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.

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).

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.

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.

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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.

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) (PubChem, NCBI, 2026) · Public domain; Pantothenic acid and biotin: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Biotinidase deficiency: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Holocarboxylase synthetase deficiency: MedlinePlus Genetics (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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