Chiral copper(I)-thiolate clusters in metallothionein and glutathione

Metallothionein (MT) is a ubiquitous mammalian protein comprising 61 or 62 nonaromatic amino acids of which 20 are cysteine residues. The high sulfhydryl content imparts to this protein a unique and remarkable ability to bind multiple metal ions in structurally significant metal-thiolate clusters. MT can bind seven divalent metal ions per protein molecule in two domains with exclusive tetrahedral metal coordination. The domain stoichiometries for the M7S20 structure are M4(Scys)11 (alpha domain) and M3(Scys)9 (beta domain). Up to 12 Cu(I) ions can displace the 7 Zn(2+) ions bound per molecule in Zn-MT. The incoming Cu(I) ions adopt a trigonal planar geometry with domain stoichiometries for the Cu12S20 structure of Cu6(Scys)11 and Cu6(Scys)9 for the alpha and beta domains, respectively. The circular dichroism (CD) spectra recorded as Cu+ is added to Zn-MT to form Cu12-MT directly report structural changes that take place in the metal binding region. The spectrum arises under charge transfer transitions between the cysteine S and the Cu(i); because the Cu(I)-thiolate cluster units are located within the chiral binding site, intensities in the CD spectrum are directly related to changes in the binding site. The CD technique clearly indicates stoichiometries of several Cu(I)-MT species. Model Cu(I)-thiolate complexes, using the tripeptide glutathione as the sulfhydryl source, were examined by CD spectroscopy to obtain transition energies and the Cu(I)-thiolate coordination geometries which correspond to these bands. Possible structures for the Cu(I)-thiolate clusters in the alpha and beta domains of Cu12-MT are proposed.