Magnetic Resonance Studies of Concanavalin A: Conformational Changes Induced by Ca2+ and α-Methyl-D-mannopyranoside

Three independent solution spectroscopic techniques (solvent proton relaxation enhancement, circular dichroism, and high resolution 220 MHz proton magnetic resonance spectroscopy) have been utilized to demonstrate metal ion- and monosaccharide inhibitor-induced structural perturbations for the dimeric form of the plant lectin concanavalin A (Con A). The results indicate that (i) the occupation of the transition metal ion site S1 by Mn2+ or Zn2+ does not detectably perturb the demetallized protein conformation, (ii) the binding of Ca2+ to the Con A – Mn2+ or Con A – Zn2+ complexes perturbs the protein structure in the vicinity of the S1 site as well as at points remote from the S1–S2 double ion site, and (iii) the binding of the monosaccharide inhibitor α-methyl-D-mannopyranoside to the fully metallized Con A complex also significantly perturbs the structural features of the protein.A detailed radio frequency dependence analysis of the Ca2+ effect on the solvent proton relaxation enhancement properties of the Con A – Mn2+ complex indicates that the considerable reduction in the observed enhancement upon Ca2+ binding principally results from an approximate 120-fold decrease in the single Mn2+ water of hydration exchange rate. The 220 MHz proton magnetic resonance spectra for Con A indicate that this form of spectroscopy is the most useful of those utilized in detailing the solution structural features of this lectin, and a tentative assignment for the C-2-H proton of histidine residue 24 (the S1 site ligand) has been proposed.