BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.2. Fat-soluble vitamins

8.2.5. Vitamin K group

Structure and physicochemical properties. Vitamin K is a generic term for a group of compounds that includes phylloquinone and related derivatives with a modified side chain (Fig. 8.7). Animal Tissues are capable of synthesizing isoprenoid chains, but they cannot synthesize 1,4-naphthoquinone.

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Fig. 8.7. Vitamin K₁

Vitamin K₁ (phylloquinone) is a light yellow liquid, unstable upon exposure to light and heating in an alkaline environment, which is synthesized in plants. K₂ (menaquinone) is a yellow crystalline substance synthesized by microorganisms, particularly the human intestinal microflora. Vitamin K₂ exists in several forms that differ in the length of their isoprenoid side chains. In addition to natural Vitamins, synthetic naphthoquinone derivatives with biological activity are also known. In 1943, a disulfide derivative, vikasol, was synthesized. It is used in medical practice as a Water-soluble substitute for vitamin K.

Biochemical Functions. Vitamin K is involved in the γ-carboxylation of glutamic acid residues in the precursors of inactive Blood clotting factors—prothrombin, proconvertin, Christmas factor, and Stuart factor—which is essential for normal Blood Coagulation. The resulting γ-carboxyglutamic acid is able to bind Ca²⁺, a process required for blood clotting (Fig. 8.8). This process is inhibited by vitamin K antagonists, such as coumarin derivatives, which are used in the Treatment and Prevention of thrombosis.

Fig. 8.8. Carboxylation reaction involving vitamin K

Furthermore, vitamin K is involved in the carboxylation of glutamic acid residues in osteocalcin, one of the primary calcium-binding Proteins in Bone tissue.

Dietary requirements and natural occurrence. Inadequate intake of vitamin K leads to subcutaneous and intramuscular hemorrhages resulting from a decreased blood clotting rate. K avitaminosis is characterized by a reduced prothrombin level in the blood. However, vitamin deficiency is quite rare because it is produced by the intestinal microflora. Hypovitaminosis and avitaminosis can also occur when intestinal Lipid Absorption is impaired.

Vitamin K is abundant in green plant tissues, such as spinach, cabbage, nettles, and tomatoes. It also accumulates in the Liver of animals. It is difficult to establish a precise daily requirement; however, it typically ranges from 1 to 2 mg.

Table 8.1

Fat-soluble vitamins