BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.2. Vitamin B₂ — Riboflavin

Structure and physicochemical properties. Riboflavin was first isolated from milk as a heat-stable factor in 1879. Later, similar yellow compounds were isolated from eggs, Muscles, and named after their sources: lactoflavin, hepatoflavin, ovoflavin, etc. Riboflavin solutions have a yellow-orange color and are capable of fluorescence. The core of the vitamin molecule is an isoalloxazine structure, a heterocycle composed of benzene, pyrazine, and pyrimidine rings. In addition, the vitamin contains the sugar alcohol ribitol. Riboflavin is sparingly soluble in Water at room Temperature, though its solubility increases in acidic environments. The vitamin is heat-stable, withstanding temperatures up to 120°, but is sensitive to light.

An important property of riboflavin is its ability to undergo reversible Oxidation and reduction (Fig. 8.11). The reduction of riboflavin to dihydroriboflavin occurs via the successive addition of hydrogen atoms at positions 1 and 5, forming an intermediate semi-reduced semiquinone radical. The reduced form of riboflavin can be oxidized by atmospheric oxygen.

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Fig. 8.11. Riboflavin and dihydroriboflavin

Biochemical Functions. Riboflavin serves as a structural component of flavin mononucleotide (FMN, FMN) and flavin adenine dinucleotide (FAD, FAD), which act as prosthetic groups for numerous oxidoreductases (dehydrogenases). FMN synthesis takes place in the intestinal mucosa through the phosphorylation of riboflavin, a reaction catalyzed by the enzyme riboflavin kinase, whereas FAD is synthesized from FMN by The addition of AMP, with ATP serving as the donor (Fig. 8.12).

Flavin NUCLEOTIDES, much like free riboflavin, can exist in both oxidized and reduced forms.

Fig. 8.12. Scheme of FMN and FAD coenzyme synthesis

Approximately 200 flavin dehydrogenases are currently known. Flavin-containing Enzymes can catalyze The oxidation of hemiacetals to lactones, alcohols to aldehydes, amines to Imines, and carbonyl or carboxylic compounds to unsaturated carbonyl compounds.

Flavin nucleotides (Fig. 8.13) are stronger oxidizing agents than nicotinamide nucleotides. Reduced flavins can be oxidized by O₂, a feature characteristic of only a few Organic compounds in The Human Body. Typically, flavin nucleotides are bound to Proteins via strong covalent bonds.

An example of a FAD-containing enzyme is glucose oxidase. It catalyzes the oxidation of glucose to gluconic acid and is widely used in clinical practice to determine Blood glucose levels.

Amino acid oxidases are highly active enzymes found, notably, in snake venoms, where they catalyze the Oxidative Deamination of Amino Acids. Succinate dehydrogenase, which catalyzes the dehydrogenation of succinate to fumarate within The Tricarboxylic Acid Cycle, also belongs to the class of Flavoproteins.

Many flavin enzymes form complexes with metals. For instance, xanthine oxidase, which catalyzes the oxidation of hypoxanthine and xanthine to uric acid, contains 2 molecules of FAD, 2 molybdenum atoms, and 8 iron atoms.

Fig. 8.13. Structure of FAD

Dietary requirement and distribution in nature. Specific severe deficiency DISEASES ASSOCIATED WITH riboflavin are virtually non-existent. In animal experiments, a deficiency leads to inflammatory processes in the mucous membranes of the Tongue and Lips (especially the corners of the Mouth), Skin epithelial changes, as well as keratitis, corneal inflammation, and cataracts. General and cardiac weakness have also been observed. The vitamin is fairly widespread in nature. Humans obtain it from dairy products and eggs, with a daily requirement ranging from 1.8 to 2.6 mg. Factors that can precipitate a deficiency include Arthritis, tuberculosis, and The Use of Antibiotics, tranquilizers, and sulfonamides.