Protein cofactors play an important role in blood coagulation. The coagulation cascade is triggered by exposure of tissue factor to circulating fVIIa. Tissue factor is a typical cofactor, since it is required for the expression of fVIIa activity, but has no catalytic activity on its own. The TF-fVIIa complex activates fVII, fIX and fX, kickstarting the proteolytic cascade that ultimately leads to formation of a blood clot and cessation of bleeding.
Factors V and VIII are large proteins (>300,000 Da) that circulate as inactive cofactors. Just like zymogens, these ‘procofactors’ are activated to fVa and fVIIIa by a proteolytic event. Thrombin cleaves off an inhibitory B-domain, allowing the enzymes and substrates to bind. Factor VIIIa binds to fIXa on membrane surfaces, and this ‘intrinsic Xase complex’ efficiently converts fX to fXa. Deficiency in either component of the Xase complex results in haemophilia. Similarly, fVa binds to fXa on cell surfaces, and the prothrombinase complex efficiently converts prothrombin to thrombin.
Thrombomodulin (TM) is a complex type-1 integral membrane protein, composed of an N-terminal lectin-like domain, six EGF-like domains, a heavily glycosylated Ser-Thr rich region, a single transmembrane helix and a short cytosolic region. EGF-domains 5 and 6 bind with high affinity to exosite I of thrombin, and the addition of the 4th EGF domain is sufficient to confer cofactor activity with respect to protein C activation. TM can also accelerate thrombin activation of thrombin-activatable fibrinolysis inhibitor (TAFI), and has other functions outside of coagulation.