Vitamin K is a fat-soluble vitamin that is synthesized by the intestinal flora. It is first activated by vitamin K epoxide reductase, after which it can participate as a cofactor for the gamma-carboxylation of glutamic acid residues on various blood clotting proteins. Vitamin K plays an important role in coagulation, as there are a number of such vitamin K-dependent factors: factor II, factor VII, factor IX, factor X, protein C, and protein S.