BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.2. Fat-soluble vitamins

8.2.1. General pathways of fat-soluble vitamin biosynthesis

Fat-soluble Vitamins include vitamins A, D, E, and K. The direct precursor in the synthesis of fat-soluble vitamins is the prenyl group of isopentenyl pyrophosphate, which is formed from acetyl-CoA. The presence of an isoprenoid Structure determines the fat-soluble nature of these vitamins. Although The Human Body is capable of synthesizing such structures, it cannot synthesize the complete vitamin entirely on its own.

The synthesis pathway is shown in Fig. 8.1. First, mevalonic acid is synthesized from three molecules of acetyl-CoA and is subsequently converted into isopentenyl pyrophosphate. The latter undergoes isomerization to dimethylallyl pyrophosphate. The polymerization of prenyl residues leads to The formation of A wide variety of BIOLOGICALLY ACTIVE SUBSTANCES. This process can yield such diverse compounds as Terpenes, carotenoids, high-polymer rubber, and others. Terpenes are predominantly found in plants and include compounds such as menthol, thymol, camphor, and carvone. Diterpenes serve as the basis for the synthesis of Gibberellins and farnesyl Abscisic acid.

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Fig. 8.1. Pathway of fat-soluble vitamin precursor synthesis

One of the polymerization pathways involves the "HEAD-to-head" Condensation of two polyprenyl (isoprenoid) derivatives. This is precisely how the precursors of carotenoids (phytoene) and sterols (squalene) are synthesized.

Phytoene is an acyclic molecule, a tetraterpene containing 40 carbon atoms, 9 double bonds, and 8 methyl groups. Through stepwise desaturation, phytoene is converted into lycopene, the red pigment of tomatoes, which already contains 13 double bonds (Fig. 8.2).

Fig. 8.2. Carotenoid synthesis pathway

The enzymatic cyclization of the two Terminal Groups of lycopene leads to the synthesis of β-carotene. The oxidation of lycopene terminal groups yields xanthophylls or lutein. The polymerization of three isoprene fragments produces farnesyl pyrophosphate, followed by squalene. The cyclization of squalene, catalyzed by squalene epoxidase (oxidosqualene cyclase), leads to the formation of lanosterol, and subsequently Cholesterol and 7-dehydrocholesterol, which serves as the precursor for vitamin D3.