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 concanava lin A complexed with the monosaccharide methyl alpha-D-glucopyranoside is described. The space group has been determined as I2(1)3 rather th an I23. The use of cadmium to replace cobalt at the transition metal-i on binding site and to replace calcium at its binding site proved to b e crucial to the successful solution of the crystal structure. The rel atively small isomorphous signals of 21e(-) for the replacement of cob alt and 28e(-) for the replacement of calcium, yielded interpretable d ifference 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 classi cal 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 concanava lin A converged to an R factor of 27.4%. A molecular-replacement analy sis, consistent with this crystal structure, and initial experiences i n the incorrect space group I23 are described as these also prove to b e instructive.