Fatty acid synthesis

Fatty acid synthesis

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  Fatty Acid Synthesis 1 Introduction Fattyacid synthesis occurs similarly to Beta-oxidation – acetyl groups are added to a growing chain, but the mechanism of the pathway is distinctly different from being simply the reverse of Beta-oxidation. Fatty acid synthesis occurs in the cytosol (not mitochondria). It uses a moiety called Acyl- carrier protein (ACP) instead of CoA and the reducing agent NADPH (not NAD/FAD). It will be noted that there are structural similarities between the Coenzyme A portion of CoA and the Phosphopantetheine moiety of ACP.
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  Fatty Acid Synthesis 2 FattyAcid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.[1] Fatty acids are usually derived from triglycerides or phospholipids. Fatty acids are important dietary sources of fuel for animals because, when metabolized, they yield large quantities of ATP. Types of Fatty Acids There are two types: 1) Saturated 2) Unsaturated  Fatty acids that possess no double bonds are saturated (have maximum number of H atoms)  Saturated fatty acids are linear in structure, originate from animal sources (i.e. fats) and are typically solid at room temperatures  Fatty acids with double bonds are unsaturated – either monounsaturated (1 double bond) or polyunsaturated (>1 double bond)  Unsaturated fatty acids are bent in structure, originate from plant sources (i.e. oils) and are typically liquid at room temperatures
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  Fatty Acid Synthesis 3 FattyAcid Synthesis 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.
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  Fatty Acid Synthesis 4 Transportof Aetyl-CoA from Mitochondria to Cytoplasm
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  Fatty Acid Synthesis 5 Stepsof Fatty Acid Synthesis Pathways of fatty acid synthesis Fatty acid biosynthesis occurs in the cytoplasm. A. Glucose. Most of the carbon from glucose enters fatty acid synthesis via glycolysis. 1. Carbon must enter the mitochondria and be converted to both OAA and Ac CoA, which form citrate. 2. The citrate exits the mitochondria and is hydrolyzed by citrate lyase (or citrate cleavage enzyme). 3. The Ac CoA is utilized for fatty acid synthesis (palmitate). 4. The OAA is reduced to malate, when then is oxidatively decarboxylated back to pyruvate generating NADPH. This cycle can produce about 1/2 the NADPH required for fatty acid biosynthesis.
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  Fatty Acid Synthesis 6 B.Acetate. Acetate is converted to Ac CoA in the cytoplasm. C. Lactate. Follows the same pathway as glucose; enters the pathway at pyruvate.  The assembly of fatty acids 1.Acetyl CoA carboxylase Glucose  2 Pyruvate  2 Acetyl CoA + 2CO2 The first committed step of fatty acid biosynthesis is catalyzed by Acetyl-CoA carboxylase. The enzyme contains biotin, and adds a CO2 (resulting in a carboxyl group) to the methyl end of acetyl CoA. Note that this reaction is an energy requiring process (1 ATP per Malonyl-CoA formed). Acetyl-CoA carboxylase is an interesting enzyme. Studies of the enzyme from birds and mammals indicate that it forms long linear polymers. The polymer appears to be the active form of the enzyme. Monomeric units are inactive. Citrate shifts the polymer – monomer equilibrium towards polymer formation. Palmitoyl-CoA shifts the equilibrium towards monomer formation. Of the two compounds affecting enzyme form, palmitoyl-CoA probably exerts the greater influence. Another regulation of Acetyl-CoA carboxylase is by hormones. Glucagon, epinephrine, and norepinephrine trigger a cAMP dependent phosphorylation (remember the cascade system) of the enzyme that shifts the equilibrium towards monomer formation. Insulin, conversely, stimulates desphosphorylation, favoring polymerization. The enzymes responsible for phosphorylating Acetyl-CoA carboxylase are cAMP- dependent protein kinase and AMP-dependent protein kinase (AMPK). E. coli’s Acetyl- CoA carboxylase is regulated by guanine nucleotides, which are a function of those cells’ growth requirements || Ac CoA carboxylase || CH3-C-S-CoA + CO2 + ATP  HOOC-CH2-C-S-CoA + ADP + Pi Acetyl CoA Malonyl CoA O O CoA CoA
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  Fatty Acid Synthesis 7 2.Fatty acid synthase This multifunctional enzyme catalyzes the seven different reactions whereby two carbon units from malonyl-CoA are linked together, ultimately to form palmitoyl-CoA. In some systems, the activities are present on separate enzyme units. In other cells, a single polypeptide chain has multiple activities that can be isolated after protease treatment. The enzyme complex can exist as both a monomer and dimer. The dimeric form is the fully functional form of the enzyme. The overall synthesis of palmitate from acetyl-CoA requires 14 NADPHs, and 7 ATPs. O O || Fatty acid synthase || HOOC-CH2-C-S-CoA + 1st ACP-SH  1st ACP-S-C-CH2CO2H + CoASH O || AcCoA + 2nd ACP  2nd ACP-S-C-CH3 CoA CoA CoA ACPs FAS ACPs FAS ACPs FAS ACPs FAS CoA
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  Fatty Acid Synthesis 8 OO || || 1st ACP-S-C-CH2CO2H + 2nd ACP-S-C-CH3  O O || || 2nd ACP-C-CH2-C-CH3 + 1st ACP + CO2 O OH O O || | || H NADPH || ACP-S-C-CH2-CH-CH3 -----------> ACP-S-C-C=C-CH3 --------> ACP-S-C-CH2-CH2-CH3 H2O H NADP+ + MalCoA, etc.  Palmitic acid (plus some lauric and myristic acids) Elongation of fatty acid synthase Elongation of Palmitate The product of fatty acid synthase action, palmitate, is but of course one of many fatty acids synthesized by cells. Elongasesare enzymes that act to lengthen palmitateto produce many of the other fatty acids. Elongases are present in mitochondria and the endoplasmic reticulum. Elongation using elongase in the mitochondrion involves a mechanism that is essentially the reverse of Beta-oxidation except substitution of NADPH for FADH2 in the last reaction. 1. Lauric acid NADPH NADP+ ACPs FAS ACPs FAS ACPs FAS ACPs FAS
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  Fatty Acid Synthesis 9 3.Palmitic acid (final product of fatty acid synthase) Desaturation of Fatty Acids:  Terminal de saturases produce unsaturated fatty acids. One such enzyme is fatty acyl-CoA desaturase. Note the unusual electron transferring pathway in which electrons from NADH are ultimately passed to oxygen, forming water.  The energy released in this process drives oxidation of stearoyl-CoA to oleyl-CoA. From the free methyl end, mammals cannot make double bonds closer to the end than the Delta-9 position (Oleic acid is a Delta-9 fatty acid). Thus, linoleic acid (Delta 9,12 double bonds) and linolenic acid (Delta 9,12,15 double bonds) must be provided in the diet of mammals, and are called essential fatty acids.  The synthesis of Arachidonic acid from Linoleic acid is depicted in the given above Figure. Note that arachidonic acid contains 4 double bonds. ACPs FASLauric acid . Myristic acid2 ACPs FAS Myristic acid ACPs FAS Palmitic acid
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  Fatty Acid Synthesis 10 Stepsof fatty acid synthesis starting with Acetyl-CoA and Malonyl-CoA are shown in in the given Figure The reactions are as follows: 1. Transfer of the malonyl group of malonyl-CoA to ACP (Reaction #2 – catalyzed by malonyl-CoA-ACP transacylase). Transfer of the acetyl group of Acetyl-CoA to ACP (Reaction #3 of – catalyzed by acetyl-CoA-ACP transacylase). 2. Addition of an acetyl group from malonyl-ACP between the thioester bond of the acetyl-ACP molecule in reaction 1 (Reaction 4 of – catalyzed by Beta-keto-ACP synthase – also called condensing enzyme). 3. Reduction of the Beta-keto group to a Beta-hydroxyl group with NADPH (Reaction 5 of – catalyzed by Beta-keto-ACP reductase). 4. Dehydration between the alpha and Beta carbons (Reaction 6 of – catalyzed by Beta- hydroxyacyl-ACP dehydrase). 5. Reduction of the trans double bond by NADPH (Reaction 7 of – catalyzed byenoyl- ACP reductase).
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  Fatty Acid Synthesis 11 6.Repetition of steps 2-6 six more times. The acetyl group of reaction 1 is replaced by the growing acyl-ACP molecule. (That is, new acetyl groups are added at the ACP end of the molecule). 7. The product of this series of reactions, palmitoyl-ACP can be cleaved to palmitate and ACP by the enzyme palmitoyl thioesterase. 8. The multiple enzymatic activities integrated into Fatty Acid Synthase complex are probably related to the growing fatty acid being “swung” into the appropriate catalytic region of the synthase.
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  Fatty Acid Synthesis 12
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  Fatty Acid Synthesis 13 Regulationof Fatty Acid Synthesis Since fatty acid biosynthesis takes the opposite direction to β-oxidation, it must be regulated to prevent the two processes from operating as a futile cycle, in particular in animals where significant amounts of energy are stored as fats and fatty acids. Bacteria make fatty acids for membrane lipids, but do not use fatty acids and β-oxidation as a primary energy source, so regulation less strict and is mainly correlated with cell growth. In plants, fatty acid biosynthesis occurs in the chloroplasts, and is dependent on photosynthesis to provide substrate and NADPH (Lehninger p. 778-780).. In animals, the bulk of stored fat is found in adipose tissue. Fatty acids are released from triacylglycerol by hormone sensitive lipase, which is actually activated by protein kinase A in response to cyclic AMP induced by the hormones epinephrine and glucagon. Fatty acid is released from adipose tissue, and circulated in blood to those muscle tissues (e.g. cardiac) which use β-oxidation of fatty acid as an energy source. In contrast, fatty acid biosynthesis occurs primarily in the liver, and involves the following modes of regulation. 1) Compartmentation b-oxidation occurs in mitochondria or in a specialized organelle called the peroxisome, whereas fatty acid biosynthesis is cytoplasmic. 2) Substrate availability Fatty acid biosynthesis requires a cytoplasmic source of acetyl CoA. Acetyl CoA produced by β-oxidation or by oxidation of amino acids or pyruvate is located in mitochondria, and is directed towards the TCA cycle. Cytoplasmic acetyl CoA is produced when more citrate is produced than is needed for the TCA cycle to generate ATP (Lehninger) If ATP is not being consumed quickly enough, ADP levels drop, and isocitrate dehydrogenase loses activity. This causes unused isocitrate to accumulate, and because the aconitase equilibrium is unfavourable (Keq = 0.075), citrate builds up to very high level .A transporter allows excess citrate to enter the cytoplasm, where it is broken down by citrate lyase. citrate + HS CoA + ATP fi ADP + Pi + oxaloacetate + acetyl-CoA
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  Fatty Acid Synthesis 14 3)Enzyme regulation Fatty acid biosynthesis is subject to hormonal and allosteric regulation. Insulin (secreted when blood glucose levels are high) stimulates citrate lyase. Glucagon (secreted when blood glucose levels are low) and epinephrine (secreted when demand for ATP is anticipated) induce cyclic AMP, which stimulates protein kinase A to phosphorylate and inactivate acetyl CoA carboxylase (Lehninger p. 780). Control of Fatty Acid Synthesis: Like all metabolic pathways, cells must have appropriate controls on fatty acid metabolism to be able to meet energy needs. Precursors for energy generation – triacyl glycerols in chylomicrons and VLDL, fatty acid/albumin complexes, ketone bodies, amino acids, lactate, and glucose – are all carried in the blood as needed for various tissues. One mechanism of regulation involves hormone release.  Signals received in the pancreas (glucose concentration) trigger production of hormones.  Low blood sugar triggers glucagon release.  High blood sugar triggers insulin release. Both of latter systems control glucose-related metabolism as well. Students should recognize that the regulatory mechanisms of controlling enzymatic reactions we have discussed to date are short-term regulation. They act in minutes (or less). Fatty acid synthesis is controlled partly by short term regulation(mechanisms include substrate availability, allosterism, covalent modification of enzymes) and partly by long term regulatory mechanisms. Long term regulation involves controlling the quantity of enzyme by controlling the rate with which a protein is synthesized and/or degraded. One of the reasons fats do not supply emergency energy is that control of their metabolism is largely bylong term regulatory mechanisms whereas control of sugar metabolism is more prominent under short term regulatory mechanisms. Insulin stimulates increased synthesis of acetyl-CoA carboxylase and fatty acid synthase (two critical enzymes for synthesizing fatty acids). Starvation, conversely decreases synthesis of these enzymes. Fatty acid oxidation is regulated by fatty acid concentration in the blood. This is controlled by the amount of hydrolysis of triacylglycerols in adipose tissue by hormone-sensitive triacylglycerol lipase (HSTL).
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  Fatty Acid Synthesis 15 Thisenzyme is phosphorylated in the hormonally-controlled cAMP-dependent phosphorylation cascade, which activates the lipase, stimulating release of fatty acids. This cascade is turned on by the cell’s binding of glucagon or epinephrine It should also be noted that the cAMP-dependent phosphorylation system also causes inactivation of acetyl-CoA carboxylase, an important control enzyme in fatty acid synthesis.
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  Fatty Acid Synthesis 16 Conclusion Plantssynthesise a huge variety of fatty acids although only a few are major and common constituents [1]. Broadly speaking, long-chain fatty acids are synthesised de novo from small precursors ultimately derived from photosynthate. Two enzyme systems are utilised, acetyl- CoA carboxylase and fatty acid synthase .The end products of this synthesis are usually the saturated fatty acids palmitate and stearate with the latter predominating (in most plants by 2- 3 times that of palmitate). Once the long-chain acids have been produced they can be subject to elongation, desaturation and further modifications.
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  Fatty Acid Synthesis 17 References https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.cliffsnotes.c om/study-guides/biology/biochemistry-ii/lipid-biosynthesis/fatty-acid- biosynthesis&ved=0ahUKEwjwj- XUupHXAhVMYo8KHQnqCrwQFghoMA4&usg=AOvVaw3glKaUx4abDnUFMala hvNp  https://www.google.com/url?sa=t&source=web&rct=j&url=https://en.m.wikipedia.or g/wiki/Fatty_acid_synthesis&ved=0ahUKEwjlzJzMu5HXAhVLLI8KHVRaC00QFg grMAI&usg=AOvVaw0j-Opq2XdmBCzeMOcj2Xh7  https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.biochemden. com/fatty-acid- biosynthesis/&ved=0ahUKEwjlzJzMu5HXAhVLLI8KHVRaC00QFghoMA8&usg= AOvVaw1syFWA3UhmwCY8hZE7yjmi  https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.slideshare.n et/mobile/namarta28/fatty-acid-synthesis- 13753491&ved=0ahUKEwjlzJzMu5HXAhVLLI8KHVRaC00QFghxMBE&usg=AO vVaw3CNK0njTbR5nlul-e0SpQL  https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.biochem.uths csa.edu/med/10-Lipid- Metabolism/LipidMetabolism10.html&ved=0ahUKEwjlzJzMu5HXAhVLLI8KHVR aC00QFgh0MBI&usg=AOvVaw3_LTy05EXKk18KwAgvMciA  https://www.google.com/url?sa=t&source=web&rct=j&url=http://lipidlibrary.aocs.or g/Biochemistry/content.cfm%3FItemNumber%3D41289&ved=0ahUKEwjlzJzMu5H XAhVLLI8KHVRaC00QFgigATAd&usg=AOvVaw1Zj-oxB1qut3lqdaEgc_rV  https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.namrata.co/st eps-of-fatty-acid-synthesis-a-quick- revision/&ved=0ahUKEwjA9oiLwZHXAhXMsY8KHbpFB- sQFgibATAe&usg=AOvVaw3SlX5EQXorHGzcjbLPvCpL  http://lipidlibrary.aocs.org/Biochemistry/content.cfm?ItemNumber=40304