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.