Biotin, a water-soluble vitamin, is used as cofactor of enzymes involved in carboxylation reactions. In humans, there are five biotin-dependent carboxylases: propionyl-CoA carboxylase; methylcrotonyl-CoA carboxylase; pyruvate carboxylase, and two forms of acetyl-CoA carboxylase. These enzymes catalyze key reactions in gluconeogenesis, fatty acid metabolism, and amino acid catabolism; thus, biotin plays an essential role in maintaining metabolic homeostasis. In recent years, biotin has been associated with several diseases in humans. Some are related to enzyme deficiencies involved in biotin metabolism. However, not all biotin-responsive disorders can be explained based on the classical role of the vitamin in cell metabolism. Several groups have suggested that biotin may be involved in regulating transcription or protein expression of different proteins. Biotinylation of histones and triggering of transduction signaling cascades have been suggested as underlying mechanisms behind these non-classical biotin-deficiency manifestation in humans.