Vitamin K is a group of compounds that share a similar structure, namely a 2-methyl 1,4-napthoquinone ring, with an isoprenoid chain. Phylloquinones (vitamin K1) are derived from plants, and some evidence suggest that they are the most important contributor to dietary intakes. In contrast, menaquinones (vitamin K2) are synthesised mainly by bacteria. A third form of vitamin K, menadione (vitamin K3), is a synthetic form that is inactive unless alkylated by enzymes in tissues. The isoprenoid chain length of phylloquinone is generally fixed and contains 4 isoprenoid residues, with one unsaturated bond. However, the menaquinone chain length can vary, altering the chemical, physical and biological properties of the variant. The designated numbers of the individual menaquinones are based on the length of isoprenoid chains giving, menaquinone 4 (MK-4) for example containing 4 isoprenoid residues, and menaquinone 7 (MK-7) containing 7 isoprenoid residues.
Animals are able to synthesis MK-4 (also called menatetrenone) from phylloquinone, suggesting that humans may also be able to undertake this conversion. Some evidence suggests that MK-4 can affect gene regulation and may be more biologically active that phylloqinone. In contrast, the longer chain menaquinones are synthesised almost exclusively by bacterial fermentation and controversy surrounds their biological activity in humans. Good food sources of MK-7 include food that have undergone bacterial fermentation such as fermented cheese (aged goat cheese, blue cheese, brie, cheddar, cultured dry cottage cheese and parmesan) and natto (a Japanese food made from fermented soybeans). Intestinal bacteria can synthesis menaquinones which is absorbed in lymph along with other fats. The bacteria Bacteroides is able to synthesis MK-10 to MK-13, Enterobacter can synthesis MK-8, Veillonella can synthesise MK-7 and Eubacterium lentum can synthesise MK-6.
Vitamin K is unlike most other vitamins because it has a single known function is humans. Reduced vitamin K is a co factor for the enzyme γ-glutamylcarboxylase which is located in the endoplasmic reticulum of various issues. This enzyme is required for the posttranslational carboxylation of specific glutamic acid residues on a number of vitamin K dependent proteins. This γ-carboxylation causes the binding of calcium, facilitates proper folding of the γ-carboxylglutamate residues and allows their binding to the cell membrane. This process is necessary for the formation of 4 of the 13 factors required for normal blood clotting. It is also required for the binding of osteocalcin to hydroxyapetite in bone formation. The fact that deficiency of vitamin K derived from insufficient dietary intake in normal healthy adults is rare, suggests that menaquinones from gut bacteria are an important source of vitamin K.
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