An extensive interaction interface between thrombin and factor V is required for factor V activation

The interaction interface between human thrombin and human factor V (FV), necessary for complex formation and cleavage to generate factor Va, was investigated using a site-directed mutagenesis strategy. Fifty-three recombinant thrombins, with a total of 78 solvent-exposed basic and polar residues substituted with alanine, were used in a two-stage clotting assay with human FV, Seventeen mutants with less than 50% of wild-type (WT) thrombin FV activation were identified and mapped to anion-binding exosite I (ABE-I), anion-binding exosite II (ABE-II), the Leu(45)-Asn(57) insertion loop, and the Na+ binding loop of thrombin, Three ABE-I mutants (R68A, R70A, and Y71A) and the ABE-II mutant R98A had less than 30% of WT activity. The thrombin Na+ binding loop mutants, E229A and R233A, and the Leu45-Asn57 insertion loop mutant, W50A, had a major effect on FV activation with 5, 15, and 29% of WT activity, respectively. The K52A mutant, which maps to the S' specificity pocket, had 29% of WT activity. SDS-polyacrylamide gel electrophoresis analysis of cleavage reactions using the thrombin ABE mutants R68A, Y71A, and R98A, the Na+ binding loop mutant E229A, and the Leu(45)-Asn(57) insertion loop mutant W50A showed a requirement for both ABEs and the Na+-bound form of thrombin for efficient cleavage at the FV residue Arg(709) Several basic residues in both ABEs have moderate decreases in FV activation (40-60% of WT activity), indicating a role for the positive electrostatic fields generated by both ABEs in enhancing complex formation with complementary negative electrostatic fields generated by FV, The data show that thrombin activation of FV requires an extensive interaction interface with thrombin, Both ABE-I and ABE-II and the S' subsite are required for optimal cleavage, and the Na+-bound form of thrombin is important for its procoagulant activity.