Crystal structure of demetallized concanavalin A: the metal-binding region.

\Ve llave determined the crystal structure of demetallized concanavalin A, at a resolution of 3.2 8, by rnolecnlar replacement using the knowd structure of native concanavalin A. Refinement of the initial model using a, constraint-restraint reciprocal-space least-squares procedure caused the conventional crystallographic agreement (11) factor t,o decrcase from 0.47 to a final valne of 0.26. There are significant, conformational changes in the metal-binding region involving residues Asp1 B and His24, which a.re snbstantislly closer to each other than in native concanava,lin X. These residues form an internal salt bridge which does not exist whe11 tbc metal ions arc attaclled to t’lle protein. The binding site for transit,ioltmet’alions is still intact, hut the calcium sitme is not, since one of its two carboxylic ligartds, 9sp I R, is unavaila.ble. Flexibility is observed for one of the transitionmet.al ligands, Glu8, as well as for some sepnlents of tile backbone. The latter couId account for t,be increased suscept,ibility of demctallieecl coneann\-alin ,4 to proteolysis.