Fig. 4. Schematic diagram of ACC reaction and of domains and/or subunits of the biotin-dependent ACC.

(A) Stepwise enzymatic reactions of ACC. (B) ACC is composed of three different components, the biotin carboxylase component (BC), the biotin carboxyl carrier protein (BCCP) and carboxyltransferase (CT). The “Swinging Arm” of biotin-BCCP transports CO₂ between the components BC and CT. Then the carboxyl group is transferred from biotin to acetyl-CoA to form malonyl-CoA. (C) The eukaryotic ACC is a multifunctional polypeptide chain that contains the three components (BC, BCCP and CT). Bacterial ACC is composed of multiple subunits. The E. coli and B. subtilis ACCs are formed by four proteins: the subunit BC, the subunit BCCP, and two proteins (α and β) that form the CT subunit. In actinomycetes the ACCase complex is made up of two main subunits: the α subunit, which contains the BC and BCCP components and the β subunit, which has the CT activity. In addition, actinomycete ACCases may include a third subunit, ε, the presence of which dramatically stimulates the specific activity of the enzyme complexes.