biotin carboxylase

**Mechanism (two-step reaction):** 1. Biotin carboxylase (BC) step: Biotin (covalently attached via an amide bond to a lysine on the biotin carboxyl carrier protein, BCCP) + ATP + HCO3⁻ → carboxybiotin + ADP + Pi. This step is ATP-dependent. 2. Carboxyltransferase (CT) step: Carboxybiotin transfers CO2 to the substrate (e.g., acetyl-CoA → malonyl-CoA).

**Structure:** In bacteria, BC, BCCP, and CT are separate proteins. In humans/eukaryotes, they are domains of a single multifunctional polypeptide (e.g., ACC1, ACC2, pyruvate carboxylase). Mitochondrial propionyl-CoA carboxylase and methylcrotonyl-CoA carboxylase are multi-subunit enzymes.

**Biotin-dependent carboxylases (all need ATP, HCO3⁻, and biotin):** - Acetyl-CoA carboxylase (ACC): acetyl-CoA → malonyl-CoA; the rate-limiting step of fatty acid synthesis (cytosol). Activated by insulin (dephosphorylation) and citrate; inhibited by glucagon/epinephrine (AMPK-mediated phosphorylation) and long-chain fatty acyl-CoA. ACC2 produces malonyl-CoA, which inhibits CPT-1 and thus carnitine shuttle/β-oxidation. - Pyruvate carboxylase: pyruvate → oxaloacetate (mitochondria; gluconeogenesis and anaplerosis). Activated by acetyl-CoA. - Propionyl-CoA carboxylase: propionyl-CoA → methylmalonyl-CoA (odd-chain fatty acids, Val, Ile, Met, Thr; cholesterol side chain). Followed by methylmalonyl-CoA mutase (requires vitamin B12). - Methylcrotonyl-CoA carboxylase (leucine catabolism).

**Clinical significance:** - Biotin deficiency: dermatitis, alopecia, conjunctivitis, enteritis, brittle nails, neurologic symptoms (lethargy, hypotonia, seizures, depression, paresthesias). Causes: excessive raw egg white ingestion (avidin binds biotin tightly), prolonged total parenteral nutrition without biotin, antibiotics/gut flora loss, long-term anticonvulsants, and inborn errors. Labs: organic aciduria (3-hydroxyisovaleric acid, 3-hydroxypropionate, methylcitrate), lactic acidosis, and hypoglycemia can occur because of impaired gluconeogenesis (pyruvate carboxylase). - Multiple carboxylase deficiency: holocarboxylase synthetase deficiency (early onset; attaches biotin to apocarboxylases) and biotinidase deficiency (late onset; impaired biotin recycling; newborn screening). Presentation: metabolic acidosis, seizures, hypotonia, skin rash, alopecia, developmental delay. Treatment: oral biotin supplementation. - Pyruvate carboxylase deficiency: lactic acidosis, hypoglycemia, neurologic impairment. - Propionic acidemia: propionyl-CoA carboxylase deficiency causing vomiting, anion gap metabolic acidosis, hyperammonemia, and lethargy in neonates. - Antibiotic target: bacterial BC (AccC) is being studied as an antibacterial drug target. Also, bacterial ACC is distinct from the eukaryotic multifunctional enzyme.

**High-yield pearls:** Carboxylation reactions requiring biotin use ATP; vitamin K-dependent carboxylation (γ-carboxylation of glutamate in clotting factors II, VII, IX, X, protein C/S) is a different reaction. Biotin is vitamin B7. Biotin transfers one-carbon unit as CO2, whereas folate carries other one-carbon groups (methyl, methylene, formyl) and SAM carries methyl groups.