ABSTRACT
Vitamin K is obtained from plant foods in the form of phylloquinone and from bacterial sources as a series of menaquinones with side chains of different length. The vitamin is essential for the activation of specific proteins involved in blood clotting (e.g., prothrombin) and bone mineralization (e.g., osteocalcin) through its role as a cofactor for g-glutamylcarboxylase. This enzyme catalyzes a unique posttranslational conversion of selected glutamate residues in the proteins to g-carboxyglutamate residues, allowing the proteins to bind calcium and thus become activated.
