Ace Hemostasis Coagulation Quiz 2026 – Unleash Your Inner Clotting Pro!

Protein C, activated to APC, works with its cofactor protein S to dampen coagulation by proteolytically cleaving the cofactors Va and VIIIa. These cofactors are essential for thrombin generation: Va is part of the prothrombinase complex that converts prothrombin (II) to thrombin (IIa), and VIIIa is part of the tenase complex that helps activate X to Xa, promoting thrombin production. By inactivating Va and VIIIa, APC markedly reduces thrombin formation and the amplification of clotting. Other coagulation enzymes like IIa, Xa, XIa, or XIIa aren’t targeted in this pathway, so their activity isn’t curtailed in the same way. The cooperation with protein S enhances APC’s ability to recognize and proteolyze Va and VIIIa on phospholipid surfaces. Thus, the inactivation targets Va and VIIIa.