Mitochondrial enzyme that catalyzes the carboxylation of acetyl-CoA to malonyl-CoA and plays a central role in fatty acid metabolism (PubMed:16854592, PubMed:19236960, PubMed:20457939, PubMed:20952656, PubMed:19900410, PubMed:26976583). Catalyzes 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:19236960, PubMed:20457939, PubMed:20952656, PubMed:26976583). Through the production of malonyl-CoA that allosterically inhibits carnitine palmitoyltransferase 1 at the mitochondria, negatively regulates fatty acid oxidation (By similarity). Together with its cytosolic isozyme ACACA, which is involved in de novo fatty acid biosynthesis, promotes lipid storage (By similarity). acetyl-CoA + ATP + hydrogencarbonate = ADP + H(+) + malonyl-CoA + phosphate Binds 2 magnesium or manganese ions per subunit. Activity is increased by oligomerization of the protein into filaments (PubMed:19900410). The oligomerization and the activity of the enzyme are inhibited by phosphorylation at Ser-222 (PubMed:12488245). Inhibited by its product, malonyl-CoA (PubMed:16854592). Activated by citrate (PubMed:16854592). Activation by MID1IP1 is citrate-dependent (PubMed:20457939). Soraphen A, inhibits the enzyme by preventing the formation of active filamentous oligomers (Probable). Optimum pH is 7.5. Lipid metabolism; malonyl-CoA biosynthesis; malonyl-CoA from acetyl-CoA: step 1/1. Monomer, homodimer, and homotetramer (PubMed:20952656, PubMed:18772397). Forms filamentous polymers (PubMed:20457939, PubMed:20952656, PubMed:19900410). Interacts with MID1IP1; interaction with MID1IP1 promotes oligomerization and increases its activity in a citrate-dependent manner (PubMed:20952656, PubMed:20457939). Widely expressed with highest levels in heart, skeletal muscle, liver, adipose tissue, mammary gland, adrenal gland and colon (PubMed:9099716). Isoform 3 is expressed in skeletal muscle, adipose tissue and liver (at protein level) (PubMed:19190759). Isoform 3 is detected at high levels in adipose tissue with lower levels in heart, liver, skeletal muscle and testis (PubMed:19190759). 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 (Probable). The C-terminal carboxyl transferase (CT) domain catalyzes the transfer of the carboxyl group from carboxylated biotin to acetyl-CoA to produce malonyl-CoA (Probable). The biotin cofactor is covalently attached to the central biotinyl-binding domain and is required for the catalytic activity. Phosphorylation at Ser-222 by AMPK inactivates the enzyme (PubMed:12488245). Required for the maintenance of skeletal muscle lipid and glucose homeostasis (By similarity). Inhibition of ACACB may prevent lipid-induced insulin resistance and type 2 diabetes, making the enzyme a potential pharmaceutical target for treatment of obesity and type 2 diabetes. Many Frameshifts and conflicts. Wrong choice of CDS.