Harper's Biochemistry Chapter 23 - Biosynthesis of Fatty Acids & Eicosanoids PDF

C H A P T E R Biosynthesis of Fatty Acids & Eicosanoids Kathleen M. Botham, PhD, DSc, & Peter A. Mayes, PhD, DSc 23 Describe the reaction cata...

C H A P T E R Biosynthesis of Fatty Acids & Eicosanoids Kathleen M. Botham, PhD, DSc, & Peter A. Mayes, PhD, DSc 23 Describe the reaction catalyzed by acetyl-CoA carboxylase and understand the OBJ EC T IVES mechanisms by which its activity is regulated to control the rate of fatty acid synthesis. After studying this chapter, Outline the structure of the fatty acid synthase multienzyme complex, indicating you should be able to: the sequence of enzymes in the two peptide chains of the homodimer. Explain how long-chain fatty acids are synthesized by the repeated condensation of two carbon units, with formation of the 16-carbon palmitate being favored in most tissues, and identify the cofactors required. Indicate the sources of reducing equivalents (NADPH) for fatty acid synthesis. Explain how fatty acid synthesis is regulated by nutritional status and identify other control mechanisms that operate in addition to modulation of the activity of acetyl-CoA carboxylase. Identify the nutritionally essential fatty acids and explain why they cannot be formed in the body. Explain how polyunsaturated fatty acids are synthesized by desaturase and elongation enzymes. Outline the cyclooxygenase and lipoxygenase pathways responsible for the formation of the various classes of eicosanoids. BIOMEDICAL IMPORTANCE thromboxanes, leukotrienes, and lipoxins. Prostaglandins mediate inflammation, pain, induce sleep, and also regulate Fatty acids are synthesized by an extramitochondrial system, blood coagulation and reproduction. Nonsteroidal anti- which is responsible for the complete synthesis of palmitate inflammatory drugs (NSAIDs) such as aspirin and ibupro- from acetyl-CoA in the cytosol. In most mammals, glucose fen act by inhibiting prostaglandin synthesis. Leukotrienes is the primary substrate for lipogenesis, but in ruminants it have muscle contractant and chemotactic properties and are is acetate, the main fuel molecule they obtain from the diet. important in allergic reactions and inflammation. 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. THE MAIN PATHWAY FOR Unsaturated fatty acids in phospholipids of the cell mem- DE NOVO SYNTHESIS OF FATTY brane are important in maintaining membrane fluidity (see ACIDS (LIPOGENESIS) OCCURS Chapter 40). A high ratio of polyunsaturated fatty acids to saturated fatty acids (P:S ratio) in the diet is considered to be IN THE CYTOSOL beneficial in preventing coronary heart disease. Animal tissues This system is present in many tissues, including liver, kidney, have limited capacity for desaturating fatty acids, and require brain, lung, mammary gland, and adipose tissue. Its cofactor certain dietary polyunsaturated fatty acids derived from plants. requirements include NADPH, ATP, Mn2+, biotin, and HCO3− These essential fatty acids are used to form eicosanoic (C20) (as a source of CO2). Acetyl-CoA is the immediate substrate, fatty acids, which give rise to the eicosanoids prostaglandins, and free palmitate is the end product. 226  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 227 Production of Malonyl-CoA the enzyme domains are believed to be linked in the sequence as shown in Figure 23–2. X-ray crystallography of the three- Is the Initial & Controlling Step dimensional structure, however, has shown that the complex in Fatty Acid Synthesis is a homodimer, with two identical subunits, each containing six The initial step in fatty acid synthesis is the carboxylation of enzymes and an ACP, arranged in an X shape (see Figure 23–2). acetyl-CoA to form malonyl-CoA by acetyl-CoA carboxylase. The use of one multienzyme functional unit has the advan- The reaction requires ATP and the B vitamin biotin. Acetyl- tages of achieving compartmentalization of the process within CoA carboxylase is a multienzyme protein containing biotin the cell without the necessity for permeability barriers, and carboxylase, and a carboxyl transferase as well as biotin carrier synthesis of all enzymes in the complex is coordinated since it protein (which binds biotin), and a regulatory allosteric site. is encoded by a single gene. The reaction takes place in two steps: Step 1 catalyzed by biotin Initially, a priming molecule of acetyl-CoA combines with carboxylase results in the carboxylation of biotin and uses a cysteine —SH group on the ACP of one monomer of the ATP and Step 2 catalyzed by carboxyl transferase results in fatty acid synthase complex (Figure 23–3, reaction 1a), while the transfer of the carboxyl group to acetyl-CoA forming the malonyl-CoA combines with the adjacent —SH on the 4′- product, malonyl-CoA (Figure 23–1). Acetyl-CoA carboxylase phosphopantetheine of ACP of the other monomer (reaction 1b). has a major role in the regulation of fatty acid synthesis These reactions are catalyzed by malonyl acetyl transacylase, (see following discussion). 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 The Fatty Acid Synthase Complex Is a enzyme (acetoacetyl enzyme) (reaction 2), freeing the cysteine Homodimer of Two Polypeptide Chains —SH group. Decarboxylation allows the reaction to go to com- pletion, pulling the whole sequence of reactions in the forward Containing Six Enzyme Activities direction. The 3-ketoacyl group is reduced (3-ketoacylreductase), & the Acyl Carrier Protein dehydrated (dehydratase), and reduced again (enoyl reductase) After the formation of malonyl-CoA, fatty acids are formed (reactions 3-5) to form the corresponding saturated acyl enzyme by the fatty acid synthase enzyme complex. The individual (product of reaction 5). The saturated acyl residue now transfers enzymes required for fatty acid synthesis are linked in this from the —SH of the 4′ phosphopantetheine to the free cysteine multienzyme polypeptide complex that incorporates the acyl —SH group as it is displaced by a new malonyl-CoA. The sequence carrier protein (ACP), which has a similar function to that of of reactions is repeated six more times until a saturated CoA in the β-oxidation pathway (see Chapter 22). It contains the 16-carbon acyl radical (palmitoyl) has been assembled. It is vitamin pantothenic acid in the form of 4′-phosphopantetheine liberated from the enzyme complex by the activity of the (see Figure 44–15). In the primary structure of the protein, sixth enzyme in the complex, thioesterase (deacylase). The free ATP ADP + Pi –OOC-CH -CO S -CoA + H+ Overall CH3-CO S-CoA + HCO3– 3 Acetyl-CoA biotin reaction Malonyl-CoA Acetyl-CoA carboxylase biotin-COO– ATP ADP + Pi Step 1 biotin Acetyl-CoA E1 E2 + HCO3– E1 E2 carboxylase Biotin enzyme complex carboxylase (E1) BCP BCP biotin-COO– biotin E1 E2 + CH3-CO S-CoA –OOC-CH -CO S -CoA + E1 E2 Step 2 3 Acetyl-CoA Malonyl-CoA Carboxyl transferase (E2) BCP BCP FIGURE 23–1 Biosynthesis of malonyl-CoA by acetyl-CoA carboxylase. Acetyl carboxylase is a multienzyme complex containing two enzymes, biotin carboxylase (E1) and a carboxyltransferase (E2), and the biotin carrier protein (BCP). Biotin is covalently linked to the BCP. The reaction proceeds in two steps. In step 1, catalyzed by E1, biotin is carboxylated as it accepts a COO− group from HCO3− and ATP is used. In step 2, catalyzed by E2, the COO− is transferred to acetyl-CoA forming malonyl-CoA. 228 SECTION V Metabolism of Lipids Malonyl/acetyl Ketoacyl Enoyl Ketoacyl N- Dehydratase ACP Thioesterase -C transacylase synthase reductase reductase Sequence of enzyme domains in primary structure of fatty acid synthase monomer Ketoacyl Ketoacyl reductase Enoyl Enoyl reductase reductase reductase ACP ACP Hydratase Thioesterase Thioesterase Ketoacyl synthase Malonyl/acetyl Malonyl/acetyl transacylase transacylase Fatty acid synthase homodimer FIGURE 23–2 Fatty acid synthase multienzyme complex. The complex is a dimer of two identical polypeptide monomers in which six enzymes and the acyl carrier protein (ACP) are linked in the primary structure in the sequence shown. X-ray crystallography of the three- dimensional structure has demonstrated that the two monomers in the complex are arranged in an X-shape. palmitate must be activated to acyl-CoA before it can proceed both metabolic pathways are found in the cytosol of the cell, via any other metabolic pathway. Its possible fates are esteri- so there are no membranes or permeability barriers against the fication into acylglycerols, chain elongation, desaturation, or transfer of NADPH. Other sources of NADPH include the reac- esterification into cholesteryl ester. In mammary gland, there tion that converts malate to pyruvate catalyzed by the NADP is a separate thioesterase specific for acyl residues of C8, C10, or malate dehydrogenase (malic enzyme) (Figure 23–4) and the C12, which are subsequently found in milk lipids. extramitochondrial isocitrate dehydrogenase reaction (a sub- The equation for the overall synthesis of palmitate from stantial source in ruminants). acetyl-CoA and malonyl-CoA is CH3 COSCoA + 7HOOCCHCOSCoA + 14NADPH + 14H+ Acetyl-CoA Is the Principal Building → CH3(CH2)14 COOH + 7CO2 + 6H2O + 8CoASH + 14NADP+ Block of Fatty Acids Acetyl-CoA is formed from glucose via the oxidation of pyru- The acetyl-CoA used as a primer forms carbon atoms vate in the matrix of the mitochondria (see Chapter 17). 15 and 16 of palmitate. The addition of all the subsequent C2 However, as it does not diffuse readily across the mitochondrial units is via malonyl-CoA. Propionyl-CoA instead of acetyl- membranes, its transport into the cytosol, the principal site CoA is used as the primer for the synthesis of long-chain fatty of fatty acid synthesis, requires a special mechanism involv- acids with an odd number of carbon atoms, which are found ing citrate. After condensation of acetyl-CoA with oxaloac- particularly in ruminant fat and milk. etate in the citric acid cycle within mitochondria, the citrate produced can be translocated into the extramitochondrial compartment via the tricarboxylate transporter, where in the The Main Source of NADPH for presence of CoA and ATP, it undergoes cleavage to acetyl- CoA and oxaloacetate by ATP-citrate lyase, which increases Lipogenesis Is the Pentose in activity in the well-fed state. The acetyl-CoA is then avail- Phosphate Pathway able for malonyl-CoA formation and synthesis of fatty acids NADPH is involved as a donor of reducing equivalents in the (see Figures 23–1 and 23–3), and the oxaloacetate can form reduction of the 3-ketoacyl and the 2,3-unsaturated acyl deriva- malate via NADH-linked malate dehydrogenase, followed by tives (see Figure 23–3, reactions 3 and 5). The oxidative reac- the generation of NADPH and pyruvate via the malic enzyme. tions of the pentose phosphate pathway (see Chapter 20) are the The NADPH becomes available in the cytosol for lipogen- chief source of the hydrogen required for the synthesis of fatty esis, and the pyruvate can be used to regenerate acetyl-CoA acids. Significantly, tissues specializing in active lipogenesis after transport into the mitochondrion (see Figure 23–4). that is, liver, adipose tissue, and the lactating mammary gland This pathway is a means of transferring reducing equivalents also possess an active pentose phosphate pathway. Moreover, from extramitochondrial NADH to NADP to form NADPH.  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 229 *CO2 Acetyl-CoA *Malonyl-CoA C2 Acetyl-CoA C3 carboxylase 1a HS Pan 1 Cys SH 1b CoA Malonyl acetyl Cn transfer from transacylase Malonyl acetyl transacylase HS Cys 2 Pan SH CoA 2 to 1 C2 Fatty acid synthase O multienzyme complex 1 Cys S C CH 3 O 2 Pan S C CH 2 *COO – (C3 ) Acyl(acetyl)-malonyl enzyme 3-Ketoacyl synthase 2 *CO 2 1 Cys SH O O 2 Pan S C CH 2 C CH 3 3-Ketoacyl enzyme (acetoacetyl enzyme) NADPH + H+ 3-Ketoacyl reductase 3 + NADP 1 Cys SH O OH 2 Pan S C CH 2 CH CH 3 NADPH generators D (–)-3-Hydroxyacyl enzyme Pentose phosphate pathway Dehydratase 4 Isocitrate H 2O dehydrogenase Malic enzyme 1 Cys SH O 2 Pan S C CH CH CH 3 2,3-Unsaturated acyl enzyme NADPH + H + Enoyl reductase 5 NADP + H 2O 1 Cys SH Thioesterase O After cycling through steps 2 – 5 seven times 2 Pan S C CH2 CH2 CH3 (Cn ) Acyl enzyme Palmitate KEY: 1 , 2 , individual monomers of fatty acid synthase FIGURE 23–3 Biosynthesis of long-chain fatty acids. After the initial priming step in which acetyl-CoA is bound to a cysteine-SH group on the fatty acid synthase enzyme (reaction 1a), the addition of a malonyl residue causes the acyl chain to grow by two carbon atoms in each cycle. (Cys, cysteine residue; Pan, 4′-phosphopantetheine.) The blocks highlighted in blue contain the C 2 unit derived from acetyl-CoA initially (as illustrated) and subsequently the Cn unit formed in reaction 5. * Shows that the carbon in the CO2 initially incorporated into malonyl-CoA is then released as CO2 in reaction 2. As the citrate (tricarboxylate) transporter in the mitochon- little ATP-citrate lyase or malic enzyme in ruminants, probably drial membrane requires malate to exchange with citrate (see because in these species acetate (derived from carbohydrate Figure 13–10), malate itself can be transported into the mito- digestion in the rumen and activated to acetyl-CoA extra- chondrion, where it is able to reform oxaloacetate. There is mitochondrially) is the main source of acetyl-CoA. 230 SECTION V Metabolism of Lipids FIGURE 23–4 The provision of acetyl-CoA and NADPH for lipogenesis. (K, α-ketoglutarate transporter; P, pyruvate transporter; PPP, pentose phosphate pathway; T, tricarboxylate transporter.) Elongation of Fatty Acid Chains Occurs the main factor regulating the rate of lipogenesis. Thus, the rate is high in the well-fed animal whose diet contains a high pro- in the Endoplasmic Reticulum portion of carbohydrate. It is depressed by restricted caloric This pathway (the “microsomal system”) elongates saturated intake, high-fat diet, or a deficiency of insulin, as in diabetes and unsaturated fatty acyl-CoAs (from C10 upward) by two mellitus. These latter conditions are associated with increased carbons, using malonyl-CoA as the acetyl donor and NADPH concentrations of plasma-free fatty acids, and an inverse rela- as the reductant, and is catalyzed by the microsomal fatty acid tionship has been demonstrated between hepatic lipogenesis elongase system of enzymes (Figure 23–5). Elongation of and the concentration of serum-free fatty acids. Lipogenesis stearyl-CoA in brain increases rapidly during myelination in is increased when sucrose (a disaccharide consisting of glu- order to provide C22 and C24 fatty acids for sphingolipids (see cose and fructose) is fed instead of glucose because fructose Figures 21–10 and 21–11). bypasses the phosphofructokinase control point in glycolysis and floods the lipogenic pathway (see Figure 20–5). THE NUTRITIONAL STATE REGULATES LIPOGENESIS SHORT- & LONG-TERM Excess carbohydrate is stored as fat in many animals in antici- pation of periods of caloric deficiency such as starvation, hiber- MECHANISMS REGULATE nation, etc., and to provide energy for use between meals in LIPOGENESIS animals, including humans, that take their food at spaced inter- Long-chain fatty acid synthesis is controlled in the short term vals. Lipogenesis converts surplus glucose and intermediates such by allosteric and covalent modification of enzymes and in the as pyruvate, lactate, and acetyl-CoA to fat, assisting the anabolic long term by changes in gene expression governing rates of phase of this feeding cycle. The nutritional state of the organism is synthesis of enzymes.  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 231 O O R CH2 C S CoA + CH2 C S CoA COOH Acyl-CoA Malonyl-CoA 3-Ketoacyl-CoA synthase CoA SH + CO2 O O R CH2 C CH2 C S CoA 3-Ketoacyl-CoA FIGURE 23–6 Regulation of acetyl-CoA carboxylase. Acetyl- NADPH + H+ CoA carboxylase is activated by citrate, which promotes the conver- 3-Ketoacyl-CoA sion of the enzyme from an inactive dimer to an active polymeric reductase form. Inactivation is promoted by phosphorylation of the enzyme and by long-chain acyl-CoA molecules such as palmitoyl-CoA. In NADP+ addition, acyl-CoA inhibits the tricarboxylate transporter, which trans- OH O ports citrate out of mitochondria into the cytosol, thus decreasing the citrate concentration in the cytosol and favoring inactivation of the R CH2 CH CH2 C S CoA enzyme. 3-Hydroxyacyl-CoA (Figure 23–6). Thus, if acyl-CoA accumulates because it is not esterified quickly enough or because of increased lipolysis or 3-Hydroxyacyl-CoA dehydrase an influx of free fatty acids into the tissue, it will automatically H2O reduce the rate of synthesis of new fatty acid. Acyl-CoA also inhibits the mitochondrial tricarboxylate transporter, thus O preventing activation of the enzyme by egress of citrate from R CH2 C CH CH C S CoA the mitochondria into the cytosol (see Figure 23–6). 2-trans-Enoyl-CoA Acetyl-CoA carboxylase is also regulated by hormones such as glucagon, epinephrine, and insulin via changes in its NADPH + H+ phosphorylation state (details in Figure 23–7). 2-trans-Enoyl-CoA reductase Pyruvate Dehydrogenase Is Also NADP+ O Regulated by Acyl-CoA Acyl-CoA causes an inhibition of pyruvate dehydrogenase, R CH2 CH2 CH2 C S CoA the enzyme which catalyzes the formation of acetyl-CoA Acyl-CoA from pyruvate to link glycolysis with the citric acid cycle (see Chapters 16 and 17), by inhibiting the ATP-ADP exchange FIGURE 23–5 Microsomal elongase system for fatty acid chain elongation. NADH may also be used by the reductases, but transporter of the inner mitochondrial membrane. This leads NADPH is preferred. to increased intramitochondrial (ATP)/(ADP) ratios and therefore to conversion of active to inactive pyruvate dehy- Acetyl-CoA Carboxylase Is the Most drogenase (see Figure 17–6), thus regulating the availability of acetyl-CoA for lipogenesis. Furthermore, oxidation of acyl- Important Enzyme in the Regulation CoA due to increased levels of free fatty acids may increase the of Lipogenesis ratios of (acetyl-CoA)/(CoA) and (NADH)/(NAD+) in mito- Acetyl-CoA carboxylase is an allosteric enzyme and is acti- chondria, which also inhibits pyruvate dehydrogenase. vated by citrate, which increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl- CoA. Citrate promotes the conversion of the enzyme from Insulin Also Regulates Lipogenesis by an inactive dimer (two subunits of the enzyme complex) to Other Mechanisms an active polymeric form, with a molecular mass of several Insulin stimulates lipogenesis by several other mechanisms million. Inactivation is promoted by phosphorylation of the as well as by increasing acetyl-CoA carboxylase activity. It enzyme and by long-chain acyl-CoA molecules, an example increases the transport of glucose into the cell (eg, in adipose of negative feedback inhibition by a product of a reaction tissue), increasing the availability of both pyruvate (and thus 232 SECTION V Metabolism of Lipids enzymes of glycolysis and lipogenesis. These mechanisms for longer-term regulation of lipogenesis take several days to become fully manifested and augment the direct and immedi- ate effect of free fatty acids and hormones such as insulin and glucagon. SOME POLYUNSATURATED FATTY ACIDS CANNOT BE SYNTHESIZED BY MAMMALS & ARE NUTRITIONALLY ESSENTIAL Certain long-chain unsaturated fatty acids of metabolic sig- nificance in mammals are shown in Figure 23–8. Other C20, C22, and C24 polyenoic fatty acids may be derived from oleic, linoleic, and α-linolenic acids by chain elongation. Palmitoleic and oleic acids are not essential in the diet because the tissues can introduce a double bond at the Δ9 position of a saturated fatty acid. Linoleic and α-linolenic acids are the only fatty acids known to be essential for the complete nutrition of many species of animals, including humans, and are termed the nutritionally essential fatty acids. In humans and most other FIGURE 23–7 Regulation of acetyl-CoA carboxylase by mammals, arachidonic acid can be formed from linoleic acid. phosphorylation/dephosphorylation. The enzyme is inactivated by phosphorylation by AMP-activated protein kinase (AMPK), which in turn is phosphorylated and activated by AMP-activated protein kinase kinase (AMPKK). Glucagon and epinephrine increase cAMP, and thus activate this latter enzyme via cAMP-dependent protein kinase. The kinase kinase enzyme is also believed to be activated by acyl-CoA. Insulin activates acetyl-CoA carboxylase via dephosphoryla- tion of AMPK. acetyl-CoA) for fatty acid synthesis and glycerol-3-phosphate for triacylglycerol synthesis via esterification of the newly formed fatty acids (see Figure 24–2), and also converts the inactive form of pyruvate dehydrogenase to the active form in adipose tissue, although not in liver. Insulin alsoby its ability to depress the level of intracellular cAMPinhibits lipolysis in adipose tissue, reducing the concentration of plasma-free fatty acids and, therefore, long-chain acyl-CoA, which are inhibitors of lipogenesis. The Fatty Acid Synthase Complex & Acetyl-CoA Carboxylase Are Adaptive Enzymes These enzymes adapt to the body’s physiologic needs via FIGURE 23–8 Structure of some unsaturated fatty acids. changes in gene expression which lead to increases in the Although the carbon atoms in the molecules are conventionally total amount of enzyme protein present in the fed state and numbered—that is, numbered from the carboxyl terminal—the ω decreases during intake of a high-fat diet and in conditions numbers (eg, ω7 in palmitoleic acid) are calculated from the reverse such as starvation, and diabetes mellitus. Insulin plays an end (the methyl terminal) of the molecules. The information in parentheses shows, for instance, that α-linolenic acid contains dou- important role, promoting gene expression and induction of ble bonds starting at the third carbon from the methyl terminal, has enzyme biosynthesis, and glucagon (via cAMP) antagonizes 18 carbons and 3 double bonds, and has these double bonds at the this effect. Feeding fats containing polyunsaturated fatty acids 9th, 12th, and 15th carbons from the carboxyl terminal. *Nutritionally coordinately regulates the inhibition of expression of key essential fatty acids in humans.  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 233 Oxygen and either NADH or NADPH are necessary for the reac- tion. The enzymes appear to be similar to a mono-oxygenase sys- tem involving cytochrome b5 (see Chapter 12). SYNTHESIS OF POLYUNSATURATED FATTY FIGURE 23–9 Microsomal Δ9 desaturase. ACIDS INVOLVES DESATURASE & ELONGASE ENZYME SYSTEMS Double bonds can be introduced at the Δ4, Δ5, Δ6, and Δ9 posi- Additional double bonds introduced into existing monounsatu- tions (see Chapter 21) in most animals, but never beyond the rated fatty acids are always separated from each other by a meth- Δ9 position. In contrast, plants are able to synthesize the nutri- ylene group (methylene interrupted) except in bacteria. Since tionally essential fatty acids by introducing double bonds at animals have a Δ9 desaturase, they are able to synthesize the ω9 the Δ12 and Δ15 positions. (oleic acid) family of unsaturated fatty acids completely by a com- bination of chain elongation and desaturation (see Figures 23–9 and 23–10) after the formation of saturated fatty acids by the MONOUNSATURATED FATTY pathways described in this chapter. However, as indicated earlier, ACIDS ARE SYNTHESIZED BY linoleic (ω6) or α-linolenic (ω3) acids are nutritionally essential, as they are required for the synthesis of the other members of the A Δ9 DESATURASE SYSTEM ω6 or ω3 families (pathways shown in Figure 23–10) and must be Several tissues including the liver are considered to be respon- supplied in the diet. Linoleic acid is converted to arachidonic acid sible for the formation of nonessential monounsaturated fatty (20:4 ω6) via γ-linolenic acid (18:3 ω6). The nutritional require- acids from saturated fatty acids. The first double bond introduced ment for arachidonate may thus be dispensed with if there is ade- into a saturated fatty acid is nearly always in the Δ9 position. An quate linoleate in the diet. Cats, however, cannot carry out this enzyme systemΔ9 desaturase (Figure 23–9)in the endo- conversion owing to the absence of Δ6 desaturase and must obtain plasmic reticulum catalyzes the conversion of palmitoyl-CoA or arachidonate in their diet. The desaturation and chain elongation stearoyl-CoA to palmitoleoyl-CoA or oleoyl-CoA, respectively. system are greatly diminished in the starving state, in response to FIGURE 23–10 Biosynthesis of the ω9, ω6, and ω3 families of polyunsaturated fatty acids. In animals, the ω9, ω6, and ω3 families of polyunsaturated fatty acids are synthesized in the endoplasmic reticulum from oleic, linoleic, and β-linolenic acids, respectively, by a series of elongation and desaturation reactions. The production of 22:5 ω6 (osbond acid) or 22:6 ω3 (docosahexanoic acid [DHA]), however, requires one cycle of β-oxidation, which takes place inside peroxisomes after the formation of 24:5 ω6 or 24:6 ω3. (AA, arachidonic acid; E, elongase; DS, desaturase; EFA, essential fatty acids; EPA, eicosapentaenoic acid; GLA, γ-linolenic acid; – , inhibition.) 234 SECTION V Metabolism of Lipids glucagon and epinephrine administration, and in the absence of the cyclooxygenase pathway, or the LT4 and LX4 series by the insulin as in type 1 diabetes mellitus. lipoxygenase pathway, with the two pathways competing for the arachidonate substrate (see Figure 23–11). THE ESSENTIAL FATTY ACIDS (EFA) HAVE IMPORTANT FUNCTIONS THE CYCLOOXYGENASE IN THE BODY PATHWAY IS RESPONSIBLE FOR Rats fed a purified nonlipid diet containing vitamins A and PROSTANOID SYNTHESIS D exhibit a reduced growth rate and reproductive deficiency which may be cured by the addition of linoleic, α-linolenic, Prostanoids (see Chapter 21) are synthesized by the pathway and arachidonic acids to the diet. These fatty acids are found summarized in Figure 23–12. In the first reaction, catalyzed by in high concentrations in vegetable oils (see Table 21–2) and cyclooxygenase (COX) (also called prostaglandin H synthase), in small amounts in animal carcasses. Essential fatty acids are an enzyme that has two activities, a cyclooxygenase and required for prostaglandin, thromboxane, leukotriene, and peroxidase, two molecules of O2 are consumed. COX is pres- lipoxin formation (see following discussion). They are found ent as two isoenzymes, COX-1 and COX-2. The product, an in the structural lipids of the cell, often in the position 2 of endoperoxide (PGH), is converted to prostaglandins D and E phospholipids, and are concerned with the structural integrity as well as to a thromboxane (TXA2) and prostacyclin (PGI2). of the mitochondrial membrane. Prostanoids are synthesized by many different cell types, but Arachidonic acid is present in membranes and accounts each one produces only one type of prostanoid. for 5 to 15% of the fatty acids in phospholipids. Docosahexae- noic acid (DHA; ω3, 22:6), which is synthesized to a lim- Prostanoids Are Potent, Biologically ited extent from α-linolenic acid or obtained directly from fish oils, is present in high concentrations in retina, cerebral Active Substances cortex, testis, and sperm. DHA is particularly needed for Thromboxanes are synthesized in platelets and on release cause development of the brain and retina and is supplied via the vasoconstriction and platelet aggregation. Their synthesis is placenta and milk. Patients with retinitis pigmentosa are specifically inhibited by low-dose aspirin. Prostacyclins (PGI2) reported to have low blood levels of DHA. In essential fatty are produced by blood vessel walls and are potent inhibitors of acid deficiency, nonessential polyenoic acids of the ω9 family, platelet aggregation. particularly Δ5,8,11-eicosatrienoic acid (ω9 20:3) (see Figure 23–10), Thus, thromboxanes and prostacyclins are antagonistic. replace the essential fatty acids in phospholipids, other com- PG3 and TX3, formed from eicosapentaenoic acid (EPA), inhibit plex lipids, and membranes. The triene:tetraene ratio in the release of arachidonate from phospholipids and, therefore, plasma lipids can be used to diagnose the extent of essential the formation of PG2 and TX2. PGI3 is as potent an antiaggre- fatty acid deficiency. gator of platelets as PGI2, but TXA3 is a weaker aggregator than TXA2, changing the balance of activity and favoring longer clot- ting times. As little as 1 ng/mL of plasma prostaglandins causes EICOSANOIDS ARE FORMED contraction of smooth muscle in animals. FROM C20 POLYUNSATURATED FATTY ACIDS Essential Fatty Acids Do Not Exert Arachidonate and some other C20 polyunsaturated fatty acids All Their Physiologic Effects via give rise to eicosanoids, physiologically and pharmaco- Prostaglandin Synthesis logically active compounds known as prostaglandins (PG), The role of essential fatty acids in membrane formation is thromboxanes (TX), leukotrienes (LT), and lipoxins (LX) unrelated to prostaglandin formation. Prostaglandins do not (see Chapter 21). Physiologically, they are considered to act as relieve symptoms of essential fatty acid deficiency, and an local hormones functioning through G-protein–linked recep- essential fatty acid deficiency is not caused by inhibition of tors to elicit their biochemical effects. prostaglandin synthesis. There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids derived from the essential fatty acids linoleate and α-linolenate, or directly from dietary arachi- Cyclooxygenase Is a “Suicide Enzyme” donate and eicosapentaenoate (Figure 23–11). Arachidonate, “Switching off ” of prostaglandin activity is partly achieved by a which may be obtained from the diet, but is usually derived remarkable property of cyclooxygenasethat of self-catalyzed from the position 2 of phospholipids in the plasma membrane destruction; that is, it is a “suicide enzyme.” Furthermore, the by the action of phospholipase A2 (see Figure 24–5), is the sub- inactivation of prostaglandins by 15-hydroxyprostaglandin strate for the synthesis of the PG2, TX2 series (prostanoids) by dehydrogenase is rapid. Blocking the action of this enzyme  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 235 FIGURE 23–11 The three groups of eicosanoids and their biosynthetic origins. 1 , cyclooxygenase pathway; 2 , lipoxygenase path- way; LT, leukotriene; LX, lipoxin; PG, prostaglandin; PGI, prostacyclin; TX, thromboxane. The subscript denotes the total number of double bonds in the molecule and the series to which the compound belongs. with sulfasalazine or indomethacin can prolong the half-life of of conjugated tetraenes also arising in leukocytes. They are prostaglandins in the body. formed by the combined action of more than one lipoxygenase (see Figure 23–13). LEUKOTRIENES & LIPOXINS ARE CLINICAL ASPECTS FORMED BY THE LIPOXYGENASE PATHWAY Essential Fatty Acids Play a Role in the The leukotrienes are a family of conjugated trienes formed Prevention of Diseases in Humans from eicosanoic acids in leukocytes, mastocytoma cells, Studies have shown that dietary intake of essential fatty acids platelets, and macrophages by the lipoxygenase pathway in is associated with a reduction in the development of a number response to both immunologic and nonimmunologic stimuli. of human diseases including cardiovascular disease, cancer, Three different lipoxygenases (dioxygenases) insert oxygen arthritis, diabetes mellitus, and various neurologic conditions. into the 5, 12, and 15 positions of arachidonic acid, giving Various mechanisms are believed to be involved, including rise to hydroperoxides (HPETE). Only 5-lipoxygenase forms changes in gene expression, prostanoid production, and mem- leukotrienes (details in Figure 23–13). Lipoxins are a family brane composition. 236 SECTION V Metabolism of Lipids FIGURE 23–12 Conversion of arachidonic acid to prostaglandins and thromboxanes of series 2. HHT, hydroxyheptadecatrienoate; PG, prostaglandin; PGI, prostacyclin; TX, thromboxane. *Both of these starred activities are attributed to the cyclooxgenase enzyme (prostaglandin H synthase). Similar conversions occur in prostaglandins and thromboxanes of series 1 and 3. Symptoms of Essential Fatty Acid to be beneficial in terms of the risk of development of coronary heart disease. Deficiency in Humans Include Skin Lesions & Impairment of Trans Fatty Acids Are Implicated Lipid Transport in Various Disorders In adults subsisting on ordinary diets, no signs of essential fatty Small amounts of trans-unsaturated fatty acids (see Chapter 21) acid deficiencies have been reported. However, infants receiv- are found in ruminant fat (eg, butter fat has 2-7%), where they ing formula diets low in fat and patients maintained for long arise from the action of microorganisms in the rumen, but the periods exclusively by intravenous nutrition low in essential main source in the human diet is from partially hydrogenated fatty acids show deficiency symptoms that can be prevented vegetable oils (eg, margarine). Trans fatty acids compete with by an essential fatty acid intake of 1 to 2% of the total caloric essential fatty acids and may exacerbate essential fatty acid requirement. deficiency. Moreover, they are structurally similar to saturated fatty acids (see Chapter 21) and have comparable effects in the Abnormal Metabolism of Essential promotion of hypercholesterolemia and atherosclerosis (see Chapter 26). Fatty Acids Occurs in Several Diseases Abnormal metabolism of essential fatty acids, which may be connected with dietary insufficiency, has been noted in cystic Nonsteroidal Anti-Inflammatory fibrosis, acrodermatitis enteropathica, hepatorenal syndrome, Drugs Inhibit COX Sjögren-Larsson syndrome, multisystem neuronal degeneration, Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) Crohn disease, cirrhosis and alcoholism, and Reye syndrome. that inhibits COX-1 and COX-2. Other NSAIDs include indo- Elevated levels of very-long-chain polyenoic acids have been methacin and ibuprofen, and these usually inhibit cyclooxy- found in the brains of patients with Zellweger syndrome (see genases by competing with arachidonate. Since inhibition of Chapter 22). Diets with a high P:S (polyunsaturated:saturated COX-1 causes the stomach irritation often associated with fatty acid) ratio reduce serum cholesterol levels and are considered taking NSAIDs, attempts have been made to develop drugs  CHAPTER 23 Biosynthesis of Fatty Acids & Eicosanoids 237 COOH 15-Lipoxygenase 12-Lipoxygenase Arachidonate COOH COOH O2 HOO OOH 1 12-HPETE 15-HPETE 5-Lipoxygenase COOH 1 COOH OH HO 15-HETE OOH OH 12-HETE COOH COOH 5-Lipoxygenase 1 5-HPETE 5-HETE OH H2O OH OH COOH COOH H2O O COOH OH 15-Lipoxygenase OH 2 Leukotriene B4 Leukotriene A4 Lipoxins, eg, LXA 4 Glutathione 3 Glutamic acid O NH2 Glycine Glycine OH O NH O NH2 NH2 O NH NH HO HO HO Cysteine Glutamic acid Cysteine Glycine Cysteine O S O S O S COOH 4 COOH 5 COOH OH OH OH Leukotriene C4 Leukotriene D4 Leukotriene E4 FIGURE 23–13 Conversion of arachidonic acid to leukotrienes and lipoxins of series 4 via the lipoxygenase pathway. Some similar conversions occur in series 3 and 5 leukotrienes. 1 , peroxidase; 2 , leukotriene A4 epoxide hydrolase; 3 , glutathione S-transferase; 4 , γ-glutamyltranspeptidase; 5 , cysteinyl-glycine dipeptidase; HETE, hydroxyeicosatetraenoate; HPETE, hydroperoxyeicosatetraenoate. that selectively inhibit COX-2 (coxibs). Unfortunately, how- thyroid, corpus luteum, fetal bone, adenohypophysis, and lung ever, the success of this approach has been limited and some but reduce cAMP in renal tubule cells and adipose tissue (see coxibs have been withdrawn or suspended from the market Chapter 25). due to undesirable side effects and safety issues. Transcrip- tion of COX-2but not of COX-1is completely inhibited by Leukotrienes & Lipoxins Are Potent anti-inflammatory corticosteroids. Regulators of Many Disease Processes Slow-reacting substance of anaphylaxis (SRS-A) is a mixture Prostanoids May Be Used of leukotrienes C4, D4, and E4. This mixture of leukotrienes is a Therapeutically potent constrictor of the bronchial airway musculature. These Potential therapeutic uses of prostanoids include prevention leukotrienes together with leukotriene B4 also cause vascular of conception, induction of labor at term, termination of permeability and attraction and activation of leukocytes and pregnancy, prevention or alleviation of gastric ulcers, control are important regulators in many diseases involving inflamma- of inflammation and of blood pressure, and relief of asthma tory or immediate hypersensitivity reactions, such as asthma. and nasal congestion. In addition, PGD2 is a potent sleep- Leukotrienes are vasoactive, and 5-lipoxygenase has been found promoting substance. Prostaglandins increase cAMP in platelets, in arterial walls. Evidence supports an anti-inflammatory role 238 SECTION V Metabolism of Lipids for lipoxins in vasoactive and immunoregulatory function, for promotes its activity, while phosphorylation (eg, by glucagon or example, as counterregulatory compounds (chalones) of the epinephrine) is inhibitory. immune response. Biosynthesis of unsaturated long-chain fatty acids is achieved by desaturase and elongase enzymes, which introduce double bonds and lengthen existing acyl chains, respectively. SUMMARY Higher animals have Δ4, Δ5, Δ6, and Δ9 desaturases but cannot The synthesis of long-chain fatty acids (lipogenesis) is carried insert new double bonds beyond the position 9 of fatty acids. out by two enzyme systems: acetyl-CoA carboxylase and fatty Thus, the essential fatty acids linoleic (ω6) and α-linolenic (ω3) acid synthase. must be obtained from the diet. The pathway converts acetyl-CoA to palmitate and requires Eicosanoids are derived from C20 (eicosanoic) fatty acids NADPH, ATP, Mn2+, biotin, and pantothenic acid as cofactors. synthesized from the essential fatty acids and make up Acetyl-CoA carboxylase converts acetyl-CoA to malonyl-CoA, important groups of physiologically and pharmacologically and then fatty acid synthase, a multienzyme complex consisting active compounds, including the prostaglandins, of two identical polypeptide chains, each containing six thromboxanes, leukotrienes, and lipoxins. separate enzymatic activities and ACP, catalyzes the formation of palmitate from one acetyl-CoA and seven malonyl-CoA molecules. REFERENCES Lipogenesis is regulated at the acetyl-CoA carboxylase step Eljamil AS: Lipid Biochemistry: For Medical Sciences. iUniverse, 2015. by allosteric modifiers, phosphorylation/dephosphorylation, Smith WL, Murphy RC: The eicosanoids: cyclooxygenase, and induction and repression of enzyme synthesis. The lipoxygenase, and epoxygenase pathways. In Biochemistry of Lipids, enzyme is allosterically activated by citrate and deactivated Lipoproteins and Membranes, 6th ed. Ridgway N, McLeod R by long-chain acyl-CoA. Dephosphorylation (eg, by insulin) (editors). Academic Press, 2015:260-296.