Abstract
In prokaryotes, vitamin K2 (menaquinone) transfers two electrons in a process of aerobic or anaerobic respiration. Respiration occurs in the cell membrane of prokaryotic cells. Electron donors transfer two electrons to menaquinone (MK). Menaquinone in turn transfers these electrons to an electron acceptor. Menaquinones are vital for the electron transport chain. In the spectrum of Gram‐positive bacteria and Mycobacterium spp., vitamin K2 serves as the only quinone molecule in their electron shuffling systems. Hence, the bacterial enzymes associated with biosynthesis of the menaquinone(s) serve as potential target molecules for the development of new antibacterial drugs. This chapter summarizes the effects of vitamin K2 in bacteria and describes in more detail the aspects of menaquinone in bacterial electron transport in general, while also featuring the discoveries of menaquinone biosynthesis inhibitors.
Keywords
- vitamin K1
- vitamin K2
- menaquinone
- menaquinone biosynthesis
- electron transport systems
- ATP biosynthesis
- Gram‐positive bacteria
- Mycobacterium tuberculosis
- antibacterial agents
1. Introduction
Vitamin K is a lipid‐soluble vitamin that facilitates the process of blood clot formation. Henrick Dam and Edward Doisy were co‐awarded the Nobel Prize in 1943 for the discovery of vitamin Ks and the discovery of their structure. Naturally existing forms of vitamin K, vitamin K1 (i.e., phylloquinone), and vitamin K2 (i.e., menaquinone) can be found in the human liver (some 10% of phylloquinone of the total vitamin K contents), as well as in other tissues in rather low amounts; vitamin K1 accumulates in the liver, while vitamin K2 is effectively distributed to other body organs [1]. Phylloquinone stems from the dietary intake, while menaquinones are synthesized by bacteria of the intestine. However, no direct evidence can be established for the biological effects of menaquinones in humans; however, it is surmised that menaquinones mainly are utilized in the synthesis of blood‐clotting factors upon the depletion of phylloquinone [2]. Additionally, menaquinones have proven to be more efficient than phylloquinone as a bioactive molecule in processes such as osteroclast differentiation, lowering of blood cholesterol levels, as well as the slowing down of atherosclerotic progression.
The menaquinones have proven to be important in several biological reactions, for example electron transport, active transport, oxidative phosphorylation, as well as endospore formation in bacteria. Furthermore, variations in the inherent molecular structures of the menaquinones and their skewed distributions among bacterial strains are considered as an important marker in bacterial taxonomy [3]. The biosynthesis of menaquinone has been subject to considerable attention in the quest for drug targets displaying multidrug resistance toward Gram‐positive pathogenic microorganisms, including
2. Vitamin K: general molecular structures
There are two naturally occurring forms of vitamin K. Plants and some cyanobacteria synthesize phylloquinone, which is also known as vitamin K1. Bacteria synthesize a range of vitamin K, but not vitamin K1, using the different lipophilic side chains derived from isoprene (5‐carbon) units. Bacterial vitamin K(s) are designated menaquinone‐n (MK‐n), where “n” stands for the number of isoprenoid (5‐carbon) units. Vitamin K1 and K2 share the common molecular structure, belonging to the 2‐methyl‐1,4‐naphthoquinone system, and they appear to be differ structurally in their number of isoprene units within their side chain, as well as their degree of unsaturation [4] (Figure 1). Menaquinones, displaying side chains containing up to 15 isoprene units have been identified. For instance, MK‐8 is predominantly seen in
3. Vitamin K in humans
The role of vitamin K as a cofactor in blood coagulation stems from the post‐translational modification of a number of plasma proteins such as factors II, VII, IX, X, proteins C, S, Gla (γ‐carboxyglutamic acid) proteins has been well‐documented. In addition to the essential role of vitamin K in the blood‐clotting cascade, the potential role in the increase of bone mass [5, 6], antioxidant mechanisms [7], the biosynthesis of cholesterol and steroid hormones [8], and anticancer effects have been reported [9, 10].
Vitamin K is taken up by organs such as the liver and bones, but abundantly distributed in other organs such as the brain, the kidneys, and gonadal tissues [11]. However, the exact role of vitamin K in the present tissues is not well described. The distribution of vitamin K moieties is varying, which depends on the molecular structure of the side chain. As for humans, vitamin K1 is distributed to all tissues and organs, with relatively large amounts to the liver, the heart, and the pancreas (some 10.6, 9.3, 28.4 pmol/g wet tissue weight, respectively). However, low levels (<2 pmol/g) were measured in the brain, kidney, and lung tissue specimens. Menaquinone‐4 (MK‐4) also seems to be distributed to a plethora of other tissues, that is, its levels exceed the levels of vitamin K1 detected in the brain and kidneys (2.8 ng/g), which equals to that found in the pancreas. However, some organs, such as the liver, heart, and lung remain low in terms of MK‐4 contents. The less known MK‐6 ∼ 11 menaquinone species are also found in the liver, while only small amounts of MK‐6 ∼ 9 are detected in organs such as the heart and pancreas. Finally, the total amount of vitamin K reported in human plasma was in the range of 0.47 ∼ 1.19 nmol/L [1, 12].
Vitamin K can be absorbed well from diet; however, total vitamin K levels are depend greatly on the gut or digestive health. The intestinal bacteria (i.e., microbiota) influence human nutrition and metabolism in diverse ways. The gut microbiota produces menaquinone; thus, it is considered that vitamin K deficiency is quite rare for healthy humans. In addition, the vitamin K1 is absolutely abundant in leafy and salad vegetables, and herbs. It is easy to understand that newborn babies are born with a vitamin K deficiency, giving newborns a vitamin K injection upon birth or oral vitamin K drops is established to prevent bleeding or a hemorrhagic disease development. In general, vitamin K deficiency results from extremely inadequate intake of fat (malabsorption) or use of coumarin anticoagulants.
4. The role of vitamin K2 in electron transport system
Function of ubiquinone (Q) (coenzyme Q10) as a component of the mitochondrial respiratory chain in human is well established (“the chemiosmotic theory,” Mitchell, 1978). In prokaryotes, especially in Gram‐positive bacteria, vitamin K2 (menaquinone) will transfer two electrons in a process of aerobic or anaerobic respiration. Respiration occurs in the cell membrane of prokaryotic cells. Electron donors transfer two electrons to menaquinone. Menaquinone in turn transfer these electrons to an electron acceptor. Schematic electron flow mediated by menaquinone in
Protons are translocated across the cell membrane (from the cytoplasm to the periplasmic space) concomitantly. Synthesis of Adenosine triphosphate (ATP) from ADP and phosphate is coupled with protons move through these complexes (Figure 2). Therefore, CoQ10 and menaquinone occupy a central and essential role in Adenosine triphosphate (ATP) synthesis [13, 14]. From the taxonomic studies, it is evident that the majority of Gram‐positive bacteria including
5. Biosynthesis of menaquinone
Menaquinones play an important role in electron transport, and oxidative phosphorylation. In addition, they are responsible for active transport, and endospore formation in some
Menaquinones are the predominant isoprenoid lipoquinones of Gram‐positive bacteria, whereas Gram‐negative bacteria and enterobacteria use menaquinone (MK), demethylmenaquinone (DMK), and ubiquinone (Q) in their electron transport chains (Figure 2). Recent studies have shown that several γ‐proteobacteria appear to share the similar electron transport system to that observed in
6. Antibacterial drug discovery by targeting menaquinone biosynthesis
Menaquinone is the sole quinone in the electron transport chain in the majority of Gram‐positive bacteria including
7. Menaquinone biosynthesis inhibitors
7.1. MenA inhibitors
Among the menaquinone biosynthesis enzymes, MenA is a membrane‐associated protein that catalyzes prenylation of demethylmenaquinone (DMK), forming 1,4‐dihydroxy‐2‐naphtoate (DHNA). Analyses of the amino acid sequence of MenA were revealed that MenA displays five transmembrane segments, and that there exists highly conserved aspartate (D), which would be localized to the inner‐plasma membrane, which was being predicted by the aid of a prediction program (Sosui) [14]. The activity is totally dependent on the presence of divalent cations, such as Mg2+. It is therefore likely that these divalent cations produce ion pairs with Asp residues contained within the catalytic site within MenA. A library of DMMK mimics possessing the amino group(s) were generated and evaluated in an enzymatic assay
7.2. MenB inhibitors
One of at least seven enzymes in menaquinone biosynthesis, MenB (1,4‐dihydroxy‐2‐naphthoyl‐CoA synthase) forms the bicyclic ring system by catalyzing the Dieckmann type reaction of
7.3. MenD inhibitors
MenD (2‐succinyl‐5‐enolpyruvyl‐6‐hydroxy‐3‐cyclohexadiene‐1‐carboxylate synthase) catal-yzes a thiamin diphosphate‐dependent decarboxylative carboligation of α‐ketoglutarate and isochorismate via a Stetter‐like conjugate addition. MenD is also essential for menaquinone biosynthesis in some bacteria and has been recognized as an antibacterial drug target. A succinylphosphonate ester, 4‐(methoxyoxidophosphoryl)‐4‐oxobutanoate (8) was reported to be a competitive inhibitor of MenD (Ki values ∼700 nM) [35]. An analog of the cofactor, thiamine diphosphate, oxythiamine 9 was reported to exhibit MenD enzyme and
7.4. MenE inhibitors
MenE (
8. Conclusion
Bacterial Adenosine triphosphate (ATP) synthase, F1F0‐ATPase, is a viable target for treatment of
Recently, a narrow‐spectrum antibiotic, siamycin I was reported to kill
Acknowledgments
I thank CORNET award (University of Tennessee Health Science Center) for generous financial support.
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