Spectroscopic Properties of Ag(I), Cd(II), Cu(I), Hg(II), and Zn(II) Metallothioneins

Abstract

Metallothioneins (MT), a class of protein characterized by a high cysteine content, low molar mass, and lack of aromatic amino acid residues, have been isolated from mammals, yeasts, fungus, and crustaceans [1–3]. The 20 cysteines out of a total of 61 or 62 amino acids in rabbit liver MT bind a remarkably wide range of metals, including, significantly, cadmium, copper, and mercuiy. The binding constants for metal binding follow the order found for inorganic thiolates, Hg(II) > Ag(I) ≈ Cu(I) > Cd(II) > Zn(II). In mammalian Cd7-MT and Zn7-MT the metals arc tetrahedrally co-ordinated in two isolated domains with stoichiometries of M4Sn and M3S9. Optical spectra, in particular circular dichroism and luminescence spectra, have provided rich details of a complicated metal binding chemistry when metals arc added directly to the metal free- or zinc- containing protein. The absence of aromatic amino acids is important because spectral data can be measured for the thiolate to metal charge transfer transitions that occur between 220 and 350 nm, a region that would be completely masked by the presence of aromatic groups. The CD technique is sensitive to changes in the orientation of the peptide chain induced by changes in the metal binding site as a result of metal binding or metal exchange. In particular, the CD spectral changes are extensive when the metal co-ordination geometry changes, for example from the tetrahedral of Zn7-MT to accommodate metals like Cu(I) and Ag(I), metals that generally exhibit trigonal or digonal coordination geometries. Absorption, emission, MCD, and CD spectra provide considerable detail about the stoichiometries of complexes that form as metal are added to either apo-MT or the Zn(II) in Zn7-MT.