The binding of polyvalent galactosides to the lectin Ricinus communis agglutinin 120 (RCA(120)): an ITC and SPR study

Mono-and polyvalent galactosides have been investigated with respect to their binding to the plant lectin Ricinus communis agglutinin 120 (RCA(120)). Thermodynamic parameters (K-a, Delta G, Delta H, Delta S and n) have been determined by isothermal titration calorimetry (ITC) and kinetics of binding (k(a) and k(d)) measured by surface plasmon resonance (SPR). ITC analysis using a single set of sites model found a non-statistical increase in avidity with increasing valency with the largest ligand displaying a greater than 20-fold increase in K-a compared to its monomeric precursor after correction for valency; binding was found to be enthalpically driven. SPR analysis supports the avidity increase but values of K-a observed were up to 100-fold greater than those measured by ITC. The large discrepancy between the two measurements is rationalized by the polyvalent-polyvalent interaction that is measured by SPR.