Biotin

Q: Which protein acts as a natural antivitamin for biotin, preventing its absorption?

The natural antivitamin for biotin is avidin. Avidin is a glycoprotein found in raw egg whites. It forms a specific complex with biotin, rendering it insoluble and unabsorbable, thereby reducing biotin assimilation. Consuming large amounts of raw egg white can lead to biotin deficiency. Heat treatment denatures and destroys avidin.

Q: What are the clinical manifestations of vitamin H (biotin) hypovitaminosis?

Clinical manifestations of biotin deficiency include: Dermatological: dermatitis with excessive sebaceous gland activity (seborrhea), hair loss (alopecia), brittle nails.Muscular: myalgia.Hematological: various types of anemia.Neurological: mental depression.

Q: What class and subclass of enzymes does biotin belong to as a coenzyme?

Biotin is a coenzyme for enzymes belonging to the ligase class. Within this class, it specifically works with the carboxylase subclass, which catalyzes carbon dioxide attachment reactions.

Q: What is the active form of biotin called, and where is carbon dioxide fixed within it?

The active form is called N-carboxybiotin. In this structure, the $CO_2$ molecule attaches to one of the nitrogen atoms located in the imidazole ring of biotin.

Biotinum

For medical students2 min readUpdated 2026-10-10

Biotin is a vital bioactive substance that acts as a coenzyme for enzymes of the ligase class. In the body, it facilitates energy-dependent carboxylation reactions, driving the binding and transfer of carbon dioxide to substrates.

Precursor VitaminsVitamin H, Vitamin B7, Biotin itself

Enzyme ClassLigases (subclass: carboxylases)

Mechanism of ActionCarboxylation (attachment of a $CO_2$ molecule)

Energy DependenceReaction requires ATP energy expenditure

Active FormN-carboxybiotin (carbon dioxide on the imidazole ring)

Role in the Body and Precursor Vitamins

Biotin functions in biochemical processes primarily as a coenzyme. This means that to perform its task, it must be physically bound to an enzyme molecule, forming a unified catalytic apparatus. In biochemical and medical literature, the precursor vitamins of this coenzyme are referred to as biotin itself, along with its traditional synonyms: vitamin H and vitamin B7.

Upon entering the body, these substances undergo necessary transformation pathways to integrate into the active sites of specific enzymes. It is important to understand that without a stable bond to the enzyme, biotin cannot carry out its coenzyme functions.

Enzyme Class: Ligases and Carboxylases

In the international enzyme classification, biotin is strictly associated with the class known as ligases. The primary task of ligases in biochemistry is joining two molecules together through the formation of new chemical bonds.

In the case of biotin, we refer to a narrower and more specialized subclass of ligases: carboxylases. As the name implies, carboxylases specialize in one specific chemical operation: the addition of a carbon dioxide ($CO_2$) molecule to a target substrate. This fundamental biochemical process is called carboxylation. An essential condition for such reactions is the mandatory presence of an energy source, universally provided here by the molecule ATP (adenosine triphosphate).

Chemical Mechanism Details

The mechanism of action of biotin-dependent carboxylases represents a strict sequence of steps aimed at activating carbon dioxide and its subsequent transfer.

The chemical reaction begins with the transition of a carbon dioxide molecule ($CO_2$) into its active form. This step is inextricably linked to the cleavage and expenditure of ATP energy. Then, the activated carbon dioxide attaches directly to the coenzyme molecule. The attachment point is a specific nitrogen atom located within the cyclic structure of the biotin molecule known as the imidazole ring.

Formation of the Active Form: N-Carboxybiotin

The result of successful interaction between the biotin molecule and activated carbon dioxide is the formation of a new structural complex: N-carboxybiotin (sometimes simply designated in diagrams as carboxybiotin).

It is important to distinguish this coenzyme from other well-known coenzymes, such as thiamine pyrophosphate, tetrahydrofolate, or pyridoxal phosphate, which perform completely different chemical functions in the cell. N-carboxybiotin should also be differentiated from molecules like carbamoyl phosphate or oxybiotin.

The formation of N-carboxybiotin can be represented by a simple yet comprehensive chemical scheme: Biotin + $CO_2$ + ATP $\rightarrow$ Carboxybiotin

It is precisely in the form of N-carboxybiotin that the coenzyme acts as a reliable donor of the carbon dioxide molecule, successfully completing the carboxylation reaction.

Mnemonic

To remember the core mechanism, use the rule of three "C's": Coenzyme biotin helps Carboxylases convert substrates, forming N-Carboxybiotin.

Frequently asked questions

Which specific biotin-dependent enzymes function in human metabolism?

The following biotin-dependent enzymes from the ligase class (carboxylase subclass) function in human metabolism:

Pyruvate carboxylase — participates in gluconeogenesis (conversion of pyruvate to oxaloacetate).
Acetyl-CoA carboxylase — drives fatty acid synthesis.
Propionyl-CoA carboxylase — ensures the oxidation of propionic acid residues in the citric acid cycle.

In their active sites, the coenzyme biotin is covalently bound to the ε-amino group of lysine, catalyzing carbon dioxide attachment reactions coupled with ATP hydrolysis.

Which protein acts as a natural antivitamin for biotin, preventing its absorption?

The natural antivitamin for biotin is avidin.

Avidin is a glycoprotein found in raw egg whites. It forms a specific complex with biotin, rendering it insoluble and unabsorbable, thereby reducing biotin assimilation. Consuming large amounts of raw egg white can lead to biotin deficiency. Heat treatment denatures and destroys avidin.

What are the clinical manifestations of vitamin H (biotin) hypovitaminosis?

Clinical manifestations of biotin deficiency include:

Dermatological: dermatitis with excessive sebaceous gland activity (seborrhea), hair loss (alopecia), brittle nails.
Muscular: myalgia.
Hematological: various types of anemia.
Neurological: mental depression.

What is the biochemical difference between the functions of biotin and tetrahydrofolate?

The main biochemical difference between biotin and tetrahydrofolate lies in the types of transferred groups and enzyme classes.

Feature	Biotin	Tetrahydrofolate (THF)
Enzyme Class	Ligases, carboxylase subclass	Transferases, methyltransferases
Primary Function	Substrate carboxylation, $CO_2$ attachment; tissue uptake of carbon dioxide/bicarbonate ions	Transfer of one-carbon units
Transferred Groups	$CO_2$	Methyl, methylene, formyl, and other one-carbon units
Coenzyme Form	$N^5$-carboxybiotin; biotin linked to the ε-$NH_2$ group of lysine in "biotin" enzymes	THF coenzyme forms: $N^5$-formyl-THF, $N^{10}$-formyl-THF, $N^5,N^{10}$-methenyl-THF, $N^5,N^{10}$-methylene-THF, $N^5$-methyl-THF

What class and subclass of enzymes does biotin belong to as a coenzyme?

Biotin is a coenzyme for enzymes belonging to the ligase class. Within this class, it specifically works with the carboxylase subclass, which catalyzes carbon dioxide attachment reactions.

Why is an ATP molecule required in biotin-dependent reactions?

Carbon dioxide attachment reactions require energy expenditure. The ATP molecule is necessary to supply energy to the process, allowing the reaction to proceed successfully and form a bond between $CO_2$ and biotin.

What is the active form of biotin called, and where is carbon dioxide fixed within it?

The active form is called N-carboxybiotin. In this structure, the $CO_2$ molecule attaches to one of the nitrogen atoms located in the imidazole ring of biotin.

Go deeper

Detailed chemical structure of the imidazole ring in the biotin molecule.
Comparative characteristics of biotin and other coenzymes (thiamine pyrophosphate, tetrahydrofolate, pyridoxal phosphate).
Energy balance of the reaction: exact mechanism of ATP hydrolysis during carboxybiotin formation.
Mechanism of activated $CO_2$ transfer from the N-carboxybiotin structure to the final target substrate.