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. ¹¹¹ Ag perturbed angular correlation of γ-rays (PAC) spectroscopy probes the metal site structure and the relaxation accompanying the instantaneous change from Ag^I to Cd^II upon ¹¹¹ Ag radioactive decay. That is, a change from Ag^I , which activates transcription, to Cd^II , which does not. In the frozen state (-196 °C) 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 °C only NQI₂ is observed, demonstrating that relaxation to this structure occurs within ≈10 ns of the decay of ¹¹¹ Ag. 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, ¹¹¹ Ag PAC spectroscopy may be applied to probe the flexibility of protein metal sites.