Fatty acid synthase | |||||||||
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Identifiers | |||||||||
EC no. | 2.3.1.85 | ||||||||
CAS no. | 9045-77-6 | ||||||||
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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fatty acid synthase | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Identifiers | |||||||||||||||||||||||||||||||||||||||||||||||||||
Aliases | fatty acid synthases | ||||||||||||||||||||||||||||||||||||||||||||||||||
External IDs | GeneCards: | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Wikidata | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Fatty acid synthase (FAS) [1] is an enzyme that in humans is encoded by the FASN gene. [2] [3] [4] [5]
Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis. It is not a single enzyme but a whole enzymatic system composed of two identical 272 kDa multifunctional polypeptides, in which substrates are handed from one functional domain to the next. [1] [6] [7] [8] [9]
Its main function is to catalyze the synthesis of palmitate (C16:0, a long-chain saturated fatty acid) from acetyl-CoA and malonyl-CoA, in the presence of NADPH. [5]
The fatty acids are synthesized by a series of decarboxylative Claisen condensation reactions from acetyl-CoA and malonyl-CoA. Following each round of elongation the beta keto group is reduced to the fully saturated carbon chain by the sequential action of a ketoreductase (KR), dehydratase (DH), and enoyl reductase (ER). The growing fatty acid chain is carried between these active sites while attached covalently to the phosphopantetheine prosthetic group of an acyl carrier protein (ACP), and is released by the action of a thioesterase (TE) upon reaching a carbon chain length of 16 (palmitic acid). [1]
There are two principal classes of fatty acid synthases.
The mechanism of FAS I and FAS II elongation and reduction is the same, as the domains of the FAS II enzymes are largely homologous to their domain counterparts in FAS I multienzyme polypeptides. However, the differences in the organization of the enzymes - integrated in FAS I, discrete in FAS II - gives rise to many important biochemical differences. [12]
The evolutionary history of fatty acid synthases are very much intertwined with that of polyketide synthases (PKS). Polyketide synthases use a similar mechanism and homologous domains to produce secondary metabolite lipids. Furthermore, polyketide synthases also exhibit a Type I and Type II organization. FAS I in animals is thought to have arisen through modification of PKS I in fungi, whereas FAS I in fungi and the CMN group of bacteria seem to have arisen separately through the fusion of FAS II genes. [10]
Mammalian FAS consists of a homodimer of two identical protein subunits, in which three catalytic domains in the N-terminal section (-ketoacyl synthase (KS), malonyl/acetyltransferase (MAT), and dehydrase (DH)), are separated by a core region (known as the interdomain) of 600 residues from four C-terminal domains (enoyl reductase (ER), -ketoacyl reductase (KR), acyl carrier protein (ACP) and thioesterase (TE)). [13] [14] The interdomain region allows the two monomeric domains to form a dimer. [13]
The conventional model for organization of FAS (see the 'head-to-tail' model on the right) is largely based on the observations that the bifunctional reagent 1,3-dibromopropanone (DBP) is able to crosslink the active site cysteine thiol of the KS domain in one FAS monomer with the phosphopantetheine prosthetic group of the ACP domain in the other monomer. [15] [16] Complementation analysis of FAS dimers carrying different mutations on each monomer has established that the KS and MAT domains can cooperate with the ACP of either monomer. [17] [18] and a reinvestigation of the DBP crosslinking experiments revealed that the KS active site Cys161 thiol could be crosslinked to the ACP 4'-phosphopantetheine thiol of either monomer. [19] In addition, it has been recently reported that a heterodimeric FAS containing only one competent monomer is capable of palmitate synthesis. [20]
The above observations seemed incompatible with the classical 'head-to-tail' model for FAS organization, and an alternative model has been proposed, predicting that the KS and MAT domains of both monomers lie closer to the center of the FAS dimer, where they can access the ACP of either subunit (see figure on the top right). [21]
A low resolution X-ray crystallography structure of both pig (homodimer) [22] and yeast FAS (heterododecamer) [23] along with a ~6 Å resolution electron cryo-microscopy (cryo-EM) yeast FAS structure [24] have been solved.
The solved structures of yeast FAS and mammalian FAS show two distinct organizations of highly conserved catalytic domains/enzymes in this multi-enzyme cellular machine. Yeast FAS has a highly efficient rigid barrel-like structure with 6 reaction chambers which synthesize fatty acids independently, while the mammalian FAS has an open flexible structure with only two reaction chambers. However, in both cases the conserved ACP acts as the mobile domain responsible for shuttling the intermediate fatty acid substrates to various catalytic sites. A first direct structural insight into this substrate shuttling mechanism was obtained by cryo-EM analysis, where ACP is observed bound to the various catalytic domains in the barrel-shaped yeast fatty acid synthase. [24] The cryo-EM results suggest that the binding of ACP to various sites is asymmetric and stochastic, as also indicated by computer-simulation studies [25]
Metabolism and homeostasis of fatty acid synthase is transcriptionally regulated by Upstream Stimulatory Factors (USF1 and USF2) and sterol regulatory element binding protein-1c (SREBP-1c) in response to feeding/insulin in living animals. [26] [27]
Although liver X receptors (LXRs) modulate the expression of sterol regulatory element binding protein-1c (SREBP-1c) in feeding, regulation of FAS by SREBP-1c is USF-dependent. [27] [28] [29] [30]
Acylphloroglucinols isolated from the fern Dryopteris crassirhizoma show a fatty acid synthase inhibitory activity. [31]
The FASN gene has been investigated as a possible oncogene. [32] FAS is upregulated in breast and gastric cancers, as well as being an indicator of poor prognosis, and so may be worthwhile as a chemotherapeutic target. [33] [34] [35] FAS inhibitors are therefore an active area of drug discovery research. [36] [37] [38] [39] [40]
FAS may also be involved in the production of an endogenous ligand for the nuclear receptor PPARalpha, the target of the fibrate drugs for hyperlipidemia, [41] and is being investigated as a possible drug target for treating the metabolic syndrome. [42] Orlistat which is a gastrointestinal lipase inhibitor also inhibits FAS and has a potential as a medicine for cancer. [43] [44]
In some cancer cell lines, this protein has been found to be fused with estrogen receptor alpha (ER-alpha), in which the N-terminus of FAS is fused in-frame with the C-terminus of ER-alpha. [5]
An association with uterine leiomyomata has been reported. [45]
Mupirocin, sold under the brand name Bactroban among others, is a topical antibiotic useful against superficial skin infections such as impetigo or folliculitis. It may also be used to get rid of methicillin-resistant S. aureus (MRSA) when present in the nose without symptoms. Due to concerns of developing resistance, use for greater than ten days is not recommended. It is used as a cream or ointment applied to the skin.
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.
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.
Cerulenin is an antifungal antibiotic that inhibits fatty acid and steroid biosynthesis. It was the first natural product antibiotic known to inhibit lipid synthesis. In fatty acid synthesis, it has been reported to bind in equimolar ratio to b-keto-acyl-ACP synthase, one of the seven moieties of fatty acid synthase, blocking the interaction of malonyl-CoA. It also has the related activity of stimulating fatty acid oxidation through the activation of CPT1, another enzyme normally inhibited by malonyl-CoA. Inhibition involves covalent thioacylation that permanently inactivates the enzymes. These two behaviors may increase the availability of energy in the form of ATP, perhaps sensed by AMPK, in the hypothalamus.
Sterol regulatory element-binding proteins (SREBPs) are transcription factors that bind to the sterol regulatory element DNA sequence TCACNCCAC. Mammalian SREBPs are encoded by the genes SREBF1 and SREBF2. SREBPs belong to the basic-helix-loop-helix leucine zipper class of transcription factors. Unactivated SREBPs are attached to the nuclear envelope and endoplasmic reticulum membranes. In cells with low levels of sterols, SREBPs are cleaved to a water-soluble N-terminal domain that is translocated to the nucleus. These activated SREBPs then bind to specific sterol regulatory element DNA sequences, thus upregulating the synthesis of enzymes involved in sterol biosynthesis. Sterols in turn inhibit the cleavage of SREBPs and therefore synthesis of additional sterols is reduced through a negative feed back loop.
The liver X receptor (LXR) is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors (LXRs) are important regulators of cholesterol, fatty acid, and glucose homeostasis. LXRs were earlier classified as orphan nuclear receptors, however, upon discovery of endogenous oxysterols as ligands they were subsequently deorphanized.
The long chain fatty acyl-CoA ligase is an enzyme of the ligase family that activates the oxidation of complex fatty acids. Long chain fatty acyl-CoA synthetase catalyzes the formation of fatty acyl-CoA by a two-step process proceeding through an adenylated intermediate. The enzyme catalyzes the following reaction,
Acetyl-CoA synthetase (ACS) or Acetate—CoA ligase is an enzyme involved in metabolism of acetate. It is in the ligase class of enzymes, meaning that it catalyzes the formation of a new chemical bond between two large molecules.
In molecular biology, Beta-ketoacyl-ACP synthase EC 2.3.1.41, is an enzyme involved in fatty acid synthesis. It typically uses malonyl-CoA as a carbon source to elongate ACP-bound acyl species, resulting in the formation of ACP-bound β-ketoacyl species such as acetoacetyl-ACP.
Squalene synthase (SQS) or farnesyl-diphosphate:farnesyl-diphosphate farnesyl transferase is an enzyme localized to the membrane of the endoplasmic reticulum. SQS participates in the isoprenoid biosynthetic pathway, catalyzing a two-step reaction in which two identical molecules of farnesyl pyrophosphate (FPP) are converted into squalene, with the consumption of NADPH. Catalysis by SQS is the first committed step in sterol synthesis, since the squalene produced is converted exclusively into various sterols, such as cholesterol, via a complex, multi-step pathway. SQS belongs to squalene/phytoene synthase family of proteins.
Sterol regulatory element-binding transcription factor 1 (SREBF1) also known as sterol regulatory element-binding protein 1 (SREBP-1) is a protein that in humans is encoded by the SREBF1 gene.
In enzymology, a [acyl-carrier-protein] S-malonyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a β-ketoacyl-[acyl-carrier-protein] synthase III (EC 2.3.1.180) is an enzyme that catalyzes the chemical reaction
Fatty-acyl-CoA Synthase, or more commonly known as yeast fatty acid synthase, is an enzyme complex responsible for fatty acid biosynthesis, and is of Type I Fatty Acid Synthesis (FAS). Yeast fatty acid synthase plays a pivotal role in fatty acid synthesis. It is a 2.6 MDa barrel shaped complex and is composed of two, unique multi-functional subunits: alpha and beta. Together, the alpha and beta units are arranged in an α6β6 structure. The catalytic activities of this enzyme complex involves a coordination system of enzymatic reactions between the alpha and beta subunits. The enzyme complex therefore consists of six functional centers for fatty acid synthesis.
Upstream stimulatory factor 1 is a protein that in humans is encoded by the USF1 gene.
Oxysterol-binding protein 1 is a protein that in humans is encoded by the OSBP gene.
Very long-chain acyl-CoA synthetase is an enzyme that in humans is encoded by the SLC27A2 gene.
Ketoacyl synthases (KSs) catalyze the condensation reaction of acyl-CoA or acyl-acyl ACP with malonyl-CoA to form 3-ketoacyl-CoA or with malonyl-ACP to form 3-ketoacyl-ACP. This reaction is a key step in the fatty acid synthesis cycle, as the resulting acyl chain is two carbon atoms longer than before. KSs exist as individual enzymes, as they do in type II fatty acid synthesis and type II polyketide synthesis, or as domains in large multidomain enzymes, such as type I fatty acid synthases (FASs) and polyketide synthases (PKSs). KSs are divided into five families: KS1, KS2, KS3, KS4, and KS5.
BI 99179 is a selective small molecule inhibitor suitable for the in vivo validation of type 1 fatty acid synthase (FAS) as a therapeutic target for lipid metabolism-related disorders which has been discovered by Boehringer Ingelheim.
UBXD8 is a protein in the Ubiquitin regulatory X (UBX) domain-containing protein family. The UBX domain contains many eukaryotic proteins that have similarities in amino acid sequence to the tiny protein modifier ubiquitin. UBXD8 engages in a molecular interaction with p97, a protein that is essential for the degradation of membrane proteins associated with the endoplasmic reticulum (ER) through the proteasome. Ubxd8 possesses a UBA domain, alongside the UBX domain, that could interact with polyubiquitin chains. Additionally, it possesses a UAS domain of undetermined function, and this protein is used as a protein sensor that detects long chain unsaturated fatty acids (FAs), having a vital function in regulating the balance of Fatty Acids within cells to maintain cellular homeostasis.