**7. Conclusions**

**6. Vitamin K2 as a target for inhibition**

270 Vitamin K2 - Vital for Health and Wellbeing

stood.

MKs are clearly critical components of many aspects of the growth and proliferation of bacterial and human cells, but most of the enzymes necessary for their biosynthesis are only bacterially encoded and are missing from humans. MK biosynthesis would appear to be an ideal target for the development of small molecule inhibitors as potent antibiotics. Among pathogenic bacteria, *Mtb* poses one of the most significant threats, as it accounts for nearly two million deaths annually. While combinatorial antibiotic therapies have been developed against *Mtb*, serious complications have arisen that compromise the efficacy of these treatments. In addition to excessive length of treatment, the side effects of these drugs can be debilitating, and antibiotic resistance has arisen at a startling rate. In addition, *Mtb* can remain for long periods of time in a dormant state in which traditional antibiotics are not effective. However, even in this quiescent state, *Mtb* requires an active electron transport chain to maintain adequate levels of ATP, and MKs therefore play a key role [84]. To this end, researchers have developed screens specifically targeting the MK biosynthetic pathway of *Mtb*. Early results show that compounds‐targeting MenE show some promise [85], and the prenylating enzyme MenA is also being developed as a target [86, 87]. Most strikingly, one MenA inhibitor (allylaminomethanone‐A) was shown to be up to 320‐times more effective in killing non‐ replicating *Mtb* than first line drugs currently prescribed for infection [84], and MenA inhibi‐ tors have been shown to inhibit growth of *Mtb* resistant to commonly used antitubercular drugs [86]. Caution must be exercised in advancing such therapies, however, as the full scope of vitamin K metabolism in the body has not been elucidated. If gut bacteria do contribute significantly to vitamin K stores in the body, then inhibitors targeting MK biosynthesis may have significant effects on blood coagulation and bone calcification, for example. MenA inhibitors are particularly noteworthy, since off‐target effects on the human homolog UBIAD1 could potentially disrupt a number of cellular processes that are only beginning to be under‐

As an inhibitor of vitamin K‐dependent reactions, warfarin has long been used as an anticoa‐ gulant that at least in part targets human VKOR. While the mycobacterial VKOR has been shown to be sensitive to warfarin, the amount necessary to inhibit the bacterial enzyme is orders of magnitudes higher than the amount needed to prevent blood coagulation [74]. This would suggest that while the human and bacterial VKORs can perform similar functions and do so by similar mechanisms, the divergence in the amino acid sequence of the two is significant enough that treatment of mycobacterial infection with anticoagulants would not be an effective therapeutic strategy. However, ferulenol, an anticoagulant, shown to be approximately 20‐fold more potent against human VKOR than warfarin, showed similar potency against the VKOR from *Synechococcus* [75]. It is therefore possible that drug discovery efforts to identify novel anticoagulants may impact the search for inhibitors of bacterial VKOR and vice versa.

Disulfide bond formation appears to be dispensable for *in vitro* aerobic growth of *E. coli* and other bacteria, although many virulence factors absolutely require disulfide bonds for proper assembly and function. Bacteria disrupted in the DSB pathway are rendered less virulent, and *E. coli* cannot grow anaerobically, suggesting that small molecule inhibitors of DsbB‐ or VKOR‐ Since their incorporation into the electron transfer pathways of ancient microbes, menaqui‐ nones have become a cornerstone of redox‐dependent reactions in almost every domain of life. Their ability to interact with a large variety of proteins, to readily accept and donate electrons, and to easily move within biological membranes have combined to make MKs flexible and efficient molecular wires. As such, organisms have evolved to integrate MKs into many metabolic processes, thus plugging into previously untapped sources of power. While researchers have seemed to only scratch the surface of the myriad uses for MKs to this point, further investigation will yield not only fascinating insights into the biochemical pathways critical to life, but may be a crucial starting point for the development of therapies designed to protect and enhance those pathways.

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