Coagulation factor V is a cofactor that participates with factor Xa to activate prothrombin to thrombin. Inhibited by SERPINA5. Factor Va, the activated form of factor V, is composed of a heavy chain and a light chain, non-covalently bound. The interaction between the two chains is calcium-dependent. Forms heterodimer with SERPINA5 (By similarity). Domain B contains 41 X 9 AA tandem repeats. Domains C1 and C2 may be involved in membrane binding. 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). Belongs to the multicopper oxidase family.