It is a human vitamin, lipophilic (i.e. soluble in lipids) and therefore hydrophobic (i.e. insoluble in water). It is needed in the synthesis of proteins required for blood coagulation.
Normally it is produced by bacteria in the intestines, and dietary deficiency is extremely rare unless the intestines are heavily damaged.
Vitamin K is involved in the formation of calcium-binding groups in proteins. These calcium-binding groups are called Gla-residues, and the proteins containing these residues are designated as Gla-proteins. The Gla-residues are essential for the biological activity of all Gla-proteins. At this time fewer than 12 human Gla-proteins have been discovered, and they play key roles in the regulation of three physiological processes:
Table of contents |
2 Discovery of Gla-proteins 3 Structures of K-vitamins |
In the late 1920s, Danish scientist Henrik Dam investigated the role of cholesterol by feeding chickens with a cholesterol-depleted diet. After several weeks, the animals developed hemorrhages and started bleeding. These defects could not be restored by adding purified cholesterol to the diet. It appeared that - together with the cholesterol - a second compound had been extracted from the food, and this compound was called the coagulation vitamin. The new vitamin received the letter K because the initial discoveries were reported in a German journal, in which it was designated as Koagulations Vitamin.
For several decades the vitamin K-deficient chick model was the only method of quantitating of vitamin K in various foods: the chicks were made vitamin K-deficient and subsequently fed with known amounts of vitamin K-containing food. The extent to which blood coagulation was restored by the diet was taken as a measure for its vitamin K content.
The precise function of vitamin K was not discovered before 1974, when the vitamin K-dependent coagulation factor prothrombin was isolated from cows which had received a high dose of the vitamin K-antagonist warfarin. It was shown that normal prothrombin contained 10 unusual amino acid residues which were identified as g-carboxyglutamate (abbreviation: Gla). Prothrombin isolated from warfarin-treated cows had normal glutamate at the Gla-positions, and was designated as descarboxyprothrombin. The extra carboxyl group in Gla made clear that vitamin K plays a role in a carboxylation reaction during which Glu is converted into Gla.Discovery of vitamin K
Figure 1: Chemical structures of vitamin K1 (phylloquinone, left structure) and vitamin K2 (menaquinones, right structure). Both contain a functional naphthoquinone ring and an aliphatic side chain. Phylloquinone has a phytyl side chain, whereas in menaquinone the side chain is composed of a varying number of isoprenoid residues. |