COMPARISON OF THE NMR SOLUTION STRUCTURE AND THE X-RAY CRYSTAL STRUCTURE OF RAT METALLOTHIONEIN-2
Refined Crystal Structure of Cd, Zn Metallothionein at 2.0 Angstroms Resolution
Crystal Structure Ofcd,Zn Metallothionein
Single Crystals of Cadmium, Zinc Metallothionein
TURN *T2* HAS A LEFT HANDED ALPHA CONFORMATION.
THE NUMBERING CONVENTION FOR THE FIVE CADMIUM ATOMS FOLLOWS THOSE USED IN THE NMR LITERATURE (SEE M. VASAK ET AL., J.MOL.BIOL. 196,711-719 (1987)). ZN1 AND ZN2 REPLACE CD2 AND CD3 IN THE CD(7) NMR STRUCTURES, RESPECTIVELY. BECAUSE NO ELECTRON DENSITY IS PRESENT AT THE ZINC POSITIONS IN AN ANOMALOUS DIFFERENCE FOURIER MAP, THE AUTHORS BELIEVE THAT LITTLE OR NO CADMIUM IS PRESENT AT THESE SITES. IN THESE MAPS, THE ELECTRON DENSITY FOR THE ISOLATED CADMIUM ATOM IN THE CDZN(2) CLUSTER IS COMPARABLE TO THAT OBSERVED AT EACH OF THE OTHER FOUR CADMIUM LOCATIONS. RESIDUE NA 1 IS OCTAHEDRALLY COORDINATED TO THE OXYGEN OF RESIDUES HOH 1, HOH 2, HOH 3 AND THE OXYGEN OF ALA 42' AND OG OF SER 45' IN A SYMMETRY RELATED MOLECULE.
THE STRUCTURE WAS SOLVED BY LOCATING THE FIVE CADMIUM ATOMS BY DIRECT METHODS. THE STARTING MODEL WAS A FIT OF THE CONSENSUS DOMAINS FROM THE 2-D NMR MODEL INTO AN ELECTRON DENSITY MAP GENERATED FORM THE LOCATION OF THE CADMIUM POSITIONS. THE ANOMALOUS SCATTERING COMPONENTS OF CADMIUM ZINC AND SULFUR WERE INCLUDED IN THE REFINEMENT. DUE TO WEAK ELECTRON DENSITY, THE POSITIONS OF RESIDUES 1-2, 51-56, AND THE C-TERMINAL ALANINE SHOULD BE TREATED AS TENTATIVE. THE SIDE CHAINS OF LYSINES 20 AND 22 ARE ALSO UNRELIABLE.