The fifth cupredoxin domain of coagulation factor VIII and similar proteins
Factor VIII functions in the factor X-activating complex of the intrinsic coagulation pathway. It facilitates blood clotting by acting as a cofactor for factor IXa. In the presence of Ca2+ and phospholipids, Factor VIII and IXa form a complex that converts factor X to the activated form Xa. A variety of mutations in the Factor VIII gene can cause hemophilia A, which typically requires replacement therapy with purified protein. Factor VIII is synthesized as a single polypeptide with six cupredoxin domains and a domain structure of 1-2-3-4-B-5-6-C1-C2, where 1-6 are cupredoxin domains, B is a domain with no known structural homologs and is dispensible for coagulant activity, and C are domains distantly related to discoidin protein-fold family members. Factor VIII is initially processed through proteolysis to generate a heterodimer consisting of a heavy chain (1-2-3-4) and a light chain (5-6-C1-C2), which circulates in a tight complex with von Willebrand factor (VWF). Further processing of the heavy chain produces activated factor VIIIa, a heterotrimer composed of polypeptides (1-2), (3-4), and the light chain. This model represents the cupredoxin domain 5 of unprocessed Factor VIII or the first cupredoxin domain of the light chain of circulating Factor VIII, and similar proteins.
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