Vitamin K is a group of structurally similar, fat-soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone (3-) derivatives. This group of vitamins includes two natural vitamers: vitamin K1 and vitamin K2.
Vitamin K1, also known as phylloquinone, phytomenadione, or phytonadione, is synthesized by plants, and is found in highest amounts in green leafy vegetables because it is directly involved in photosynthesis. It may be thought of as the "plant form" of vitamin K. It is active in animals since animals can easily convert it to vitamin K2.
Vitamin K2, the form used by enzymes in animals, has several subtypes, which differ in isoprenoid chain length. These vitamin K2 homologs are called menaquinones, and are characterized by the number of isoprenoid residues in their side chains. Menaquinones are abbreviated MK-n, where M stands for menaquinone, the K stands for vitamin K, and the n represents the number of isoprenoid side chain residues. For example, menaquinone-4 (abbreviated MK-4) has four isoprene residues in its side chain. Menaquinone-4 (also known as menatetrenone from its four isoprene residues) is the most common type of vitamin K2 in animal products since it is normally directly synthesized by desaturation from vitamin K1 in certain animal tissues (arterial walls, pancreas, and testes). It derives its four isoprenoid residues from the fact that phylloquinone/vitamin K1 from plants has sufficient carbon atoms (20) in its side chain to construct four isoprene units, thus yielding menaquinone-4. This homolog of vitamin K2 is responsible for the classic functions of vitamin K in animals, including its activity in the production of blood clotting proteins.
Bacteria in the colon (large intestine) can also convert K1 into vitamin K2. In addition, bacteria typically lengthen the isopreneoid side chain of vitamin K2 to produce a range of vitamin K2 forms, most notably the MK-7 to MK-11 homologs of vitamin K2. All forms of K2 other than MK-4 can only be produced by bacteria, which use these forms in anaerobic respiration. The MK-7 and other bacteria-derived form of vitamin K2 exhibit vitamin K activity in animals, but MK-7's extra utility over menaquinone-4 (MK-4), if any, is unclear and is presently a matter of investigation.
Three synthetic types of vitamin K are known: vitamins K3, K4, and K5. Although the natural K1 and all K2 homologs have proven nontoxic, the synthetic form K3 (menadione) has shown toxicity.
Read more about Vitamin K: Discovery of Vitamin K1, Subtypes of Vitamin K2, Chemical Structure, Physiology, Vitamin K Absorption and Dietary Need, Recommended Amounts, Anticoagulant Drug Interactions, Sources, Deficiency, Toxicity, Vitamin K Injection in Newborns, Vitamin K and Bone Health, Vitamin K and Alzheimer's Disease, Vitamin K Used Topically, Vitamin K and Cancer, Vitamin K As Antidote For Poisoning By 4-hydroxycoumarin Drugs, Vitamin K in Rats, History of Discovery