Human Factor V Native Protein. Coagulation factor V (or Proaccelerin) is an essential factor of the blood coagulation cascade. This factor circulates in plasma, and is converted to the active form by the release of the activation peptide by thrombin during coagulation. This generates a heavy chain and a light chain which are held together by calcium ions. The active factor V is a cofactor that participates with activated coagulation factor X to activate prothrombin to thrombin. Defects in the Factor V gene can result various diseases including Factor V deficiency, thrombophilia due to activated protein C resistance, Budd-Chiari syndrome, Ischemic stroke, and recurrent pregnancy loss.