Is Ag(I) an adequate probe for Cu(I) in structural copper-metallothionein studies?: The binding features of Ag(I) to mammalian metallothionein 1

The binding abilities of silver(I) silver(l) to mammalian MT 1 have been studied and compared with those of copper(I), recently reported [Bofill et al. (2001) J Biol Inorg Chem 6:408-417], with the aim of analyzing the suitability of Ag(I) as a Cu(I) probe in Cu-MT studies. The Zn/Ag replacement in recombinant mouse Zn7-MT 1 and corresponding Zn4-α MT 1 and Zn3-βMT 1 fragments, as well as the stepwise incorporation of Ag(I) to the corresponding apo-MTs, have been followed in parallel by various spectroscopic techniques including electronic absorption (UV-vis), circular dichroism (CD) and electrospray mass spectrometry coupled to capillary zone electrophoresis (CZE-ESI-MS). A comparative analysis of the sets of data obtained in the titration of Zn7-MT 1, Zn4-α MT 1 and Zn3-βMT 1 with AgClO4 at pH 7.5 and 2.5 has led to the reaction pathways followed during the incorporation of silver to these proteins under these specific conditions, disclosing unprecedented stoichiometries and structural features for the species formed. Thus, the Zn/Ag replacement in Zn7-MT 1 at pH 7.5 has revealed the subsequent formation of Ag4Zn5-MT, Ag7Zn3-MT, Ag8Zn3-MT, Ag10Zn2-MT, Ag 12Zn1-MT, Agx-MT, x = 14-19, whose structure consists of two additive domains only if Zn(II) remains coordinated to the protein. A second structural role for Zn(II) has been deduced from the different folding found for the Agx-MT species of the same stoichiometry formed at pH 7.5 or 2.5. Comparison of the binding features of Cu(I) and Ag(I) to the entire MT at pH 7.5 shows that, among all the M xZny-MT (0 ≤ y < 7) species found, only M 4IZn5-MT [(Zn4)α(M 4Zn1)β] and M7IZn3-MT [(M3Zn2)α (M4Zn1)β], which form during the first stages of the Zn(II)/M(I) metal replacement, show comparable 3D structures; thus, they are the only species where Ag(I) ions can be predicted to be an adequate probe for Cu(I).