acetyl-CoA carboxylase | |||||||||
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Identifiers | |||||||||
EC no. | 6.4.1.2 | ||||||||
CAS no. | 9023-93-2 | ||||||||
Databases | |||||||||
IntEnz | IntEnz view | ||||||||
BRENDA | BRENDA entry | ||||||||
ExPASy | NiceZyme view | ||||||||
KEGG | KEGG entry | ||||||||
MetaCyc | metabolic pathway | ||||||||
PRIAM | profile | ||||||||
PDB structures | RCSB PDB PDBe PDBsum | ||||||||
Gene Ontology | AmiGO / QuickGO | ||||||||
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Acetyl-CoA carboxylase alpha | |||||||
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Identifiers | |||||||
Symbol | ACACA | ||||||
Alt. symbols | ACAC, ACC1, ACCA | ||||||
NCBI gene | 31 | ||||||
HGNC | 84 | ||||||
OMIM | 601557 | ||||||
RefSeq | NM_198839 | ||||||
UniProt | Q13085 | ||||||
Other data | |||||||
EC number | 6.4.1.2 | ||||||
Locus | Chr. 17 q21 | ||||||
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Acetyl-CoA carboxylase beta | |||||||
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Identifiers | |||||||
Symbol | ACACB | ||||||
Alt. symbols | ACC2, ACCB | ||||||
NCBI gene | 32 | ||||||
HGNC | 85 | ||||||
OMIM | 200350 | ||||||
RefSeq | NM_001093 | ||||||
UniProt | O00763 | ||||||
Other data | |||||||
EC number | 6.4.1.2 | ||||||
Locus | Chr. 12 q24.1 | ||||||
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Acetyl-CoA carboxylase (ACC) is a biotin-dependent enzyme (EC 6.4.1.2) 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. [1] 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 [2] — ACACA [3] and ACACB . [4]
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. [1] Humans and most eukaryotes have evolved an ACC with CT and BC catalytic domains and BCCP domains on a single polypeptide. Most plants also have this homomeric form in cytosol. [5] 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. [1]
The polypeptides composing the multi-subunit ACCs of prokaryotes and plants are encoded by distinct genes. In Escherichia coli , accA encodes the alpha subunit of the acetyl-CoA carboxylase, [6] and accD encodes its beta subunit. [7]
The overall reaction of ACAC(A,B) proceeds by a two-step mechanism. [8] 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. [9] 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. [9] The carboxybiotin translocates to the carboxyl transferase (CT) active site, where the carboxyl group is transferred to acetyl-CoA. In contrast to the BC domain, little is known about the reaction mechanism of CT. A proposed mechanism is the release of CO2 from biotin, which subsequently abstracts a proton from the methyl group from acetyl-CoA carboxylase. The resulting enolate attacks CO2 to form malonyl-CoA. In a competing mechanism, proton abstraction is concerted with the attack of acetyl-CoA.
The function of ACC is to regulate the metabolism of fatty acids. When the enzyme is active, the product, malonyl-CoA, is produced which is a building block for new fatty acids and can inhibit the transfer of the fatty acyl group from acyl-CoA to carnitine with carnitine acyltransferase, which inhibits the beta-oxidation of fatty acids in the mitochondria.
In mammals, two main isoforms of ACC are expressed, ACC1 and ACC2, which differ in both tissue distribution and function. ACC1 is found in the cytoplasm of all cells but is enriched in lipogenic tissue, such as adipose tissue and lactating mammary glands, where fatty acid synthesis is important. [10] 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. [11] 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. [12] [13]
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. [14]
Mammalian ACC1 and ACC2 are regulated transcriptionally by multiple promoters which mediate ACC abundance in response to the cells nutritional status. Activation of gene expression through different promoters results in alternative splicing; however, the physiological significance of specific ACC isozymes remains unclear. [11] The sensitivity to nutritional status results from the control of these promoters by transcription factors such as sterol regulatory element-binding protein 1, controlled by insulin at the transcriptional level, and ChREBP, which increases in expression with high carbohydrates diets. [15] [16]
Through a feed-forward loop, citrate allosterically activates ACC. [17] 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. [18] Long and short chain fatty acyl-CoAs are negative feedback inhibitors of ACC. [19] One such negative allosteric modulator is palmitoyl-CoA. [20]
Phosphorylation can result when the hormones glucagon [21] or epinephrine [22] 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. [23] 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. [24] 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. [25]
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.[ citation needed ]
This protein may use the morpheein model of allosteric regulation. [26]
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. [27] 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. [14] 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. [28]
Firsocostat (formerly GS-976, ND-630, NDI-010976) is a potent allosteric ACC inhibitor, acting at the BC domain of ACC. [29] Firsocostat is under development in 2019 (Phase II) [30] 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. [31]
In addition, plant-selective ACC inhibitors are in widespread use as herbicides, [32] which suggests clinical application against Apicomplexa parasites that rely on a plant-derived ACC isoform, [33] 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). [34]
Acetyl-CoA is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production.
Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It raises the concentration of glucose and fatty acids in the bloodstream and is considered to be the main catabolic hormone of the body. It is also used as a medication to treat a number of health conditions. Its effect is opposite to that of insulin, which lowers extracellular glucose. It is produced from proglucagon, encoded by the GCG gene.
Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids from the cytosol into mitochondria to be oxidized for free energy production, and also participates in removing products of metabolism from cells. Given its key metabolic roles, carnitine is concentrated in tissues like skeletal and cardiac muscle that metabolize fatty acids as an energy source. Generally individuals, including strict vegetarians, synthesize enough L-carnitine in vivo.
Ketogenesis is the biochemical process through which organisms produce ketone bodies by breaking down fatty acids and ketogenic amino acids. The process supplies energy to certain organs, particularly the brain, heart and skeletal muscle, under specific scenarios including fasting, caloric restriction, sleep, or others.
5' AMP-activated protein kinase or AMPK or 5' adenosine monophosphate-activated protein kinase is an enzyme that plays a role in cellular energy homeostasis, largely to activate glucose and fatty acid uptake and oxidation when cellular energy is low. It belongs to a highly conserved eukaryotic protein family and its orthologues are SNF1 in yeast, and SnRK1 in plants. It consists of three proteins (subunits) that together make a functional enzyme, conserved from yeast to humans. It is expressed in a number of tissues, including the liver, brain, and skeletal muscle. In response to binding AMP and ADP, the net effect of AMPK activation is stimulation of hepatic fatty acid oxidation, ketogenesis, stimulation of skeletal muscle fatty acid oxidation and glucose uptake, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipogenesis, inhibition of adipocyte lipolysis, and modulation of insulin secretion by pancreatic β-cells.
Pyruvate carboxylase (PC) encoded by the gene PC is an enzyme of the ligase class that catalyzes the physiologically irreversible carboxylation of pyruvate to form oxaloacetate (OAA).
Malonyl-CoA is a coenzyme A derivative of malonic acid.
In biochemistry, lipogenesis is the conversion of fatty acids and glycerol into fats, or a metabolic process through which acetyl-CoA is converted to triglyceride for storage in fat. Lipogenesis encompasses both fatty acid and triglyceride synthesis, with the latter being the process by which fatty acids are esterified to glycerol before being packaged into very-low-density lipoprotein (VLDL). Fatty acids are produced in the cytoplasm of cells by repeatedly adding two-carbon units to acetyl-CoA. Triacylglycerol synthesis, on the other hand, occurs in the endoplasmic reticulum membrane of cells by bonding three fatty acid molecules to a glycerol molecule. Both processes take place mainly in liver and adipose tissue. Nevertheless, it also occurs to some extent in other tissues such as the gut and kidney. A review on lipogenesis in the brain was published in 2008 by Lopez and Vidal-Puig. After being packaged into VLDL in the liver, the resulting lipoprotein is then secreted directly into the blood for delivery to peripheral tissues.
Biotin carboxyl carrier protein (BCCP) refers to proteins containing a biotin attachment domain that carry biotin and carboxybiotin throughout the ATP-dependent carboxylation by biotin-dependent carboxylases. The biotin carboxyl carrier protein is an Acetyl CoA subunit that allows for Acetyl CoA to be catalyzed and converted to malonyl-CoA. More specifically, BCCP catalyzes the carboxylation of the carrier protein to form an intermediate. Then the carboxyl group is transferred by the transcacrboxylase to form the malonyl-CoA. This conversion is an essential step in the biosynthesis of fatty acids. In the case of E. coli Acetyl-CoA carboxylase, the BCCP is a separate protein known as accB. On the other hand, in Haloferax mediterranei, propionyl-CoA carboxylase, the BCCP pccA is fused with biotin carboxylase.
Malonyl-CoA decarboxylase, is found in bacteria and humans and has important roles in regulating fatty acid metabolism and food intake, and it is an attractive target for drug discovery. It is an enzyme associated with Malonyl-CoA decarboxylase deficiency. In humans, it is encoded by the MLYCD gene.
Methylcrotonyl CoA carboxylase is a biotin-requiring enzyme located in the mitochondria. MCC uses bicarbonate as a carboxyl group source to catalyze the carboxylation of a carbon adjacent to a carbonyl group performing the fourth step in processing leucine, an essential amino acid.
In biochemistry, fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. This process takes place in the cytoplasm of the cell. Most of the acetyl-CoA which is converted into fatty acids is derived from carbohydrates via the glycolytic pathway. The glycolytic pathway also provides the glycerol with which three fatty acids can combine to form triglycerides, the final product of the lipogenic process. When only two fatty acids combine with glycerol and the third alcohol group is phosphorylated with a group such as phosphatidylcholine, a phospholipid is formed. Phospholipids form the bulk of the lipid bilayers that make up cell membranes and surrounds the organelles within the cells. In addition to cytosolic fatty acid synthesis, there is also mitochondrial fatty acid synthesis (mtFASII), in which malonyl-CoA is formed from malonic acid with the help of malonyl-CoA synthetase (ACSF3), which then becomes the final product octanoyl-ACP (C8) via further intermediate steps.
Carnitine palmitoyltransferase I (CPT1) also known as carnitine acyltransferase I, CPTI, CAT1, CoA:carnitine acyl transferase (CCAT), or palmitoylCoA transferase I, is a mitochondrial enzyme responsible for the formation of acyl carnitines by catalyzing the transfer of the acyl group of a long-chain fatty acyl-CoA from coenzyme A to l-carnitine. The product is often palmitoylcarnitine, but other fatty acids may also be substrates. It is part of a family of enzymes called carnitine acyltransferases. This "preparation" allows for subsequent movement of the acyl carnitine from the cytosol into the intermembrane space of mitochondria.
The Randle cycle, also known as the glucose fatty-acid cycle, is a metabolic process involving the competition of glucose and fatty acids for substrates. It is theorized to play a role in explaining type 2 diabetes and insulin resistance.
In enzymology, a biotin carboxylase (EC 6.3.4.14) is an enzyme that catalyzes the chemical reaction
5'-AMP-activated protein kinase catalytic subunit alpha-2 is an enzyme that in humans is encoded by the PRKAA2 gene.
5'-AMP-activated protein kinase subunit beta-1 is an enzyme that in humans is encoded by the PRKAB1 gene.
5'-AMP-activated protein kinase subunit gamma-1 is an enzyme that in humans is encoded by the PRKAG1 gene.
Acetyl-CoA carboxylase 1 also known as ACC-alpha or ACCa is an enzyme that in humans is encoded by the ACACA gene.
Acetyl-CoA carboxylase 2 also known as ACC-beta or ACC2 is an enzyme that in humans is encoded by the ACACB gene.