Location of the Saccharide Binding Site of Concanavalin A

Crystals of the lectin concanavalin A containing the saccharide β-(o-iodophenyl)-d-glucopyranoside have been prepared. Difference electron density projections at 2.8-A resolution have revealed the positions of the iodine atoms, directly within prominent cavities on the surface of the protein. One saccharide is bound to each 27,000 molecular weight subunit, and thus 4 sugar molecules are bound to symmetry equivalent positions on the tetrameric concanavalin A molecule. The iodine atoms of the four bound saccharides are separated by distances of 24.6, 25.1, and 35.1 A.