Biotin-dependent carboxylase deficiencies (multiple carboxylase deficiency) present with organic acidaemia, skin rash, and alopecia. All four biotin-dependent carboxylases in humans share the common mechanism of:

• A Using biotin as a direct electron carrier between substrates and NADH
• B Using biotin to activate acyl-CoA substrates via thioester bond formation
• C Using biotin as a lysine-crosslinking agent for enzyme quaternary structure stabilisation
• D Using biotin to shuttle CO2 as carboxybiotin-enzyme intermediate via an ATP-dependent carboxylation mechanism ✓

Explanation

All four human biotin-dependent carboxylases—pyruvate carboxylase (PC), propionyl-CoA carboxylase (PCC), 3-methylcrotonyl-CoA carboxylase (MCC), and acetyl-CoA carboxylase (ACC)—use the same two-step mechanism: (1) biotin covalently linked to a lysine residue of the enzyme is carboxylated using HCO3- and ATP (forming N1'-carboxybiotin-enzyme + ADP + Pi); (2) the carboxyl group is transferred from carboxybiotin to the specific substrate (pyruvate → OAA, propionyl-CoA → methylmalonyl-CoA, etc.). The carboxybiotin arm acts as a flexible 'swinging arm' carrying activated CO2 between the two active sites of the bifunctional carboxylase enzyme. Biotin functions solely as a CO2 carrier, not as an electron carrier, thioester activator, or structural crosslinker.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.