The effect of binding of metal ions on the three-dimensional structure of demetallized concanavalin A.

Concanavalin A (con A) is a saccharide-binding protein of the Jack bean [ 11. The protein contains two different metal-binding sites, which must be occupied in order for saccharide binding to be possible [2,3]. When the metal ions are removed by acid treatment, the demetallized protein can bind only divalent transition metal ions (Mn”, Co”, Ni2+, Zn”, or Cd23 at a site designated S 1, but it cannot bind Ca2+ or saccharide. When Sl is occupied by one of the metal ions listed above, the protein can bind Ca” at a second metalbinding site designated S2 [4]. We have made an X-ray crystallographic determination of the three-dimensional structure of demetallized concanavalin A and we report here the important structural differences in the metal-binding region between the demetallized protein and the native metalloprotein [5-71 (fig.1). We postulate a detailed mechanism whereby binding of the transition metal ion at Sl engenders the Ca2+-binding site (S2). The experimental details, refinement procedure and electron density maps upon which our structure is based will be reported fully [8].