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Structural Biochemistry/Chemistry of important organic molecules in Biochemistry/Vitamin K

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Vitamin K is a fat-soluble compound that plays an essential role in the physiology of mammals. Its primary function pertains to the blood-clotting process. Vitamin K has an aromatic structure that undergoes oxidation-reduction during the formation of prothrombin, a protein that is crucial in blood clotting. Prothrombin converts fibrinogen to fibrin, which holds blood clots together. Deficiency in vitamin K slows blood clotting, which could ultimately lead to death.[1]

Vitamin K is a group of fat-soluble vitamins. The letter, “K,” derives from its German name, Koagulationsvitamin, which signifies its ability to help blood clot or coagulate. Not only does it contribute to blood clotting, some studies have also proven its capability of maintaining strong bones in the elderly. All Vitamin K group members are composed of a methylated naphthoquinone ring, which is also known as menadione. Vitamin K1, phylloquinone, contains four isoprenoid residues, in which one is unsaturated, in its side chain. Vitamin K2, menaquinone, can be composed of varying numbers of unsaturated isoprenoid residues.

Shown is Vitamin K1, or phylloquinone, which is composed of a methylated naphthoquinone ring. The naphthoquinone ring has been widely accepted as a functional group that gives rise to the function and distinctive nature of all vitamin K group. In this phyloquinone, a phytyl side chain is also presented
Vitamin K2, also known as menaquinone, comprises of varying numbers of unsaturated of isoprenoid residues. Similar to vitamin K1, a methylated naphthoquinone ring is presented. The common numbers of isoprenoid resides are usually 4 and 7, giving rise to MK-4 and MK7, respectively.

Vitamin K can be categorized into two natural vitamers: vitamin K1 and vitamin K2. Because they vitamin K1 and vitamin K2 are both 2-methyl-1,4-naphthoquinone (3-) derivatives, they have similar metabolic pathways during reactions. Overall, vitamin K contains a functional naphthoquinone ring and an aliphatic side chain. Vitamin K1 has a phytyl side chain attached to the homolog while vitamin K2 are composed of menaquinones that are made up of varying numbers of isoprenoid residues. In 1929, Henrik Dam, a Danish scientist, first discovered vitamin K1 while he performed an experiment to study the role of cholesterol. He gave chickens a cholesterol-deplete diet and found out that these livestock developed hemorrhages. This symptom cannot be reversed even if purified cholesterol was added back to the diet. Later, he found out that it was because coagulation vitamin was missing in the diet. This coagulation vitamin became essential to animal health, and was later identified to be vitamin K1.

Vitamin K1, also known as, phylloquinone, is synthesized by green plants. It is found primarily within photosynthetic tissues of plant such as, lettuce, cabbage, collard greens, broccoli, spinach, and turnip greens. Not only involved in plants, vitamin K1 can also be active within animal tissues although it operates in a completely different way in terms of its biochemical reactions.

Vitamin K2 can be classified into several menaquinone forms, or MK-n, where n signifies the number of 5-carbon units. The common forms of vitamin K2 are MK4 and MK7. While MK4 is generally found in meat or dairy products, MK7 is present is fermented, natural products such as, cheese or natto. Through transformation of vitamin K1 in the testes, pancreas and arterial walls, MK4 can be produced and synthesized. In animals, vitamin K2 can be synthesized in the large intestine by bacteria. This synthesis pathway, however, is commonly carried out in germ-free rats, and is proven to be gut bacteria-independent. Studies have also shown that that the amount of MK4 present is cell tissues has direct influence over the conversion of K1 into MK4. While scientists are still attempting to better understand the production of MK7, some have reported that it can be synthesized from vitamin K1 in the colon with the help of E-coli.

Understanding of how vitamin K can contribute to blood clotting is crucial for severe bleeding can be life threatening. In many proteins, gamma-carboxylation of specific glutamic acid residues allows them to bind to calcium ions (Ca2+). The main cascade of coagulation is formed by Factor II, VII, IX, and X primarily. These factors are proteins specialized in binding to calcium ions. The activated form of Factor II can be enhanced when protein Z is involved in the mechanism since protein Z stimulates a higher interaction between the phospholipids in cell membranes. To counterbalance this blood clotting mechanism, protein C, protein Z, and protein S can be occasionally utilized to control the coagulation cascade for they are both anticoagulant proteins. In face, Vitamin K factor originates from liver. Therefore, a healthy liver is essential in delivering and carrying out the complete function of vitamin K and its coagulation cascade. In addition, this is why liver disorders can give rise to problematic vitamin K-dependent clotting formation, resulting in severe hemorrhage.

Besides its indispensable contribution to blood clotting formation, vitamin K also helps bone development. Studies have shown that higher levels of vitamin K in human bodies can furnish higher bone density, providing better structure and higher level of rigidness. Osteoporosis is a common disorder in which patients whose levels of vitamin K do not meet the standard. Osteoporosis patients, especially postmenopausal women, tend to develop fragile bone structures that can easily contribute to bone fractures.

Other than Vitamin K’s two natural vitamers, vitamin K1 and vitamin K2, synthetic forms of vitamin K are also available in the market. Vitamin K1 is a common U.S. supplement customers can obtain from pharmacies. It can be found in tablet capsule, and liquid forms, depending on the preference of the customer. A healthy, average diet do not generally lack in vitamin K. Vitamin K deficiency is rather rare in health adults. As mentioned earlier, postmenopausal women and newborn infants are at a higher risk of deficiency. Those who suffer from liver disorders can also face higher likelihood of developing vitamin K deficiency. Beside hemorrhage, common symptoms of vitamin K deficiency also includes anemia, bruising, bleeding of the gum of nose, and heavy menstrual bleeding in women.

References

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  1. ↑ Cox, Michael M. and Nelson, David L. Principles of Biochemistry. 5th ed. New York: W.H. Freeman, 2008. Print.

2. "Disclaimer." University of Maryland Medical Center. University of Maryland Medical Center, 2011. Web. 22 Nov. 2012. <http://www.umm.edu/altmed/articles/vitamin-k-000343.htm>.

3. Higdon, Jane. "Linus Pauling Institute Micronutrient Research for Optimum Health." Linus Pauling Institute at Oregon State University. N.p., 19 Dec. 2011. Web. 22 Nov. 2012. <http://lpi.oregonstate.edu/infocenter/vitamins/vitaminK/>.

4. NG, ClueBot. "Vitamin K Deficiency." Wikipedia. Wikimedia Foundation, 22 Nov. 2012. Web. 22 Nov. 2012. <w:en:Vitamin K deficiency>.

5. Nordqvist, Christian. "What Is Vitamin K?" Medical News Today. MediLexicon International, 22 Mar. 2011. Web. 22 Nov. 2012. <http://www.medicalnewstoday.com/articles/219867.php>.

6. Vitamin K." Vitamin K. American Cancer Society, 21 Oct. 2010. Web. 22 Nov. 2012. <http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/herbsvitaminsandminerals/vitamin-k>.

7. Vitamin K." Vitamin K. American Cancer Society, 21 Oct. 2010. Web. 22 Nov. 2012. <http://www.cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/herbsvitaminsandminerals/vitamin-k>.

8. Wikipedia. Wikimedia Foundation, 27 Dec. 1997. Web. 22 Nov. 2012. <http://en.wikipedia.org/wiki/File:Phylloquinone_structure.svg>.

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