In order to identify the forces involved in the binding and to understand the mechanism involved, equilibrium and kinetic studies were performed on the binding of the winged bean acidic lectin to human erythrocytes. The magnitudes of DELTA-S and DELTA-H were positive and negative respectively, an observation differing markedly from the lectin-simple sugar interactions where DELTA-S and DELTA-H are generally negative. Analysis of the sign and magnitudes of these values indicate that ionic and hydrogen bonded interactions prevail over hydrophobic interactions resulting in net - ve DELTA-H ( - 37.12 kJ. mole-1) and + ve DELTA-S (14.4 J. mole-1 K-1 at 20-degrees-C), thereby suggesting that this entropy driven reaction also reflects conformational changes in the lectin and/or the receptor. Presence of two kinds of receptors for WBA II on erythrocytes, as observed by equilibrium studies, is consistent with the biexponential dissociation rate constants (at 20-degrees-C K1 = 1.67 x 10(-3) M-1 sec-1 and K2 = 11.1 X 10(-3) M-1 sec-1). These two rate constants differed by an order of magnitude accounting for the difference in the association constants of the two receptors of WBA 11. However, the association process remains monoexponential suggesting no observable difference in the association rates of the lectin molecule with both the receptors, under the experimental conditions studied. The thermodynamic parameters calculated from kinetic data correlate well with those observed by equilibrium. A two-step binding mechanism is proposed based on the kinetic parameters for WBA II-receptor interaction. The first step is diffusion controlled followed by the second step involving fitting of the receptor and the binding site of the lectin, which proceeds through a highly ordered activation state.
