Acetyl-CoA acetyltransferase, mitochondrial, also known as acetoacetyl-CoA thiolase, is an enzyme that in humans is encoded by the ACAT1 (Acetyl-Coenzyme A acetyltransferase 1) gene. [5]
Acetyl-Coenzyme A acetyltransferase 1 is an acetyl-CoA C-acetyltransferase enzyme.
The gene is located on chromosome 11q22.3-q23.1, spanning approx. 27 kb and contains twelve exons interrupted by eleven introns. [6] The region flanking the 5′ end of the gene lacks a TATA box, but contains many GC's and also has two CAAT boxes. The gene also may have a binding site for the transcription factor Sp1, and has sequences resembling the binding sites of several other transcription factors. Additionally, there is a 101-bp DNA fragment immediately upstream from the cap site that has promoter activity. [7]
The human ACAT1 gene produces a chimeric mRNA through trans-splicing, a process in which separate transcripts from chromosomes 1 and 7 are spliced together. The chimeric mRNA transcript uses two sections to initiate translation: AUG(1397-1399) and GGC(1274-1276). Initiation of the first codon (AUG) results in the translation of a 50-kDa ACAT1, and initiation of the other (GGC) produces another enzymatically active 56-kDa isoform respectively; the 56kDa isoform is naturally present in human cells, including human monocyte-derived macrophages. [8]
The resulting transcript encodes ACAT1, which is a 45.1 kDa protein composed of 427 amino acids. [9] [10] It is also a homotetrameric protein that has nine transmembrane domains (TMDs). One active residue is a Histidine at the 460th position, which is in the 7th TMD. ACAT1 has seven free Cysteine residues, but they do not affect catalytic activity. There are two functional sections of this protein, TMD7 and TMD8; one side is involved in substrate binding and catalysis, while the other is involved in subunit interactions and binding [11]
This gene encodes a mitochondrially localized enzyme that catalyzes the reversible formation of acetoacetyl-CoA from two molecules of acetyl-CoA. [5] The ACAT1 enzyme has a few unique properties. First, it is activated by potassium ions binding near the CoA binding site and the catalytic site. This binding causes a structural change in the active site loop. Additionally, this enzyme is able to use 2-methyl-branched acetoacetyl-CoA as a substrate, making it a unique thiolase. [12] ACAT1 is regulated at both transcriptional and translational levels. ACAT1 enzyme activity is enhanced ACAT1's expression is promoted transcriptionally by leptin, [13] angiotensin II, [14] and insulin in human monocytes/macrophages. [15] Insulin-mediated regulation also involves ERK, p38MAPK, and JNK signaling pathways. [16]
Mutations of the ACAT1 gene are associated with a deficiency in the encoded protein mitochondrial acetoacetyl-CoA thiolase (T2); this is also known as Beta-ketothiolase deficiency. Many mutations have been identified in specific populations, and large scale studies have been performed to determine the allelic and genotypic frequency for the defective gene. [17] As mitochondrial acetoacetyl-CoA thiolase is involved in beta-oxidation, a deficiency in this enzyme is marked by an increased amount of cholesterol compounds. Additionally, the isoleucine amino acid pathway is affected, such that proper metabolism of it is halted. This deficiency belongs to a more general class of disorders known as organic acidemias, in which the dysfunction of a specific step of amino acid catabolism results in the excretion of non-amino acids in the urine. This deficiency specifically presents as ketosis, acidosis, as well as hypoglycemia, but there are other clinical manifestations as well. The characteristics of organic acidemia disorders are vomiting, poor feeding, neurologic symptoms such as seizures and abnormal tone, and lethargy progressing to coma, which are all manifestations of toxic encephalopathy. The clinical outcome of infants with these disorders is largely determined by the time of diagnosis, with the potential outcome greatly improving if the disease is diagnosed in the first ten days of life. Ketothiolase deficiency is diagnosed by performing GC-MS and quantitative amino acid analysis in the urine; the diagnostic markers are 2-methyl-3-hydroxybutyric acid, 2-methylacetoacetic acid, and tiglylglycine. The disease is managed by trying to restore biochemical and physiologic homeostasis; common therapies include restricting diet to avoid the precursor amino acids and use of compounds to either dispose of toxic metabolites or increase enzyme activity. This disease is inherited in an autosomal recessive manner, meaning that carriers of the gene do not show symptoms of the disease. [6]
Additionally, expression of ACAT1 has been associated with manifestations of prostate cancer, in that ACAT1 is more significantly expressed in aggressive prostate cancer tissue samples when compared to its expression in benign cells. [18] [19]
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.
In biochemistry and metabolism, beta oxidation (also β-oxidation) is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA. Acetyl-CoA enters the citric acid cycle, generating NADH and FADH2, which are electron carriers used in the electron transport chain. It is named as such because the beta carbon of the fatty acid chain undergoes oxidation and is converted to a carbonyl group to start the cycle all over again. Beta-oxidation is primarily facilitated by the mitochondrial trifunctional protein, an enzyme complex associated with the inner mitochondrial membrane, although very long chain fatty acids are oxidized in peroxisomes.
Malonyl-CoA is a coenzyme A derivative of malonic acid.
Mitochondrial trifunctional protein (MTP) is a protein attached to the inner mitochondrial membrane which catalyzes three out of the four steps in beta oxidation. MTP is a hetero-octamer composed of four alpha and four beta subunits:
Acyl-CoA is a group of coenzymes that metabolize carboxylic acids. Fatty acyl-CoA's are susceptible to beta oxidation, forming, ultimately, acetyl-CoA. The acetyl-CoA enters the citric acid cycle, eventually forming several equivalents of ATP. In this way, fats are converted to ATP, the common biochemical energy carrier.
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.
Trifunctional enzyme subunit beta, mitochondrial (TP-beta) also known as 3-ketoacyl-CoA thiolase, acetyl-CoA acyltransferase, or beta-ketothiolase is an enzyme that in humans is encoded by the HADHB gene.
Acetoacetyl CoA is the precursor of HMG-CoA in the mevalonate pathway, which is essential for cholesterol biosynthesis. It also takes a similar role in the ketone bodies synthesis (ketogenesis) pathway of the liver. In the ketone bodies digestion pathway, it is no longer associated with having HMG-CoA as a product or as a reactant.
Thiolases, also known as acetyl-coenzyme A acetyltransferases (ACAT), are enzymes which convert two units of acetyl-CoA to acetoacetyl CoA in the mevalonate pathway.
Butyryl-CoA is an organic coenzyme A-containing derivative of butyric acid. It is a natural product found in many biological pathways, such as fatty acid metabolism, fermentation, and 4-aminobutanoate (GABA) degradation. It mostly participates as an intermediate, a precursor to and converted from crotonyl-CoA. This interconversion is mediated by butyryl-CoA dehydrogenase.
In enzymology, an acetyl-CoA C-acetyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a [acyl-carrier-protein] S-acetyltransferase is an enzyme that catalyzes the reversible chemical reaction
In biochemistry, hydroxymethylglutaryl-CoA synthase or HMG-CoA synthase EC 2.3.3.10 is an enzyme which catalyzes the reaction in which acetyl-CoA condenses with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This reaction comprises the second step in the mevalonate-dependent isoprenoid biosynthesis pathway. HMG-CoA is an intermediate in both cholesterol synthesis and ketogenesis. This reaction is overactivated in patients with diabetes mellitus type 1 if left untreated, due to prolonged insulin deficiency and the exhaustion of substrates for gluconeogenesis and the TCA cycle, notably oxaloacetate. This results in shunting of excess acetyl-CoA into the ketone synthesis pathway via HMG-CoA, leading to the development of diabetic ketoacidosis.
Sterol O-acyltransferase is an intracellular protein located in the endoplasmic reticulum that forms cholesteryl esters from cholesterol.
Sterol O-acyltransferase 1, also known as SOAT1, is an enzyme that in humans is encoded by the SOAT1 gene.
Sterol O-acyltransferase 2, also known as SOAT2, is an enzyme that in humans is encoded by the SOAT2 gene.
3-oxoacid CoA-transferase 1 (OXCT1) is an enzyme that in humans is encoded by the OXCT1 gene. It is also known as succinyl-CoA-3-oxaloacid CoA transferase (SCOT). Mutations in the OXCT1 gene are associated with succinyl-CoA:3-oxoacid CoA transferase deficiency. This gene encodes a member of the 3-oxoacid CoA-transferase gene family. The encoded protein is a homodimeric mitochondrial matrix enzyme that plays a central role in extrahepatic ketone body catabolism by catalyzing the reversible transfer of coenzyme A (CoA) from succinyl-CoA to acetoacetate.
Acetyl-CoA acetyltransferase, cytosolic, also known as cytosolic acetoacetyl-CoA thiolase, is an enzyme that in humans is encoded by the ACAT2 gene
3-Ketoacyl-CoA thiolase, peroxisomal also known as acetyl-Coenzyme A acyltransferase 1 is an enzyme that in humans is encoded by the ACAA1 gene.
3-Ketoacyl-CoA thiolase, mitochondrial also known as acetyl-Coenzyme A acyltransferase 2 is an enzyme that in humans is encoded by the ACAA2 gene.
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This article incorporates text from the United States National Library of Medicine, which is in the public domain.
