Structure solution of a cubic crystal of concanavalin a complexed with methyl alpha-D-glucopyranoside

The solution of the cubic crystal form (a=167.8 Angstrom) of concanavalin A complexed with the monosaccharide methyl alpha-D-glucopyranoside is described. The space group has been determined as I2(1)3 rather than I23. The use of cadmium to replace cobalt at the transition metal-ion binding site and to replace calcium at its binding site proved to be crucial to the successful solution of the crystal structure. The relatively small isomorphous signals of 21e(-) for the replacement of cobalt and 28e(-) for the replacement of calcium, yielded interpretable difference Patterson maps. The electron-density map calculated in space group I2(1)3 at 5.4 Angstrom resolution, based on phases derived from single- and double-substituted cadmium differences, revealed a classical concanavalin A tetramer of 222 point symmetry, as seen in all the known crystal structures of concanavalin A. Rigid-body refinement at 3.6 Angstrom using the refined coordinates of saccharide-free concanavalin A converged to an R factor of 27.4%. A molecular-replacement analysis, consistent with this crystal structure, and initial experiences in the incorrect space group I23 are described as these also prove to be instructive.