Concanavalin A binding various numbers of calcium and manganese ions

pH-dependent changes in properties of concanavalin A in the acid pH range

Properties characteristic of the structure and function of dimeric concanavalin A have been studied as a function of pH in the acid pH range using preparations comprising intact subunits or enriched in fragmented chains. For intact subunits, the glycogen binding ability falls to zero with a midpoint of pH 4.7, the release of Mn+2, Ca+2 and the fluorescent ligand 4-methylumbelliferyl-alpha-D-mannopyranoside from the lectin coincides over a pH range centered at pH 3.9, and the CD spectra of the aromatic amino acid residues increase sharply in amplitude between pH 4.0 and 1.5. Nevertheless, the sedimentation coefficient and peptide CD spectrum change insignificantly in the pH range 5 to 2, indicating that dimeric concanavalin A retains its secondary structure and overall hydrodynamic shape essentially unchanged upon acidification. The behavior of concanavalin comprising primarily fragmented chains is not significantly different from that of intact subunits, although it precipitates glycogen less efficiently. It is concluded that dimeric concanavalin A does not undergo a concerted change in structure upon acidification, but rather that it passes through a series of states differing from one another in their local conformations. The distinction in binding between the monosaccharide and the polysaccharide is attributed to participation of a secondary binding site in the latter case. A change in optical activity at 283 nm in the pH range 5-6 is ascribed to disruption of intersubunit interactions of Tyr 67 as the protein undergoes the dimer-tetramer equilibrium.

Effects of manganese and calcium on conformational stability of concanavalin A: a differential scanning calorimetric study

The effect of degree of saturation of concanavalin A with Mn2+ or Ca2+, or both, on its thermal denaturation was investigated by differential scanning calorimetry. Acid-demetallized concanavalin A was partly or fully remetallized in acetate buffer (pH 5.0) containing 0.4 to 0.5 M NaCl. Under these conditions, native dimeric concanavalin A is highly stable, undergoing heat denaturation at 101 degrees C, with an enthalpy of denaturation of 7.4 cal/g. Removal of metal ions lowered stability considerably; concanavalin A with 0.06 Mn2+/monomer and 0.23 Ca2+/monomer (mol/mol) was denatured at 74 degrees C with an enthalpy of denaturation of 3.2 cal/g. Added Mn2+ stabilized demetallized concanavalin A, but added Ca2+ alone (up to 2 mol/mol monomer) did not. The Ca2+/ concanavalin A ratio influenced stabilization by Mn2+. In the presence of 1 to 2 Mn2+/ monomer and 0.5 or less Ca2+/monomer (mol/mol), stabilized concanavalin A was denatured at 85-88 degrees C and at 94-97 degress C, indicating presence of two stabilized metallo-concanavalin A species. At 1.0 or more mole each of Mn2+ and Ca2+ per monomer, one endotherm was observed at or above 98 degrees C and the enthalpy of denaturation was increased to 5.3 cal/g from less than 3.6 cal/g at lower metal ion/protein ratios. Stabilization was greater with Mn2+ plus Ca2+ than with Mn2+ alone, consistent with intrasubunit cooperativity in metal ion-induced stabilization of concanavalin A.