Lysine residue 114 in human antithrombin III is required for heparin pentasaccharide-mediated activation

Recombinant native antithrombin III (ATIII) and two genetic variants with glutamine substitutions at lysine residues 114 and 139 were expressed in insect cells using a baculovirus-driven expression system. The purified proteins were used to evaluate the potential role(s) of these residues in the pentasaccharide-mediated activation of ATIII. The second order rate constants for the inhibition of factor Xa by both of the genetic variants were nearly identical to those of recombinant native ATIII, indicating that the glutamine substitutions did not result in serious protein conformational changes. The glutamine substitution at lysine 139 had no effect on the pentasaccharide-mediated activation of ATIII toward factor Xa. In contrast, lysine 114 was found to be critical in the activation of ATIII toward factor Xa. No activation was observed, even at a pentasaccharide concentration 10 times higher than that required to activate recombinant native ATIII. These data are the first to demonstrate a pivotal role for lysine 114 in the pentasaccharide-mediated activation of ATIII.