Biotin = vitamin B7, cofactor for the FOUR mammalian carboxylases: pyruvate carboxylase (PC, gluconeogenesis anaplerosis), acetyl-CoA carboxylase (ACC, fatty acid synthesis rate-limiting), propionyl-CoA carboxylase (PCC, BCAA + odd-chain FA), methylcrotonyl-CoA carboxylase (MCC, leucine catabolism). Biotin is covalently attached to apo-carboxylases by HCS (holocarboxylase synthetase) on a specific lysine. Recycled by BIOTINIDASE that cleaves biotin from biotinyl-lysine (biocytin). Biotinidase deficiency = newborn-screened multiple carboxylase deficiency presenting with alopecia + seizures + cutaneous rash + metabolic acidosis; treated with oral biotin. Biotin supplementation (high doses for "hair + nails") interferes with biotin-streptavidin immunoassays — causes spurious lab results (TSH, troponin, hCG).
Organ Systems
endocrine
integumentary
Pathway Steps
biotin → biotinyl-apocarboxylase — via holocarboxylase synthetase (HCS) — covalent attachment to PC/ACC/PCC/MCC. Holocarboxylase synthetase covalently attaches biotin to the four carboxylases (pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, methylcrotonyl-CoA carboxylase); its deficiency is multiple carboxylase deficiency, treatable with biotin.
biocytin → biotin — via biotinidase — recycles cofactor; deficiency is screened in newborn screen. Biotinidase recycles biotin from degraded carboxylases (biocytin); its deficiency — on newborn screens — causes a treatable neurocutaneous syndrome. Raw egg white (avidin) tightly binds biotin and can induce deficiency.
Known Modulators
biotin (substrate) — B7 vitamin; carboxylase prosthetic group