Six plasma proteins— prothrombin, factor VII, factor IX, factor X, protein C, and protein S—require vitamin K for their biosynthesis in the liver. Each contains 9 to 12 γ-carboxyglutamic acid residues in the N-terminal portion of their molecule, and these residues are formed by vitamin K-dependent carboxylation of specific glutamic acid residues. The γ-carboxyglutamic acid residues are required for the calcium-dependent binding of the vitamin K-dependent proteins to phospholipid surfaces.
Prothrombin, factor VII, factor IX, factor X, and protein C circulate in blood as precursor molecules to serine proteases. When the blood coagulation cascade is initiated by exposure to tissue factor, prothrombin, factor VII, factor IX, and factor X are converted to serine proteases by minor proteolysis. Each protease in turn then cleaves a specific protein substrate(s). These reactions eventually lead to the generation of thrombin and fibrin at the site of vascular injury. Protein C is also converted to a serine protease by minor proteolysis during the coagulation cascade. This enzyme, however, plays a regulatory role in blood coagulation in that it inactivates factor Va and factor VIIIa in the presence of protein S, which helps bring the coagulation cascade to a halt.
Reduced levels of prothrombin (MIM 176930), factor VII (MIM 227500), factor IX (MIM 306900), and factor X (MIM 227600) result in bleeding complications in patients with these deficiencies. Reduced levels of protein C or protein S, however, may be associated with thrombotic risk because the regulation of the coagulation pathway is impaired.
The genes for prothrombin, factor VII, factor IX, protein C, and protein S have been well characterized and their chromosomal locations established. Abnormalities in the genes for the vitamin K-dependent proteins have been identified and range from partial gene deletions to single nucleotide changes or deletions. These abnormalities were observed in the exons coding for the mature protein and the leader sequence, as well as in the intron-exon boundaries and regulatory regions of the genes for these proteins.
GENERAL INTRODUCTION TO HEMOSTASIS
Hemostasis in humans involves a number of plasma proteins and platelets, and their interaction with the vascular endothelium. Initially, a platelet plug is formed followed by a fibrin clot at the site of vascular injury. Platelet plug formation requires von Willebrand factor, a plasma protein that forms a bridge between the activated platelet and the subendothelium. This reaction, which is called platelet adhesion, involves a specific platelet membrane receptor (glycoprotein Ib) that binds to von Willebrand factor. Platelet adhesion is immediately followed by platelet aggregation. In this reaction, fibrinogen forms a bridge between platelets by binding one activated platelet to another. Another platelet membrane receptor, called glycoprotein IIb/IIIa, is involved in this reaction. During platelet plug formation, phospholipid is made available and the blood coagulation cascade is initiated. The precise events that trigger the coagulation cascade are not known, but it appears that tissue factor, a subendothelial-cell surface glycoprotein, plays an important role in the ...