Refined crystal structure of Cd, Zn metallothionein at 2.0 A resolution

The crystal structure of Cd5,Zn2-metallothionein from rat liver has been refined at 2.0 A resolution of a R-value of 0.176 for all observed data. The five Cd positions in the asymmetric unit of the crystal create a pseudo-centrosymmetric constellation about a crystallographic 2-fold axis. Consequently, the distribution of anomalous differences is almost ideally centrosymmetric. Therefore, the previously reported metal positions and the protein model derived therefrom are incorrect. Direct methods were applied to the protein amplitudes to locate the Cd positions. The new positions were used to calculate a new electron density map based on the Cd anomalous scattering and partial structure to model the metal clusters and the protein. Phases calculated from this model predict the positions of three sites in a (NH4)2WS4 derivative. Single isomorphous replacement phases calculated with these tungsten sites confirm the positions of the Cd sites from the new direct methods calculations. The refined metallothionein structure has a root-mean-square deviation of 0.016 A from ideality of bonds and normal stereochemistry of phi, phi and chi torsion angles. The metallothionein crystal structure is in agreement with the structures for the alpha and beta domains in solution derived by nuclear magnetic resonance methods. The overall chain folds and all metal to cysteine bonds are the same in the two structure determinations. The handedness of a short helix in the alpha-domain (residues 41 to 45) is the same in both structures. The crystal structure provides information concerning the metal cluster geometry and cysteine solvent accessibility and side-chain stereochemistry. Short cysteine peptide sequences repeated in the structure adopt restricted conformations which favor the formation of amide to sulfur hydrogen bonds. The crystal packing reveals intimate association of molecules about the diagonal 2-fold axes and trapped ions of crystallization (modeled as phosphate and sodium). Variation in the chemical and structural environments of the metal sites is in accord with data for metal exchange reactions in metallothioneins.