Acetyl-CoA carboxylase

Enzyme that regulates the metabolism of fatty acids

Acetyl CoA carboxylase (ACC) is a biotin dependent enzyme that catalyzes the irreversible carboxylation of acetyl CoA to produce malonyl CoA through its two catalytic activities, biotin carboxylase (BC) and carboxyltransferase (CT). ACC is a multi subunit enzyme in most prokaryotes and in the chloroplasts of most plants and algae, whereas it is a large, multi domain enzyme in the cytoplasm of most eukaryotes. The most important function of ACC is to provide the malonyl CoA substrate for the biosynthesis of fatty acids. The activity of ACC can be controlled at the transcriptional level as well as by small molecule modulators and covalent modification. The human genome contains the genes for two different ACCs—ACACA and ACACB.

Prokaryotes and plants have multi subunit ACCs composed of several polypeptides. Biotin carboxylase (BC) activity, biotin carboxyl carrier protein (BCCP), and carboxyl transferase (CT) activity are each contained on a different subunit. The stoichiometry of these subunits in the ACC holoenzyme differs amongst organisms. ACC functional regions, starting from the N terminus to C terminus are the biotin carboxylase (BC), biotin binding (BB), carboxyl transferase (CT), and ATP binding (AB). AB lies within BC. Biotin is covalently attached through an amide bond to the long side chain of a lysine reside in BB. As BB is between BC and CT regions, biotin can easily translocate to both of the active sites where it is required. In mammals where two isoforms of ACC are expressed, the main structural difference between these isoforms is the extended ACC2 N terminus containing a mitochondrial targeting sequence. and accD encodes its beta subunit.

The overall reaction of ACAC(A,B) proceeds by a two step mechanism. The first reaction is carried out by BC and involves the ATP dependent carboxylation of biotin with bicarbonate serving as the source of CO2. The carboxyl group is transferred from biotin to acetyl CoA to form malonyl CoA in the second reaction, which is catalyzed by CT. In the active site, the reaction proceeds with extensive interaction of the residues Glu296 and positively charged Arg338 and Arg292 with the substrates. Two Mg2+ are coordinated by the phosphate groups on the ATP, and are required for ATP binding to the enzyme. Bicarbonate is deprotonated by Glu296, although in solution, this proton transfer is unlikely as the pKa of bicarbonate is 10.3. The enzyme apparently manipulates the pKa to facilitate the deprotonation of bicarbonate. The pKa of bicarbonate is decreased by its interaction with positively charged side chains of Arg338 and Arg292. Furthermore, Glu296 interacts with the side chain of Glu211, an interaction that has been shown to cause an increase in the apparent pKa. Following deprotonation of bicarbonate, the oxygen of the bicarbonate acts as a nucleophile and attacks the gamma phosphate on ATP. The carboxyphosphate intermediate quickly decomposes to CO2 and PO43−. The PO43− deprotonates biotin, creating an enolate, stabilized by Arg338, that subsequently attacks CO2 resulting in the production of carboxybiotin. In oxidative tissues, such as the skeletal muscle and the heart, the ratio of ACC2 expressed is higher. ACC1 and ACC2 are both highly expressed in the liver where both fatty acid oxidation and synthesis are important. The differences in tissue distribution indicate that ACC1 maintains regulation of fatty acid synthesis whereas ACC2 mainly regulates fatty acid oxidation (beta oxidation). A mitochondrial isoform of ACC1 (mACC1) plays a partially redundant role in lipoic acid synthesis and thus in protein lipoylation by providing malonyl CoA for mitochondrial fatty acid synthesis (mtFASII) in tandem with ACSF3.

The regulation of mammalian ACC is complex, in order to control two distinct pools of malonyl CoA that direct either the inhibition of beta oxidation or the activation of lipid biosynthesis. Through a feed forward loop, citrate allosterically activates ACC. Citrate may increase ACC polymerization to increase enzymatic activity; however, it is unclear if polymerization is citrate's main mechanism of increasing ACC activity or if polymerization is an artifact of in vitro experiments. Other allosteric activators include glutamate and other dicarboxylic acids. Long and short chain fatty acyl CoAs are negative feedback inhibitors of ACC. One such negative allosteric modulator is palmitoyl coA. Phosphorylation can result when the hormones glucagon or epinephrine bind to cell surface receptors, but the main cause of phosphorylation is due to a rise in AMP levels when the energy status of the cell is low, leading to the activation of the AMP activated protein kinase (AMPK). AMPK is the main kinase regulator of ACC, able to phosphorylate a number of serine residues on both isoforms of ACC. On ACC1, AMPK phosphorylates Ser79, Ser1200, and Ser1215. Protein kinase A also has the ability to phosphorylate ACC, with a much greater ability to phosphorylate ACC2 than ACC1. Ser80 and Ser1263 on ACC1 may also serve as a site of phosphorylation as a regulatory mechanism. However, the physiological significance of protein kinase A in the regulation of ACC is currently unknown. Researchers hypothesize there are other ACC kinases important to its regulation as there are many other possible phosphorylation sites on ACC. When insulin binds to its receptors on the cellular membrane, it activates a phosphatase enzyme called protein phosphatase 2A (PP2A) to dephosphorylate the enzyme; thereby removing the inhibitory effect. Furthermore, insulin induces a phosphodiesterase that lowers the level of cAMP in the cell, thus inhibiting PKA, and also inhibits AMPK directly. This protein may use the morpheein model of allosteric regulation.

At the juncture of lipid synthesis and oxidation pathways, ACC presents many clinical possibilities for the production of novel antibiotics and the development of new therapies for diabetes, obesity, and other manifestations of metabolic syndrome. Researchers aim to take advantage of structural differences between bacterial and human ACCs to create antibiotics specific to the bacterial ACC, in efforts to minimize side effects to patients. Promising results for the usefulness of an ACC inhibitor include the finding that mice with no expression of ACC2 have continuous fatty acid oxidation, reduced body fat mass, and reduced body weight despite an increase in food consumption. These mice are also protected from diabetes. A lack of ACC1 in mutant mice is lethal already at the embryonic stage. However, it is unknown whether drugs targeting ACCs in humans must be specific for ACC2. Firsocostat (formerly GS 976, ND 630, NDI 010976) is a potent allosteric ACC inhibitor, acting at the BC domain of ACC. Firsocostat is under development in 2019 (Phase II) by the pharmaceutical company Gilead as part of a combination treatment for non alcoholic steatohepatitis (NASH), believed to be an increasing cause of liver failure. In addition, plant selective ACC inhibitors are in widespread use as herbicides, which suggests clinical application against Apicomplexa parasites that rely on a plant derived ACC isoform, including malaria. The heterogeneous clinical phenotypes of the metabolic disease combined malonic and methylmalonic aciduria (CMAMMA) due to ACSF3 deficiency are thought to result from partial compensation of a mitochondrial isoform of ACC1 (mACC1) for deficient ACSF3 in mitochondrial fatty acid synthesis (mtFASII).