Comparison of the structures of the metal-thiolate binding site in Zn(II)-, Cd(II)-, and Hg(II)-metallothioneins using molecular modeling techniques

The first fully energy-minimized structures for a series of structurally related metal complexes of the important mammalian metal binding protein metallothionein are described. The structures were calculated based on structural information obtained from existing spectroscopic and crystallographic data, and minimized using molecular mechanics (MM2) techniques. A two domain structure, with stoichiometries of M(II)(3)-(S-cys)(9) and M(II)(4)-(S-cys)(11) where M = zinc(II), cadmium(II), and mercury(II), was assembled and minimized. The resultant three-dimensional structure closely resembled that of rat liver Cd5Zn2-MT 1 obtained by analysis of x-ray diffraction data [A. H. Robbins, D. E. McRee, M. Williamson, S. A. Collett, N. H. Xuong, W. F. Furey, B. C. Wang and C. D. Stout, J. Mol. Biol. 221, 1269-1293 (1991)]. Minimized structures for Zn-7-MT, Cd-7-MT, and Hg-7-MT are reported. Deep crevices that expose the metal-thiolate clusters are seen in each structure. However, for the mercury-containing protein, much of the mercury-thiolate structure is visible and it is proposed that this provides access for extensive interaction between solvent water molecules and the mercury(II), resulting in the observed distortion away from tetrahedral geometry for Hg-7-MT. Volume calculations are reported for the protein metallated with 7 Zn(II), Cd(II), or Hg(II). A series of structural changes calculated for the step-wise isomorphous replacement of Zn(II) by Cd(II) and Hg(II) in the Zn4S11 alpha domain are shown.