CRYSTALLOGRAPHIC STRUCTURE OF METAL-FREE CONCANAVALIN-A AT 2.5 ANGSTROM RESOLUTION

The three-dimensional structure of demetallized concanavalin A has bee n determined at 2.5 Angstrom resolution and refined to a crystallograp hic R-factor of 18%. The lectin activity of concanavalin A requires th e binding of both a transition metal ion, generally Mn2+ and a Ca2+ io n in two neighboring sites in close proximity to the carbohydrate bind ing site, Large structural differences between the native and the meta l-free lectin are observed in the metal-binding region and consequentl y for the residues involved in the specific binding of saccharides. Th e demetallization invokes a series of conformational changes in the pr otein backbone, apparently initiated mainly by the loss of the calcium ion. Most of the Mn2+ ligands retain their position, but the Ca2+ bin ding site is destroyed, The Ala207-Asp208 peptide bond, in the beta-st rand neighboring the metal-binding sites, undergoes a cis to trans iso merization. The cis conformation for this bond is a highly conserved f eature among the leguminous lectins and is critically maintained by th e Ca2+ ion in metal-bound concanavalin A. A further and major change a djacent to the isomerized bond is an expansion of the loop containing the monosaccharide ligand residues Leu99 and Tyr100. The dispersion of the ligand residues for the monosaccharide binding site (Asn14, Agr22 8, Asp208, Leu99, and Tyr100) in metal-free concanavalin A abolishes t he lectin's ability to bind saccharides. Since the quaternary structur e of legume lectins is essential to their biological role, the tetrame r formation was analyzed. In the crystal (pH 5), the metal-free concan avalin A dimers associate into a tetramer that is similar to true nati ve one, but with a drastically reduced number of inter-dimer interacti ons. This explains the tetramer dissociation into dimers below pH valu es of 6.5. (C) 1995 Wiley-Liss, Inc.