from Ag^I to Cd^II

Selectivity for monovalent metal ions is an important facet of the function of the metalloregulatory protein CueR. ¹¹¹Ag perturbed angular correlation of gamma-rays (PAC) spectroscopy probes the metal site structure and the relaxation accompanying the instantaneous change from Ag^I to Cd^II upon ¹¹¹Ag  radioactive decay. That is, a change from Ag^I, which activates transcription, to Cd^II, which does not. In the frozen state (-196 ºC) two nuclear quadrupole interactions (NQIs) are observed; one (NQI₁) agrees well with two coordinating thiolates and an additional longer contact to the S77 backbone carbonyl, and the other (NQI₂) reflects that Cd^II has attracted additional ligand(s). At 1 ºC only NQI₂ is observed, demonstrating that relaxation to this structure occurs within approximate to 10 ns of the decay of ¹¹¹Ag. Thus, transformation from Ag^I to Cd^II rapidly disrupts the functional linear bis(thiolato)Ag^I metal site structure. This inherent metal site flexibility may be central to CueR function, leading to remodelling into a non-functional structure upon binding of non-cognate metal ions. In a broader perspective, ¹¹¹Ag PAC spectroscopy may be applied to probe the flexibility of protein metal sites.