The gamma-carboxyglutamic acid and epidermal growth factor-like domains of factor X. Effect of isolated domains on prothrombin activation and endothelial cell binding of factor X

Factor Xa is the enzymatically active constituent of the prothrombinase complex, which catalyzes the conversion of prothrombin to thrombin. We have isolated fragments, from tryptic digests of factor X, that consists of the gamma-carboxyglutamic acid (Gla) region linked to one or two epidermal growth factor (EGF)-like domains. Calcium ion binding measurements indicated that these fragments have a native conformation. The factor X-GlaEGF fragments inhibit factor Xa-induced blood clotting in a manner suggesting that they compete with factor Xa for phospholipid binding sites. The same conclusion was reached when thrombin generation was studied in a system of purified components (factor Xa, factor Va, prothrombin, phospholipid, and Ca2+). There was no evidence for a strong interaction between the EGF-like domains of factor Xa and factor Va in either system. However, experiments in the purified system without phospholipid indicated a direct, albeit weak, interaction between the Gla region of factor Xa and factor Va and between the COOH-terminal EGF-like domain of factor Xa and factor Va. Using domain-specific Fab fragments, we have confirmed that the conformation of the serine protease region alters dramatically upon activation of factor X. Furthermore, we have demonstrated that the conformation of the Gla region is affected by the activation, whereas the EGF-like domains appear to be unaltered. The association constant for factor X binding to endothelial cells was two orders of magnitude lower than that for binding of factor IX to these cells. Binding of the Gla and GlaEGF fragments suggested Gla-mediated binding to phospholipid rather than binding to a specific receptor.