BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.7. Vitamin B₁₂ — Cobalamin

Structure and physicochemical properties. Cobalamins are coordination compounds in which a central cobalt atom is linked to the nitrogen atoms of four reduced pyrrole rings forming a corrin Nucleus, and to the nitrogen atom of 5,6-dimethylbenzimidazole (Fig. 8.20). The structure of the vitamin was elucidated in 1955 by Hodgkin, who was awarded the Nobel Prize in 1964. This vitamin is synthesized exclusively by microorganisms. Dietary sources include Liver, meat, eggs, and milk; it is completely absent from plant foods. The vitamin is absorbed only in the presence of an endogenous glycoprotein, the so-called intrinsic factor, which binds the vitamin in The Stomach and ensures its attachment to specific receptor sites on the apical membrane of enterocytes. In the Blood, cobalamin also binds to specific Proteins known as transcobalamins. In humans and animals, vitamin B₁₂ is stored in the liver. Vitamin deficiency or impaired absorption of vitamin B₁₂ leads to The Development of pernicious anemia.

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Fig. 8.20. Methylcobalamin

Biochemical Functions. Cobalamin Coenzymes contain Two Types of ligands: a methyl group and 5'-deoxyadenosine. Methylcobalamin participates in Transmethylation reactions, whereas deoxyadenosylcobalamin is involved in isomerization reactions. An example of a transmethylation reaction is the resynthesis of Methionine from homocysteine. The methyl group donor is methyl-THF (see Chapter 6).