Cd-111 NMR studies of the domain specificity of Ag+ and Cu+ binding to metallothionein

Metal displacement reactions of Cd(7)MT with Ag+ or Cu+ and interprotein metal exchange reactions between Cd(7)MT and Ag(12)MT or Cu(12)MT were studied by Cd-111 NMR. Titration of (111)Cd(7)MT with Ag+ indicates that Ag+ binds preferentially to the beta-domain of the protein to form the metal hybrid species, (Cd-4)(alpha)(Ag-6)(beta)MT. Once the beta-domain is filled, additional Ag+ ions displace Cd2+ from the alpha-domain to form (Ag-6)(alpha)(Ag-6)(beta)MT. The metal displacement reaction is cooperative and the two domains react independently of one another. The (Cd-4)(alpha)(Ag-6)(beta)MT hybrid protein is also formed as the major product of direct interprotein metal exchange between Cd(7)MT and Ag(12)MT. Cu+ reacts with Cd(7)MT in a manner similar to Ag+, with addition of 6 equiv of Cu+ leading to preferential formation of (Cd-4)(alpha)(Cu-6)(beta)MT, and 12 equiv of Cu+ to formation of (Cu-6)(alpha)(Cu-6)(beta)MT. However, unlike Ag+, Cu+ appears to produce intermediate species that may contain mixed-metal clusters. Interprotein metal exchange between Cu(12)MT and Cd(7)MT leads to the net transfer of Cd2+ into the alpha-domain and Cu+ into the beta-domain. The differential affinities of the two domains for monovalent and divalent metal ions plus the availability of facile pathways for metal exchange may be features that enable MT to function simultaneously in the metabolism of different metal ions.