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.
Highlights
Autoproteolysis of blood coagulation factor Xa (FXa) results in the excision of a 4-kDa fragment (-peptide) from the intact subform, factor Xa␣ (FXa␣), to yield factor Xa (FXa)
In the current study we evaluated the kinetics of FXa generation, which showed that autoproteolysis of FXa␣ followed a second order mechanism where FXa␣ and FXa behaved as identical enzymes
A mechanism describing the autoproteolytic conversion of FXa␣ to FXa was derived
Summary
(22) in 20 mM HEPES, 150 mM NaCl, 2 mM CaCl2, pH 7.2 (HBS) at 37 °C in the presence of a 100-fold molar excess of the covalent inhibitor, EGRck (EGR-FXa) or 5 mM of the noncovalent inhibitor, pAB The EGR-FXa␣ was loaded onto a DEAE-Cellulose (Whatman) column equilibrated in the same buffer to remove the RVV and was eluted by including 1.0 M NaCl. The RVV was separated from the pAB-FXa␣ in the same way except that all buffers contained 5 mM pAB, and the activation mixture was diluted 15-fold in start buffer and loaded directly onto DEAE-Cellulose. The protein peak that eluted from DEAE-cellulose was concentrated by precipitation with 75% saturated ammonium sulfate and resuspended in a minimum volume of HBS (containing 5 mM pAB for the pAB-FXa␣ preparation). FXa were dialyzed into HBS containing 80 M pAB at 4 °C
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