Poster Board II-98
The coagulation cascade culminates in the production of thrombin from prothrombin by the prothrombinase complex (factor Xa (fXa), factor Va (fVa), Ca2+, on a membrane surface). Once enough thrombin is produced to stop bleeding when there is an injury to the vasculature, down-regulation of the coagulation cascade begins. Thrombin is a procoagulant (promotes clotting by aiding in the formation of the fibrin plug) and also an anti-coagulant (initiates the down-regulation of the coagulation cascade). The anti-coagulant function of thrombin is to activate protein C (PC), to its active form, activated protein C (APC). APC down regulates the coagulation cascade by inactivating the protein cofactors, fVa and factor VIIIa (fVIIa), following proteolysis. APC inactivates factor Va following proteolytic cleavages at Arg306, Arg506, and Arg679. APC also inactivates fV following cleavages at Arg306, Arg506, Arg679 and Arg994. In order to examine the molecular mechanism by which APC down-regulates fV/Va in the presence of a membrane surface, recombinant fV molecules were constructed with R → Q mutation at the APC cleavage sites as follows: fV306 (R306Q), fV506 (R506Q), and fV306/506 (R306Q and R506Q). We first analyzed the fV and fVa recombinant molecules for clotting activity. Clotting times were recorded before the addition of a membrane surface and APC and following 15 and 30 minutes of APC/membrane incubation. Following 15 minutes of APC inactivation in the presence of a membrane surface, fVaiWT and fViWT had no clotting activity. This data demonstrate that the procofactor and activated cofactor behave similarly during inactivation. FVi306 behave like the wild type molecule. Conversely, fV506 and fVa506 have comparable clotting times when exposed to APC and a membrane surface. These recombinant molecules retain partial clotting activity even following prolonged incubation with APC. These data demonstrate that cleavage at Arg306 has minimal consequences on clotting activity. FVa306/506 is not susceptible to APC inactivation as evidenced by unchanging clotting times following extended incubation with APC and a membrane surface. We next assayed the recombinant molecules for their capability to promote thrombin formation in an assay using purified reagents and a chromogenic substrate. Prothrombinase assembled with factor Vai506 or factor Vai306/506, were found to have Km values close to prothrombinase assembled with factor VaWT. These results suggest that cleavage at Arg306 does not have an effect on binding the substrate of prothrombinase. Prothrombinase assembled with factor VaiWT has a 40-fold reduction in the kcat, while prothrombinase assembled with factor Vai506had only a 2-fold reduction in the catalytic efficiency of prothrombin activation as compared to prothrombinase assembled with factor VaWT. Prothrombinase assembled with factor Vai306 had similar kcat value as the prothrombinase assembled with fVaWT. Our data demonstrate that 1) cleavage at Arg506 doesn't completely inactivate the cofactor, and 2) the activity of the corresponding cofactor molecule depends on the assay used. In a clotting assay, where there is limiting amounts of fVa, the effect of the mutations is more pronounced. In contrast, when cofactor activity is measure in an assay using saturating concentrations of fVa to determine thrombin formation, the effect of the Arg506 cleavage appears less important than cleavage at Arg306. In addition, our data also demonstrate that regardless of the assay, in the absence of the APC inactivation cleavage sites at Arg306 and Arg506, the procofactor and active cofactor molecules are unable to be proteolytically inactivated by APC in the presence of a membrane surface.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.