Central regulator of hemostasis. It serves as a critical cofactor for the prothrombinase activity of factor Xa that results in the activation of prothrombin to thrombin (By similarity). Heterodimer of a light and a heavy chains; non-disulfide-linked. The interaction between the two chains is calcium-dependent (By similarity). Plasma; synthesized in the liver. Thrombin activates factor V proteolytically to the active cofactor, factor Va (formation of a heavy chain at the N-terminus and a light chain at the C-terminus). Activated protein C inactivates factor V and factor Va by proteolytic degradation. Belongs to the multicopper oxidase family.