The binding of polyvalent galactosides to the lectin Ricinus communis agglutinin 120 (RCA120): 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, ΔG, ΔH, ΔS 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.