Vitamin K in Humans: Nutritional Aspects and Functions

Although vitamin K (VK) was discovered decades ago and its fundamental function in humans as a cofactor required for γ-carboxylation of VK-dependent proteins is recognized, significant progress in the research on this lipid-soluble vitamin has been made only recently. For example, the molecular structure of γ-glutamyl carboxylase (GGCX) from humans has been resolved, as well as that of the cyanobacterial vitamin K epoxide reductase (VKOR) homologue, thereby providing structural information for all enzymes of the VK epoxide cycle that are engaged in carboxylation of VK-dependent γ-carboxyglutamic acid (Gla) proteins. These proteins are responsible for the best-known functions of VK in blood clotting and the prevention of osteoporosis, as well as vascular calcification. Moreover, it has recently been shown that all organisms, from bacteria to mammals and higher plants, share common, phylogenetically related VKOR-type enzymes engaged in the formation of disulfide bridges in various target proteins. The intriguing function of UBIAD1 prenyltransferase is presented and discussed, suggesting a specific role for MK-4 in humans, although such a function has not yet been identified. The nutritional aspects of VK are described, along with numerous reports of its positive effects, mostly unresolved at the molecular level, such as those related to cellular signaling, regulation of gene expression or the prevention of many diseases.