This protein is a component of the acetyl coenzyme A carboxylase complex; first, biotin carboxylase catalyzes the carboxylation of the carrier protein and then the transcarboxylase transfers the carboxyl group to form malonyl-CoA. ATP + hydrogencarbonate + N(6)-biotinyl-L-lysyl-[protein] = ADP + H(+) + N(6)-carboxybiotinyl-L-lysyl-[protein] + phosphate Binds 2 magnesium or manganese ions per subunit. Optimum pH is 8.3-8.9. 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 two subunits each of ACCase subunit alpha and ACCase plastid-coded subunit beta (accD). A number of isoforms are produced. According to EST sequences. Accumulates in fatty acids synthesizing tissues. Mostly expressed in siliques, developing leaves, and flowers, present in roots and embryos (especially at torpedo stage), and, to a lower extent, in mature leaves.