The function of Vitamin K is achieved through its role in a carboxylation reaction that converts glutamate (Glu) residues in prothrombin to gamma-carboxylglutamate residues (Gla). These modified residues are often found in the Gla domains of certain proteins involved in calcium binding, which are referred to as Gla proteins. Currently, there are sixteen Gla proteins known to exist in the body and these are described below.

• Gla proteins involved in blood coagulationinclude prothrombin, factors VII, IX and X, protein C, protein S, and protein Z.
• Gla proteins involved in bone metabolisminclude osteocalcin, matrix Gla protein, periostin and Gla-rich protein.
• A Gla protein known to be involved in vascular biology is growth arrest-specific protein 6. This protein acts as a growth factor, either by preventing apoptosis or by activating a receptor tyrosine kinase and stimulating the proliferation of cells.
• Other Gla proteins that have been discovered but have unknown functions include transmembrane g-carboxyl glutamyl proteins 3 and 4 and proline rich g-carboxy glutamyl proteins 1 and 2.

In all cases where the function of Gla proteins is known, the proteins have been found to be essential to bodily functions. Without vitamin K, bleeding would become uncontrolled, bones would weaken (particularly after menopause) and the arteries and other soft tissues would be susceptible to calcification and atherosclerosis, particularly in diabetics and patients with chronic renal disease.