Figure 1.

Role of factor V (FV) on blood coagulation and effect of FV Leiden (FVL). Thrombin (IIa) cleaves FV at R709, R1018, and R1545 to activate FV (FVa). FVa together with factor Xa, forms the prothrombinase complex, which promotes prothrombin (FII) conversion to thrombin on the platelet surface. Thrombin generates the fibrin clot and also activates FXIII, which by means of its transglutaminase activity crosslinks fibrin fibers. Moreover, thrombin also activates the thrombin activatable fibrinolysis inhibitor (TAFI). These two effects contributed to the strength and resistance of the clot. Cleavage of FVa at R506 by activated protein C (APC) and protein S (PS) initiates the inactivation of FVa (FVi). Patients with FV Leiden (FVL) are resistant to FVa inactivation. Therefore, FVL activated has a longer half-life in plasma that results in increased thrombin generation. FV can also be cleaved by APC at R506, resulting in a molecule that acts as an anticoagulant (FVac) by stimulating APC- and PS–mediated inactivation of FVIIIa. FVL carriers also have this anticoagulant mechanism. Both FVL effects certainly contribute to the increased risk of venous thrombosis. However, it is necessary to identify any possible relationship of these two FVL-mediated mechanisms with a potential reduced embolization risk that could explain the minor incidence of pulmonary embolism in FVL-carriers.