Factor V

Factor V is a coagulation factor whose activated form, factor Va, is a protein cofactor in the prothrombinase complex. Limited proteolysis, including cleavage by thrombin, converts Factor V into the more active cofactor. In the assembled complex, factor Xa performs the cleavage of prothrombin to thrombin; factor Va helps the reaction occur efficiently on a phospholipid surface in the presence of calcium ions.^[ref]

The distinction between precursor and activated cofactor matters experimentally. Investigators mixed purified coagulation factors, separated the products by gel chromatography, and identified components with specific antibodies. Pretreating Factor V with small amounts of thrombin increased the rate and yield of subsequent thrombin generation. A little product could therefore promote production of more product: a positive feedback mechanism within coagulation.^[ref]

A useful reaction sketch is “Factor V becomes factor Va; factor Xa plus factor Va helps convert prothrombin into thrombin”. The two activated factors have different jobs: adding a cofactor changes the performance of a catalytic assembly without making the cofactor itself the protease.