Copper-Thiolate Proteins (Metallothioneins)
Exclusive metal-sulfur coordination is found in iron-sulfur proteins, the noncatalytic zinc-binding center of alcohol dehydrogenase, and in the metallothioneins (MT's), including copper-thionein. The control of the levels of biochemically active metals, including Zn or Cu, could be the attractive function of the MT's. Cu-thionein could undergo a reversible redox reaction where the sulfur is exclusively involved. The metal content of MT is variable and depends in vivo on nutritional levels of the corresponding metal ions including Cd, Zn, and Cu. Cd, Zn-binding proteins from the hepatopancreas of invertebrates and a prokaryotic blue-green alga have been isolated and characterized as MT. Yeast cells grown on a Cd-enriched medium were able to incorporate the metal ion. The physicochemical properties of yeast Cu-thionein encouraged the proposal of a substantial degree of copper-sulfur coordination. The Cu-thioneins isolated from untreated yeast proved identical with 1-Cu-thionein from Cu-supplemented cells.