Vitamin K

Vitamin K is not a single compound but a family of fat-soluble vitamins essential for normal blood-clotting function and comprises two vitamers that are found naturally: phytomenadione (also known as phylloquinone or K1) and menaquinone (or K2).

Menaquinone (K2) is synthesized by normal flora in the intestine although the amount produced in vivo in the human gut is likely negligible.

Physiology

The specific molecular mechanism responsible for the action of vitamin K is well-understood. It acts as a cofactor for gammaglutamate carboxylase (GGCX), an endoplasmic reticulum enzyme, found in many human (and mammalian) cells. Vitamin K accelerates the γ-glutamyl carboxylation of glutamate residues on its target proteins to form γ-carboxyglutamate (Gla). These target compounds are therefore called Gla proteins .

Historically, the only Gla proteins known were the clotting factors II, VII, IX and X, hence the importance of vitamin K for normal function of the coagulation pathway. However many other Gla proteins are now known, including osteocalcin (important for osteogenesis), matrix-Gla protein (important for the antagonism of calcification in the body), and the coagulation molecules, protein C and protein S .

Related pathology

• hypovitaminosis K (vitamin K deficiency)
  • ​leads to jaundice and anemia in neonates
  • bleeding tendency in adults
• toxicity to natural vitamin K has not been found, even with massive supraphysiological doses
  • however menadione, a synthetic water-soluble form of vitamin K (a.k.a. K3) which is not found in nature has been banned due to toxicity at higher doses