**1. Introduction**

Vitamin K occurs naturally in two biologically active forms. Vitamin K1, also called phyllo‐ quinone (PK), is abundant in leafy green vegetables, such as cabbage, spinach, and lettuce [1]. The other form, vitamin K2, is called menaquinone (MK) and is predominantly of microbial origin [2, 3]. Vitamin K2 is mainly present in fermented food such as cheese and natto (fermented soybeans), but gut microbiota are also able to synthesize vitamin K2 [4]. One exception, menaquinone‐4 (MK‐4), is formed in humans and animals by tissue‐specific

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

conversion of PK and/or menadione [5]. However, in the literature, all MKs are mostly grouped under the term vitamin K2 resulting in the assumption that all MKs are similar in origin and function. Moreover, despite the knowledge that MKs are present in the food supply, little is known about their individual synthesis, growth conditions, and interactions of the producing bacteria and the total amounts of the different MKs in fermented foods. Regarding the findings that MKs play an important role in health aspects beyond coagulation, study of the interaction of MKs with other nutrients may lead to a better understanding of the effect of different food items on health aspects, such as bone health or cardiovascular health.

oxidation [12]. Menaquinones are also necessary for sporulation and proper regulation of cytochrome formation in some Gram‐positive bacteria such as *Lactobacillus subtilis* [13]. In particular, the food industry uses various lactic acid bacteria (LAB) as starter cultures to produce fermented milk products, meat products, and vegetables. As many LAB lack a heme biosynthesis pathway, which results in an incomplete electron transport chain, the addition of menaquinone to the media facilitates aerobic growth, improves yield, and reduces production

Menaquinones, Bacteria, and Foods: Vitamin K2 in the Diet

http://dx.doi.org/ 10.5772/63712

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Menaquinone synthesis has mostly been described in *Escherichia coli*, *Mycobacterium phlei*, and *Bacillus subtilis*. In *E. coli*, chorismate from the shikimate pathway is converted into the naphthoquinone ring by six enzymes (MenFDHCEB) [11, 15]. The isoprenoid side chain is synthesized separately and is joined to the naphthoquinone ring to form demethylmenaqui‐ none. Prenylation and methylation catalyzed by polyprenyltransferase (MenA) and methyl‐ transferase (MenG) are the last steps of the synthesis of menaquinone [16, 17]. An alternative pathway, called the futalosine pathway, was described in microorganisms that lack *men* genes. In this pathway, chorismate is converted to menaquinone with four enzymes encoded by *mqnABCD* genes and unknown enzymes [17–19]. The majority of the bacteria containing the classical menaquinone pathway are obligately or facultatively aerobic, and the majority of menaquinone in anaerobic bacteria is synthesized via the futalosine pathway [17]. For example, the metabolic pathway of *Lactoccocus lactis*, which is used as a cheese starter, can function through aerobic and anaerobic reactions, and the *men* genes for the synthesis of menaquinone

Menaquinones have side chains of different sizes in different organisms and sometimes even within the same organism. Depending on the growing conditions, the basic structure can be modified by demethylation of the naphthoquinone ring to reform DMK or by saturation of the

Bacterially synthesized menaquinones that contribute to human vitamin K2 requirements may be produced by the gut microbiota or by bacteria present in food. In humans, the most important genera of intestinal flora are *Bacteroides* and *Bifidobacteria*. However, only *Bacteroides* can synthesize menaquinone. The major forms produced by *Bacteroides* are MK‐10 and MK‐11. MK‐6 produced by *Eubacterium lentum*, MK‐7 produced by *Veillonella*, and MK‐8 produced by *Enterobacter* were also found in isolates from intestinal flora [2, 7, 8, 20]. Most menaquinones are present in the distal colon, but the most promising site of absorption is the terminal ileum, where there are menaquinone‐producing bacteria and bile salts that are needed for solubili‐ zation of menaquinones [7, 21]. Therefore, although intestinal microflora synthesize large amounts of menaquinones, the bioavailability of bacterial menaquinone is poor, and diet is the major source of functionally available vitamin K2 [3, 7, 8]. Recent studies also showed that a short‐term decrease in dietary vitamin K intake is not compensated by intestinal menaqui‐

costs [14].

were detected in its genome.

isoprenoid side chain [2, 19].

nones [22–24].

**4. Non-dietary sources of menaquinones**

Such a global view could be essential for guiding the development of dietary intake recom‐ mendations for vitamin K.
