text: The synthesis of Malonyl-CoA consists of two half reactions. The first being the carboxylation of biotin with bicarbonate and the second being the transfer of the CO group to acetyl-CoA from carboxybiotin to allow for the formation of malonyl-CoA. Two different protein subassemblies, along with BCCP, are required for this two step reaction to be successful: biotin carboxylase (BC) and carboxyltransferase (CT). BCCP contains the biotin cofactor which is covalently bound to a lysine residue.
  In fungi, mammals, and plant cytosols, all three of these components (BCCP, BC, and CT) exist on one polypeptide chain. However, most studies of this protein have been conducted on the E. coli form of the enzyme, where all three components exist as three separate complexes rather than being united on one polypeptide chain.