vitamin k2 vitamin k2 vitamin k2 or menaquinone (mk) (/ˌmɛnəˈkwɪnoʊn/) is one of three types of vitamin k, the other two being vitamin k1 (phylloquinone) and k3 (menadione). k2 is both a tissue and bacterial product (derived from vitamin k1 in both cases) and is usually found in animal products or fermented foods. general structure of vitamin k2 (mk-n) the number n of isoprenyl units in their side chain differs and ranges from 4 to 13, hence vitamin k2 consists of various forms. it is indicated as a suffix (-n), e. g. mk-7 or mk-9. the most common in the human diet is the short-chain, water-soluble menatetrenone (mk-4), which is usually produced by tissue and/or bacterial conversion of vitamin k1, and is commonly found in animal products. it is known that production of mk-4 from dietary plant vitamin k1 can be accomplished by animal tissues alone, as it proceeds in germ-free rodents. however, at least one published study concluded that "mk-4 present in food does not contribute to the vitamin k status as measured by serum vitamin k levels. mk-7, however significantly increases serum mk-7 levels and therefore may be of particular importance for extrahepatic tissues." long-chain menaquinones (longer than mk-4) include mk-7, mk-8 and mk-9 and are more predominant in fermented foods such as natto. longer-chain menaquinones (mk-10 to mk-13) are produced by anaerobic bacteria in the colon, but they are not well absorbed at this level and have little physiological impact. when there are no isoprenyl side chain units, the remaining molecule is vitamin k3. this is usually made synthically, and is used in animal feed. it was formerly given to premature infants, but due to inadvertent toxicity in the form of hemolytic anemia and jaundice, it is no longer used for this purpose. k3 is now known to be a circulating intermediate in the animal production of mk-4. description