High-resolution structures of single-metal-substituted concanavalin A: the Co,Ca-protein at 1.6 A and the Ni,Ca-protein at 2.0 A.
The molecular structures of cobalt- and nickel-substituted concanavalin A have been refined at 1.6 and 2.0 A resolution, respectively. Both metal derivatives crystallize in space group I222 with approximate cell dimensions a = 89, b = 87 and c = 63 A and one monomer in the asymmetric unit. The final R factor for Co-substituted concanavalin A is 17.8% for 29 211 reflections with F > 1.0sigma(F) between 8.0 and 1.6 A. For Ni-substituted concanavalin A the final R factor is 15.9% for 16 128 reflections with F > 1.0sigma(F) between 8.0 and 2.0 A resolution. Both structures contain a transition-metal binding site and a calcium-binding site but, unlike Cd-substituted concanavalin A, do not have a third metal-binding site. The Co-substituted concanavalin A structure diffracts to the highest resolution of any concanavalin A structure reported to date. A comparison of the structures of Ni-, Co-, Cd-substituted and native concanavalin A gives an indication of coordinate errors, which is a useful baseline for comparisons with saccharide complexes of concanavalin A described in other work. We also give a detailed account of multiple conformations which were found for five side-chain residues.