Prothrombin activator is a complex enzyme (components within red circle) required for the 'final common pathway' leading to the formation of a blood clot. There are two pathways to prothrombin activator; in this pathway all components required are present in--intrinsic to-- the blood.
Blood contains a variety of compounds essential for clotting. They were assigned roman numerals in the order of their discovery. Most are produced by the liver in an inactive form represented as unshaded hexagons. Activated factors are represented as shaded hexagons.
The presence of a platelet plug is essential to begin this pathway. Platelet factor III, the yellow complex on the surface of platelets, activates factor XII by binding with it. Active factor XII binds with and activates factor XI. Active XI, in the presence of calcium ions, binds with and activates factor IX. Factor VIII attaches to the growing chain of factors but is only fully activated after thrombin begins to form. (Factor VIII is the factor that is deficient of missing in hemophelia.) Factor X, the main component of 'prothrombin activator', is activated when it attaches to the growing chain. Factor V is also required for the activation of factor X but, like factor VIII, it is only fully activated in the presence of thrombin as indicated by the solid arrow.
The combination of platelet factor III (yellow complex), calcium ions, active factor X (shaded hexagon), and factor V (shaded hexagon) constitutes 'prothrombin activator' (encircled in red). Active factor X is the actual enzyme that will convert prothrombin to thrombin; the other components are referred to as cofactors.
Last update: 7/19/2005