The structural features of concanavalin A governing non-proline peptide isomerization

The reversible binding of manganese and calcium to concanavalin A determine s the carbohydrate binding of the lectin by inducing large conformational c hanges. These changes are governed by the isomerization of a non-proline pe ptide bond, Ala-207-Asp-208, positioned in a beta-strand in between the cal cium binding site 52 and the carbohydrate specificity-determining loop. The replacement of calcium by manganese allowed us to investigate the structur es of the carbohydrate binding, locked state and the inactive, unlocked sta te of concanavalin A, both with and without metal ions bound. Crystals of u nlocked metal-free concanavalin A convert to the locked form with the bindi ng of two Mn2+ ions. Removal of these ions from the crystals traps metal-fr ee concanavalin A in its locked state, a minority species in solution. The ligation of a metal ion in 52 to unlocked concanavalin A causes bending of the beta-strand foregoing the 52 ligand residues Asp-10 and Tyr-12. This be nding disrupts conventional beta-sheet hydrogen bonding and forces the Thr- 11 side chain against the Ala-207-Asp-208 peptide bond. The steric strain e xerted by Thr-ll is presumed to drive the trans-to-cis isomerization, Upon isomerization, Asp-208 flips into its carbohydrate binding position, and th e conformation of the carbohydrate specificity determining loop changes dra matically.