Section 12: Fatty Acid and Lipid Metabolism

Section12: FattyAcidandLipidMetabolismChapter27:FattyAcidDegradationChapter28:FattyAcidSynthesis

Bytheendofthissection,youshouldbeableto:Ø Identifytherepeatedstepsoffattyaciddegradation.

ØDescribeketonebodiesandtheirroleinmetabolism.

Ø Explainhowfattyacidsaresynthesized.Ø Explainhowfattyacidmetabolismisregulated.

Lecture:Ch27-28-29

CHAPTER27FattyAcidSynthesis

CHAPTER28FattyAcidDegradation

Chapter27Outline

Fattyaciddegradationisakeyenergysourceformammalsduringhibernation.

• Fattyacidsarestoredinadiposetissueastriacylglycerols (TAG)inwhichfattyacidsarelinkedtoglycerolwithesterlinkages.

• Adiposetissueislocatedthroughoutthebody,withsubcutaneous(belowtheskin)andvisceral(aroundtheinternalorgans)depositsbeingmostprominent.

Thefattyacidsincorporatedintotriacylglycerolsinadiposetissuearemadeaccessibleinthreestages.

1. DegradationofTAG toreleasefattyacidsandglycerolintothebloodfortransporttoenergy-requiringtissues.

2. Activationofthefattyacidsandtransportintothemitochondriaforoxidation.

3. DegradationofthefattyacidstoacetylCoAforprocessingbythecitricacidcycle.

Lipiddegradation

• Triacylglycerolsarestoredinadipocytesasalipiddroplet.• Epinephrineandglucagon,actingthrough7TMreceptors,stimulatelipid

breakdownorlipolysis.• ProteinkinaseAphosphorylatesperilipin,whichisassociatedwiththelipid

droplet,andhormone-sensitivelipase.• Phosphorylationofperilipin resultsintheactivationofadipocytetriacylglyceride

lipase(ATGL).• ATGLinitiatesthebreakdownoflipids.Chanarin-Dorfmam syndrome,

characterizedbydryskin,enlargedliverandmuscle,andmildcognitivedisability,resultsifATGLactivityiscompromised.

• Theglycerolreleasedduringlipolysisisabsorbedbytheliverforuseinglycolysisorgluconeogenesis.

Triacylglycerols inadiposetissueareconvertedintofreefattyacidsinresponsetohormonalsignals

ThephosphorylationofperilipinrestructuresthelipiddropletandreleasesthecoactivatorofATGL.TheactivationofATGLbybindingwithitscoactivatorinitiatesthemobilization.Hormone-sensitivelipasereleasesafattyacidfromdiacylglycerol.Monoacylglycerol lipasecompletesthemobilizationprocess.

Abbreviations:7TM,seventransmembrane;ATGL,adiposetriglyceridelipase;CA,coactivator;HSlipase,hormone-sensitivelipase;MAGlipase,monoacylglycerol lipase;DAG,diacylglycerol;TAG,triacylglycerol.

Lipolysisgeneratesfattyacidsandglycerol

AcylCoAisanactivatedformoffattyacid

AfterbeingactivatedbylinkagetoCoA,thefattyacidistransferredtocarnitine,areactioncatalyzedbycarnitineacyltransferaseI,fortransportintothemitochondria.Atranslocasetransportstheacylcarnitineintothemitochondria.

Inthemitochondria,carnitineacyltransferaseIItransfersthefattyacidtoCoA.ThefattyacylCoAisnowreadytobedegraded.

Acylcarnitinetranslocase

ClinicalInsights:Muscle,kidney,andheartusefattyacidsasafuel.Pathologicalconditionsresultsiftheacyltransferaseorthetranslocasearedeficient.

Carnitinedeficienciescanbetreatedbycarnitinesupplementation.

Fattyaciddegradationconsistsoffourstepsthatarerepeated.1. Oxidationoftheβcarbon,catalyzedbyacyl

CoAdehydrogenase,generatestrans-Δ2-enoylCoAandFADH2.

2. Hydrationoftrans-Δ2-enoylCoAbyenoylCoAhydrataseyieldsL-3-hydroxyacylCoA.

3. OxidationofL-3-hydroxyacylCoAbyL-3-hydroxyacyldehydrogenasegenerates2-ketoacylCoAandNADH.

4. Cleavageofthe3-ketoacylCoAbythiolaseformsacetylCoAandafattyacidchaintwocarbonsshorter.

Fattyaciddegradationisalsocalledβ-oxidation.

Thereactionsequenceforthedegradationoffattyacids

Twocarbonunitsaresequentiallyremovedfromthecarboxylendofthefattyacid

Fattyacidsaredegradedbytherepetitionofafour-reactionsequenceconsistingofoxidation,hydration,oxidation,andthiolysis.

Thereactionforoneroundofβ-oxidationis:

ThecompletereactionforC16 palmitoylCoAis:

Processingoftheproductsofthecompletereactionbycellularrespirationwouldgenerate106moleculesofATP.

Answer:The steps are (1) oxidation by FAD; (2) hydration; (3) oxidation by NAD+; (4) thiolysis to yield acetyl CoA. In symbolic notation, the β-carbon atom is oxidized.

• β-oxidationalonecannotdegradeunsaturatedfattyacids.Whenmonounsaturatedfattyacidsaredegradedbyβ-oxidation,cis-Δ3-enoylCoAisformed,whichcannotbeprocessedbyacylCoAdehydrogenase.

• Cis-Δ3-enoylCoAisomerase convertsthedoublebondintotrans-Δ2-enoylCoA,anormalsubstrateforβ-oxidation.

• Whenpolyunsaturatedfattyacidsaredegradedbyβ-oxidation,cis-Δ3-enoylCoAisomeraseisalsorequired.2,4-DienoylCoAisalsogenerated,butcannotbeprocessedbythenormalenzymes.

• 2,4-DienoylCoAisconvertedintotrans-Δ3-enoylCoAby2,4-dienoylCoAreductase,andtheisomeraseconvertsthisproducttotrans-Δ2-enoylCoA,anormalsubstrate.

• Unsaturatedfattyacidswithoddnumbersofdoublebondsrequireonlytheisomerase.Evennumberofdoublebondsrequireboththeisomeraseandreductase.

β-OxidationoffattyacidswithoddnumbersofcarbonsgeneratespropionylCoAinthelastthiolysisreaction.

Propionylcarboxylase,abiotinenzyme,addsacarbontopropionylCoAtoformmethylmalonylCoA

SuccinylCoA,acitricacidcyclecomponent,issubsequentlyformedfrommethylmalonylCoAbymethylmalonylCoAmutase,avitaminB12 requiringenzyme.

• Ketonebodies—acetoacetate,3-hydroxybutyrate,andacetone— aresynthesizedfromacetylCoAinlivermitochondriaandsecretedintothebloodforuseasafuelbysometissuessuchasheartmuscle.

• 3-Hydroxybutyrateisformeduponthereductionofacetoacetate.Acetoneisgeneratedbythespontaneousdecarboxylationofacetoacetate.

• Intissuesusingketonebodies,3-hydroxybutyrateisoxidizedtoacetoacetate,whichisultimatelymetabolizedtotwomoleculesofacetylCoA.

Theformationofketonebodies

Theketonebodies—acetoacetate,D-3-hydroxybutyrate,andacetone—areformedfromacetylCoAprimarilyintheliver.Enzymescatalyzingthesereactionsare(1)3-ketothiolase,(2)hydroxymethylglutaryl CoAsynthase,(3)hydroxymethylglutaryl CoAcleavageenzyme,and(4)D-3-hydroxybutyratedehydrogenase.Acetoacetatespontaneouslydecarboxylates toformacetone

TheutilizationofD-3-hydroxybutyrateandacetoacetateasafuel

Ketogenicdiets, richinfatsandlowincarbohydratesbutwithadequateproteins,leadtoformationofsubstantialamountsofketonebodies.

Ketogenicdietsmayhavetherapeuticproperties:Forreasonsnotyetestablished,suchdietsreducetheseizuresinchildrensufferingfromdrug-resistantepilepsy.

FatsareconvertedintoacetylCoA,whichisthenprocessedbythecitricacidcycle.

Oxaloacetate,acitricacidcycleintermediate,isaprecursortoglucose.

However, acetylCoAderivedfromfatscannotleadtothenetsynthesisofoxaloacetateorglucosebecausealthoughtwocarbonsenterthecyclewhenacetylCoAcondenseswithoxaloacetate,twocarbonsarelostasCO2beforeoxaloacetateisregenerated.

High levels of acetoacetate in the blood signify an abundance of acetyl units and lead to a decrease in the rate of lipolysis in adipose tissue.

Answer:D-3-Hydroxybutyrate is more energy rich because its oxidation potential is greater than that of acetoacetate. After having been absorbed by a cell, d-3-hydroxybutyrate is oxidized to acetoacetate, generating high-energy electrons in the form of NADH. The acetoacetate is then cleaved to yield to acetyl CoA.

• Ketonebodiesaremoderatelystrongacids,andexcessproductioncanleadtoacidosis.

• Anoverproductionofketonebodiescanoccurwhendiabetes,aconditionresultingfromalackofinsulinfunction,isuntreated.Theresultingacidosisiscalleddiabeticketosis.

• Ifinsulinisabsentornotfunctioning,glucosecannotentercells.Allenergymustbederivedfromfats,leadingtotheproductionofacetylCoA.

• AcetylCoAbuildsupbecauseoxaloacetate,whichcanbegeneratedfromglucose,isnotavailabletoreplenishthecitricacidcycle.

• Moreover,fattyacidreleasedfromadiposetissueisenhancedintheabsenceofinsulinfunction.

Diabeticketosisresultswheninsulinisabsent

Intheabsenceofinsulin,fatsarereleasedfromadiposetissue,andglucosecannotbeabsorbedbytheliveroradiposetissue.Theliverdegradesthefattyacidsbyb-oxidation,butcannotprocesstheacetylCoAbecauseofalackofglucose-derivedoxaloacetate(OAA).Excessketonebodiesareformedandreleasedintotheblood. Abbreviation:CAC,citricacidcycle.

• Glucoseisthepredominantfuelforthebrain.

• Duringstarvation,proteindegradationisinitiallythesourceofcarbonsforgluconeogenesisintheliver.Theglucoseisthenreleasedintothebloodforthebraintouse.

• Afterseveraldaysoffasting,thebrainbeginstouseketonebodiesasafuel.

• Ketonebodyusecurtails(reduces)proteindegradationandthuspreventstissuefailure.Moreover,ketonebodiesaresynthesizedfromfats,thelargestenergystoreinthebody.

Fuelchoiceduringstarvation

Saturatedandtransunsaturatedfattyacids aresynthesizedcommerciallytoenhancetheshelflifeandheatstabilityoffatsforfoodpreparation.

Studiessuggestthatexcessconsumptionofthesefatsresultsinobesity,heartdiseaseandtype2diabetes.

Lecture:Ch27&Ch28

CHAPTER28FattyAcidSynthesis

Outline

1. ThefirststageoffattyacidsynthesisistransferofacetylCoAoutofthemitochondriaintothecytoplasm.CitrateistransportedintothecytoplasmandcleavedintooxaloacetateandacetylCoA.

2. ThesecondstateistheactivationofacetylCoAtoformmalonylCoA.

3. ThethirdstageistherepetitiveadditionandreductionoftwocarbonunitstosynthesizeC16 fattyacid.Synthesisoccursonanacylcarrierprotein,amolecularscaffold.

Citrate,synthesizedinthemitochondria,istransportedtothecytoplasmandcleavedbyATP-citratelyasetogenerateacetylCoAforfattyacidsynthesis.

ThetransferofacetylCoAtothecytoplasm

AcetylCoAistransferredfrommitochondriatothecytoplasm,andthereducingpotentialofNADHisconcomitantlyconvertedintothatofNADPHbythisseriesofreactions

PFK

• FattyacidsynthesisrequiresreducingpowerintheformofNADPH.

• SomeNADPHcanbeformedfromtheoxidationofoxaloacetate,generatedbyATP-citratelyase,bythecombinedactionofcytoplasmicmalatedehydrogenaseandmalicenzyme.

• Pyruvateformedbymalicenzymeentersthemitochondriawhereitisconvertedintooxaloacetatebypyruvatecarboxylase.

• Thesumofthereactionscatalyzedbymalatedehydrogenase,malicenzyme,andpyruvatecarboxylaseis:

• AdditionalNADPHissynthesizedbythepentosephosphatepathway.

PATHWAYINTEGRATION:Fattyacidsynthesis

Fattyacidsynthesisrequiresthecooperationofvariousmetabolicpathwayslocatedindifferentcellularcompartments.

• FattyacidsynthesisstartswiththecarboxylationofacetylCoAtomalonylCoA,theactivatedformofacetylCoA

• Malonyl CoAissynthesizedbyacetylCoAcarboxylase(ACC),abiotin-requiringenzyme.

TheformationofmalonylCoAoccursintwosteps:

(theactivatedformofacetylCoA)

(theactivatedformofCO2)

ACC

• Fattyacidsynthase,acomplexofenzymes,catalyzestheformationoffattyacidsfromfromacetylCoA,malonyl CoA,andNADPHiscalledfattyacidsynthase.

• Fattyacidsynthesisoccursontheacylcarrierprotein(ACP),apolypeptidelinkedtoCoA.IntermediatesarelinkedtothesulfhydrylgroupofthepantothenateattachedtoACP.

• AcetyltransacylaseandmalonyltransacylaseattachsubstratestotheACP.

• Thenextthreesteps—areduction,dehydration,andanotherreduction—converttheketogroupatcarbon3toamethylenegroup(-CH2-),formingbutyrylACP.

• NADPHisthesourceofreducingpower.

β-KetoacylsynthasecatalyzesthecondensationofacetylACPandmalonylACPtoformacetoacetylACP.

• ThesecondroundofsynthesisbeginswiththecondensationofmalonylCoAwiththenewlysynthesizedbutyrylACP,formingC6-β-ketoacylACP.

• Thereduction,dehydration,reductionsequenceisrepeated.

• SynthesiscontinuesuntilC16-acylACP,whichiscleavedbythioesterase toyieldpalmitate.

Thestoichiometryforthesynthesisofpalmitateis:

ThesynthesisoftherequiredmalonylCoAisdescribedbythefollowingreaction:

Thus,thestoichiometryforthesynthesisofpalmitatefromacetylCoAis:

• ThereactionsoffattyacidsynthesisaresimilarinE.coliandanimals.

• Inanimals,alloftheenzymesrequiredforfattyacidsynthesisarecomponentsofasinglepolypeptidechain.

• Thefunctionalenzymeiscomposedoftwoidenticalchains.

• Theenzymeconsistsoftwodistinctcompartments.

Ø Theselectingandcondensingcompartment,whichbindstheacetylandmalonylsubstratesandcondensesthem.

Ø Themodificationcompartment,whichcarriesoutthereductionanddehydrationactivitiesrequiredforelongation.

Animalfattyacidsynthasedomainstructure

Bindsacetylandmalonyl substrates

Answer:Acetyl CoA is the basic substrate for fatty acid synthesis. It is transported out of mitochondria in the form of citrate. After the formation of acetyl CoA, the resulting pyruvate is transported back into the mitochondria with a concomitant formation of NADPH, the reducing power for fatty acid synthesis. Additional NADPH can be generated by the pentose phosphate pathway. Malonyl CoA, the ultimate substrate for fatty acid synthesis is formed by the carboxylation of acetyl CoA.

• Tumorsrequirelargeamountsoffattyacidsynthesistoproduceprecursorsformembranesynthesis.

• β-Ketoacyl synthaseinhibitorsretardtumorgrowth.

• Micetreatedwithβ-Ketoacyl synthaseinhibitorsalsoshoweddramaticweightloss,suggestingthatsuchdrugsmaybeusedtotreatobesity.

• AcetylCoAcarboxylaseinhibitorsmayalsobepotentialchemotherapyagents.

• β-Hydroxybutyricacid,whenattachedtoACPorCoA,isasubstrateinfattyacidsynthesisanddegradation,andisaketonebodyaswell.

• Anisomerofthiskeybiochemical,γ-hydroxybutyricacidisapotent,illegaldrug.

• FattyacidsynthasecannotgeneratefattyacidslongerthanC16palmitate.

• Longerfattyacidsaresynthesizedbyenzymesattachedtotheendoplasmicreticulum.

• Theseenzymesextendpalmitatebyaddingtwocarbonunits,usingmalonylCoAasasubstrate.

• Enzymesboundtotheendoplasmicreticulumintroducedoublebondsintosaturatedfattyacids.

Forinstance:

• Mammalslacktheenzymesthatintroducedoublebondsbeyondcarbon9.

• Linoleateandlinolenateareessentialfattyacidsthatmustbeobtainedinthediet.

• Arachidonate,a20-carbonfattyacidwithfourdoublebonds,isderivedfromlinoleate.

• Arachidonateisaprecursorforavarietyofsignalmolecules20carbonslong,collectivelycalledtheeicosanoids.

• Thesesignalmolecules,whichincludeprostaglandins,arelocalhormonesbecausetheyareshortlivedandonlyaffectnearbycells.

Arachidonate isthemajorprecursorofeicosanoidhormones

Structuresofseveraleicosanoids

• AspirinpreventstheuseofarachidonateasasubstratefortheenzymethatgeneratesprostaglandinH2 .

• Blockingthisstepeffectsmanysignalingpathways,accountingforthewiderangeeffectsofaspirin.

AcetylCoAcarboxylase1and2aresubjecttoregulationonseverallevels.

Carboxylase1, acytoplasmicenzyme,isinhibitedwhenphosphorylatedbyAMP-activatedkinase(AMPK).Inhibitionduetophosphorylationisreversedbyproteinphosphatase2A.

Citrate activescarboxylasebyfacilitatingtheformationofactivepolymersofthecarboxylase.Citratemitigatesinhibitionduetophosphorylation.

PalmitoylCoA,theend-productoffattyacidsynthase,inhibitscarboxylasebycausingdepolymerizationoftheenzyme.

Carboxylase2,amitochondrialenzyme,inhibitsfattyaciddegradationbecauseitsproduct,malonylCoA,preventstheentryoffattyacylCoAintothemitochondriabyinhibitingcarnitineacyltransferase1.

ThecontrolofacetylCoAcarboxylase

DependenceofthecatalyticactivityofacetylCoAcarboxylaseontheconcentrationofcitrate.

• GlucagonandepinephrineinhibitcarboxylasebyenhancingAMPKactivity,bywhichtheypreventfattyacidsynthesis.

• Insulinstimulatesthedephosphorylationandactivationofcarboxylase,bywhichitstimulatesfattyacidsynthesis.

• Theenzymesoffattyacidsynthesisareregulatedbyadapativecontrol.Ifadequatefatsarenotpresentinthediet,thesynthesisofenzymesrequiredforfattyacidsynthesisisenhanced.

AnswerMalonyl CoA, the substrate for fatty acid synthesis, inhibits carnitine acyl transferase I, thus preventing the transport of fatty acids into mitochondria for degradation. Palmitoyl CoA inhibits acetyl CoA carboxylase, the transport of citrate into the cytoplasm, and glucose 6-phosphate dehydrogenase, the controlling enzyme of the pentose phosphate pathway.

• OnepathwayforethanolprocessingconsistsoftwostepsandleadstoexcessproductionofNADH:

• ExcessNADHinhibitsgluconeogenesisandenhanceslactateproduction,whichmayresultinlacticacidosis.

• ExcessNADHinhibitsfattyaciddegradationandstimulatesfattyacidsynthesis,leadingtotheaccumulationoffatsintheliver.

• LivercanconvertsomeoftheacetategeneratedbyaldehydedehydrogenaseintoacetylCoA,buttheacetylCoAcannotbeprocessedbythecitricacidcyclebecauseofthepaucityorlackofNAD+.

• Thebuild-upofacetylCoAcanleadtoketonebodysecretionbytheliver,whichexacerbatestheacidosiscausedbylactateaccumulation.

• Ifacetatecannotbeprocessed,acetaldehydeaccumulates.Acetaldehydeisveryreactiveandmodifiesreactivegroupsofproteins,causingalossofproteinfunction.

• Asproteindamageaccumulates,liverfunctioncanfail.