Biotin is a water soluble
enzyme cofactor that belongs to the
vitamin B complex. In humans,
biotin is involved in important metabolic pathways such as gluconeogenesis,
fatty acid synthesis, and
amino acid catabolism by acting a as prosthetic group for
pyruvate carboxylase,
propionyl-CoA carboxylase, beta-methylcrotinyl-
CoA carboxylase, and
acetyl-CoA carboxylase. Carboxylases are synthesized as apo-carboxylases without
biotin and the active form is produced by their covalent binding of
biotin to the epsilon-amino group of a
lysine residue of the apocarboxylases. This reaction is catalyzed by the holo-carboxylase
synthetase. The last step in the degradation of carboxylases, the cleavage of the biotinyl moiety from the epsilon-amino group
lysine residues, is catalyzed by
biotinidase and results in the release of free
biotin, which can be recycled.
Biotin regulates the catabolic
enzyme propionyl-CoA carboxylase at the posttranscriptional level whereas the holo-carboxylase
synthetase is regulated at the transcriptional level. Aside from its role in the regulation of gene expression of carboxylases,
biotin has been implicated in the induction of the receptor for the
asialoglycoprotein, glycolytic
enzymes and of egg yolk
biotin binding proteins.
Biotin deficiency in humans is extremely rare and is generally associated with prolonged
parenteral nutrition, the consumption of large quantities of
avidin, usually in the form of raw eggs, severe
malnutrition and, inherited metabolic disorders. In humans, there are autosomal recessive disorders of
biotin metabolism that result from the disruption of the activity of
biotinidase or holo-carboxylase
synthetase.