Vitamin K is a structurally a napthoquinone. Several compounds share the biological activity of vitamin K. These compounds are phylloquinones, menaquinones and menadiones. Phylloquinones are derived from plants, menaquinones are derived from bacteria, and menadione is a synthetic form of the vitamin. Vitamin K deficiency is rare and so generally the role of vitamin K in human nutrition has been neglected by the nutritional science. The most well known role for vitamin K in human nutrition is its function in the clotting of blood, where vitamin K takes part in the carboxylation of glutamic acid residues in a process described as the vitamin K cycle. Here vitamin K is required for the synthesis of the blood coagulation proteins prothrombin, as well as factors II, VII, IX and X. However, more recently a role for vitamin K in correct bone metabolisms has also been extensively researched. As with blood clotting proteins, vitamin K is involved with the carboxylation of glutamic acid residues on the bone proteins osteocalcin (bone GLA protein; BGP) and matrix GLA protein (MGP). Another function of vitamin K might be the carboxylation of glutamic acid residues in the kidney Gla protein (KGP) in the cortex of the kidney.