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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Biotin (vitamin B7)

Biotin is the coenzyme of the carboxylases, the enzymes that add CO₂ in gluconeogenesis and fatty acid synthesis.

Updated 2026-11-01

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## Key facts

- Coenzyme of four carboxylases: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase and 3-methylcrotonyl-CoA carboxylase.
- It is covalently bound to a lysine of the enzyme (biocytin) and carries activated CO₂ at the cost of ATP.
- Avidin in raw egg white binds biotin and blocks its absorption.
- Deficiency: dermatitis around the body openings, alopecia, conjunctivitis, hypotonia and lactic acidosis.

## In detail

- Sources: egg yolk, liver, peanuts, soy, mushrooms; colonic bacteria also make it.
- Reaction: bicarbonate binds biotin using ATP energy (carboxybiotin) and is then transferred to the substrate.
- Pyruvate carboxylase: pyruvate → oxaloacetate (gluconeogenesis and refilling the Krebs cycle). Acetyl-CoA carboxylase: acetyl-CoA → malonyl-CoA (fatty acid synthesis).
- Propionyl-CoA carboxylase: propionyl-CoA → methylmalonyl-CoA (branched-chain amino acids and odd-chain fatty acids). 3-methylcrotonyl-CoA carboxylase: leucine breakdown.
- Holocarboxylase synthetase attaches biotin to the enzymes; biotinidase releases it when they are degraded, so it can be recycled.
- Causes of deficiency: eating many raw egg whites for a long time, parenteral nutrition without biotin, anticonvulsants, genetic biotinidase deficiency.
- Excess: not toxic, but high-dose supplements interfere with immunoassays that use biotin–streptavidin (TSH, troponin, hormones) and give false results.
- Lab: biotinidase activity (newborn screening); urine organic acids (raised 3-hydroxyisovalerate).

**Also called**
Vitamin B7, Vitamin H

**Formula**
C₁₀H₁₆N₂O₃S

**PubChem**
[CID 171548](https://pubchem.ncbi.nlm.nih.gov/compound/171548)

## Structure

- Two fused rings: a ureido (imidazolidone) ring and a sulphur-containing tetrahydrothiophene, with a valeric acid chain.
- The chain's carboxyl links to a lysine of the carboxylase; CO₂ is fixed on a ureido nitrogen.

## Clinical notes

- Biotinidase deficiency is found on expanded newborn screening and treated with lifelong biotin.
- Before thyroid hormone or troponin tests, people are asked about biotin supplements, which disturb some assays.
- Eating large amounts of raw egg is the classic cause of acquired deficiency; cooking destroys avidin.

## Related

- [Acetyl-CoA](https://durgical.com/en/biochemistry/molecules/acetyl-coa/)
- [Pantothenic acid (vitamin B5)](https://durgical.com/en/biochemistry/molecules/pantothenic-acid/)
- [Cobalamin (vitamin B12)](https://durgical.com/en/biochemistry/topics/cobalamin/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which type of reaction depends on biotin?

ATP-dependent carboxylations: pyruvate, acetyl-CoA, propionyl-CoA and 3-methylcrotonyl-CoA carboxylases.

## Why can raw egg white cause biotin deficiency?

It contains avidin, which binds biotin very tightly and prevents its absorption; cooking denatures it.

## Which enzyme recycles biotin and is checked on newborn screening?

Biotinidase, which frees biotin from degraded carboxylases.

## Why do biotin supplements disturb some lab tests?

Many immunoassays use the biotin–streptavidin pair; free biotin competes and gives falsely high or low values.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Biotin (CID 171548)](https://pubchem.ncbi.nlm.nih.gov/compound/171548) (PubChem, NCBI, 2026) · Public domain; [Pantothenic acid and biotin: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002410.htm) (MedlinePlus, US National Library of Medicine, 2026); [Biotinidase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/biotinidase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [Holocarboxylase synthetase deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/holocarboxylase-synthetase-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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