Cadmium binding and metal cluster formation in metallothionein: a differential modification study

Mammalian metallothioneins (MT) contain 20 Cys in a total of 61 amino acid residues and bind 7 Cd and/or Zn ions. The metal is localized in two clusters made up of three and four metal-thiolate complexes in the NH2- and COOH-terminal half of the chain, respectively. The formation of these oligonuclear complexes designated as Cd4- and Cd3-cluster has now been monitored in MT reconstituted with varying amounts of Cd using differential modification of Cys with 14C-iodoacetamide. At ratios below 3 moles Cd/mole MT bound, no differential protection of Cys by the metal and, hence, no preferred binding is detectable. At Cd-to-protein ratios between 3 and 5 moles Cd/mole MT the modification profiles reveal preferred and cooperative binding in the COOH-terminal half of the chain indicating formation of the Cd4-cluster. At still higher ratios formation of the Cd3-cluster is initiated in the NH2-terminal section of the polypeptide chain. Comparison of the differential modification data of Cd6-MT and Cd7-MT suggests that the last Cd to be bound is coordinated to Cys ligands located mainly between positions 20 and 30 of the sequence. The extent of labelling of the different Cys in Cd7-MT indicates that the ligands of the Cd3-cluster are three times as accessible to iodoacetamide as those of the Cd4-cluster, suggesting a greater thermodynamic stability of the latter.