protrombinasecomplex

Prothrombinase complex, also called the prothrombin activator complex, is a multiprotein enzyme assembly responsible for converting prothrombin (factor II) to thrombin (factor IIa) in hemostasis. It forms on phospholipid surfaces, especially negatively charged membranes of activated platelets, in the presence of calcium ions. The core components are activated factor X (Xa) and activated factor V (Va). Xa is the protease; Va is a non-enzymatic cofactor that substantially increases the rate of prothrombin activation by bringing prothrombin into the Xa active site.

Formation: Xa is generated from factor X by the tissue factor–VIIa complex (extrinsic pathway) or by the intrinsic tenase complex (IXa-VIIIa). Va arises from factor V when cleaved by thrombin or Xa. In the prothrombinase complex, Xa and Va assemble on a phospholipid surface with Ca2+, anchoring the complex through the Gla domains to the membrane.

Function: The prothrombinase complex cleaves prothrombin to thrombin. Thrombin then converts fibrinogen to fibrin and activates platelets and other coagulation factors, amplifying the coagulation cascade.

Regulation and clinical relevance: Activity is localized to sites of injury and regulated by anticoagulant pathways, including antithrombin and the protein C pathway. Deficiencies or mutations affecting the components can lead to bleeding or thrombotic disorders; a well-known example is factor V Leiden, which impairs inactivation of Va by activated protein C, increasing thrombosis risk.