Evidence of new cadmium binding sites in recombinant horse L-chain ferritin by anomalous Fourier difference map calculation.

We refined the structure of the tetragonal form of recombinant horse L-chain apoferritin to 2.0 A and we compared it with that of the cubic form previously refined to the same resolution. The major differences between the two structures concern the cadmium ions bound to the residues E130 at the threefold axes of the molecule. Taking advantage of the significant anomalous signal (f" = 3.6 e-) of cadmium at 1.375 A, the wavelength used here, we performed anomalous Fourier difference maps with the refined model phases. These maps reveal the positions of anomalous scatterers at different locations in the structure. Among these, some are found near residues that were known previously to bind metal ions, C48, E57, C126, D127, E130, and H132. But new cadmium binding sites are evidenced near residues E53, E56, E57, E60, and H114, which were suggested to be involved in the iron loading process. The quality of the anomalous Fourier difference map increases significantly with noncrystallographic symmetry map averaging. Such maps reveal density peaks that fit the positions of Met and Cys sulfur atoms, which are weak anomalous scatterers (f" = 0.44 e-).