BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.9. Vitamin H — biotin

Structure and physicochemical properties. Biotin was isolated from dried egg yolk in 1935. The vitamin is stable at high temperatures and is found in the Liver, egg yolk, legumes, and other

foods; in addition, it is synthesized by the intestinal microflora. There are 8 stereoisomers of biotin, of which only one exhibits biological activity. In most organisms, biotin is present in a bound form as biocytin (biotinyllysine). The binding occurs via the s-amino group of the Lysine residue to enzyme Proteins (Fig. 8.22).

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Fig. 8.22. Structure of biocytin

Biotin binds with high affinity and Specificity to the egg white protein Avidin, forming a Water-insoluble complex. Avidin is a tetramer, and each of its subunits contains a biotin-binding site. Avidin is denatured upon boiling; therefore, vitamin H deficiency can occur only when consuming raw eggs.

Biochemical Functions. Biotin-dependent Enzymes catalyze carboxylation reactions (1) coupled with ATP Hydrolysis, and transcarboxylation reactions (2).

The former include Pyruvate carboxylase, which catalyzes The formation of oxaloacetate from pyruvate for the Krebs cycle.

The reaction proceeds in two stages. In The First stage, carboxybiotin is formed utilizing ATP energy. In the second stage, the carboxyl group is transferred to pyruvate. A similar mechanism is involved in the synthesis of malonyl-CoA from acetyl-CoA (see fatty acid synthesis). The second type includes, for example, propionic acid formation reactions during Propionic Acid Fermentation:

Nutritional requirement. The daily requirement for the vitamin is negligible—only 150–300 mcg. Deficiency may develop due to The Use of Antibiotics and sulfonamides. This condition manifests as dermatitis, increased activity of the Sebaceous Glands, Hair loss, weight loss, and, in severe cases, paralysis.

Summary information on all Water-Soluble Vitamins is presented in Table 8.1.

Table 8.1

Water-soluble vitamins

Vitamin	Daily requirement	Coenzyme form	Biological role	Characteristic symptoms of avitaminosis
B₁ - thiamine	2-3 mg	TPP	Decarboxylation of α-keto acids, α-Cleavage	polyneuritis
B₂ - riboflavin	1.8-2.6 mg	FMN, FAD	Hydrogen transfer	eye lesions, inflammation of the lip mucosa
PP - nicotinamide (niacin)	15-25 mg	NAD, NADP	Hydrogen transfer	symmetrical dermatitis, diarrhea, dementia
pantothenic acid	10-12 mg	CoA-SH	activation and transfer of acyl groups	dystrophic Changes in the Adrenal Glands
B₆ - pyridoxine	2-6 mg	PLP (Pyridoxal phosphate)	Amino acid METABOLISM: Transamination and decarboxylation	dermatitis, hyperexcitability of Nervous Tissue
B_c - folic acid	50-150 mcg	H₄FA (tetrahydrofolic acid)	Transfer of single-carbon groups	impaired hematopoiesis, anemia
H - biotin	10 mcg	biotin	Carboxylation of α-keto acids	dermatitis, hyperactive sebaceous glands
B₁₂ - cobalamin	1-2 mcg	methylcobalamin, deoxyadenosylcobalamin	Methyl group transfer	Macrocytic anemia
C - ascorbic acid	50-75 mg	Ascorbic acid	hydroxylation	scurvy, bleeding Gums, subcutaneous hemorrhages

Last update: 06/08/2026

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