Prothrombinase is the protein complex which consist of negatively charged phospholipid (phosphatidylserine), prothrombin, Va and Xa. The prothrombinase complex converts prothrombin, an inactive protease, into thrombin which has proteolytic activity.
Assembly of the prothrombinase complex occurs on activated platelets. In normal conditions platelets do not expose negatively charged phospholipids on the surface. However binding to collagen sets a sequence of biochemical reactions in motion that activate the membrane protein scramblase. Scramblase transports negatively charged phospholipids from the inner leaflet of the membrane to the outer leaflet. The presence of negatively charged phospholipids allows the binding and the formation of the prothrombinase complex.