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.