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 Angstrom and we compared it with that of the cubic form previously refined to the same resolution, The major diffe rences between the two structures concern the cadmium ions bound to th e residues E130 at the threefold axes of the molecule, Taking advantag e of the significant anomalous signal (f'' = 3.6 e(-)) of cadmium at 1 .375 Angstrom, the wavelength used here, we performed anomalous Fourie r difference maps with the refined model phases. These maps reveal the positions of anomalous scatterers at different locations in the struc ture, Among these, some are found near residues that were known previo usly 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 pro cess. The quality of the anomalous Fourier difference map increases si gnificantly with noncrystallographic symmetry map averaging. Such maps reveal density peaks that fit the positions of Met and Cys sulfur ato ms, which are weak anomalous scatterers (f'' = 0.44 e(-)). (C) 1998 Wi ley-Liss, Inc.