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

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Abstract
Conversion of human α-thrombin to γ-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 × 103 and 7 ± 0.5 × 103 M−1 s−1 for α- and γ-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 α-thrombin to γ-thrombin. In addition, γ-thrombin failed to displace α-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 α-thrombin to γ-thrombin and/or by the resulting conformational changes.