 Prothrombinase complex

The prothrombinase complex is a membrane-associated assembly that converts prothrombin into thrombin. Its catalytic enzyme is factor Xa, assisted by the protein cofactor factor Va, calcium ions, and a phospholipid surface. The protease and its assembled reaction system are distinct: factor Va helps organize efficient prothrombin conversion without replacing the catalytic role of factor Xa.^[ref]

Experiments in 1975 combined purified coagulation components and used chromatography to separate the material formed. Tests with specific antibodies helped identify the factors in the collected fractions, and kinetic measurements established that the assembled preparation converted purified prothrombin into thrombin. These supplied different kinds of evidence: association of components, and catalytic activity.^[ref]

Pretreating Factor V with trace amounts of thrombin increased the subsequent rate and yield of thrombin production. This illustrates positive feedback: a product of the reaction helps activate a component that promotes further product formation. A single observed rate increase would not establish complex composition; the fractionation and identification experiments addressed that separate question.^[ref]