Factor Xa Activation of Factor V Is of Paramount Importance in Initiating the Coagulation System

Abstract

Background Generation of active procoagulant cofactor FVa and its subsequent association with the enzyme FXa to form the prothrombinase complex is a pivotal initial event in blood coagulation and has been the subject of investigative effort, speculation and controversy. The current paradigm assumes that FV activation is initiated by limited proteolysis by traces of (meizo) thrombin. Methods and Results Recombinant tick salivary protein TIX-5 was produced and anticoagulant properties were studied using plasma, whole blood and purified systems. Here we report that TIX-5 specifically inhibits FXa-mediated FV activation involving the B-domain of FV and show that FXa activation of FV is pivotal for plasma and blood clotting. In line, tick feeding is impaired on TIX-5 immune rabbits displaying the in vivo importance of TIX-5. Conclusions Our data elucidate a unique molecular mechanism by which ticks inhibit the host's coagulation system. Based on our data we propose a revised blood coagulation scheme wherein direct FXa-mediated FV activation occurs in the initiation phase during which thrombin-mediated FV activation is restrained by fibrinogen and inhibitors.

Tissue factor pathway inhibitor-alpha inhibits prothrombinase during the initiation of blood coagulation

“…7A). The essential nature of FXa activation of FV during the initiation of coagulation, before thrombin-mediated FV activation, was recently demonstrated by Schuijt and coworkers using a tick salivary protein that specifically blocks this reaction (38), supporting the physiological importance of TFPIα-mediated inhibition of prothrombinase assembled with FXa-activated FVa. Additionally, Vincent and coworkers (39) have described a FV mutation that results in circulating FV that contains the acidic region but lacks the basic region, binds tightly to TFPIα, and is associated with significant bleeding.…”

TFPIα interacts with FVa and FXa to inhibit prothrombinase during the initiation of coagulation

“…3,4 FVa generated through limited proteolysis by FXa 6 and forms of FVa released from activated platelets 7,8 retain the acidic region, which remains only during the initiation phase of coagulation, as thrombin catalyzes rapid removal of the entire B-domain. 6,9 …”

Biology of tissue factor pathway inhibitor

“…The first, performed by Schuijt and colleagues, used a tick anticoagulant protein (TIX-5) that specifically inhibits FXa from activating FV to demonstrate that FXa-catalyzed activation of FV is a critical event during the initiation of blood coagulation. 130 The second, performed by Vincent and colleagues, characterized the east Texas bleeding disorder (mentioned in the "TFPI and bleeding" section), finding that the moderately severe bleeding in these patients is caused by tight binding of TFPIa to the acidic region of the B domain in the short form of FV produced in this disorder. 68 Furthermore, TFPIa-mediated For personal use only.…”