FAD The key enzymatic step of fatty acid syn thesis is the carboxylation of acetyl-CoA to form malonyl-CoA. The carboxyl of biotin is covalently attached to an E-amino acid group of a lysine residue of acetyl-CoA carboxylase. The reaction occurs in two stages. In the first step, a carboxybiotin is formed: HCO3- + biotin-enzyme + ATP CO2 - biotin-enzyme+ ADP + Pi In the second step, the CO2 is transferred to acetyl CoA to produce malonyl CoA: CO2-biotin-enzyme + acetyl CoA malonyl CoA+ biotin-enzyme None of the other cofactors listed are involved in this reaction.