Metallothionein: an exceptional metal thiolate protein.
Metallothioneins are unusual, low molecular weight proteins of extremely high sulphur and metabl content. They occur in substantial quantity and in multiple variant forms in parenchymatous tissues (liver, kidney, intestines) of vertebrates and certain microorganisms (Neurospora crassa, yeast). They are though to play a central role in the cellular metabolism of metals such as zinc, copper and cadmium. All mammalian forms studied are single chains with 20 cysteinyl residues among a total of 61 amino acid residues and highly characteristic amino acid sequences. Their most conspicuous common features are seven -Cys-X-Cys- sequences where X stands for an alphatic residue other than Cys. Together with additional cysteinyl residues located elsewhere in the chain and brought into juxtaposition by appropriate chain folding, these dithiol sequences are believed to form the basis of the trithiolate chelating structures typical of most of the six or seven metal-binding sites of the mammalian cadium- and/or zinc-containing metallothioneins. The positions of the cysteinyl residues are preserved in evolution: the copper-containing metallothionein from Neurospora crassa, containing only 25 amino acid residues, has a distribution of metal-binding cysteinyl residues identical to that of the N-terminal portion of the mammalian chains. The detailed physiological role of metallothionein remains to be clarified but its biosynthesis is known to be modulated by nutritional and endocrine factors. Recent evidence suggests that metallothionein is a critical determinant in the homeostasis of zinc.