Metabolism III. Lipids

� Metabolism Part 111: Lipids Three of the four major classes of Ьiomolecules are defined on the basis of similarities in their structures: proteins con­sist of amino acids; carЪohydrates 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 Ьiomolecules that are soluЫe in nonpolar solvents such as benzene, chloroform, or ether. Lipids therefore exhiЬit а wide variety of structures charac­terized Ьу the presence of hydrophoЬic groups. Lipids are often 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 Fatty acids are long-chain carboxylic acids such as stearic acid, СНз{СН2)15СО2Н, that contain а nonpolar, hydropho­ Ьic hydrocarbon tail and а polar, hydrophilic carboxylic acid head. Fatty acids in terrestial organisms usually have an even number of carbon 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) 12СО2 Н СНз{СН2)14СО2 Н СНз{СН2)1 5СО2Н and unsaturated fatty acids such as palmitoleic acid СНз{СН2)5СН=СН{СН2)1СО2Н oleic acid СН3{СН2)7СН=СН{СН2)1СО2 Н linoleic acid СН3{СН 2)4СН=СНСН2СН= СН{СН2)1СО2Н linolenic acid СН3СН=СНСН2СН= СНСН2СН=СН{СН2)1СО2 Н in which the С=С douЫe bonds are usually present in а cis orientation. The presence of а douЫe bond between С9 and С 10 or С1 2 and С 1 з in these unsaturated fatty acids is indicat­ ed with the symbols д9 and Л 12 • This reaction is called saponification {from the Latin sa­ soap, and facere, to make) because it has been used for more than 2,000 years to make soap. The first step in the cataЪolism of triacylglycerols involves an enzyme-catalyzed hydrolysis of the triester to glycerol and three fatty acids. The glycerol produced in 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. ponis, АТР ADP 2 \, j ' снон СН 0РО3 - о О СН 2О�(СН 2)1вСНз 11 1 СН: 1(СН 2 )1вС09Н СН 20С(СН2) 16СН:1 � NADH, н + \, j 2 2 СН 2ОН 1 с-о 6Н 20Р0 32 - Neutral Fats, Trlglycerldes, or Trlacylglycerols The concentration of free fatty acids in а cell is negligiЫe, 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" on the basis of whether they are solids or liquids at room temperature. NAD+ Н ОН ? сна 1 - НС-ОН СН 20РОз 2 - Careful consideration of the reactions involved in glycolysis, the ТСА cycle, electron transport, and oxidative phosphory­ lation suggests that а total of 22 АТР can Ъе synthesized from the total oxidation of glycerol to СО2 and Н20. Actlvatlon and Transport Fats and oils are broken down to fatty acids and glycerol in the soluЫe portion of the cytoplasm. Oxidation of the fatty acids to СО2 and Н20, however, is catalyzed Ьу enzymes found on the inner walls of the mitochondria. Before fatty acids сап Ье metaЪolized, they must first Ъе activated and then transported across а membrane into the mitochondria. о tions are reversiЬ!e. Thiolysis of the l'/-ketothioester сап Ье thought of as а retrograde Claisen condensation. о о 11 11 RCCH2C-S-CoA + HS-CoA о о 11 11 RC-S-CoA + СНзС-S-СоА Fatty Acld Degradatlon as а Source of Blochemlcal Energy Eight turns around the l'/-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 FADH2 , eight NAD+ coenzymes are reduced to NADH, and nine molecules of acetyl СоА are produced. о 11 СНз(СН2) 1 вС-S-СоА + 8 HS-CoA + 8 FAD + 8 NAD+ + 8 Н2О - 9 СНзС-S-СоА + 8 FADH2 + 8 NADH + 8 н+ The FADH2 and NADH coenzymes are regenerated Ьу elec­ tron transport coupled with oxidative phosphorylation. Two АТР are formed for each FADH2 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 СО 2 and Н20 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. 8 (FADH2 -FAD) 8 (NADH -NдD+) 9 (acetyl СоА - СO 2 + Н20) 8Х 2АТР = 8Х ЗАТР = 9Х 12АТР = total = 16АТР 24АТР 108АТР 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 сап 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 l'/-oxidation spiral is therefore по more or less efficient at capturing energy than glycolysis or the ТСА cycle. The oxidation of stearic acid to С0 2 and Н 2 0, 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 NumЬer of CarЬon Atoms The l'/-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. Н 11 C-S-CoA "- "Н / с�с / RCH 2 cis д 3 trans д 2 Polyunsaturated fatty acids present another proЬ!em. Af­ ter the isomerase enzyme operates оп the first С=С douЫe bond, two more passes around the spiral leaves а cis д 2 intermediate. The douЬ!e bond is in the right position (д 2 ), and the enol hydratase enzyme adds water across this bond. But because the enzyme operates оп а cis instead of а trans С=С bond, the о instead of L stereoisomer of 3-hydroxyacyl СоА is formed. An epimerase enzyme then inverts the con­ figuration of the hydroxyl group at С 3 , so that the product of this reaction сап 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 СО 2 at the expense of АТР to form methylmalonyl СоА, which is then isomerized Ьу а mutase enzyme to form succinyl СоА which сап enter the ТСА cycle. о 11 АТР ADP, НРО4 2 - \ j CH 2CH 2C-S-CoA + СО2 о � CH3CH�-S-CoA -- но,ссн,сн,с-s-соА 1 со,н Ketone Вodles When the degradation of carbohydrates and fatty acids is in balance, the acetyl СоА from fatty acids enters the tricar­ boxylic acid сус\е where it is oxidized to С0 2 and Н2О. Low carbohydrate diets, or metabolic disorders such as diabetes, сап lead to conditions where the amount of acetyl СоА pro­ duced from the degradation of fatty acids exceeds the abllity of the ТСА сус\е to oxidize it. When this happens, the last step in the l'/-oxidation spiral is reversed, and two molecules of acetyl СоА condense to form acetoacetyl СоА. � fi fi 2 СНзС-S-СоА - CH 3CCH 2C-S-CoA + 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. о 11 о 11 о 11 CH3CCH2C-S-CoA + Н2О - СНзССН 2СО,- + HS-CoA Acetoacetate сап then undergo either decarboxylation to give acetone, Fatty acids are activated Ьу forming а thioester to coen­ zyme A. NAD + coenzyme to oxidize this alcohol to the corresponding ketone. СН3(СН2)"С0 2Н + HS-CoA - CHз(CH2)nCO-S-CoA Stвp 4: Тhiolysis The /3-ketothioester now reacts with coenzyme А to form acetyl СоА and а new fatty acyl thioester from which two carbon atoms have been cleaved. The /3-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 /3-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 /3-diketones can Ье prepared Ьу а Claisen condensation, 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 (Р20 74 -) ion is used to form а mixed anhydride between the fatty acid and АМР. i СНз(СН2)пС-О-АМР + Р2О1•The energy released when the pyrophosphate ion is hydro­ lyzed to а pair of phosphate ions then drives the transfer of the fatty асу! group to form а thioester. о о 11 11 СН3СОСН2 СН3 + СН3СОСН2 СН3 о 2 2 2 о 11 11 СН3ССН.СОСН.СН3 + СН3СН20Н о СН3(СН )}-О-АМР + HS-CoA-CH3(CH )nCO-S-CoA + АМР Р2О14- + Н 20-2 HP04 - - 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. CH 2C02 О 1 II СНз(СН2)пС-S-СоА + НО-СН 1 СН2 N(СНз)з + � ?Н 2СО2 СН3(СН2)п С-0-СН + HS-CoA 1 СН2 N(СНз)з + Once the fatty асу! carnitine ester has been transported across the membrane, а second acyltransferase enzyme transfers the fatty асу! group back to coenzyme А in а regen­ eration step. он 1 Тhе �-Oxldatlon Splral Once inside the mitochondrion, the fatty асу! coenzyme А thioester enters the /3-oxidation spiral shown in the figure, which degrades the fatty acid, two carbon atoms at а time. Stвp 1: Oxidat/on The first enzyme in this spiral is an асу! 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 douЫe bonds that are well insulated from the carboxylate group. Stвp 2: Нydration An enoyl СоА hydratase enzyme catalyzes the stereospe­ cific addition of water across the С=С douЫe bond to form а 3- or /3-hydroxyacyl СоА derivative in the L configuration. Stвp З: Oxidation of thв Alcohol А 3-hydroxyacyl СоА dehydrogenase enzyme then uses an о11 RCH СН C-S-CoA + �NдD Step 3• t'----NADн,н + о11 о11 RCCH2C-S-CoA нs- од � Step 4• О 11 RC-S-CoA Тhе l'J-oxldatlon spiral. CH3C-S-CoA