Sulfur K-Edge EXAFS Studies of Cadmium-, Zinc-, Copper-, and Silver-Rabbit Liver Metallothioneins

Abstract

The structures of metal-thiolate clusters in Zn(7)-MT, Cd(7)-MT, Cu(12)-MT, Ag(12)-MT, and Ag(17)-MT from rabbit liver have been investigated by sulfur K-edge X-ray absorption spectroscopy (XAS). In addition to providing metal-cysteinyl sulfur bond lengths, the sulfur K-edge EXAFS data provide the first direct evidence for mixtures of bridging and terminal sulfurs in Cu-MT and Ag-MT. The Zn-S and Cd-S bond lengths for tetrahedrally coordinated Zn(4)(SPh)(10)(2-) and Cd(4)(SPh)(10)(2-) compounds obtained from sulfur K-edge EXAFS data are 2.35 +/- 0.03 and 2.52 +/- 0.02 Å, respectively. Zn-S and Cd-S bond distances of 2.34 +/- 0.03 Å for Zn(7)-MT and 2.54 +/- 0.02 Å for Cd(7)-MT, respectively, calculated from sulfur K-edge EXAFS measurements, are consistent with the previously reported results from metal K-edge EXAFS data. Analysis of the sulfur K-edge EXAFS data for Cu(12)-MT indicates that Cu(I) is trigonally coordinated to sulfurs at a distance of 2.25 +/- 0.01 Å. Significant changes in CD spectra observed between Ag(12)-MT 1 and Ag(17)-MT 1 indicate that the modification of the three-dimensional structure occurs when Ag(17)-MT 1 is formed from Ag(12)-MT 1 as Ag(I) is added to the Ag(12)-MT 1. The Ag-S bond distances of 2.45 +/- 0.02 and 2.44 +/- 0.03 Å in Ag(1)(2)-MT 1 and Ag(1)(7)-MT 1, respectively, calculated from the sulfur K-edge EXAFS measurements, lead us to conclude that the Ag(I) in both Ag(1)(2)-MT 1 and Ag(1)(7)-MT 1 is digonally coordinated by thiolates. The number of metals bonded to sulfur in both model compounds and metal-containing metallothioneins is estimated from sulfur K-edge EXAFS measurements to be in the range 1.2-1.7, depending on the fraction of bridging sulfurs present in compounds. Unlike the spectral data recorded during Cu(I) binding, where sharp changes take place past 12 Cu(I), the CD data for Ag-MT 1 show little variation over the entire range of Ag(I):MT molar ratios. This result, established by both low- and high-energy optical methods, suggests that the three-dimensional structure of the metal-binding sites in metallothioneins is strongly influenced by the fraction of bridging sulfur. This analysis is the first to provide direct support for the presence of a clustered Ag-S structure for the Ag(17)-MT 1 species. These data also suggest that the structures in Ag(I) and Cu(I) metallothioneins are probably quite different.

The building blocks of metallothioneins: heterometallic Zn2+ and Cd2+ clusters from first-principles calculations

“…58 and Zn 7 -MT 58 structures, and the heterometallic rat Cd 5 Zn 2 -MT structure. 17 The metal sites in experimental protein structures are poorly determined, based on a priori fixed parameters for bonds and angles, 27,57,58 so direct comparison is made only to inorganic complexes.…”

Residue Modification and Mass Spectrometry for the Investigation of Structural and Metalation Properties of Metallothionein and Cysteine-Rich Proteins

“…The lack of a rigid structure allows MTs to coordinate a number of metals including essential metals like zinc and copper [11], toxic metals like cadmium [12] and mercury [13], and exotic metals like uranium [14] and technetium [15]. This flexibility, while fundamental to their function, makes MTs extremely difficult to characterize structurally.…”

Sulfur K-edge extended X-ray absorption fine structure spectroscopy of homoleptic thiolato complexes with Zn(II) and Cd(II)

“…However, this method can provide direct information about sulfur atom environment in proteins and metal complexes that cannot be obtained by the complementary metal K-edge measurements. The sulfur K-edge EXAFS has been applied already to inorganic materials [24][25][26][27][28][29] and metalloprotein [14,30]. However, no sulfur K-edge EXAFS studies of Zn(II) and Cd(II) complexes have been reported yet to our knowledge, in contrast, various Zn(II) and Cd(II) complexes mimicking biological systems do exist.…”