Factor IX of the Blood Coagulation System: a Review

L. D. Taran¹

¹Palladin Institute of Biochemistry, Ukrainian National Academy of Sciences, ul. Leontovicha 9, Kiev, 252030 Ukraine; fax: (044) 225-51-72.

Submitted June 10, 1996; revision submitted March 16, 1997.

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Factor IX is a factor of the blood coagulation system. Its activation occurs on the surface of phospholipid membranes. It can be activated by the factor VIIa--TF (tissue factor)--Ca²⁺ complex via an extrinsic pathway and by factor XIa in the presence of Ca²⁺ via the intrinsic pathway of blood coagulation system activation. The activated factor IXa is a serine proteinase. The main function of the activated factor IXa in complex with factor VIIIa and phospholipids in presence of the Ca²⁺ consists of the activation of factor X. Factor IX is synthesized in the liver and is subject to a number of posttranslational modifications including gamma-carboxylation, beta-hydroxylation, and glycosylation. It forms a subgroup of vitamin K-dependent plasma proteins including factors VII and X and protein C characterized by identical domain structures having high levels of homology. Factor IX consists of an NH₂-terminal Gla domain, two epidermal growth factor (EGF)-like domains, and a C-terminal domain containing Ser in its active site. Factor IX deficiency in human plasma results in the disease known as hemophilia B.

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KEY WORDS: factor IX, blood coagulation system, activation, enzyme--cofactor complex, proteolysis, domain structure, hemophilia.

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