Blood Coagulation Factor IX: Structure, Function, and Regulation

ABSTRACT Blood coagulation factor IX plays a crucial role in the intrinsic pathway of coagulation by generating factor Xa, ultimately leading to thrombin formation. Over the past 50 years, extensive research has deepened our understanding of the biology, physiology, pathology, biochemistry, and molecular genetics of factor IX. This wealth of knowledge has revealed how the factor IX gene and protein evolved, how factor IX is regulated, how it functions within the coagulation cascade, and how structural changes affect its function. In this review, we will summarize current knowledge on the biology of factor IX, with a focus on its structure–function relationships, gene structure, and regulation.