Introduction
Hemophilia A, the most common of the severe, inherited bleeding disorders, results from a deficiency or defect in the plasma protein factor VIII. The activated form of the protein serves as an essential cofactor for factor IXa in the conversion of factor X to factor Xa. This surface-bound complex of enzyme and cofactor is referred to as the intrinsic factor Xase. Factor VIIIa dramatically increases the catalytic rate constant for substrate conversion by an unclear mechanism. The activity and stability of the factor Xase appears to be regulated by the integrity of the cofactor. Factor VIIIa possesses a labile structure, and subunit dissociation results in the decay of Xase activity. Furthermore, factor VIIIa is a substrate for proteolytic inactivation by several enzymes, including factor IXa, the enzyme for which it serves as a cofactor. Although interest in the structure, function, and metabolism of factor VIII is commensurate with its biochemical and clinical importance, the molecular basis for its role in coagulation and the regulation of function through complex intramolecular and intermolecular interactions remain poorly understood.
