Factor V (aka proaccelerin, labile factor) is a protein of the coagulation system. In contrast to most other coagulation factors, it is not enzymatically active but functions as a cofactor. Deficiency leads to predisposition for hemorrhage, while some mutations (most notably factor V Leiden) predispose for thrombosis.
The gene for factor V is located on the first chromosome (1q23). It is genomically related to the family of multicopper oxidases.
Factor V is bound to platelets and is activated by thrombin. On activation, factor V is spliced in two chains (heavy and light chain) which are nonconvalently bound to each other by calcium.
Factor Va, the activated form, is a cofactor for factor X in its reciprocal activation of prothrombin (factor II) into thrombin.
Factor Va is degraded by activated protein C, one of the principal physiological inhibitors of coagulation.
Various hereditary disorders of factor V are known. Deficiency is associated with a rare mild form of hemophilia (termed parahemophilia or Owren parahemophilia), the incidence of which is about 1:1,000,000. It inherits in an autosomal recessive fashion.
Other mutations of factor V are associated with venous thrombosis. They are the most common hereditary causes for thrombophilia (a tendency to form blood clots). The most common one of these, factor V Leiden, is due to the replacement of an arginine residue with glutamine at amino acid position 506 (R506G). All prothrombotic factor V mutations (factor V Leiden, factor V Cambridge, factor V Hong Kong) make it resistant to cleavage by activated protein C ("APC resistance"). It therefore remains active and increases the rate of thrombin generation.
It was discovered in 1947 by Dr Paul Owren (1905-1990). The complete amino acid sequence of the protein was published in 1987 by Jenny et al.
Sources: DiaPharma, Wikipedia, PubMed