Component of the acetyl coenzyme A carboxylase (ACC) complex. Biotin carboxylase (BC) catalyzes the carboxylation of biotin on its carrier protein (BCCP) and then the CO(2) group is transferred by the transcarboxylase to acetyl-CoA to form malonyl-CoA. acetyl-CoA + N(6)-carboxybiotinyl-L-lysyl-[protein] = malonyl-CoA + N(6)-biotinyl-L-lysyl-[protein] Binds 1 zinc ion per subunit. Lipid metabolism; malonyl-CoA biosynthesis; malonyl-CoA from acetyl-CoA: step 1/1. Acetyl-CoA carboxylase is a heterohexamer composed of biotin carboxyl carrier protein, biotin carboxylase and 2 subunits each of ACCase subunit alpha and ACCase plastid-coded subunit beta (accD). RNA expressed in leaf, root and stem; the least expression occurs in stems. Overexpression of the accD operon increases the specific activity of acetyl-coenzyme A carboxylase, increases leaf longevity and seed production. The accD operon consists of accD, psaI, ycf4, cemA, and petA in this order. The protein in tobacco runs as 53 kDa; thus this may not be the correct start site. Belongs to the AccD/PCCB family. Extended N-terminus.