A new insight into the Ag⁺ and Cu⁺ binding sites in the metallothionein β domain

The copper(I) and silver(I) binding properties of the β fragment of recombinant mouse metallothionein 1 have been studied by electronic absorption and circular dichroism spectroscopy. When possible, the stoichiometry of the species formed was confirmed by electrospray mass spectrometry. The behaviour observed differs from that reported for the native protein. Titration of either Zn3-βMT at pH 7 or apo-βMT at pH 3 with Cu+ leads to the formation of species having the same stoichiometry and structure: Cu6-βMT, Cu7-βMT and Cu10-βMT. In the first stage of the titration of Zn3-βMT with Cu+ at pH 7 one additional species of formula Cu4Zn1-βMT was detected. In contrast, the titration of Zn3-βMT at pH 7.5 and of apo-βMT at pH 2.5 with Ag+ proceeds through different reaction pathways, affording Zn(x)Ag3-βMT, Ag6-βMT and Ag9-βMT or Ag3-βMT Ag6-βMT and Ag9-βMT, respectively. The CD envelope corresponding to species with the same stoichiometric ratio, Ag6-βMT and Ag9-βMT, indicates that they have a different structure at each pH value. On the basis of the different observed, the postulated similarity between copper and silver binding to metallothionein may be questioned.