Incorporation of sulfide ions into the cadmium(II)-thiolate cluster of Cicer arietinum metallothionein2

Abstract

The plant metallothionein2 from <i>Cicer arietinum</i> (chickpea), cic-MT2, is known to coordinate five divalent metal ions such as ZnII or CdII, which are arranged in a single metal thiolate cluster. When the ZnII form of the protein is titrated with CdII ions in the presence of sulfide ions, an increased CdII binding capacity and concomitant incorporation of sulfide ions into the cluster are observed. The exact stoichiometry of this novel cluster, its spectroscopic properties, and the significantly increased pH stability are analyzed with different techniques, including UV and circular dichroism spectroscopy and calorimetric assays. Limited proteolytic digestion provides information about the spacial arrangement of the cluster within the protein. Increasing the CdII scavenging properties of a metallothionein by additionally recruiting sulfide ions might be an economic and very efficient detoxification strategy for plants.