Cytosolic enzyme that catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the first and rate-limiting step of de novo fatty acid biosynthesis (PubMed:20952656). This is a 2 steps reaction starting with the ATP-dependent carboxylation of the biotin carried by the biotin carboxyl carrier (BCC) domain followed by the transfer of the carboxyl group from carboxylated biotin to acetyl-CoA (PubMed:20952656). acetyl-CoA + ATP + hydrogencarbonate = ADP + H(+) + malonyl-CoA + phosphate Binds 2 magnesium or manganese ions per subunit. Inhibited by phosphorylation (By similarity). Citrate promotes oligomerization of the protein into filaments that correspond to the most active form of the carboxylase (PubMed:20952656). Lipid metabolism; malonyl-CoA biosynthesis; malonyl-CoA from acetyl-CoA: step 1/1. Monomer, homodimer, and homotetramer (PubMed:20952656). Can form filamentous polymers (PubMed:20952656). Interacts in its inactive phosphorylated form with the BRCT domains of BRCA1 which prevents ACACA dephosphorylation and inhibits lipid synthesis (PubMed:12360400). Interacts with MID1IP1; interaction with MID1IP1 promotes oligomerization and increases its activity (PubMed:20952656, PubMed:20457939). Up-regulated by endocannabinoid anandamide/AEA. Consists of an N-terminal biotin carboxylation/carboxylase (BC) domain that catalyzes the ATP-dependent transient carboxylation of the biotin covalently attached to the central biotinyl-binding/biotin carboxyl carrier (BCC) domain. The C-terminal carboxyl transferase (CT) domain catalyzes the transfer of the carboxyl group from carboxylated biotin to acetyl-CoA to produce malonyl-CoA. Phosphorylation on Ser-1262 is required for interaction with BRCA1. Phosphorylation at Ser-79 by AMPK inactivates enzyme activity. The biotin cofactor is covalently attached to the central biotinyl-binding domain and is required for the catalytic activity.