Importance of the Arg-Gly-Asp Triplet in Human Thrombin for Maintenance of Structure and Function

Site-directed mutagenesis was employed to assess the importance of the Arg-Gly-Asp triplet that comprises residues 197 to 199 in the B-chain of thrombin. Properties of the R197E and the D199E variants were compared with those of ζ-thrombin and the inactive S205A variant wherein the active site Ser is replaced by Ala. Relative to ζ-thrombin, the R197E thrombin variant under the assay conditions used exhibits 26% activity toward a small chromogenic substrate, 13% activity in the activation of protein C in the presence of thrombomodulin, <3% activity in processing fibrinogen, and 1% activity in inducing platelet activation. Thus, the substrate specificity of thrombin was altered by the R197 → E replacement. The D199E variant was essentially inactive. It exhibited only 0.02% of the activity of thrombin toward the chromogenic substrate and its reactivity toward the active site-directed alkylating agent D-Phe-Pro-Arg-CH2Cl was 10,000-fold lower than that of thrombin. Like the inactive S205A thrombin variant, the D199E variant antagonized the interactions of thrombin with hirudin and thrombomodulin, but was a less effective antagonist. The dependence of the antagonism of the thrombin-thrombomodulin interaction on the concentration of D199E thrombin variant provided evidence suggesting the presence of two or more domains in thrombin that independently interact with their counterparts in thrombomodulin. Although the S205A thrombin variant antagonized the action of thrombin on platelets no such activity could be demonstrated for the D199E variant in the concentration range studied (<800 nM). Comparison of the circular dichroism spectra of ζ-thrombin, the D199E, R197E, and S205A variants indicated that subtle differences in conformation exist between the Dl 99E variant and the other thrombins. These differences in conformation might well account for the altered behavior of the D199E variant with respect to its interactions toward thrombomodulin, hirudin, and platelets.