ABSTRACT
That biotin should constitute an essential requirement for all organisms would be expected from its obligatory involvement in carbohydrate, lipid, and amino acid metabolism as the prosthetic group of the carboxylases. There are only four biotincontaining enzymes in higher organisms. They are acetyl-coenzyme A (CoA) carboxylase (ACC), propionyl-CoA carboxylase (PCC), pyruvate carboxylase (PC), and β-methyl-crotonyl-CoA carboxylase (MCC). Biotin is covalently bound to a lysine residue of the carboxylase protein in ACC, PCC, MCC, and PC. Each of the biotindependent carboxylases catalyzes an adenosine triphosphate (ATP)–dependent CO2 xation reaction. Biotin functions as a CO2 carrier on the surface of the enzyme. There are two variants of ACC: ACC1 is located in the cytosol and is a rate-limiting enzyme of fatty acid synthesis, whereas ACC2 is present on the outer mitochondrial membrane. Malonyl CoA produced by ACC2 has an inhibitory effect on fatty acid transport into mitochondria and thus controls fatty acid oxidation. PC, PCC, and MCC are mitochondrial enzymes. PC catalyzes the incorporation of bicarbonate into oxaloacetate, an intermediate of the tricarboxylic acid cycle. MCC catalyzes a crucial step in the degradation of branched-chain amino acid leucine. PCC catalyzes the incorporation of bicarbonate into propionyl CoA to form methylmalonyl CoA, which enters the tricarboxylic acid cycle after isomerization to succinyl CoA (1-4).
