Biotin as the cofactor for four carboxylases
Biotin (vitamin B7) works as a covalently bound cofactor for the carboxylase enzymes, which attach a carbon dioxide group to their substrates. In humans there are four: acetyl-CoA carboxylase (fatty acid synthesis), pyruvate carboxylase (gluconeogenesis), propionyl-CoA carboxylase (amino acid and odd-chain fatty acid metabolism), and methylcrotonyl-CoA carboxylase (leucine breakdown). The enzyme holoenzyme synthetase attaches biotin to each one, which is why a single vitamin underpins several distinct pathways.
The diagram traces biotin's reach across fatty acid, amino acid, and glucose metabolism through these four enzymes, illustrating why true deficiency disrupts multiple systems at once.
Deficiency, supplementation, and the hair-and-nails claim
Genuine biotin deficiency is uncommon because the vitamin is widespread in food and produced by gut bacteria; it appears mainly with prolonged raw-egg-white consumption (avidin binds biotin), certain genetic disorders, or specific medications, causing rash, hair loss, and neurological symptoms. The popular use of high-dose biotin for hair and nail growth has limited evidence outside of correcting a real deficiency.
High-dose biotin can also distort some laboratory blood tests, including thyroid and cardiac markers. This is general educational information, not medical advice; tell a qualified professional about biotin supplements before blood work.