Section12:
FattyAcidandLipidMetabolism
Chapter27:FattyAcidDegradation
Chapter28:FattyAcidSynthesis
Bytheendofthissection,youshouldbeableto:
Ø Identifytherepeatedstepsoffattyacid
degradation.
Ø Describeketonebodiesandtheirrolein
metabolism.
Ø Explainhowfattyacidsaresynthesized.
Ø Explainhowfattyacidmetabolismisregulated.
Lecture:Ch27-28-29
CHAPTER27
FattyAcidSynthesis
CHAPTER28
FattyAcidDegradation
Chapter27Outline
Fattyaciddegradationisakeyenergysourceformammalsduringhibernation.
•
Fattyacidsarestoredin
adiposetissueas
triacylglycerols (TAG)in
whichfattyacidsare
linkedtoglycerolwith
esterlinkages.
•
Adiposetissueislocated
throughoutthebody,
withsubcutaneous
(belowtheskin)and
visceral(aroundthe
internalorgans)deposits
beingmostprominent.
Lipiddegradation
Thefattyacidsincorporatedinto
triacylglycerolsinadiposetissuearemade
accessibleinthreestages.
1. DegradationofTAG toreleasefattyacids
andglycerolintothebloodfortransportto
energy-requiringtissues.
2. Activationofthefattyacidsandtransport
intothemitochondriaforoxidation.
3. Degradationofthefattyacidstoacetyl
CoAforprocessingbythecitricacidcycle.
• Triacylglycerolsarestoredinadipocytesasalipiddroplet.
• Epinephrineandglucagon,actingthrough7TMreceptors,stimulatelipid
breakdownorlipolysis.
• ProteinkinaseAphosphorylatesperilipin,whichisassociatedwiththelipid
droplet,andhormone-sensitivelipase.
• Phosphorylationofperilipin resultsintheactivationofadipocytetriacylglyceride
lipase(ATGL).
• ATGLinitiatesthebreakdownoflipids.Chanarin-Dorfmam syndrome,
characterizedbydryskin,enlargedliverandmuscle,andmildcognitive
disability,resultsifATGLactivityiscompromised.
• Theglycerolreleasedduringlipolysisisabsorbedbytheliverforuseinglycolysis
orgluconeogenesis.
Triacylglycerols inadiposetissueareconvertedintofreefattyacids
inresponsetohormonalsignals
Thephosphorylationofperilipin
restructuresthelipiddroplet
andreleasesthecoactivatorof
ATGL.TheactivationofATGLby
bindingwithitscoactivator
initiatesthemobilization.
Hormone-sensitivelipase
releasesafattyacidfrom
diacylglycerol.
Monoacylglycerol lipase
completesthemobilization
process.
Abbreviations:7TM,seven
transmembrane;ATGL,adipose
triglyceridelipase;CA,
coactivator;HSlipase,
hormone-sensitivelipase;MAG
lipase,monoacylglycerol lipase;
DAG,diacylglycerol;TAG,
triacylglycerol.
Lipolysisgeneratesfattyacidsandglycerol
AcylCoAisanactivatedformoffattyacid
Acylcarnitinetranslocase
Afterbeingactivatedbylinkageto
CoA,thefattyacidistransferredto
carnitine,areactioncatalyzedby
carnitineacyltransferaseI,for
transportintothemitochondria.A
translocasetransportstheacyl
carnitineintothemitochondria.
Inthemitochondria,carnitine
acyltransferaseIItransfersthe
fattyacidtoCoA.Thefattyacyl
CoAisnowreadytobedegraded.
ClinicalInsights:Muscle,kidney,andheartusefattyacidsasafuel.Pathological
conditionsresultsiftheacyltransferaseorthetranslocasearedeficient.
Carnitinedeficienciescanbetreatedbycarnitinesupplementation.
Fattyaciddegradationconsistsoffourstepsthat
arerepeated.
1. Oxidationoftheβcarbon,catalyzedbyacyl
CoAdehydrogenase,generatestrans-Δ2-enoyl
CoAandFADH2.
2. Hydrationoftrans-Δ2-enoylCoAbyenoylCoA
hydrataseyieldsL-3-hydroxyacylCoA.
3. OxidationofL-3-hydroxyacylCoAbyL-3hydroxyacyldehydrogenasegenerates2ketoacylCoAandNADH.
4. Cleavageofthe3-ketoacylCoAbythiolase
formsacetylCoAandafattyacidchaintwo
carbonsshorter.
Fattyaciddegradationisalsocalledβ-oxidation.
Thereactionsequenceforthedegradationoffattyacids
Fattyacidsaredegradedbythe
repetitionofafour-reaction
sequenceconsistingofoxidation,
hydration,oxidation,andthiolysis.
Twocarbonunitsaresequentially
removedfromthecarboxylendof
thefattyacid
Thereactionforoneroundofβ-oxidationis:
ThecompletereactionforC16 palmitoylCoAis:
Processingoftheproductsofthecompletereactionbycellularrespiration
wouldgenerate106moleculesofATP.
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.Whenmonounsaturated
fattyacidsaredegradedbyβ-oxidation,cis-Δ3-enoylCoAisformed,whichcannot
beprocessedbyacylCoAdehydrogenase.
•
Cis-Δ3-enoylCoAisomerase convertsthedoublebondintotrans-Δ2-enoylCoA,a
normalsubstrateforβ-oxidation.
•
Whenpolyunsaturatedfattyacidsaredegradedbyβ-oxidation,cis-Δ3-enoylCoA
isomeraseisalsorequired.2,4-DienoylCoAisalsogenerated,butcannotbe
processedbythenormalenzymes.
•
2,4-DienoylCoAisconvertedintotrans-Δ3-enoylCoAby2,4-dienoylCoAreductase,
andtheisomeraseconvertsthisproducttotrans-Δ2-enoylCoA,anormalsubstrate.
•
Unsaturatedfattyacidswithoddnumbersofdoublebondsrequireonlythe
isomerase.Evennumberofdoublebondsrequireboththeisomeraseandreductase.
β-Oxidationoffattyacidswithoddnumbersof
carbonsgeneratespropionylCoAinthelastthiolysis
reaction.
Propionylcarboxylase,abiotinenzyme,addsa
carbontopropionylCoAtoformmethylmalonylCoA
SuccinylCoA,acitricacidcyclecomponent,is
subsequentlyformedfrommethylmalonylCoAby
methylmalonylCoAmutase,avitaminB12 requiring
enzyme.
•
Ketonebodies—acetoacetate,3-hydroxybutyrate,andacetone— are
synthesizedfromacetylCoAinlivermitochondriaandsecretedintothe
bloodforuseasafuelbysometissuessuchasheartmuscle.
•
3-Hydroxybutyrateisformeduponthereductionofacetoacetate.Acetoneis
generatedbythespontaneousdecarboxylationofacetoacetate.
•
Intissuesusingketonebodies,3-hydroxybutyrateisoxidizedtoacetoacetate,
whichisultimatelymetabolizedtotwomoleculesofacetylCoA.
Theformationofketonebodies
Theketonebodies—acetoacetate,D-3-hydroxybutyrate,and
acetone—areformedfromacetylCoAprimarilyintheliver.Enzymes
catalyzingthesereactionsare(1)3-ketothiolase,(2)
hydroxymethylglutaryl CoAsynthase,(3)hydroxymethylglutaryl CoA
cleavageenzyme,and(4)D-3-hydroxybutyratedehydrogenase.
Acetoacetatespontaneouslydecarboxylates toformacetone
Ketogenicdiets, richinfatsandlowincarbohydratesbutwith
adequateproteins,leadtoformationofsubstantialamounts
ofketonebodies.
Ketogenicdietsmayhavetherapeuticproperties:
Forreasonsnotyetestablished,suchdietsreducetheseizures
inchildrensufferingfromdrug-resistantepilepsy.
TheutilizationofD-3hydroxybutyrateand
acetoacetateasafuel
FatsareconvertedintoacetylCoA,whichisthenprocessedbythecitric
acidcycle.
Oxaloacetate,acitricacidcycleintermediate,isaprecursortoglucose.
However, acetylCoAderivedfromfatscannotleadtothenetsynthesisof
oxaloacetateorglucosebecausealthoughtwocarbonsenterthecycle
whenacetylCoAcondenseswithoxaloacetate,twocarbonsarelostasCO2
beforeoxaloacetateisregenerated.
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 highenergy electrons in the form of NADH. The
acetoacetate is then cleaved to yield to
acetyl CoA.
•
Ketonebodiesaremoderatelystrongacids,andexcessproductioncanlead
toacidosis.
•
Anoverproductionofketonebodiescanoccurwhendiabetes,acondition
resultingfromalackofinsulinfunction,isuntreated.Theresultingacidosisis
calleddiabeticketosis.
•
Ifinsulinisabsentornotfunctioning,glucosecannotentercells.Allenergy
mustbederivedfromfats,leadingtotheproductionofacetylCoA.
•
AcetylCoAbuildsupbecauseoxaloacetate,whichcanbegeneratedfrom
glucose,isnotavailabletoreplenishthecitricacidcycle.
•
Moreover,fattyacidreleasedfromadiposetissueisenhancedintheabsence
ofinsulinfunction.
Diabeticketosisresultswheninsulinisabsent
Intheabsenceofinsulin,fatsarereleasedfromadiposetissue,andglucose
cannotbeabsorbedbytheliveroradiposetissue.Theliverdegradesthefatty
acidsbyb-oxidation,butcannotprocesstheacetylCoAbecauseofalackof
glucose-derivedoxaloacetate(OAA).Excessketonebodiesareformedand
releasedintotheblood. Abbreviation:CAC,citricacidcycle.
•
Glucoseisthepredominantfuelforthebrain.
•
Duringstarvation,proteindegradationisinitiallythesourceofcarbonsfor
gluconeogenesisintheliver.Theglucoseisthenreleasedintotheblood
forthebraintouse.
•
Afterseveraldaysoffasting,thebrainbeginstouseketonebodiesasa
fuel.
•
Ketonebodyusecurtails(reduces)proteindegradationandthusprevents
tissuefailure.Moreover,ketonebodiesaresynthesizedfromfats,the
largestenergystoreinthebody.
Fuelchoiceduringstarvation
Saturatedandtransunsaturatedfattyacids aresynthesizedcommerciallytoenhance
theshelflifeandheatstabilityoffatsforfoodpreparation.
Studiessuggestthatexcessconsumptionofthesefatsresultsinobesity,heartdisease
andtype2diabetes.
Lecture:Ch27&Ch28
CHAPTER28
FattyAcidSynthesis
Outline
1. ThefirststageoffattyacidsynthesisistransferofacetylCoAoutofthe
mitochondriaintothecytoplasm.Citrateistransportedintothecytoplasm
andcleavedintooxaloacetateandacetylCoA.
2. ThesecondstateistheactivationofacetylCoAtoformmalonylCoA.
3. Thethirdstageistherepetitiveadditionandreductionoftwocarbonunits
tosynthesizeC16 fattyacid.Synthesisoccursonanacylcarrierprotein,a
molecularscaffold.
Citrate,synthesizedinthemitochondria,istransportedtothecytoplasm
andcleavedbyATP-citratelyasetogenerateacetylCoAforfattyacid
synthesis.
ThetransferofacetylCoAtothecytoplasm
PFK
AcetylCoAistransferredfrommitochondriatothecytoplasm,andthereducingpotential
ofNADHisconcomitantlyconvertedintothatofNADPHbythisseriesofreactions
•
FattyacidsynthesisrequiresreducingpowerintheformofNADPH.
•
SomeNADPHcanbeformedfromtheoxidationofoxaloacetate,
generatedbyATP-citratelyase,bythecombinedactionofcytoplasmic
malatedehydrogenaseandmalicenzyme.
•
Pyruvateformedbymalicenzymeentersthemitochondriawhereit
isconvertedintooxaloacetatebypyruvatecarboxylase.
•
Thesumofthereactionscatalyzedbymalatedehydrogenase,
malicenzyme,andpyruvatecarboxylaseis:
•
AdditionalNADPHissynthesizedbythepentosephosphatepathway.
PATHWAYINTEGRATION:Fattyacidsynthesis
Fattyacidsynthesisrequiresthecooperationofvariousmetabolicpathwayslocated
indifferentcellularcompartments.
•
FattyacidsynthesisstartswiththecarboxylationofacetylCoAtomalonyl
CoA,theactivatedformofacetylCoA
•
Malonyl CoAissynthesizedbyacetylCoAcarboxylase(ACC),abiotinrequiringenzyme.
TheformationofmalonylCoAoccursintwosteps:
ACC
(theactivated
formofCO2)
(theactivatedform
ofacetylCoA)
• Fattyacidsynthase,acomplexofenzymes,catalyzestheformationoffattyacids
fromfromacetylCoA,malonyl CoA,andNADPHiscalledfattyacidsynthase.
• Fattyacidsynthesisoccursontheacylcarrierprotein(ACP),apolypeptidelinked
toCoA.Intermediatesarelinkedtothesulfhydrylgroupofthepantothenate
attachedtoACP.
• AcetyltransacylaseandmalonyltransacylaseattachsubstratestotheACP.
β-KetoacylsynthasecatalyzesthecondensationofacetylACPandmalonylACPto
formacetoacetylACP.
•
Thenextthreesteps—areduction,dehydration,and
anotherreduction—converttheketogroupatcarbon3toa
methylenegroup(-CH2-),formingbutyrylACP.
•
NADPHisthesourceofreducingpower.
• Thesecondroundofsynthesisbegins
withthecondensationofmalonylCoA
withthenewlysynthesizedbutyryl
ACP,formingC6-β-ketoacylACP.
• Thereduction,dehydration,reduction
sequenceisrepeated.
• SynthesiscontinuesuntilC16-acylACP,
whichiscleavedbythioesterase to
yieldpalmitate.
Thestoichiometryforthesynthesisofpalmitateis:
ThesynthesisoftherequiredmalonylCoAisdescribedbythefollowingreaction:
Thus,thestoichiometryforthesynthesisofpalmitatefromacetylCoAis:
•
ThereactionsoffattyacidsynthesisaresimilarinE.coliandanimals.
•
Inanimals,alloftheenzymesrequiredforfattyacidsynthesisare
componentsofasinglepolypeptidechain.
•
Thefunctionalenzymeiscomposedoftwoidenticalchains.
•
Theenzymeconsistsoftwodistinctcompartments.
Ø Theselectingandcondensingcompartment,whichbindstheacetyland
malonylsubstratesandcondensesthem.
Ø Themodificationcompartment,whichcarriesoutthereductionand
dehydrationactivitiesrequiredforelongation.
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.
•
Tumorsrequirelargeamountsoffattyacidsynthesistoproduce
precursorsformembranesynthesis.
•
β-Ketoacyl synthaseinhibitorsretardtumorgrowth.
•
Micetreatedwithβ-Ketoacyl synthaseinhibitorsalsoshoweddramatic
weightloss,suggestingthatsuchdrugsmaybeusedtotreatobesity.
•
AcetylCoAcarboxylaseinhibitorsmayalsobepotentialchemotherapy
agents.
• β-Hydroxybutyricacid,whenattachedtoACPorCoA,isasubstrate
infattyacidsynthesisanddegradation,andisaketonebodyaswell.
• Anisomerofthiskeybiochemical,γ-hydroxybutyricacidisapotent,
illegaldrug.
•
FattyacidsynthasecannotgeneratefattyacidslongerthanC16
palmitate.
•
Longerfattyacidsaresynthesizedbyenzymesattachedtothe
endoplasmicreticulum.
•
Theseenzymesextendpalmitatebyaddingtwocarbonunits,using
malonylCoAasasubstrate.
•
Enzymesboundtotheendoplasmicreticulum
introducedoublebondsintosaturatedfatty
acids.
Forinstance:
•
Mammalslacktheenzymesthatintroducedoublebondsbeyondcarbon9.
•
Linoleateandlinolenateareessentialfattyacids
thatmustbeobtainedinthediet.
•
Arachidonate,a20-carbonfattyacidwithfourdoublebonds,isderived
fromlinoleate.
•
Arachidonateisaprecursorforavarietyofsignalmolecules20carbons
long,collectivelycalledtheeicosanoids.
•
Thesesignalmolecules,whichincludeprostaglandins,arelocalhormones
becausetheyareshortlivedandonlyaffectnearbycells.
Arachidonate isthemajorprecursorofeicosanoidhormones
Structuresofseveraleicosanoids
• Aspirinpreventstheuseofarachidonateasasubstrateforthe
enzymethatgeneratesprostaglandinH2 .
• Blockingthisstepeffectsmanysignalingpathways,accountingfor
thewiderangeeffectsofaspirin.
AcetylCoAcarboxylase1and2aresubjecttoregulationonseverallevels.
Carboxylase1, acytoplasmicenzyme,isinhibitedwhenphosphorylatedbyAMPactivatedkinase(AMPK).Inhibitionduetophosphorylationisreversedbyprotein
phosphatase2A.
Citrate activescarboxylasebyfacilitatingtheformationofactivepolymersofthe
carboxylase.Citratemitigatesinhibitionduetophosphorylation.
PalmitoylCoA,theend-productoffattyacidsynthase,inhibitscarboxylasebycausing
depolymerizationoftheenzyme.
Carboxylase2,amitochondrialenzyme,inhibitsfattyaciddegradationbecauseits
product,malonylCoA,preventstheentryoffattyacylCoAintothemitochondriaby
inhibitingcarnitineacyltransferase1.
ThecontrolofacetylCoAcarboxylase
DependenceofthecatalyticactivityofacetylCoA
carboxylaseontheconcentrationofcitrate.
•
GlucagonandepinephrineinhibitcarboxylasebyenhancingAMPKactivity,
bywhichtheypreventfattyacidsynthesis.
•
Insulinstimulatesthedephosphorylationandactivationofcarboxylase,by
whichitstimulatesfattyacidsynthesis.
•
Theenzymesoffattyacidsynthesisareregulatedbyadapativecontrol.If
adequatefatsarenotpresentinthediet,thesynthesisofenzymesrequired
forfattyacidsynthesisisenhanced.
Answer
Malonyl 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 6phosphate dehydrogenase, the controlling
enzyme of the pentose phosphate
pathway.
•
Onepathwayforethanolprocessingconsistsoftwostepsandleadsto
excessproductionofNADH:
• ExcessNADHinhibitsgluconeogenesisandenhanceslactateproduction,which
mayresultinlacticacidosis.
• ExcessNADHinhibitsfattyaciddegradationandstimulatesfattyacidsynthesis,
leadingtotheaccumulationoffatsintheliver.
• Livercanconvertsomeoftheacetategeneratedbyaldehydedehydrogenase
intoacetylCoA,buttheacetylCoAcannotbeprocessedbythecitricacidcycle
becauseofthepaucityorlackofNAD+.
• Thebuild-upofacetylCoAcanleadtoketonebodysecretionbytheliver,which
exacerbatestheacidosiscausedbylactateaccumulation.
• Ifacetatecannotbeprocessed,acetaldehydeaccumulates.Acetaldehydeisvery
reactiveandmodifiesreactivegroupsofproteins,causingalossofprotein
function.
• Asproteindamageaccumulates,liverfunctioncanfail.