Contribution of Residue 192 in Factor Xa to Enzyme Specificity and Function

Mutation of residue 192 (chymotrypsin numbering) from Glu to Gln in thrombin and activated protein C has been shown to dramatically alter substrate and inhibitor specificity, in large part by allowing these enzymes to accept substrates with acidic residues in the P3 and/or P3' positions. In factor Xa, residue 192 is already a Gln. We now compare the properties of a Q192E mutant of Gla-domainless factor X (GDFX). Kinetic analysis of prothrombin activation indicates similar affinity of factor Va for GDFXa and GDFXa Q192E (Kd(app) = 3.6 and 3.7 microM, respectively). Prothrombin activation rates are similar for both enzymes with factor Va, but are approximately 10-fold slower for the Q192E mutant without factor Va. This defect is in the activation of prethrombin 2 and is corrected by factor Va only in the presence of fragment 2. Without factor Va, fragment 2 has no influence on bovine prethrombin 2 activation by GDFXa, but fragment 2 enhances prethrombin 2 activation by the Q192E mutant at least 10-fold. These results indicate that the fragment 2 domain of prothrombin probably alters the conformation of the prethrombin 2 domain, selectively improving its presentation to GDFXa Q192E. With respect to inhibition, tissue factor pathway inhibitor and bovine pancreatic trypsin inhibitor are > or = 30 times poorer inhibitors of GDFXa Q192E than of GDFXa, but these enzymes are inhibited at comparable rates by antithrombin. These results indicate that Gln-192 in factor Xa is a key determinant of substrate/inhibitor specificity.