factor Va

Factor V is synthesized in the liver and circulates as an inactive precursor. It is activated to Factor Va primarily by thrombin (and to a lesser extent by Factor Xa) through proteolytic cleavage, creating a heterodimer of heavy and light chains linked by calcium ions. Factor Va functions as a non-enzymatic cofactor that binds to Factor Xa on phospholipid membrane surfaces (typically activated platelets), forming the prothrombinase complex. This complex massively accelerates thrombin generation, which is essential for fibrin clot formation and platelet activation.

Clinically, Factor Va is central to the Protein C anticoagulant pathway: activated Protein C (APC), with Protein S as a cofactor, degrades Factor Va (and Factor VIIIa) by cleaving it at specific arginine residues (Arg306, Arg506, Arg679), thereby downregulating thrombin generation and limiting clot propagation.

Factor V Leiden is a classic high-yield mutation (Arg506Gln) that renders Factor Va resistant to inactivation by APC, leading to APC resistance. This is the most common inherited hypercoagulable (thrombophilia) disorder in Caucasians, increasing risk for venous thromboembolism (DVT/PE), especially with additional risk factors (OCP use, pregnancy, surgery, immobility).

Factor V deficiency (Owren disease) is a rare autosomal recessive bleeding disorder causing prolonged PT and PTT due to impaired common pathway function.

Understanding Factor Va is essential for grasping coagulation cascade regulation, thrombophilia workups, and the mechanism of various anticoagulants and bleeding/clotting disorders tested on USMLE Step 1 and Step 2.