High affinity interaction between a synthetic, highly negatively charged pentasaccharide and alpha- or beta-antithrombin is predominantly due to nonionic interactions.

Idraparinux is a synthetic O-sulfated, O-methylated pentasaccharide that binds tightly to antithrombin (AT) and thereby specifically and efficiently induces the inactivation of the procoagulant protease, factor Xa. In this study, the affinity and kinetics of the interaction of this high-affinity pentasaccharide with alpha- and beta-AT were compared with those of a synthetic pentasaccharide comprising the natural AT-binding sequence of heparin. Dissociation equilibrium constants, Kd, for the interactions of Idraparinux with alpha- and beta-AT were approximately 0.4 and 0.1 nM, respectively, corresponding to an over 100-fold enhancement in affinity compared with that of the normal pentasaccharide. This large enhancement was due to a approximately 400-fold tighter conformationally activated complex formed in the second binding step, whereas the encounter complex established in the first step was approximately 4-fold weaker. The high-affinity and normal pentasaccharides both made a total of four ionic interactions with AT, although the high-affinity saccharide only established one ionic interaction in the first binding step and was compensated by three in the second step, whereas the normal pentasaccharide established two ionic interactions in each step. In contrast, the affinities of the nonionic interactions (Kd approximately 450 and 90 nM for the binding to alpha- and beta-AT, respectively) were considerably higher than those for the normal pentasaccharide and the highest of all AT-saccharide interactions reported so far. The nonionic contribution to the total free energy of the high-affinity pentasaccharide binding to AT thus amounted to approximately 70%. These findings show that nonionic interactions can play a predominant role in the binding of highly charged saccharide ligands to proteins and can be successfully exploited in the design of such biologically active ligands.