Interaction of human α‐thrombin and γ‐thrombin with antithrombin III, protein C and thrombomodulin

Conversion of human alpha-thrombin to gamma-thrombin by limited proteolysis resulted in a decrease in the inactivation rate of the enzyme by antithrombin III. The second-order rate constants were similar but significantly different: 11 +/- 1.7 X 10(3) and 7 +/- 0.5 X 10(3) M-1 s-1 for alpha- and gamma-thrombin respectively. This difference is probably related to a slight change in reactivity of the catalytic site, rather than to a structural alteration of the recognition site for antithrombin III. The rate of protein C activation, measured in the absence of thrombomodulin, was greatly reduced by conversion of alpha-thrombin to gamma-thrombin. In addition, gamma-thrombin failed to displace alpha-thrombin from its complex with thrombomodulin, as demonstrated by measuring either the rate of protein C activation by thrombin-thrombomodulin, or the fibrinogen clotting activity of thrombin-thrombomodulin, in the presence of competing diisopropylphospho-thrombin. It is concluded that the recognition sites involved in protein-C-thrombin and thrombomodulin-thrombin interactions are both dramatically affected by the loss of peptide material occurring during the conversion of alpha-thrombin to gamma-thrombin and/or by the resulting conformational changes.