The membrane-bound Va:Xa (prothrombinase) complex rapidly activates large amounts of thrombin. Factor Xa (aka Factor X heavy chain), a cleavage product of coagulation factor X (F10), is a vitamin K-dependent glycoprotein able to convert prothrombin to thrombin during the blood clotting process (Mann et al. 1988, Orfeo et al. 2004). Factor Xa is a target for direct oral anticoagulant (DOAC) drugs that are direct factor Xa inhibitors (the so-called 'xabans') and used in the treatment and prevention of thromboembolic disorders (Galanis et al. 2014).