Identification of critical molecular interactions mediating heparin activation of antithrombin: implications for the design of improved heparin anticoagulants.

The serpin, antithrombin, and its polysaccharide activator, heparin, are essential anticoagulant regulators of blood-clotting cascade proteases and thereby critical for maintaining hemostasis. The relative importance of the molecular interactions that mediate heparin binding to and activation of antithrombin, and the dynamics of how they are established, have recently been revealed from the effects of mutagenesis of heparin-binding residues of antithrombin and of modifications of the specific pentasaccharide-binding region in heparin. One residue, Lys 125, is pivotal for antithrombin to recognize and bind the nonreducing-end trisaccharide of the pentasaccharide in an initial low-affinity complex. Two other residues, Lys 114 and Arg 129, then cooperate with Lys 125 to induce the low-affinity complex into an activated, high-affinity complex, in which a network of electrostatic interactions between antithrombin and the entire pentasaccharide is established. The identification of three critical basic residues in antithrombin and a trisaccharide in heparin as principal mediators of heparin activation of antithrombin may stimulate the design of small-molecule anticoagulants that mimic the action of heparin and are orally active.