Heparia-dependent modification of the reactive center arginine of antithrombin and consequent increase in heparin binding affinity

Antithrombin, the principal plasma inhibitor of coagulation proteinases, circulates in a form with low inhibitory activity due to partial insertion of its reactive site loop into the A-beta-sheet of the molecule. Recent crystallographic structures reveal the structural changes that occur when antithrombin is activated by the heparin pentasaccharide, with the exception of the final changes, which take place at the reactive center itself. Here we show that the side chain of the P-1 Arg of alpha-antithrombin is only accessible to modification by the enzyme peptidylarginine deiminase on addition of the heparin pentasaccharide, thereby inactivating the inhibitor, whereas the natural P-1 His variant, antithrombin Glasgow, is unaffected, indicating that only the P-1 Arg becomes accessible. Furthermore, the deimination of P-1 Arg converts antithrombin to a form with 4-fold higher affinity for the heparin pentasaccharide, similar to the affinity found for the P-1 His variant, due to a lowered dissociation rate constant for the antithrombin-pentasaccharide complex. The results support the proposal that antithrombin circulates in a constrained conformation, which when released, in this study by perturbation of the bonding of P-1 Arg to the body of the molecule, allows the reactive site loop to take up the active inhibitory conformation with exposure of the P-1 Arg.