Biosynthesis of Fatty Acids & Eicosanoids PDF

Biosynthesis of Fatty Acids & Eicosanoids Cecille Camille N. Init-Ubod, RN, MD Biomedical Importance Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In most mammals, glucose is the p...

Biosynthesis of Fatty Acids & Eicosanoids Cecille Camille N. Init-Ubod, RN, MD Biomedical Importance Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate, the main fuel molecule they obtain from the diet. Critical diseases of the pathway have not been reported in humans. However, inhibition of lipogenesis occurs in type 1 (insulin-dependent) diabetes mellitus, and variations in the activity of the process affect the nature and extent of obesity. Unsaturated fatty acids in phospholipids of the cell membrane are important in maintaining membrane fluidity A high ratio of polyunsaturated fatty acids to saturated fatty acids (P:S ratio) in the diet is considered to be beneficial in preventing coronary heart disease. Animal tissues have limited capacity for desaturating fatty acids, and require certain dietary polyunsaturated fatty acids derived from plants. These essential fatty acids are used to form eicosanoic (C20) fatty acids, which give rise to the eicosanoids prostaglandins, thromboxanes, leukotrienes, and lipoxins. Prostaglandins mediate inflammation, pain, and induce sleep and also regulate blood coagulation and reproduction. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and ibuprofen act by inhibiting prostaglandin synthesis. Leukotrienes have muscle contractant and chemotactic properties and are important in allergic reactions and inflammation. THE MAIN PATHWAY FOR DE NOVO SYNTHESIS OF FATTY ACIDS LIPOGENESIS OCCURS IN THE CYTOSOL This system is present in many tissues, including liver, kidney, brain, lung, mammary gland, and adipose tissue. Its cofactor requirements include ○ NADPH, ATP, Mn2+, biotin,and HCO3− (as a source of CO2). Acetyl-CoA is the immediate substrate, and free palmitate is the end product. Production of Malonyl-CoA Is the Initial & Controlling Step in Fatty Acid Synthesis Bicarbonate as a source of CO2 is required in the initial reaction for the carboxylation of acetyl-CoA to malonyl-CoA in the presence of ATP and acetyl-CoA carboxylase. This enzyme has a major role in the regulation of fatty acid synthesis Acetyl-CoA carboxylase has a requirement for the B vitamin biotin and is a multienzyme protein containing biotin, biotin carboxylase, biotin carboxyl carrier protein, and a carboxyl transferase, as well as a regulatory allosteric site One subunit of the complex contains all the components, and variable number of subunits form polymers in the active enzyme The reaction takes place in two steps: ○ (1) carboxylation of biotin involving ATP ○ (2) transfer of the carboxyl group to acetylCoA to form malonyl-CoA The Fatty Acid Synthase Complex Is a Homodimer of Two Polypeptide Chains Containing Six Enzyme Activities After the formation of malonyl-CoA, fatty acids are formed by the fatty acid synthase enzyme complex. It contains the vitamin pantothenic acid in the form of 4′-phosphopantetheine In the primary structure of the protein, the enzyme domains are linked in the sequence X-ray crystallography of the three-dimensional structure has shown that the complex is a homodimer, with two identical subunits, each containing 6 enzymes and an ACP, arranged in an X shape The position of the ACP and thioesterase domains cannot be resolved as yet by x-ray crystallography ○ possibly because they are too flexible, but they are thought to lie close to the 3-ketoacylreductase enzyme. The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. Initially, a priming molecule of acetyl-CoA combines with a cysteine ´SH group (Figure 23–3, reaction 1a), while malonyl-CoA combines with the adjacent ´SH on the 4′-phosphopantetheine of ACP of the other monomer (reaction 1b). These reactions are catalyzed by malonyl acetyl transacylase, to form acetyl (acyl)-malonyl enzyme. The acetyl group attacks the methylene group of the malonyl residue, catalyzed by 3-ketoacyl synthase, and liberates CO2, forming 3-ketoacyl enzyme (acetoacetyl enzyme) (reaction 2), freeing the cysteine —SH group. Decarboxylation allows the reaction to go to completion, pulling the whole sequence of reactions in the forward direction. The 3-ketoacyl group is reduced, dehydrated, and reduced again (reactions 3-5) to form the corresponding saturated acyl-S-enzyme A new malonyl-CoA molecule combines with the ´SH of 4′-phosphopantetheine, displacing the saturated acyl residue onto the free cysteine ´SH group. The sequence of reactions is repeated six more times until a saturated 16-carbon acyl radical (palmitoyl) has been assembled. It is liberated from the enzyme complex by the activity of the sixth enzyme in the complex, thioesterase (deacylase). The free palmitate must be activated to acyl-CoA before it can proceed via any other metabolic pathway. Its possible fates are esterification into acylglycerols, chain elongation or desaturation, or esterification into cholesteryl ester. In mammary gland, there is a separate thioesterase specific for acyl residues of C8, C10, or C12, which are subsequently found in milk lipids. The acetyl-CoA used as a primer forms carbon atoms 15 and 16 of palmitate. The addition of all the subsequent C2 units is via malonyl-CoA. Propionyl CoA acts as primer for the synthesis of long-chain fatty acids having an odd number of carbon atoms, found particularly in ruminant fat and milk. The Main Source of NADPH for Lipogenesis Is the Pentose Phosphate Pathway NADPH is involved as a donor of reducing equivalents in both the reduction of the 3-ketoacyl and of the 2,3-unsaturated acyl derivatives The oxidative reactions of the pentose phosphate pathway are the chief source of the hydrogen required for the reductive synthesis of fatty acids. Significantly, tissues specializing in active lipogenesis—ie, liver, adipose tissue, and the lactating mammary gland— also possess an active pentose phosphate pathway. Moreover, both metabolic pathways are found in the cytosol of the cell there are no membranes or permeability barriers against the transfer of NADPH. Other sources of NADPH include the reaction that converts malate to pyruvate catalyzed by the “malic enzyme” (NADP malate dehydrogenase) the extramitochondrial isocitrate dehydrogenase reaction (probably not a substantial source, except in ruminants). Acetyl-CoA Is the Principal Building Block of Fatty Acids Acetyl-CoA is formed from glucose via the oxidation of pyruvate in the matrix of the mitochondria. as it does not diffuse readily across the mitochondrial membranes its transport into the cytosol, the principal site of fatty acid synthesis, requires a special mechanism involving citrate. After condensation of acetyl-CoA with oxaloacetate in the citric acid cycle within mitochondria the citrate produced can be translocated into the extramitochondrial compartment via the tricarboxylate transporter where in the presence of CoA and ATP undergoes cleavage to acetyl-CoA and oxaloacetate catalyzed by ATP-citrate lyase increases in activity in the well-fed state. The acetyl-CoA is then available for malonyl-CoA formation and synthesis of fatty acids The resulting oxaloacetate can form malate via NADH-linked malate dehydrogenase followed by the generation of NADPH via the malic enzyme NADPH becomes available for lipogenesis pyruvate can be used to regenerate acetyl-CoA after transport into the mitochondrion. This pathway is a means of transferring reducing equivalents from extramitochondrial NADH to NADP. Alternatively, malate itself can be transported into the mitochondrion, where it is able to re-form oxaloace-tate. Note that the citrate (tricarboxylate) transporter in the mitochondrial membrane requires malate to exchange with citrate There is little ATP-citrate lyase or malic enzyme in ruminants because in these species acetate (derived from carbohydrate digestion in the rumen activated to acetyl-CoA extramitochondrially) is the main source of acetyl-CoA. Elongation of Fatty Acid Chains Occurs in the Endoplasmic Reticulum This pathway (the “microsomal system”) elongates saturated and unsaturated fatty acyl-CoAs (from C10 upward) by two carbons using malonyl-CoA as the acetyl donor and NADPH as the reductant is catalyzed by the microsomal fatty acid elongase system of enzymes THE NUTRITIONAL STATE REGULATES LIPOGENESIS Excess carbohydrate is stored as fat in many animals in anticipation of periods of caloric deficiency such as starvation, hibernation, etc provide energy for use between meals in animals, including humans, that take their food at spaced intervals. Lipogenesis converts surplus glucose and intermediates such as pyruvate, lactate, and acetyl-CoA to fat, assisting the anabolic phase of this feeding cycle. The nutritional state of the organism is the main factor regulating the rate of lipogenesis. Thus, the rate is high in the well-fed animal whose diet contains a high proportion of carbohydrate. It is depressed by restricted caloric intake, high-fat diet or a deficiency of insulin, as in diabetes mellitus. These latter conditions are associated with increased concentrations of plasma-free fatty acids, and an inverse relationship has been demonstrated between hepatic lipogenesis and the concentration of serum-free fatty acids. Lipogenesis is increased when sucrose is fed instead of glucose because fructose bypasses the phosphofructokinase control point in glycolysis and floods the lipogenic pathway SHORT & LONGTERM MECHANISMS REGULATE LIPOGENESIS Long-chain fatty acid synthesis is controlled in the short term by allosteric and covalent modification of enzymes and in the long term by changes in gene expression governing rates of synthesis of enzymes. Acetyl-CoA Carboxylase Is the Most Important Enzyme in the Regulation of Lipogenesis Acetyl-CoA carboxylase is an allosteric enzyme and is activated by citrate which increases in concentration in the well-fed state is an indicator of a plentiful supply of acetyl-CoA Citrate promotes the conversion of the enzyme from an inactive dimer (two subunits of the enzyme complex) to an active polymeric Inactivation is promoted by phosphorylation of the enzyme and by long-chain acyl-CoA molecules Pyruvate Dehydrogenase Is Also Regulated by Acyl-CoA Acyl-CoA causes an inhibition of pyruvate dehydrogenase by inhibiting the ATP-ADP exchange transporter of the inner mito-chondrial membrane leads to increased intramitochon-drial (ATP)/(ADP) ratios therefore to conversion of active to inactive pyruvate dehydrogenase regulating the availability of acetyl-CoA for lipogenesis. Insulin Also Regulates Lipogenesis by Other Mechanisms Insulin stimulates lipogenesis by several other mechanisms as well as by increasing acetyl-CoA carboxylase activity. It increases the transport of glucose into the cell (eg, in adipose tissue), increasing the availability of both pyruvate for fatty acid synthesis and glycerol-3-phosphate converts the inactive form of pyruvate dehydrogenase to the active forms adipose tissue, but not in liver. Insulin also—by its ability to depress the level of intracel-lular cAM inhibits lipolysis in adipose tissue reducing the concentration of plasma-free fatty acids long-chain acyl-CoA, which are inhibitors of lipogenesis.  DEFICIENCY SYMPTOMS OCCUR WHEN THE ESSENTIAL FATTY ACIDS EFA ARE ABSENT FROM THE DIET Rats fed a purified nonlipid diet containing vitamins A and D exhibit a reduced growth rate and reproductive deficiency which may be cured by the addition of linoleic, α-linolenic, and arachidonic acids to the diet. These fatty acids are found in high concentrations in vegetable oils (see Table 21–2) and in small amounts in animal carcasses. Essential fatty acids are required for prostaglandin, thromboxane, leukotriene, and lipoxin formation and they also have various other functions that are less well defined. They are found in the structural lipids of the cell, often in the position 2 of phospholipids, and are concerned with the structural integrity of the mitochondrial membrane. Arachidonic acid is present in membranes and accounts for 5% to 15% of the fatty acids in phospholipids. Docosahexaenoic acid (DHA; ω3, 22:6), which is synthesized to a limited extent from α-linolenic acid or obtained directly from fish oils, is present in high concentrations in retina, cerebral cortex, testis, and sperm. DHA is particularly needed for development of the brain and retina and is supplied via the placenta and milk. Patients with retinitis pigmentosa are reported to have low blood levels of DHA. In essential fatty acid deficiency, nonessential polyenoic acids of the ω9 family, particularly Δ5,8,11-eicosatrienoic acid (ω9 20:3) replace the essential fatty acids in phospholipids, other complex lipids, and membranes. The triene:tetraene ratio in plasma lipids can be used to diagnose the extent of essential fatty acid deficiency. EICOSANOIDS ARE FORMED FROM C20 POLYUNSATURATED FATTY ACIDS Arachidonate and some other C20 polyunsaturated fatty acids give rise to eicosanoids, physiologically and pharmacologically active compounds known as ○ prostaglandins (PG), thromboxanes (TX), leukotrienes (LT), and lipoxins (LX) ○ Physiologically they are considered to act as local hormones functioning through G-protein-linked receptors to elicit their biochemical effects. There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids derived from the essential fatty acids linoleate and `-linolenate, or directly from dietary arachidonate and eicosapentaenoate Arachidonate, which may be obtained from the diet but is usually derived from the position 2 of phospholipids in the plasma membrane by the action of phospholipase A2 is the substrate for the synthesis of the PG2, TX2 series (prostanoids) by the cyclooxygenase pathway or the LT4 and LX4 series by the lipoxygenase pathway, with the two pathways competing for the arachidonate substrate THE CYCLOOXYGENASE PATHWAY IS RESPONSIBLE FOR PROSTANOID SYNTHESIS Prostanoid synthesis involves the consumption of two molecules of O2 catalyzed by cyclooxygenase (COX) (also called prostaglandin H synthase), an enzyme that has two activities, a cyclooxygenase and peroxidase. COX is present as two isoenzymes, COX-1 and COX-2. The product, an endoperoxide (PGH) ○ is converted to prostaglandins D and E as well as to a thromboxane (TXA2) and prostacyclin (PGI2). Each cell type produces only one type of prostanoid. The NSAID aspirin inhibits COX-1 and COX-2. Other NSAIDs include indomethacin and ibuprofen, and usually inhibit cyclooxygenases by competing with arachi-donate. Since inhibition of COX-1 causes the stomach irritation often associated with taking NSAIDs, attempts have been made to develop drugs which selectively inhibit COX-2 (coxibs). Unfortunately, however, the success of this approach has been limited and some coxibs have been withdrawn or suspended from the market due to undesirable side effects and safety issues. Transcription of COX-2—but not of COX-1—is completely inhibited by anti-inflammatory corticosteroids. Essential Fatty Acids Do Not Exert All Their Physiologic Effects via Prostaglandin Synthesis The role of essential fatty acids in membrane formation is unrelated to prostaglandin formation. Prostaglandins do not relieve symptoms of essential fatty acid deficiency an essential fatty acid deficiency is not caused by inhibition of prostaglandin synthesis. Cyclooxygenase Is a “Suicide Enzyme” Switching off” of prostaglandin activity is partly achieved by a remarkable property of cyclooxygenase—that of self-catalyzed destruction; that is, it is a “suicide enzyme.” the inactivation of prostaglandins by 15-hydroxyprostaglandin dehydrogenase is rapid. Blocking the action of this enzyme with sulfasalazine or indomethacin can prolong the half-life of prostaglandins in the body. LEUKOTRIENES & LIPOXINS ARE FORMED BY THE LIPOXYGENASE PATHWAY The leukotrienes are a family of conjugated trienes formed from eicosanoic acids in leukocytes, mastocytoma cells, platelets, and macrophages by the lipoxygenase pathway in response to both immunologic and nonimmunologic stimuli. Three different lipoxygenases (dioxygenases) insert oxygen into the 5, 12, and 15 positions of arachidonic acid, giving rise to hydroperox-ides (HPETE). Only 5-lipoxygenase forms leukotrienes Lipoxins are a family of conjugated tetraenes also arising in leukocytes. They are formed by the combined action of more than one lipoxygenase CLINICAL ASPECTS Symptoms of Essential Fatty Acid Deficiency in Humans Include Skin Lesions & Impairment of Lipid In adultsTransport subsisting on ordinary diets, no signs of essential fatty acid deficiencies have been reported. infants receiving formula diets low in fat and patients maintained for long periods exclusively by intravenous nutrition low in essential fatty acids show deficiency symptoms that can be prevented by an essential fatty acid intake of 1% to 2% of the total caloric requirement. Abnormal Metabolism of Essential Fatty Acids Occurs in Several Diseases Abnormal metabolism of essential fatty acids, which may be connected with dietary insufficiency noted in ○ cystic fibrosis, acrodermatitis enteropathica, hepatorenal syndromeSjögren-Larsson syndrome, multisystem neuronal degeneration, Crohn disease, cirrhosis and alcoholism, and Reye syndrome. Elevated levels of very long chain polyenoic acids have been found in the brains of patients with Zellweger syndrome Diets with a high P:S (polyunsaturated:saturated fatty acid) ratio reduce serum cholesterol levels are considered to be beneficial in terms of the risk of development of coronary heart disease. Trans Fatty Acids Are Implicated in Various Disorders Small amounts of transunsaturated fatty acids are found in ruminant fat (eg, butter fat has 2%-7%) they arise from the action of microorganisms in the rumen the main source in the human diet is from partially hydrogenated vegetable oils (eg, margarine) Trans fatty acids compete with essential fatty acids may exacerbate essential fatty acid deficienc are structurally similar to saturated fatty acids) and have com-parable effects in the promotion of hypercholesterolemia and atherosclerosis Prostanoids Are Potent, Biologically Active Substances Thromboxanes are synthesized in platelets and upon release cause vasoconstriction and platelet aggregation. Their synthesis is specifically inhibited by low-dose aspirin. Prostacyclins (PGI2) are produced by blood vessel walls and are potent inhibitors of platelet aggregatio thromboxanes and prostacyclins are antagonistic. PG3 and TX3, formed from eicosapentaenoic acid (EPA) inhibit the release of arachido-nate from phospholipids and the formation of PG2 and TX2. PGI3 is as potent an antiaggregator of platelets as PGI2 TXA3 is a weaker aggregator than TXA2 changing the balance of activity and favoring longer clotting times. As little as 1 ng/mL of plasma prostaglandins causes contraction of smooth muscle in animals. Potential therapeutic uses include preventtion of conception induction of labor at term termination of pregnancy prevention or alleviation of gastric ulcers, control of inflammation and of blood pressure relief of asthma and nasal congestion Several products of the arachidonate series are of current clini-cal importance. Alprostadil (PGE 1 ) may be used for its smooth muscle relaxing effects to maintain the ductus arteriosus patent in some neonates awaiting cardiac surgery and in the treatment of impotence. Misoprostol, a PGE 1 derivative, is a cytoprotective prostaglandin used in preventing peptic ulcer and in combination with mifepristone (RU-486) for terminating early pregnancies. PGE 2 and PGF 2 are used in obstetrics to induce labor. Latanoprost and several similar compounds are topically active PGF 2α derivatives used in ophthalmology to treat open-angle glaucoma. Prostacyclin (PGI 2 , epoprostenol) is synthesized mainly by the vascular endothelium and is a powerful vasodilator and inhibitor of platelet aggregation. It is used clinically to treat pulmonary hypertension and portopulmonary hypertension. In contrast, thromboxane (TXA 2 ) has undesirable properties (aggre-gation of platelets, vasoconstriction). PGD2 is a potent sleep-pro-moting substance. Prostaglandins increase cAMP in platelets, thyroid, corpus luteum, fetal bone, adenohypophysis, and lung reduce cAMP in renal tubule cells and adipose tissue Effects of Prostaglandins and Thromboxanes A. 1. Smooth Muscle Vascular TXA 2 is a potent vasoconstrictor also a smooth muscle cell mitogen and is the only eicosanoid that has convincingly been shown to have this effect. PGF 2α is also a vasoconstrictor but is not a smooth muscle mitogen. Gastrointestinal tract Most of the prostaglandins and thromboxanes activate gastrointestinal smooth muscle. Longitudinal muscle is contracted by PGE 2 (via EP 3 ) and PGF 2α (via FP), circular muscle is contracted strongly by PGF 2α and weakly by PGI 2 , and is relaxed by PGE 2 (via EP 4 ) Administration of either PGE 2 or PGF 2α results in colicky cramps The leukotrienes also have powerful contractile effects. Airways Respiratory smooth muscle is relaxed by PGE 2 and PGI 2 contracted by PGD 2 , TXA 2 , and PGF 2α The cysteinyl leukotrienes are also bronchoconstrictors ○ They act principally on smooth muscle in peripheral airways and are a thousand times more potent than histamine, both in vitro and in vivo. ○ They also stimulate bronchial mucus secretion and cause mucosal edema. ○ Bronchospasm occurs in about 10% of people taking NSAIDs, possibly because of a shift in arachidonate metabolism from COX metabolism to leukotriene formation. Female reproductive organ Animal studies demonstrate a role for PGE 2 and PGF 2α in early reproductive processes such as ovulation, luteolysis, and fertilization. Uterine muscle is ○ contracted by PGF 2α , TXA 2 , and low concentrations of PGE 2PGI 2 ○ high concentrations of PGE 2 cause relaxation. PGF 2α , together with oxytocin, is essential for the onset of parturition. Male reproductive organ Despite the discovery of prostaglandins in seminal fluid, and their uterotropic effects, the role of prostaglandins in semen is still conjectural. The major source of these prostaglandins is the seminal vesicle; the prostate, despite the name “prostaglandin,” and the testes synthesize only small amounts. The factors that regulate the concentration of prostaglandins in human seminal plasma are not known in detail, but testosterone does promote prostaglandin production. Central and Peripheral Nervous Systems 1.. Fever— PGE 2 increases body temperature, predominantly via EP 3 , although EP 1 also plays a role, especially when administered directly into the cerebral ventricles. Exogenous PGF 2α and PGI 2 induce fever, whereas PGD 2 and TXA 2 do not. Endogenous pyrogens release interleukin-1, which in turn promotes the synthesis and release of PGE 2. This synthesis is blocked by aspirin and other antipyretic compounds. 2. Sleep When infused into the cerebral ventricles, PGD 2 induces natural sleep (as determined by electroencephalographic analysis) via activation of DP 1 receptors and secondary release of adenosine. PGE 2 infusion into the posterior hypothalamus causes wakefulness. 3. Neurotransmission PGE compounds inhibit the release of norepinephrine from postganglionic sympathetic nerve endings NSAIDs increase norepinephrine release in vivo, suggesting that the prostaglandins play a physiologic role in this process vasoconstriction observed during treatment with COX inhibitors may be due, in part, to increased release of norepinephrine as well as to inhibition of the endothelial synthesis of the vasodilators PGE 2 and PGI 2 PGE 2 and PGI 2 sensitize the peripheral nerve endings to painful stimuli by increasing their terminal membrane excitability. Prostaglandins also modulate pain centrally. F. Inflammation and Immunity PGE 2 and PGI 2 are the predominant prostanoids associated with inflammation. Both markedly enhance edema formation and leukocyte infiltration by promoting blood flow in the inflamed region. PGE 2 and PGI 2 , through activation of EP 2 and IP increase vascular permeability and leukocyte infiltration. Thromboxane and leukotrienes have not been found in seminal plasma. Men with a low seminal fluid concentration of prostaglandins are relatively infertile. Smooth muscle-relaxing prostaglandins such as PGE 1 enhance penile erection by relaxing the smooth muscle of the corpora cavernosa Leukotrienes & Lipoxins Are Potent Regulators of Many Disease Processes Slow-reacting substance of anaphylaxis (SRS-A) is a mixture of leukotrienes C4, D4, and E4. This mixture of leukotrienes is a potent constrictor of the bronchial airway musculature. These leukotrienes together with leukotriene B4 also cause vascular permeability and attraction and activation of leukocytes and are important regulators in many diseases involving inflammatory or immediate hypersensitivity reactions, such as asthma. Leukotrienes are vasoactive, and 5-lipoxygenase has been found in arterial walls. Evidence supports an anti-inflammatory role for lipoxins in vasoactive and immunoregulatory functio The End