ABSTRACT
The X-ray structure of a Cd/Zn rat liver MTII has been determined [57] and shows that the molecule is composed of two asymmetrical domains, each with a diameter of roughly 20 A and showing essentially similar folding of the polypeptide chain around the metal clusters. The two domains can bind metal ions independently and there is little chemical interaction between them. No single-crystal X-ray information is avail able for a copper MT, but using NMR techniques Peterson et al. [58] have undertaken three-dimensional solution structural calculations for a yeast Ag(I)-substituted MT and a native Cu(I) MT (PDB code: 1AQR). These authors used experimentally deter mined NOE and dihedral angle constraints in conjunction with experimentally derived metal-to-cysteine connectivities for AgMT, which were assumed identical for CuMT. In these structures the first 40 residues of the polypeptide backbone fold into two large parallel loops separated by a deep cleft and the metal cluster is bound within the cleft as shown in Fig. 7; the nearest-neighbor Cu-Cu distances range between 2.55 and 3.31 A. After residue 40 the polypeptide chains appear to be some what disordered.
