Physicochemical characterization of Cajanus cajan lectin: effect of pH and metal ions on lectin carbohydrate interaction.

The association constant of Cajanus cajan lectin for methyl alpha-D-mannopyranoside was studied by equilibrium dialysis method. An attempt was also made to understand the metal ion requirements and to establish that ionizable groups are responsible for lectin-carbohydrate interaction. The N-terminal sequence up to 27 amino acid residues was found to be more than 80% homologous with other mannose-specific legume lectins of the tribe Viceae. Like concanavalin A and pea lectin it also exhibits high affinity for the sugar alpha-methyl mannose and at 37 degrees C the association constant was found to be 1.4x104 M-1. The lectin required one Ca2+ and one Mg2+ per mole and during the lectin sugar interaction two ionizable groups with pK of 3.75 and 8.3 are ionized. Whether the secondary structure is similarly affected with pH changes and presence or absence of metal ion was investigated by circular dichroism studies. Results suggested that changes in carbohydrate binding properties of the Cajanus cajan lectin due to change in pH and addition of metal ions are not accompanied by any significant change in secondary structure.