Metal ion binding to tetrameric lima bean lectin.

The binding of Mn2+ and Ca2+ to tetrameric lima bean lectin has been examined by equilibrium dialysis and magnetic resonance techniques. Demetalized lectin prepared by acid treatment binds either 1 Mn2+ or 2 Ca2+/monomer. When demetalized lectin is presaturated with Ca2+, only 1 Mn2+ binds per dimer. Water proton relaxation rate enhancements and Mn2+ electron spin resonance spectra were used to monitor metal ion association processes. Following Mn2+ binding to demetalized lectin, a conformational change with activation energy of 16 kcal/mol was detected; this is similar in magnitude to that observed for a conformational change with the lectin concanavalin A. The pH dependence suggests that a histidine residue is involved. ESR spectroscopy shows clearly that 1 Mn2+ binds to each demetalized subunit, but that Ca2+ induces dissociation of half the Mn2+; this result is in agreement with the equilibrium dialysis studies.