Metabolism. Lipids

� Metabolism Part 111: Lipids Three of the four major classes of biomolecules are defined on the basis of similarities in their structures: proteins con­sist of amino acids; carbohydrates are polyhydroxyalde­hydes or ketones; and nucleic acids are polynucleotides. Lip­ids are defined on the basis of similarities in their physical properties, they are the class of biomolecules that are soluЫe in ponpolar solvents such as benzene, chloroform, or ether. Lipids therefore exhibit а wide variety of structures charac­terized Ьу the presence of hydrophobic groups. Lipids are ofteп divided into two classes. Complex lipids such as "fats" and "oils" contain fatty acids; simple lipids such as vitamin А or cholesterol do not. These compounds were once called neutral fats, they are now called triglycerides, or more accurately, triacylglycerols. Their primary function is the long-term storage of Ьiological energy. Saponlflcatlon One of the characteristic reactions of triacylglycerols is the alkaline hydrolysis of the ester linkage to give glycerol and sodium or potassium salts of the corresponding carboxylate ions. Fatty Aclds acid, з(СНare contain а nonpolar, СНacids Fatty long-chain acids suchhydropho­ as stearic 2 16СО 2Н, that carboxylic ) Ьic hydrocarbon tailinand а polar, organisms hydrophilicusually carboxylic acid head. Fatty acids terrestial have an even number of саrЬоп atoms, with chains of 16 to 18 being most common, although milk and butter fat have unusually high amounts of fatty acids with chain lengths between 4 and 12. Marine organisms have а greater abundance of fatty acids with an odd number of carbon atoms. Fatty acids are divided into two classes, saturated fatty acids such as lauric acid myristic acid palmitic acid stearic acid СНз(СН2 )10СО2Н СНз(СН2 ) 1 2СО2Н СНз(СН 2) 1 4СО2Н СН3(СН 2 )1вСО2Н and unsaturated fatty acids such as palmitoleic acid СН3(СН 2 ) 5СН=СН(СН 2) 1СО2Н oleic acid СН3(СН2 ) 7СН=СН(СН 2 ) 7СO2Н linoleic acid СН3(СН2 ) 4 СН=СНСН2СН= СН(СН2) 7СО 2Н linolenic acid СН3СН=СНСН2СН= СНСН2СН=СН(СН2 ) 1СО2Н in which the С=С douЫe bonds are usually present in а cis orientation. The preseпce of а douЫe bond between Cg and С10 or С12 and С 1 3 in these unsaturated fatty acids is indicat­ ed with the symbols д9 and д 12. СН,,ОН 1 - 1нон СН2ОН This reaction is called saponification (from the Latin sa­ ponis, soap, and facere, to make ) because it has been used for more than 2,000 years to make soap. The first step in the catabolism of triacylglycerols involves an eпzyme-catalyzed hydrolysis of the triester to glycerol and three fatty acids. The glycerol produced iп this reaction is phosphorylated Ьу glycerol kinase, oxidized to dihydroxy­ acetone phosphate Ьу glycerol phosphate dehydrogenase, isomerized to glyceraldehyde 3-phosphate Ьу triose phos­ phate isomerase, and finally injected into the glycolytic pathway. СН;ОН tнон 1 СН2ОН АТР ADP \, j . Н ОН ? 2 снон о О СН2ОС(СН2) 16СНа 11 1 СНз(СН2) 16СОСН 11 6н2О (СН2 ) ,вСН3 � о Nдо+ 1 СН 2ОРО/- СН2 ОН 1 С=О NАDн,н+ \ ) сно - НС-ОН 1 1 2 СН2ОРОз СН2ОРОз2 - Neutral Fats, Trlglycerldes, or Trlacylglycerols The concentration of free fatty acids in а cell is negligiЬle, almost all of the fatty acids are bound to glycerol Ьу ester linkages. The most abundant lipids are the triesters of glyc­ erol with fatty acids that are typically divided into "fats" and "oils" оп the basis of whether they are solids or liquids at room temperature. + 3 \Na + ]ICH3(CHz),,CO2 -J 1 Careful consideration of the reactions involved in glycolysis, the ТСА cycle, electron transport, and oxidative phosphory­ lation suggests that а total of 22 А ТР can Ье synthesized from the total oxidation of glycerol to СО2 and Н2 0. Actlvatlon and Transport Fats and oils are broken down to fatty acids and glycerol in the soluЬ\e portion of the cytoplasm. Oxidatioп of the fatty acids to СО2 and Н2О, however, is catalyzed Ьу enzymes fouпd on the inner walls of the mitochondria. Before fatty acids can Ье metabolized, they must first Ье activated and then transported across а memb_rane into the mitochondria. Fatty acids are activated Ьу forming а thioester to coen­ zyme A. СН3(СН 2 )пСО2Н + HS-CoA - СНз(СН2)"СО-S-СоА Because thioesters are themselves high-energy compounds, hydrolysis of АТР to ADP and the phosphate ion cannot provide quite enough energy to drive this reaction to com­ pletion. The energy released when АТР is hydrolyzed to АМР and the pyrophosphate (Р2O74 -) ion is used to form а mixed anhydride between the fatty acid and АМР. The energy released when the pyrophosphate ion is hydro­ lyzed to а pair of phosphate ions then drives the transfer of the fatty acyl group to form а thioester. NAD + coenzyme to oxidize this alcohol to the corresponding ketone. Step 4: Thiolysis The ,6-ketothioester now reacts with coenzyme А to form acetyl СоА and а new fatty асу! thioester from which two carbon atoms have been cleaved. The ,6-oxidation spiral then begins again and continues until the last pass through the spiral converts а four-carbon fatty асу! group to two mole­ cules of acetyl СоА. The fourth step in the ,6-oxidation spiral is called thiolysis because the net effect is the addition of а thiol (HS-CoA) across а С-С single bond. This reaction can Ье understood Ьу noting that ,6-diketones can Ье prepared Ьу а Claisen condensation, о о 11 11 СНзСОСН 2СНз + СН3 СОСН 2 СН 3 о 11 о о 11 СНзССН2СОСН2СНз + СН3 СН2 OН 11 СНз(СН 2 )"С-О-АМР + HS-CoA-. СНз(СН 2)"СО-S-СоА + АМР Р 2074- + Н2 0 - 2 HPO/- - and carbonyl addition reactions such as Claisen condensa- The fatty асу! СоА formed in this activitation step cannot Ье transported across the mitochondrial membrane because the coenzyme А fragment carries а significant net ionic charge due to the presence of phosphate linkages. The fatty асу! group is therefore transferred to the -ОН group of а molecule of carnitine Ьу an acyltransferase enzyme to form а zwitterion that carries no net electrical charge, and can therefore cross this membrane. Once the fatty асу! carnitine ester has been transported across the membrane, а second acyltransferase enzyme transfers the fatty acyl group back to coenzyme А in а regen­ eration step. RCH СН C-S-CoA Тhе ,8-Oxldatlon Splral Once inside the mitochondrion, the fatty acyl coenzyme А thioester enters the ,6-oxidation spiral shown in the figure, which degrades the fatty acid, two carbon atoms at а time. о11 он 1 �NдD Step 3: �NADH,H + Step 1: Oxidation The first enzyme in this spiral is an acyl СоА dehydrogen­ ase that uses а tightly bound FAD coenzyme to oxidize the fatty асу! thioester to form а trans С=С douЫe bond. This reaction produces а conjugated, trans enoyl intermediate which is very different from the naturally occurring unsatu­ rated fatty acids that contain cis С=С douЬ!e bonds that are well insulated from the carboxylate group. Step 2: Hydration An enoyl СоА hydratase enzyme catalyzes the stereospe­ cific addition of water across the С=С douЬ!e bond to form а 3- or ,6-hydroxyacyl СоА derivative in the L configuration. о11 о11 RC CH 2 C-S-CoA HS-CoA Step 4: о11 RC-S-CoA Step 3: Oxidation of the Alcohol А 3-hydroxyacyl СоА dehydrogenase enzyme then uses an + The /3-oxidation spiral. о11 CH 3C-S-CoA tions are reversiЫe. Thiolysis of the JЗ-ketothioester can Ье thought of as а retrograde Claisen condensation. о 11 о 11 RCCH 2C-S-CoA + HS-CoA о 11 Fatty Acld Degradatlon as а Source of Blochemlcal Energy Eight turns around the JЗ-oxidation spiral are necessary to oxidize stearic acid to acetyl СоА. In the course of this sequence of reactions, eight FAD coenzymes are reduced to FADH 2 , eight NAD+ coenzymes are reduced to NADH, and nine molecules of acetyl СоА are produced. о + 11 8 Н2О - 9 СНзС-S-СоА + 8 FADH2 + 8 NADH + 8 н+ The FADH 2 and NADH coenzymes are regenerated Ьу elec­ tron transport coupled with oxidative phosphorylation. Two АТР are formed for each FADH 2 coenzyme recycled, and three АТР are produced each time an NADH coenzyme is oxidized. The acetyl СоА produced in this sequence of reac­ tions is oxidized to СО2 and Н2 O in the ТСА cycle, with the net effect of generating 12 АТР per acetyl СоА consumed. The degradation of stearic acid therefore yields а total of 148АТР. - FAD) 8 (FADH2 8 (NADH - NAD +) 9 (acetyl СоА - СO 2 + Н2O) � Н -- CH 2C-S-CoA 8Х 2АТР= 16АТР 8Х ЗАТР= 24АТР 9Х12АТР=108АТР total = 148АТР However, one АТР was consumed in the activation of stearic acid, and а second АТР is consumed converting the АМР produced in the activation reaction to ADP, which can Ье cycled back through oxidative phosphorylation. Thus, а net total of 146 АТР are produced for each molecule of stearic acid consumed. Under standard state conditions, 146 АТР corresponds to the capture of slightly more than 40% of the energy given off during the combustion of stearic acid. The JЗ-oxidation spiral is therefore no more or less efficient at capturing energy than glycolysis or the ТСА cycle. The oxidation of stearic acid to СО2 and Н 2 O, however, gives off almost four times as much energy per mole, or almost 2½ times as much energy per gram, as the complete oxidation of glucose, and complex lipids are an excellent medium for the long-term storage of food energy. Degradatlon of Unsaturated Fatty Aclds and Fatty Aclds wlth an Odd Number of Carbon Atoms The JЗ-oxidation spiral cleaves two carbon atoms at а time from the carboxylic acid end of the fatty acid. Thus, when unsaturated fatty acids are catabolized, the third pass through the spiral leaves а cis д 3 instead of а trans д 2 intermediate. Metabolism of unsaturated fatty acids there­ fore requires an enoyl СоА isomerase enzyme, which not only shifts the position of the С=С douЫe bond but also converts the naturally occurring cis isomer into the trans intermediate recognized Ьу the enoyl hydratase enzyme. 'С=С ' 11 C-S-CoA / / RCH 2 Н trans Л2 cis Л3 RC-S-CoA + СНзС-S-СоА о " = / i C'-. о СНз(СН2)1 6С-S-СоА + 8 HS-CoA + 8 FAD + 8 NAD+ о н R 11 11 н Polyunsaturated fatty acids present another proЫem. Af­ ter the isomerase enzyme operates on the first С=С douЫe bond, two more passes around the spiral leaves а cis д 2 intermediate. The douЫe bond is in the right position (д 2 ), and the enol hydratase enzyme adds water across this bond. But because the enzyme operates on а cis instead of а trans С=С bond, the D instead of L stereoisomer of 3-hydroxyacyl СоА is formed. An epimerase enzyme then inverts the con­ figuration of the hydroxyl group at С3, so that the product of this reaction can Ье recognized Ьу the 3-hydroxyacyl dehy­ drogenase enzyme which catalyzes step 3. Fatty acids with an odd number of carbon atoms present yet another proЫem. Carbon atoms are cleaved from these fatty acids, two at а time, until the spiral leaves а three­ carbon proprionyl СоА thioester. А carboxylase enzyme then catalyzes the addition of СО2 at the expense of АТР to form methylmalonyl СоА, which is then isomerized Ьу а mutase enzyme to form succinyl СоА which can enter the ТСА cycle. АТР ADP, HPQ 42- \ ) Ketone Bodles When the degradation of carbohydrates and fatty acids is in balance, the acetyl СоА from fatty acids enters the tricar­ boxylic acid cycle where it is oxidized to СО2 and Н 2 O. Low carbohydrate diets, or metabolic disorders such as diabetes, can lead to conditions where the amount of acetyl СоА pro­ duced from the degradation of fatty acids exceeds the abllity of the ТСА cycle to oxidize it. When this happens, the last step in the JЗ-oxidation spiral is reversed, and two molecules of acetyl СоА condense to form acetoacetyl СоА. о 11 о 11 Q 11 . 2 СНзС-S-СоА - СНзССН2С-S-СоА + HS-CoA Acetoacetyl СоА then picks up а third acetyl СоА to form а 3-hydroxy-3-methylglutaryl СоА intermediate which de­ composes to the acetoacetate ion and acetyl СоА. The net effect of these reactions is the hydrolysis of acetoacetyl СоА to acetoacetate and coenzyme А. Acetoacetate can then undergo either decarboxylation to give acetone, or reduction to give the 3-hydroxybutyrate ion. NАDн.н+ NдD+ он --'-�}�--• СНз�НСН2СО2- Acetoacetate, acetone, and 3-hydroxybutyrate are often referred to as the ketone bodies. In anima\s, they are primar­ ily generated within the mitochondria of liver cells. Acetyl СоА cannot cross the mitochondrial membrane, but these water-soluЫe ketone bodies easily diffuse through the mem­ brane to enter the Ыооd stream. They сап then Ье transport­ ed to peripheral tissues such as the skeletal muscles where they сап Ье activated to form acetoacetate СоА, cleaved to acetyl СоА, and then used as an alternative to g\ucose as а source of energy. The ketone bodies are important sources of energy even when carbohydrate and fatty acid metabolism are in Ьаlапсе. Under certain conditions, however, such as low carbohydrate diets or diabetes, they сап become the principal source of Ьiological energy. Diabetes is the result of impaired transport of Ыооd g\u­ cose across cell membranes into the skeletal muscles. Be­ cause glucose cannot Ье used as а fuel, the body turns to burning fat for energy. The acetyl СоА produced Ьу the oxidation of fats cannot enter the ТСА сус\е because the level of oxaloacetate is depleted as the body tries to make more g\ucose to overcome the proЫem with the transport of Ыооd glucose. The excess acetyl СоА is therefore converted to ketone bodies which appear in the Ыооd stream and urine of diabetics. Diabetes is treated with insulin, which increases the rate at which Ыооd glucose is transported into the peripheral tissues, increases the concentrations of enzymes such as glu­ cokinase, phosphofructokinase, and pyruvate kinase, which are involved in glycolysis, and inhiblts the oxidation of fatty acids. In recent years, low carbohydrate diets have become pop­ ular as а means of burning excess fat. Some have even gone so far as to recommend restricting carbohydrates in the diet until the urine tests positive for ketone bodies, in other words until the body produces the symptoms of ketosis that mimic diabetes. Although significant weight loss can occur, the \ong-term effects of artificially inducing а state of ketosis are unknown.