Reactivity of Cd7-metallothionein with Cu(II) ions: evidence for a cooperative formation of Cd3,Cu(I)5-metallothionein.

Reaction of Cd7-metallothionein-2 (MT) with Cu(II) ions has been studied by a variety of spectroscopic techniques including UV-absorption, circular dichroism (CD) and luminescence spectroscopy. The addition of up to 5 Cu(II) equivalents to Cd7-MT resulted in a cooperative formation of the monomeric Cd3,Cu5-MT form, as revealed by the analytical data and the presence of isosbestic or isodichroic points in the respective UV and CD spectra. The presence of Cu(I) luminescence and the absence of Cu(II) EPR signal indicated that copper is bound in the Cu(I) oxidation state, i.e., Cd3,Cu(I)5-MT. Consequently, the reduction of Cu(II) ions is accompanied by the oxidation of thiolate ligands of the protein. The absorption features and the luminescence data at 77 K are consistent with the presence of an air-stable Cu(I)-cluster in Cd3,Cu(I)5-MT. The participation of other ligands, besides cysteine thiolates, in metal coordination cannot be ruled out. With more than 5 Cu(II) equivalents added a mixture of unstable MT metalloforms were formed. The concomitant reduction and binding of copper ions by metallated MT represent a new aspect of the MT structure.