Changes in the Three-Dimensional Structure of Concanavalin A upon Demetallization

When the Mn²⁺ and Ca²⁺ ions normally present in concanavalin A are removed, the protein becomes incapable of binding saccharides. To explore the structural differences between the native and demetallized forms and their effects on the saccharide-binding properties of the protein, we have refined and compared the crystal structures of both forms. Refinement, carried out by automated difference Fourier methods, has revealed a number of differences between the two structures as well as minor differences between the two crystallographically independent monomers in the demetallized structure. Significant differences between the holo- and apoproteins are confined to the region where the metals are bound. These differences include a reorganization and disordering of the loop, consisting of residues 7-25, that contains all of the direct metal ligands of the protein. In some molecules, the side chain of arginine-228 appears to move into the metal-binding region, possibly compensating in part for the absence of the metal's positive charge. The cis peptide observed in the native protein at alanine-207 is apparently not present in the demetallized protein. The conformational differences affect many of the residues currently thought to be involved in the specific binding of saccharides.