| Fatty Acid Synthesis | |
|---|---|
 Fatty acids are synthesized via multi-step reactions involving acetyl-CoA, malonyl-CoA, and Fatty Acid Synthase (FAS). | |
| Biochemical Reaction | |
| Part of | Cell |
| Located | Cytoplasm,Mitochondria |
| Category | Metabolic Pathway |
| Central Functions | |
Regulation of Lipid Metabolism | |
Production of Cell Membrane Components | |
Production of Cell Signaling Molecules | |
Cellular Energy Homeostasis | |
| Key Enzymes | |
| Primary Products | |
Saturated Fatty Acids | |
Monounsaturated Fatty Acids | |
Polyunsaturated Fatty Acids | |
| Used to Produce | |
Phospholipids, Glycerolipids, Sphingolipids | |
Triacylglycerols, Cholesteryl Esters | |
Eicosanoids, Prostaglandins | |
| Discovered | |
1813 | Michel Eugene Chevreul Introduces the term "fatty acid" |
1840 | Justus von Liebig Proposes "sugar-to-fat" conversion |
1953 | Jim Mead & Colleagues Uncover pathways for elongation of polyunsaturated fatty acids |
1965 | Diederik Nugteren Uncovers enzymatic chain elongation of fatty acids in rat liver microsomes |
1965 | Diederik Nugteren Postulates acetyl-CoA carboxylase as the rate-limiting step in elongation |
1979 | Bernert & Sprecher Purifies β-hydroxyacyl-CoA dehydrase for the first time |
1997 | Miller & Kunst Uncover enzymes involved in very-long chain fatty acid biosynthesis |
Inbiochemistry,fatty acid synthesis is the process by whichfatty acids, the fundamental building blocks offats, are derived from metabolic intermediates through the coordinated actions ofenzymes.
Fatty acids (FAs), comprise a large group of chemicallyheterogeneous compounds.[1] Each fatty acid is composed of a carboxylic acid attached to an aliphatic hydrocarbon chain, of which is eithersaturated orunsaturated. FAs prove crucial within the cell, as these molecules serve as the indispensable building blocks ofcell membranes. FAs also provide dense, long-term energy sources for the cell, and the biosynthetic production of FAs is essential for maintainingcellular homeostasis.[1]
Fatty acid biosynthetic pathways are highlyevolutionarily conserved across species, though different enzymes and genetic organizations have evolved to reach similarities about the general pathway.[1] In both animals and fungi, all fatty acid synthetic pathways utilize one multifunctional protein complex,type-I fatty acid synthase (FAS), which is divided into the type-Ia FAS (infungi) and the type-Ib FAS (inanimals).[1][2][3] In mostprokaryotes and in theplastids ofplants, fatty acid synthesis occurs viatype-II fatty acid synthase (FAS).[1][4][5][6]
Fatty acid synthesis occurs in thecytosol, where there is a highNADPH/NADP+ ratio available to drive the reactions forward.[7] FAs can be further processed in theendoplasmic reticulum, where they are joined to aglycerol backbone in groups of three to formtriacylglycerol (TAG), or in pairs (with the addition of apolar head group on the C1 of glycerol) to form aphospholipid.[7]
Fatty acids are classified according to the number of carbondouble bonds present about the aliphatic hydrocarbon chain.[7]Saturated fatty acids have no double bonds.[7]Monounsaturated fatty acids have one double bond, whilepolyunsaturated fatty acids have two or more double bonds present about the hydrocarbon chain.[7]
Saturated fatty acids are a primary constituent ofglycerolipids, as well as thephospholipids andsphingolipids found in cellular membranes.[1][8] Common saturated fatty acids includepalmitic acid,butyric acid, andstearic acid, all of which contribute toLDL cholesterol levels and increase the risk of obesity, heart disease, and stroke.[7]Monounsaturated fatty acids are also primary constituent ofglycerolipids and cellular membrane structures.[1][8] Monounsaturated fatty acids includeoleic acid,palmitoleic acid, andvaccenic acid, which help lowerLDL cholesterol and reduce the risk of heart disease and stroke.[7]Polyunsaturated fatty acids are found in thephospholipids of cell membranes, and are also known to act as precursors for a variety oflipid signaling molecules.[1][8] Common polyunsaturated fatty acids include the essentialomega-3 andomega-6 fatty acids, which are crucial for brain, heart, and immune health.[7][8]
While the degree ofsaturation is used to differentiatefatty acids and their chemical composition, it is important to note that the length of thealiphatic hydrocarbon chain also influences fatty acids and their biological roles.[8] Carbon chain lengths can vary greatly within each class of fatty acids, with some having as few as 12-carbons (ie.,dodecanoic acid) and others having as many as 30-carbons (ie.,triacontanoic acid).[8] Fatty acid chain length is known to shape several biophysical properties of thecellular membrane, such as membrane fluidity, microdomain formation, and the assembly of membrane-associated signaling platforms.[8] Chain length can also alter cellular susceptibility todeath or survival through modulation of membrane properties.[8]
Interestingly, mammals are unable to synthesizepolyunsaturated fatty acidsde novo.[8] While mammalian cells express the enzymes necessary for the conversion of carbohydrate-derived and protein-derived carbons intosaturated andmonounsaturated fatty acids, they lack thedesaturase enzymes required for production of the limiting-reagent in polyunsaturated fatty acid synthesis.[8] Thus, polyunsaturated fatty acids are consideredessential, and must be acquired through dietary consumption.[8][9]
Thus, for allnon-essential fatty acids, FA synthesis occursdenovo, wherein fats are derived from simple precursors like carbohydrates (ie., glucose) via acetyl-CoA.[8]De novo fatty acid synthesis primarily occurs in the cytosol ofhepatocytes (ie.,in the liver) andadipocytes (ie.,in adipose tissue or fat).[8][9] Fatty acids are also synthesizedde novo within tissues with high metabolic demands, such as mammary glands (ie.,for milk fat production duringlactation), immune cells (ie.,macrophages,B cells,T cells), and even within the brain (ie.,duringneurogenesis).[10] Notably, although saturated and monounsaturated fatty acids are considered non-essential, both saturated and monounsaturated fatty acids can be synthesizedde novo alongside polyunsaturated fatty acids.[8]
De novo fatty acid synthesis is separated intotwo groups based on the compartment wherein fatty acid synthesis takes place:cytosolic fatty acid synthesis (FAS/FASI) andmitochondrial fatty acid synthesis (mtFAS/mtFASII).[8][9]
Forpalmitoleic acid (C16:1), shorthand notation (red), ω-x notation (blue), and Δ notation (green) are shown. Forpalmitic acid (C16:0),only shorthand notation (red) need be shown.
Generally,fatty acids (FAs) can be defined as organic compounds containing acarboxyl group (–COOH) at one end of an aliphatic hydrocarbon chain (also called the "front" end of the molecule), and amethyl group (–CH3) at the opposite end (also called the "methyl" end of the molecule).[1][11]Fatty acid chains commonly contain anywhere from 4 to 24 carbon atoms, but they are known to reach up to 44 carbon atoms in length.[11] Fatty acid chains typically contain aneven number of carbon atoms (ie., 10, 12, 16) and a linear structure about theacyl chain, though both odd-numbered and branched fatty acids also exist.[11]
Fatty acids can be categorized according to the degree of saturation (number of double bonds) present about the aliphatic hydrocarbon chain.[8][11]Saturated fatty acids (SFAs) have no double bonds.[8]Monounsaturated fatty acids (MUFAs) have one double bond, whilepolyunsaturated fatty acids (PUFAs) have two or more double bonds present about the acyl chain.[8]
Two discretenomenclature systems are used for describing fatty acids:omega (ω–x)nomenclature anddelta (Δ) nomenclature, in addition to simple common names.[11] Within the fields ofmedicine andnutrition, both common names and omega nomenclature are frequently used to discuss fatty acids. Inbiochemistry andlipidsresearch, fatty acid common names, omega nomenclature, and delta nomenclature are all widely used, but delta nomenclature is more typically seen in discussions on specific fatty acids.
Additionally, common fatty acid names are often denoted alongside ashorthand notation (ie., palmitic acid(C16:0)). Herein, the total number of carbons about the fatty acid is listed prior to the total number of double bonds present within the molecule, and is generally formatted as is seen inExample 1. forpalmitic acid (C16:0) andpalmitoleic acid (C16:1).[11]
Inbiochemistry, the most prevalent, internationally-accepted system of nomenclature for fatty acids is defined by theInternational Union of Pure and Applied Chemistry (IUPAC).[11] Herein, theω-x system (also known as theomega x orn-x system) establishes that fatty acids can be identified through the following formation:[11]C:Dω–x
whereC is the total number of carbons,D is the number of double bonds, andω–x indicates the position of the first double bond, counting from the –CH3 end of the fatty acid.[11] The ω–x system curbs ambiguity regardingSFAs andMUFAs, as it specifically denotes both the carbon and unsaturation numbers, as well as the location of the double bond (ie., for MUFAs).[11] Equally, ω–x nomenclature is widely used forPUFAs, particularly for those wherein two consecutive double bonds are consistently separated by amethylene group (–CH2–).[11] However, several PUFAs contain double bonds that are not always interrupted by a methylene group, and thus the ω–x systemfails to establish the specific position of all double bonds aboutPUFA chains.[11] As such, these atypical PUFAs are generically referred to as"non-methylene interrupted" (NMI) fatty acids, and are commonly found in lipids frommarine invertebrates.[11]
Using fundamental principles established by IUPAC, thedelta (Δ) system for lipid notation was developed by researchers within the mid-20th century, to address the need for distinction between fatty acids with the same acyl chain length but different double bond positions. Thus, the Δ system defines the position of double bonds by virtue of the carboxyl (–COOH) carbon using the general formula:C:DΔy
whereC is the total number of carbons,D is the number of double bonds, andΔy indicates the position of each double bond, counting from the –COOH end of the fatty acid.[11] Unlike the omega nomenclature system, each double bond of the fatty acid must be described in the delta system.
Whileessential fatty acids (ie.,linoleic acid) are obtained exclusively through diet, allnon-essential fatty acids must be synthesizeddenovo.[8][12]Acetyl-CoA is the precursor used for fatty acid synthesis in thecytosol; therefore, fatty acid synthesis requires those reactions which produce acetyl-CoA—namely,glycolysis oramino acid metabolism.[12] Regardless of the metabolic source of the product, allde novo lipogenesis relies on the production and availability of cytosolic acetyl-CoA, and thereafter on its conversion into malonyl-CoA.[12]
Acetyl-CoA is generated in the mitochondria frompyruvate molecules derived from glucose viaglycolysis.[12] Within the mitochondria, acetyl-CoA typically combines withoxaloacetate and serves as asubstrate for the synthesis ofcitrate as part of the well knowncitric acid cycle.[12] Notably, theinner mitochondrial membrane isimpermeable to acetyl-CoA, and as such, a specializedshuttle system must be used to import acetyl-CoA into the cytosol for fatty acid production.[12] This process, known as thecitrate–malate shuttle, relies on thetricarboxylate transport protein to import citrate into the cytosol, where it is then split into acetyl-CoA and oxaloacetate by the enzymeATP citrate lyase (ACL).[12] Cytosolic acetyl-CoA is then available for use in fatty acid and cholesterol synthesis, but oxaloacetate must be reduced tomalate in order to reenter the mitochondria.[12]Malate dehydrogenase reduces cytosolic oxaloacetate by couplingNADH oxidation toNAD+, and malate produced by this reaction can be transported back into the mitochondria, thus completing the namesake of thecitrate–malate shuttle.[12]
In the first reaction of fatty acid synthesis, acetyl-CoA is irreversibly carboxylated by acetyl-CoA carboxylase (ACC) to form malonyl-CoA. The formation of malonyl-CoA is the rate-limiting step of fatty acid synthesis.
This stepprovides the malonyl-CoA substrate for use in fatty acid synthesis.
In the second reaction of fatty acid synthesis, acetyl transacylase and malonyl transacylase catalyze the formation of acetyl-ACP and malonyl-ACP, respectively. The transacylase enzymes use the sulfhydryl group of ACP to release CoA and form acetyl-ACP and malonyl-ACP.
This stepactivates acetyl-CoA and malonyl-CoA for use in fatty acid synthesis.
During the third step of fatty acid synthesis, acetyl-ACP and malonyl-ACP undergo a condensation reaction catalyzed by β-ketoacyl-ACP synthase, which produces a four-carbon acetoacetyl-ACP molecule and one molecule of CO2. The condensation reaction is shown in (c).
This is thefirst condensation reaction of fatty acid synthesis.
During the fourth reaction of fatty acid synthesis, acetoacetyl-ACP is reduced by 3-ketoacyl-ACP reductase to form one molecule of β-hydroxyacyl-ACP. The reaction scheme for the reduction is shown in (d). NADPH is used as the reducing agent.
This is thefirst reduction reaction of fatty acid synthesis.
During the fifth reaction of fatty acid synthesis, β-hydroxyacyl-ACP undergoes a dehydration reaction catalyzed by 3-hydroxyacyl-ACP dehydratase. The reaction scheme for the dehydration is shown in (e). One molecule of water is removed from a β-hydroxyacyl intermediate to form a double bond, saturating the chain and producing enoyl-ACP.
This is thefirst dehydration reaction of fatty acid synthesis.
During the sixth reaction of fatty acid synthesis, enoyl-ACP is reduced by enoyl-ACP reductase to form butyryl-ACP. The reaction scheme is shown in (f). Herein, the double bond of the trans-2-enoyl-ACP molecule is reduced to a saturated acyl-ACP using NADPH as the reducing agent.
This is thesecond reduction reaction of fatty acid synthesis.
Fatty acid synthesis begins in the cytosol. During the first reaction, irreversiblecarboxylation of acetyl-CoA tomalonyl-CoA is catalyzed by thebiotin-dependent enzymeacetyl-CoA carboxylase (ACC).[12] Notably, the conversion of acetyl-CoA to malonyl-CoA is therate-limiting step of fatty acid synthesis. Acetyl-CoA carboxylase (ACC) thus represents therate-limiting enzyme in fatty acid synthesis; ACC activity is stimulated by increasing concentrations of cytosolic citrate, and inhibited by increasing concentrations of the fatty acid palmitate.[12]
After malonyl-CoA becomes available by virtue of ACC,fatty acid synthase (FAS) is then able to complete a series of reactions to form the 16-carbon moleculepalmitate. FAS is a complex, multifunctional protein containingseven differentcatalytic sites:acetyl transacylase,malonyl transacylase, β-ketoacyl synthase, β-ketoacyl carrier protein (ACP) reductase,3-hydroxyacyl-ACP dehydratase,enoyl-ACP reductase, andthioesterase.[12] These different enzymes are covalently linked within theFAS complex, allowing for intermediates to be handled efficiently from oneactive site to another without leaving the assembly.[12] After the completion of the first reaction by ACC, fatty acid synthesis thus continues on the FAS complex.
During the second reaction of fatty acid synthesis, acetyl transacylase and malonyl transacylase catalyze the formation of acetyl-ACP and malonyl-ACP, respectively.[12] Acetyl transacylase transfers the acetyl group of acetyl-CoA onto the sulfhydryl group of Acyl Carrier Protein (ACP), releasing CoA and forming acetyl-ACP.[12] An equivalent reaction occurs for malonyl-CoA, in which malonyl transacylase transfers the malonyl group from malonyl-CoA to the sulfhydryl group of Acyl Carrier Protein (ACP), releasing CoA and forming malonyl-ACP.[12] These two reactions are essential, as they prime the acetyl and malonyl groups for condensation in the subsequent chain elongation reaction step.[12]
After the production of acetyl-ACP and malonyl-ACP, fatty acid synthesis begins to cycle through repetitions of the following reaction sequence:condensation →reduction →dehydration →reduction.[12] Ultimately, this elongation reaction sequence repeats through 7 cycles to form one molecule of (16C)palmitate, as malonyl-CoA (the carbon donor) adds 2 carbons to the growing chain per cycle.
During the third reaction of fatty acid synthesis, acetyl-ACP and malonyl-ACP undergo a condensation reaction catalyzed by the enzymeβ-ketoacyl-ACP synthase (also known as acyl-malonyl-ACP condensing enzyme), which produces the four-carbon acetoacetyl-ACP molecule and one molecule of CO2.[12] Notably, the reaction of two-carbon acetyl-ACP with three-carbon malonyl-ACP is more favorable than that of two, two-carbon acetyl-ACP molecules reacting together.[12]
The fourth step of fatty acid elongation is the reduction of acetoacyl-ACP to β-hydroxyacyl-ACP, in a reaction catalyzed by3-ketoacyl-ACP reductase.[12] Herein, the electron donorNADPH is used as the reducing agent, ultimately converting the β-keto group of β-ketoacyl-ACP into the β-hydroxyl group of β-hydroxyacyl-ACP.[12]
The fifth step of fatty acid elongation is the dehydration of β-hydroxyacyl-ACP to enoyl-ACP, in a reaction catalyzed by3-hydroxyacyl-ACP dehydratase.[12] 3-hydroxyacyl-ACP dehydratase removes one molecule of H2O to form a double bond between the C2–C3 carbons of β-hydroxyacyl-ACP, thereby saturating the chain and producing enoyl-ACP.[12]
The sixth step of fatty acid elongation is the reduction of enoyl-ACP to butyryl-ACP, in a reaction catalyzed byenoyl-ACP reductase.[12] Herein, enoyl-ACP reductase reduces the C2–C3 double bond of enoyl-ACP into a saturated acyl-ACP using one molecule NADPH as the electron donor.[12] The production of butyryl-ACP thus marks the completion of the first cycle of fatty acid elongation, and the reaction sequence thereafter repeats again (condensation →reduction →dehydration →reduction).[12]
At the beginning of the second cycle, butyryl-ACP condenses with a molecule of malonyl-ACP, forming the six-carbon β-ketoacyl-ACP molecule and one molecule of CO2.[12] The next three reactions within the second cycle (reduction →dehydration →reduction) convert the six-carbon β-ketoacyl-ACP into a six-carbon ACP molecule, which thus marks the completion of the second cycle of fatty acid elongation, and a third cycle can thereafter begin.[12] These elongation cycles continue (x7) until a (16C) acyl-ACP molecule is formed. Thereafter, the (16C) acyl-ACP is hydrolyzed by athioesterase to form one molecule ofpalmitate and one molecule of ACP.[12]
Stoichiometry for the synthesis ofpalmitate is described by the following equation:[12]
8 Acetyl-CoA + 7 ATP + 14 NADPH + 13 H+ → Palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ADP + 7 Pi
Throughout the cycle, seven ATP molecules are used for the conversion of seven acetyl-CoA molecules into seven molecules of malonyl-CoA, which is used as the substrate for chain elongation. Hence, the seven molecules of malonyl-CoA are omitted from the stoichiometric equation, given that they were originally derived from acetyl-CoA.[12]
Palmitate produced by FAS can be used in the generation of even longer fatty acids, in a process unsurprisingly catalyzed byelongase enzymes, which lengthen palmitate to yieldlong chain fatty acids.[12] Alternatively, palmitate can undergodesaturation reactions, in a process catalyzed bydesaturase enzymes, which ultimately generateunsaturated fatty acids.[12] Elongation of palmitate requires the addition of a CoA thioester to palmitate in anATP-dependent reaction, which is catalyzed byacyl-CoA synthetase.[12] Further elongation occurs through the subsequent additions of malonyl-CoA molecules onto palmitate, or onto other saturated or unsaturated fatty acyl-CoA substrates.[12] These further elongation reactions are catalyzed byfatty acyl synthase enzyme, which is located on the cytosolic face of theendoplasmic reticulum (ER).[12] Herein, thesecondensation reactions are driven by thedecarboxylation of the additional malonyl-CoA substrates.[12] Unlike the former elongation cycles, which produced the sixteen-carbon palmitate substrate, the further elongation of palmitatedoes not involve ACP and does not rely on a multifunctional enzyme (ie.,FAS).[12]
In humans, fatty acids are formed from carbohydrates predominantly in theliver andadipose tissue, as well as in themammary glands during lactation. The pyruvate produced by glycolysis is an important intermediary in the conversion of carbohydrates into fatty acids and cholesterol.[13] This occurs via the conversion of pyruvate into acetyl-CoA in the mitochondrion. However, this acetyl-CoA needs to be transported into cytosol where the synthesis of fatty acids and cholesterol occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, citrate (produced by the condensation of acetyl-CoA with oxaloacetate) is removed from thecitric acid cycle and carried across the inner mitochondrial membrane into the cytosol.[13] There it is cleaved byATP citrate lyase into acetyl-CoA and oxaloacetate. The oxaloacetate can be used forgluconeogenesis (in the liver), or it can be returned into mitochondrion as malate.[14] The cytosolic acetyl-CoA is carboxylated byacetyl-CoA carboxylase intomalonyl-CoA, the first committed step in the synthesis of fatty acids.[14][15]
The main fuel stored in the bodies of animals is fat. A young adult human's fat stores average between about 15–20 kg (33–44 lb), but varies greatly depending on age, sex, and individual disposition.[16] In contrast, the human body stores only about 400 g (0.9 lb) ofglycogen, of which 300 g (0.7 lb) is locked inside the skeletal muscles and is unavailable to the body as a whole. The 100 g (0.2 lb) or so of glycogen stored in the liver is depleted within one day of starvation.[17] Thereafter the glucose that is released into the blood by the liver for general use by the body tissues, has to be synthesized fromthe glucogenic amino acids and a few othergluconeogenic substrates, which do not include fatty acids.[18]
Fatty acids are broken down to acetyl-CoA by means ofbeta oxidation inside the mitochondria, whereas fatty acids are synthesized from acetyl-CoA outside the mitochondrion, in the cytosol. The two pathways are distinct, not only in where they occur, but also in the reactions that occur, and the substrates that are used. The two pathways are mutually inhibitory, preventing the acetyl-CoA produced by beta-oxidation from entering the synthetic pathway via theacetyl-CoA carboxylase reaction.[18] It can also not be converted topyruvate as thepyruvate decarboxylation reaction is irreversible.[17] Instead it condenses withoxaloacetate, to enter thecitric acid cycle. During each turn of the cycle, two carbon atoms leave the cycle as CO2 in the decarboxylation reactions catalyzed byisocitrate dehydrogenase andalpha-ketoglutarate dehydrogenase. Thus each turn of the citric acid cycle oxidizes an acetyl-CoA unit while regenerating the oxaloacetate molecule with which the acetyl-CoA had originally combined to formcitric acid. The decarboxylation reactions occur beforemalate is formed in the cycle. Malate is the only substance that can be removed from the mitochondrion to enter thegluconeogenic pathway to form glucose or glycogen in the liver or any other tissue.[18] There can therefore be no net conversion of fatty acids into glucose.
Only plants possess the enzymes toconvert acetyl-CoA into oxaloacetate from which malate can be formed to ultimately be converted to glucose.[18]
Acetyl-CoA is formed into malonyl-CoA byacetyl-CoA carboxylase, at which point malonyl-CoA is destined to feed into the fatty acid synthesis pathway. Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, and is subject to bothphosphorylation andallosteric regulation. Regulation by phosphorylation occurs mostly in mammals, while allosteric regulation occurs in most organisms. Allosteric control occurs as feedback inhibition by palmitoyl-CoA and activation by citrate. When there are high levels of palmitoyl-CoA, the final product of saturated fatty acid synthesis, it allosterically inactivates acetyl-CoA carboxylase to prevent a build-up of fatty acids in cells. Citrate acts to activate acetyl-CoA carboxylase under high levels, because high levels indicate that there is enough acetyl-CoA to feed into theKrebs cycle and conserve energy.[19]
High plasma levels ofinsulin in the blood plasma (e.g. after meals) cause the dephosphorylation of acetyl-CoA carboxylase, thus promoting the formation of malonyl-CoA from acetyl-CoA, and consequently the conversion of carbohydrates into fatty acids, whileepinephrine andglucagon (released into the blood during starvation and exercise) cause the phosphorylation of this enzyme, inhibitinglipogenesis in favor of fatty acid oxidation viabeta-oxidation.[13][15]
Many bacteria use the anaerobic pathway for synthesizing unsaturated fatty acids. This pathway does not utilize oxygen and is dependent on enzymes to insert the double bond before elongation utilizing the normal fatty acid synthesis machinery. InEscherichia coli, this pathway is well understood.
Most bacteria that undergo anaerobic desaturation contain homologues of FabA and FabB.[22] Clostridia are the main exception; they have a novel enzyme, yet to be identified, that catalyzes the formation of the cis double bond.[21]
This pathway undergoestranscriptional regulation byFadR and FabR. FadR is the more extensively studied protein and has been attributed bifunctional characteristics. It acts as an activator offabA andfabB transcription and as arepressor for the β-oxidationregulon. In contrast, FabR acts as a repressor for the transcription of fabA and fabB.[20]
Aerobic desaturation is the most widespread pathway for the synthesis of unsaturated fatty acids. It is utilized in all eukaryotes and some prokaryotes. This pathway utilizesdesaturases to synthesize unsaturated fatty acids from full-length saturated fatty acid substrates.[23] All desaturases require oxygen and ultimately consume NADH even though desaturation is an oxidative process. Desaturases are specific for the double bond they induce in the substrate. InBacillus subtilis, the desaturase, Δ5-Des, is specific for inducing a cis-double bond at the Δ5 position.[14][23]Saccharomyces cerevisiae contains one desaturase, Ole1p, which induces the cis-double bond at Δ9.[14]
In mammals the aerobic desaturation is catalyzed by a complex of three membrane-bound enzymes (NADH-cytochrome b5 reductase, cytochrome b5, and adesaturase). These enzymes allow molecular oxygen,O
2, to interact with the saturated fatty acyl-CoA chain, forming a double bond and two molecules of water,H
2O. Two electrons come from NADH +H+
and two from the single bond in the fatty acid chain.[13] These mammalian enzymes are, however, incapable of introducing double bonds at carbon atoms beyond C-9 in the fatty acid chain.[nb 1].) Hence mammals cannot synthesizelinoleate orlinolenate (which have double bonds at the C-12 (= Δ12), or the C-12 and C-15 (= Δ12 and Δ15) positions, respectively, as well as at the Δ9 position), nor the polyunsaturated, 20-carbonarachidonic acid that is derived from linoleate. These are all termedessential fatty acids, meaning that they are required by the organism, but can only be supplied via the diet. (Arachidonic acid is the precursor ofprostaglandins which fulfill a wide variety of functions aslocal hormones.)[13]
Odd-chain fatty acids (OCFAs) are those fatty acids that contain an odd number of carbon atoms. The most common OCFAs are the saturated C15 and C17 derivatives, respectivelypentadecanoic acid andheptadecanoic acid.[24] The synthesis of even-chained fatty acid synthesis is done by assemblingacetyl-CoA precursors, however,propionyl-CoA instead of acetyl-CoA is used as the primer for the biosynthesis of long-chain fatty acids with an odd number of carbon atoms.[25]
InB. subtilis, this pathway is regulated by atwo-component system: DesK and DesR. DesK is a membrane-associated kinase and DesR is a transcriptional regulator of thedes gene.[14][23] The regulation responds to temperature; when there is a drop in temperature, this gene is upregulated. Unsaturated fatty acids increase the fluidity of the membrane and stabilize it under lower temperatures. DesK is the sensor protein that, when there is a decrease in temperature, will autophosphorylate. DesK-P will transfer its phosphoryl group to DesR. Two DesR-P proteins will dimerize and bind to the DNA promoters of thedes gene and recruit RNA polymerase to begin transcription.[14][23]
Pseudomonas aeruginosa
In general, both anaerobic and aerobic unsaturated fatty acid synthesis will not occur within the same system, howeverPseudomonas aeruginosa andVibrio ABE-1 are exceptions.[26][27][28]WhileP. aeruginosa undergoes primarily anaerobic desaturation, it also undergoes two aerobic pathways. One pathway utilizes a Δ9-desaturase (DesA) that catalyzes a double bond formation in membrane lipids. Another pathway uses two proteins, DesC and DesB, together to act as a Δ9-desaturase, which inserts a double bond into a saturated fatty acid-CoA molecule. This second pathway is regulated by repressor protein DesT. DesT is also a repressor offabAB expression for anaerobic desaturation when in presence of exogenous unsaturated fatty acids. This functions to coordinate the expression of the two pathways within the organism.[27][29]
Branched chain fatty acids are usually saturated and are found in two distinct families: the iso-series and anteiso-series. It has been found thatActinomycetales contain unique branch-chain fatty acid synthesis mechanisms, including that which forms tuberculostearic acid.
The branched-chain fatty acid synthesizing system usesα-keto acids as primers. This system is distinct from the branched-chain fatty acid synthetase that utilizes short-chain acyl-CoA esters as primers.[30] α-Keto acid primers are derived from thetransamination anddecarboxylation ofvaline,leucine, andisoleucine to form 2-methylpropanyl-CoA, 3-methylbutyryl-CoA, and 2-methylbutyryl-CoA, respectively.[31] 2-Methylpropanyl-CoA primers derived from valine are elongated to produce even-numbered iso-series fatty acids such as 14-methyl-pentadecanoic (isopalmitic) acid, and 3-methylbutyryl-CoA primers from leucine may be used to form odd-numbered iso-series fatty acids such as 13-methyl-tetradecanoic acid. 2-Methylbutyryl-CoA primers from isoleucine are elongated to form anteiso-series fatty acids containing an odd number of carbon atoms such as 12-Methyl tetradecanoic acid.[32] Decarboxylation of the primer precursors occurs through thebranched-chain α-keto acid decarboxylase (BCKA) enzyme. Elongation of the fatty acid follows the same biosynthetic pathway inEscherichia coli used to produce straight-chain fatty acids where malonyl-CoA is used as a chain extender.[33] The major end products are 12–17 carbon branched-chain fatty acids and their composition tends to be uniform and characteristic for many bacterial species.[32]
BCKA decarboxylase and relative activities of α-keto acid substrates
The BCKA decarboxylase enzyme is composed of two subunits in a tetrameric structure (A2B2) and is essential for the synthesis of branched-chain fatty acids. It is responsible for the decarboxylation of α-keto acids formed by the transamination of valine, leucine, and isoleucine and produces the primers used for branched-chain fatty acid synthesis. The activity of this enzyme is much higher with branched-chain α-keto acid substrates than with straight-chain substrates, and inBacillus species its specificity is highest for the isoleucine-derived α-keto-β-methylvaleric acid, followed byα-ketoisocaproate and α-ketoisovalerate.[32][33] The enzyme's high affinity toward branched-chain α-keto acids allows it to function as the primer donating system for branched-chain fatty acid synthetase.[33]
| Substrate | BCKA activity | CO2 Produced (nmol/min mg) | Km (μM) | Vmax (nmol/min mg) |
|---|---|---|---|---|
| L-α-keto-β-methyl-valerate | 100% | 19.7 | <1 | 17.8 |
| α-Ketoisovalerate | 63% | 12.4 | <1 | 13.3 |
| α-Ketoisocaproate | 38% | 7.4 | <1 | 5.6 |
| Pyruvate | 25% | 4.9 | 51.1 | 15.2 |
Factors affecting chain length and pattern distribution
α-Keto acid primers are used to produce branched-chain fatty acids that, in general, are between 12 and 17 carbons in length. The proportions of these branched-chain fatty acids tend to be uniform and consistent among a particular bacterial species but may be altered due to changes in malonyl-CoA concentration, temperature, or heat-stable factors (HSF) present.[32] All of these factors may affect chain length, and HSFs have been demonstrated to alter the specificity of BCKA decarboxylase for a particular α-keto acid substrate, thus shifting the ratio of branched-chain fatty acids produced.[32] An increase in malonyl-CoA concentration has been shown to result in a larger proportion of C17 fatty acids produced, up until the optimal concentration (≈20μM) of malonyl-CoA is reached. Decreased temperatures also tend to shift the fatty-acid distribution slightly toward C17 fatty-acids inBacillus species.[30][32]
This system functions similarly to the branch-chain fatty acid synthesizing system, however it uses short-chain carboxylic acids as primers instead of alpha-keto acids. In general, this method is used by bacteria that do not have the ability to perform the branch-chain fatty acid system using alpha-keto primers. Typical short-chain primers include isovalerate, isobutyrate, and 2-methyl butyrate. In general, the acids needed for these primers are taken up from the environment; this is often seen in ruminal bacteria.[34]
The overall reaction is:
The difference between (straight-chain) fatty acid synthase and branch-chain fatty acid synthase is substrate specificity of the enzyme that catalyzes the reaction of acyl-CoA to acyl-ACP.[30]
Omega-alicyclic fatty acids typically contain an omega-terminal propyl or butyryl cyclic group and are some of the major membrane fatty acids found in several species of bacteria. The fatty acid synthetase used to produce omega-alicyclic fatty acids is also used to produce membrane branched-chain fatty acids. In bacteria with membranes composed mainly of omega-alicyclic fatty acids, the supply of cyclic carboxylic acid-CoA esters is much greater than that of branched-chain primers.[30] The synthesis of cyclic primers is not well understood but it has been suggested that mechanism involves the conversion of sugars toshikimic acid which is then converted to cyclohexylcarboxylic acid-CoA esters that serve as primers for omega-alicyclic fatty acid synthesis[34]
Tuberculostearic acid (D-10-Methylstearic acid) is a saturated fatty acid that is known to be produced byMycobacterium spp. and two species ofStreptomyces. It is formed from the precursor oleic acid (a monounsaturated fatty acid).[35] After oleic acid is esterified to a phospholipid,S-adenosyl-methionine donates a methyl group to the double bond of oleic acid.[36] This methylation reaction forms the intermediate 10-methylene-octadecanoyal. Successive reduction of the residue, with NADPH as a cofactor, results in 10-methylstearic acid[31]
In addition to fatty acid synthesis incytosol (FAS/FASI), there is also anotherde novo fatty acid synthesis inmitochondria (mtFAS/mtFASII) ineukaryotes. This pathway was first described in 1990 inNeurospora crassa.[37][38] Mitochondrial fatty acid synthesis is essential forcellular respiration and mitochondrialbiogenesis.[39] It is also required for respiratory growth inyeast and forembryonic survival inmammals.[40]
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The mtFAS pathway consists of at least six individually present enzymes, all encoded by separate genes.[41] This sets it apart from cytosolic fatty acid synthesis, where themultifunctional enzymefatty acid synthase (FASN) contains all enzymatic activities within a singlepolypeptide chain and is encoded by a single gene.[41] Despite this structural difference, mtFAS and FAS use the same chemistry to build fatty acids.[37]
In mtFAS, mitochondrialacyl carrier protein (ACP) serves as a solublescaffold protein in themitochondrial matrix,covalently attaching the growing fatty acyl chains.[37]Malonyl-CoA—formed bymtACC1 (a mitochondrialisoform ofacetyl-CoA carboxylase 1) fromacetyl-CoA and byacyl-CoA synthetase family member 3 (ACSF3) frommalonate—serves as the chain-extender unit.[42][43] However, the precise mitochondrial source of malonyl-CoA remains under debate.[44]
In each round of chain elongation,malonyl-CoA is first transferred to ACP bymalonyl‑CoA:ACP transacylase (MCAT) to formmalonyl-ACP, which then undergoescondensation with the growing acyl-ACP (with acetyl-ACP in the first round) catalyzed by3-oxoacyl-ACP synthase (OXSM), releasingCO2 and extending the chain by twocarbons.[41] Next, the newly extended fatty acyl chain on ACP (3-ketoacyl-ACP) undergoesreduction byestradiol-17β-dehydrogenase 8 (HSD17B8) andcarbonyl reductase 4 (CBR4),dehydration by3-hydroxyacyl-ACP dehydratase 2 (HTD2), and a final reduction bytrans-2-enoyl-CoA reductase (MECR), yielding asaturated fatty acid on ACP (acyl-ACP) once again, which is then available as the substrate for the next elongation round.[41]
These steps repeat until an eight-carbon saturated fatty acid on ACP—known asoctanoyl-ACP (C8)—is formed.[37] At that point, thismedium-chain fatty acid bound to ACP can either exit the mtFAS pathway or remain for further elongation intolong-chain fatty acids (C14-C16).[37] Since no mitochondrialthioesterase has been identified in any animal species, the final product of mtFAS remains bound to ACP rather than being released as afree fatty acid.[37]
Mitochondrial fatty acid synthesis plays a crucial role in cellularenergy metabolism by generating octanoyl‑ACP (C8), which serves as the directprecursor forlipoic acid biosynthesis.[45] Lipoic acid is an essentialcofactorcovalently attached to specificlysineresidues on target enzymes in a process calledlipoylation.[46] Thispost‑translational modification is essential for the activity of key mitochondrial enzyme complexes—namely, thepyruvate dehydrogenase complex (PDC), theα‑ketoglutarate dehydrogenase complex (OGDC), the2‑oxoadipate dehydrogenase complex (2‑OADHC), thebranched‑chain α‑ketoacid dehydrogenase complex (BCKDC), and theglycine cleavage system (GCS).[46][47]
In parallel, mtFAS and its acyl‑ACP products provide a metabolic feedback mechanism, regulating mitochondrialacetyl‑CoA consumption and thereby integrating lipid synthesis with broader metabolic control.[44]
Beyond octanoyl‑ACP, mtFAS also produces longer‑chain acyl‑ACP species such asmyristoyl‑ACP (C14) andpalmitoyl‑ACP (C16), which interact with members of theleucine‑tyrosine‑arginine motif (LYRM) protein family.[37] These LYRM proteins are vital for the correct assembly and stability of theelectron‑transport chain (ETC) complexes and foriron–sulfur (Fe–S) cluster biogenesis within mitochondria.[37]
In addition to these enzymatic and structural roles, mtFAS has also been implicated as a mediator of intracellularsignal transduction. This is supported by observations that the levels of bioactive lipids—such aslysophospholipids andsphingolipids—correlate with mtFAS activity.[48] For instance,knockdown of ACP reducesceramide levels, whereas loss of the terminal mtFAS enzyme MECR results in ceramide accumulation.[48][49]
Importantly, mtFAS function extends to the regulation ofimmune cell metabolism.CRISPR/Cas9 screens have identified mtFAS genes—especiallyMecr,Mcat, andOxsm—as key regulators ofT cell metabolism.[50] While MECR is not required fornaive T cell maintenance, its loss inactivated T cells impairsproliferation,survival, anddifferentiation.[50] MECR deficiency disruptsmitochondrial respiration, altersTCA cycle activity, and increasesferroptosis sensitivity, ultimately reducing T cell fitness andinflammatory capacity.[50]
Disorders in mtFAS pathway lead to the followingmetabolic diseases:
In the following, similarities and differences between cytosolic and mitochondrial fatty acid synthesis are shown:
| Feature | Cytosolic fatty acid synthesis (FAS/FASI) | Mitochondrial fatty acid synthesis (mtFAS/mtFASII) | |
|---|---|---|---|
| Place of synthesis | Cytosol | Mitochondrial matrix[37] | |
| Enzyme system | FAS type I (multifunctional enzyme) | FAS type II (single enzymes)[37] | |
| Regulation | Key enzyme | Acetyl-CoA carboxylase | Unknown |
| Activation | Allosteric:citrate | Unknown | |
| Inhibition | Allosteric:palmitoyl-CoA | Unknown | |
| Primer | Acetyl-CoA (from mitochondria viacitrate–malate shuttle) | Acetyl-CoA (directly present in the matrix) | |
| Extender units | Malonyl-CoA (fromcarboxylation of acetyl-CoA) | Malonyl-CoA (mainly from thecarboxylation of acetyl-CoA, but also from thethioesterification ofmalonic acid) | |
| Cofactors | Reducing agent | NADPH | NADPH |
| Other | ATP,biotin (both for conversion tomalonyl-CoA) | ATP,biotin (both also formalonyl-CoA) | |
| Thioesterase | Available in cytosol | None known in mitochondria[37] | |
| Endproduct(s) | Mainlypalmitate (C16:0) | Octanoyl-ACP (C8),myristoyl-ACP (C14),palmitoyl-ACP (C16)[37] | |
| Function | Lipid storage, energy balance, membrane structure | Precursors for cofactors such aslipoic acid (forPDH complex,αKGDH complex,2-oxoadipate dehydrogenase complex,BCKDH complex andglycine cleavage system);[37][45] assembly of theelectron transport chain (ETC);[37]iron-sulfur (FeS) cluster biogenesis;[37] role inceramide metabolism[37] | |
| Participation inlipid synthesis | Central role inde novolipogenesis | Supplementary role only | |
| Phylogenetic similarity | Eukaryote-specific | Bacteria-like (evolutionary conserved)[37] | |
The positions of the double bonds in a fatty acid chain can, therefore, be indicated in two ways, using the C-n or the ω-n notation. Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω-7) and C-13 (or ω-6) is reported either as Δ12 if counted from the –COOH end (indicating only the "beginning" of the double bond), or as ω-6 (or omega-6) if counting from the-CH
3 end. The "Δ" is the Greek letter "delta", which translates into "D" (forDouble bond) in the Roman alphabet. Omega (ω) is the last letter in the Greek alphabet, and is therefore used to indicate the "last" carbon atom in the fatty acid chain. Since the ω-n notation is used almost exclusively to indicate the positions of the double bonds close to the-CH
3 end inessential fatty acids, there is no necessity for an equivalent "Δ"-like notation – the use of the "ω-n" notation always refers to the position of a double bond.
Metabolism map | ||
|---|---|---|
 Majormetabolic pathways inmetro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article. Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).  Orange nodes:carbohydrate metabolism. Violet nodes:photosynthesis. Red nodes:cellular respiration. Pink nodes:cell signaling. Blue nodes:amino acid metabolism. Grey nodes:vitamin andcofactor metabolism. Brown nodes:nucleotide andprotein metabolism. Green nodes:lipid metabolism. | ||
