[33] Vitamin K-Dependent Plasma Proteins

This chapter discusses vitamin K-dependent plasma proteins—prothrombin, blood clotting factors X, IX, VII, and protein C. It provides generalizations about these proteins with regard to their biological roles and discusses the role of γ-carboxyglutamic acid and new methods for the study of structure–function relationships. The isolation of this family of proteins uses barium citrate or barium sulfate adsorption. This simple process produces preparations in nearly quantitative yields that are virtually free of other plasma proteins. The vitamin K-dependent plasma proteins display a close sequence homology in the portions of their structures. The sequence data for the amino-terminal 42 residues are highly homologous, which suggests that the genetic material for these segments is descended from a common ancestral gene. The only known function of γ-carboxyglutamic acid is to serve in calcium binding. Calcium binding by the plasma proteins is necessary for subsequent binding to appropriately constituted phospholipid membranes where the activation of coagulation cascade occurs.