Vitamin K Action

The importance of vitamin K to coagulation is well known as it activates coagulation factors synthesized in the liver. Vitamine K is an essential cofactor for the posttranslational conversion of glutamate (Glu) residues into gamma-carboxyglutamate (Gla). In these reactions the reduced form of vitamin K (KH2 – hydroxyquinone or quinol) de-protonates glutamate via gamma-glutamylcarboxylase enzyme. The epoxide (KO) formed is recycled via vitamin K epoxide reductase and quinone reductase, and glutamic acid containing proteins such as coagulation factors II, VII, IX and X, protein C, and protein S, are carboxylated.

Vitamin K Cycle

Carboxylation of vitamin K-dependent proteins conveys ability to bind calcium ions, which is essential for their biological activity. Compared to other vitamin K analogues, vitamin K2 has the most potent gamma-carboxylation activity. Elevated levels of undercarboxylated osteocalcin (OC), a vitamin K-dependent protein involved in bone metabolism, may result from subclinical vitamin K deficiency and are frequently observed in the elderly. It is hypothesized that an inadequate dietary intake of vitamin K may similarly result in undercarboxylation of vascular matrix Gla protein (MGP) leading to enhanced calcification of atherosclerotic lesions and, consequently, an increased risk of coronary heart disease.