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The biotin cycle

Free biotin enters the cycle from dietary sources or from the cleavage of biocytin or biotinyl-peptides by the action of biotinidase. The free biotin is then covalently attached to the various apocarboxylases, propionyl-CoA carboxylase (PCC), beta-methylcrotonyl-CoA carboxylase (MCC), pyruvate carboxylase (PC), and acetyl-CoA carboxylase (ACC) by the action of biotin holocarboxylase synthetase, thereby forming active holocarboxylases. The holocarboxylases are subsequently proteolyzed to biocytin and/or biotinyl-peptides, which are then further cleaved by biotinidase, thus recycling the biotin. The liberated biotin can then enter the free biotin pool. Isolated deficiencies of each of the carboxylases and deficiencies of both holocarboxylase synthetase and biotinidase can occur.