title: Kinetics of blood coagulation factor Xaalpha autoproteolytic conversion to factor Xabeta. Effect on inhibition by antithrombin, prothrombinase assembly, and enzyme activity.
abstract: Autoproteolysis of blood coagulation factor Xa (FXa) results in the excision of a 4-kDa fragment (beta-peptide) from the intact subform, factor Xaalpha (FXaalpha), to yield factor Xabeta (FXabeta). In the preceding paper, we showed that generation of FXabeta leads to expression of a plasminogen binding site. FXabeta may consequently participate in fibrinolysis; therefore, the timing of subform conversion compared with thrombin production is important. In the current study we evaluated the kinetics of FXabeta generation, which showed that autoproteolysis of FXaalpha followed a second order mechanism where FXaalpha and FXabeta behaved as identical enzymes. Rate constants of 9 and 172 M-1 s-1 were derived, respectively, in the absence and presence of FXaalpha binding to procoagulant phospholipid. Under identical conditions the latter is estimated to be 6 orders of magnitude slower than thrombin generation by prothrombinase. Since heparin binding and prothrombin recognition have been previously attributed to a region of FXaalpha proximal to the beta-peptide, functional comparisons were conducted using homogeneous and stabilized preparations of FXaalpha and FXabeta. Comparisons included 1) the recognition of small substrates; 2) the rate of interaction with antithrombin/heparin; 3) the assembly of prothrombinase; and 4) the activation of prothrombin by prothrombinase. Although the beta-peptide neighbors a probable functional region in FXaalpha, conversion to FXabeta was not observed to influence these functions. The data support a model where FXaalpha is predominantly responsible for thrombin generation and where slow conversion to FXabeta coordinates coagulation and the initiation of fibrinolysis at sites of prothrombinase assembly.
authors.affiliation: Research Department, The Canadian Red Cross Society, Ottawa, Ontario K1G 4J5, Canada.
authors.forename: E L
authors.initials: EL
authors.lastname: Pryzdial
authors.forename: G E
authors.initials: GE
authors.lastname: Kessler
mesh_terms: D000990:Antithrombin III / Q000494:pharmacology*; D015951:Factor Xa / Q000096:biosynthesis / Q000378:metabolism*; D065427:Factor Xa Inhibitors; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D013379:Substrate Specificity; D013925:Thromboplastin / Q000378:metabolism*
chemical_list: D065427:Factor Xa Inhibitors; D000990:Antithrombin III; D013925:Thromboplastin; D015951:Factor Xa