Distinct metal-binding configurations in metallothionein

Abstract

In a study of the binding stoichiometry of various metals to rat liver metallothionein, the protein appears to coordinate metals in 2 distinct configurations. Ions of at least 18 different metals were shown to associate with the protein suggesting that there is little specificity in binding. Most metals exhibited saturation binding at 7 mol eq forming M7-metallothionein. These included Bi(III), Cd(II), Co(II), Hg(II), In(III), Ni(II), Pb(II), Sb(III), and Zn(II). Others metals including Os(III), Pd(II), Pt(IV), Re(V), Rh(III), and Tl(III) give a positive indication of binding, but stoichiometries were unclear. Ag(I) and Cu(I) bound in clusters as M12-metallothionein. This binding stoichiometry was determined in 3 ways: (a) by determining the equivalence point in Cu- and Ag-titrated samples where resistance to proteolysis is maximal; (b) by determining the point where Zn ions are completely displaced from Zn7-metallothionein; and (c) by direct binding studies. Ag-reconstituted protein, recovered from gel filtration, had an average Ag content of 11.5 g atoms/mol of protein. A similar stoichiometry for the Cu-protein resulted from displacement of Zn from Zn7-metallothionein by Cu(I). The M12-protein was converted to the M7-protein by displacement of Ag(I) or Cu(I) with 7 mol eq of Hg(II). Whereas the distribution of metals in the 2 domains of M7-metallothionein is M4 alpha and M3 beta, the arrangement in the M12-molecule is probably M6 alpha and M6 beta. We propose that metallothionein ligates Ag(I) and Cu(I) in a trigonal geometry by bridging thiolates. This is in contradistinction to a tetrahedral binding geometry in the M7-protein. Distinct binding configurations may result in different tertiary structures for M7- and M12-proteins which may relate to metabolic specificity of Zn-metallothionein and Cu-metallothionein, respectively.