Component of a biotin-dependent acyl-CoA carboxylase complex. This subunit transfers the CO2 from carboxybiotin to the CoA ester substrate (PubMed:17114269). When associated with the alpha3 subunit AccA3, is involved in the carboxylation of acetyl-CoA and propionyl-CoA, with a preference for acetyl-CoA (PubMed:17114269). acetyl-CoA + N(6)-carboxybiotinyl-L-lysyl-[protein] = malonyl-CoA + N(6)-biotinyl-L-lysyl-[protein] N(6)-carboxybiotinyl-L-lysyl-[protein] + propanoyl-CoA = methylmalonyl-CoA + N(6)-biotinyl-L-lysyl-[protein] Activity of the AccA3/AccD6 complex is inhibited by interaction with the epsilon subunit AccE5 (PubMed:17114269). Inhibited by dimethyl itaconate, C75, the herbicide haloxyfop, cerulenin, and 1,2-cyclohexanedione (PubMed:17114269, PubMed:25092705). Lipid metabolism; fatty acid biosynthesis. Lipid metabolism; mycolic acid biosynthesis. Homodimer (PubMed:25092705). The biotin-dependent acyl-CoA carboxylase complex is composed of AccA3, which contains the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains, and AccD6, which contains the carboxyl transferase (CT) domain (PubMed:17114269). The AccA3/AccD6 complex forms a dodecamer (PubMed:17114269). Expressed at higher levels during the exponential growth phase. Essential. Cannot be deleted. Limited expression level affects growth, mycolic acid content and cell morphology. AccD6 is dispensable in nonpathogenic strains. Belongs to the AccD/PCCB family.