Sulfide ions as modulators of metal–thiolate cluster size in a plant metallothionein

Abstract

Metallothioneins are small cysteine-rich proteins coordinating various transition metal ions preferably with the electron configuration d¹⁰. They are ubiquitously present in all phyla, and next to phytochelatins they represent a successful molecular concept for high-capacity metal ion binding. Recent studies showed the incorporation of sulfide ions into the metal–thiolate cluster of metallothionein 2 from the plant Cicer arietinum (cicMT2) increasing the cadmium binding capacity and stability of the cluster. In the present work, the sulfide-induced structural changes accompanying the cluster formation and the sulfide-modulated increase in cluster size are analyzed in detail with a variety of analytical and spectroscopic techniques. Evaluation of the mechanism of sulfide containing Cd^II–thiolate cluster formation in cicMT2 reveals a strong dependence on the sequence of metal and sulfide additions for successful sulfide incorporation. To probe the general ability of metallothioneins to form sulfide containing larger metal–thiolate clusters, analogous experiments were performed with a mammalian metallothionein. The observation that the cadmium binding ability of rabbit liver MT2A was only slightly increased led to the development of a hypothesis in which the long cysteine-free linker regions present in certain plant metallothioneins may contribute to the accommodation of the respective larger cluster assemblies.