清华大学生物化学脂肪分解代谢

.,Chapter17FattyAcidCatabolism,.,1.Thegoodandbadsidesofusingtriacylglycerolsasanenergystorage,Highlyreduced,withanenergyofcompleteoxidationmorethantwicethatforthesameweightofcarbohydratesorproteins(38kJ/gvs18kJ/g).Highlyhydrophobic:doesnotraiseosmolarityofcytosol,noraddextraweight;butmustbeemulsifiedbeforedigestionandtransportedbyspecialproteinsinblood.Chemicallyinert:noundesiredchemicalreactionswithotherconstituents;butmustbeactivated(byattachingthecarboxylgrouptocoenzymeA)tobreakthestableC-Cbonds.,.,Fattyacidsisacentralenergysourceinanimals(especiallytheliver,heart,andrestingskeletalmuscle),manyprotists,andsomebacteria;theonlyenergysourceforhibernatinganimalsandmigratingbirds;notamajorenergysourceinplants.Sourcesforcellstoobtainfattyacids:diet;storedlipiddroplets;synthesizedfromtheexcesscarbohydratesandaminoacidsbysomespecialorgans(e.g.,liver).,.,2.Dietarytriacylglycerolsareemulsifiedandabsorbedbytheintestine,Bilesalts(e.g.,taurocholate牛黄胆酸andglycocholate甘胆酸),synthesizedfromcholesterolinliver,emulsifiesmacroscopicfatparticlesintomicroscopicmixedmicellesforbetterlipaseactionandabsorption.Fattyacidsgeneratedfromtriacylglycerol(catalyzedbytheintestinallipase)diffuseintointestinalepithelialcells,bereconvertedintotriacylglycerol,andpackedwithcholesterolestersandspecificapolipoproteinsinchylomicrons(乳糜微滴).,.,Processingofdietarylipidsinvertebrates,.,Processingofdietarylipidsinvertebrates,.,Molecularstructureofachylomicron,.,Molecularstructureofachylomicron,.,TriacylglycerolsareconvertedintofattyacidsandglycerolsinthecapillariesbytheactionoflipoproteinlipasesactivatedbyapoC-IIonchylomicrons,whichinturnareabsorbedmainlybyadipocytesandmyocytesforstorageandenergyconsumption.Theleftoverofthechylomicrons(containingmainlycholesterolandapolipoproteins)willbetakenupbytheliverbyendocytosis;triacylglycerolswillbeusedastheenergysourceforthelivercells,convertedtoketonebodiesortransportedtoadiposetissuesafterbeingpackedwithapolipoproteins.,.,3.Mobilizationofstoredtriacylglycerolsinadipocytesneedshormonetosignalthedemand,Thehormonesepinephrineandglucagon,signalingalackofglucoseintheblood,willbindtoreceptorsonadipocytesurfaceandactivatethehormone-sensitivelipaseandperilipinmoleculesonthesurfaceofthelipiddroplets.Fattyacidsreleasedarecarriedtoenergy-demandingtissues(e.g.,skeletalmuscle,heart,andrenalcortex)viathe62kDmonomericserumalbumin(eachbindingasmanyas10fattyacids).,.,Mobilizationoftriacylglycerolsstoredinadiposetissue,.,Entryofglycerolintotheglycolyticpathway,.,4.Earlylabelingexperiments(1904):fattyacidsaredegradedbysequentialremovaloftwo-carbonunits,Whendogswerefedwithodd-numberedfattyacidsattachedtoaphenylgroup,benzoatewasexcreted;andwhenfedwitheven-numbered,phenylacetatewasexcreted.Hypothesis:theb-carbonisoxidized,withtwo-carbonunitsreleasedbyeachroundofoxidation.Theseexperimentsarealandmarkinbiochemistry,inusingsyntheticlabel(thephenylgrouphere)toelucidatereactionmechanism,andwasdonelongbeforeradioisotopeswasusedinbiochemistry!,.,FranzKnoopslabelingexperiments(1904):fattyacidsaredegradedbyoxidationatthebcarbon,i.e.,boxidation,b,b,a,a,.,5.Fattyacidoxidationwasfoundtooccurinmitochondria,Enzymesoffattyacidoxidationinanimalcellswerelocalizedinthemitochondriamatrix.RevealedbyEugeneKennedyandAlbertLehningerin1948.,.,6.Fattyacidsareactivatedontheoutermembraneofmitochondria,Fattyacidsareconvertedtofattyacyl-CoA(ahighenergycompound)viaafattyacyl-adenylateintermediate(enzyme-bound,mixedanhydride)bytheactionoffattyacyl-CoAsynthetases(alsocalledfattyacidthiokinase).Pyrophosphateishydrolyzedbyinorganicpyrophosphatase,thustwoanhydridebondsofATPareconsumedtoformonehigh-energythioesterbond,thuspullingthereactionforward:acommonphenomenainbiosyntheticreactions.Acyl-adenylatesareoftenformedwhenCOOHgroupsareactivatedinbiochemistry.,.,Conversionofafattyacidtoafattyacyl-CoA,.,7.Activated(longchain)fattyacidsarecarriedintothematrixbycarnitine,Thefattyacylgroupisattachedtocarnitine(肉碱)viatransesterificationbytheactionofcarnitineacyltransferaseIlocatedontheoutermembraneofmitochondria,formingfattyacyl-carnitine,leavingtheCoAinthecytosol.Theacylcarnitine/carnitinetransportermovesacyl-carnitineacrosstheinnermembraneofmitochondriaviafacilitateddiffusion.Medium-chainacyl-CoAsseemtoenterthematrixbythemselves,withoutbeingcarriedbycarnitine.,.,.,Fattyacidentryintomitochondriaviatheacyl-carnitine/carnitinetransporter,.,TheacylgroupisthentransferredbacktoCoAtoformfattyacyl-CoAbytheactionofcarnitineacyltransferaseIIlocatedontheinnerfaceoftheinnermembrane.Thisenteringstepseemstoberate-limitingforfattyacidoxidationinmitochondriaanddiseaseshavebeenfoundtobecausedbyadefectofthisstep(withachingmusclecramp,especiallyduringfasting,exerciseorwhenonahigh-fatdiet).,.,8.Fattyacyl-CoAisoxidizedtoacetyl-CoAviamultipleroundsofboxidation,Theboxidationconsistsoffourreactions:dehydrogenationhydrationdehydrogenationthiolyticcleavage,.,Theb-oxidationpathway,.,The1stdehydrogenationiscatalyzedbythemembrane-boundacyl-CoAdehydrogenase,convertingacyl-CoAtotrans-2-enoyl-CoAwithelectronscollectedbyFADandfurthertransferredintotherespiratorychainviatheelectron-transferringflavoprotein(ETF),alsoboundtotheinnermembraneofthemitochondria.Acyl-CoAdehydrogenaseexistsinthreeisozymesactingontheacyl-CoAsoflong(12-18C),medium(4-14C)andshort(4-8C)chains.,.,.,Thehydrationstep,stereospecificallycatalyzedbyenoyl-CoAhydratase,convertsthetrans-2-enoyl-CoAtoL-b-hydroxylacyl-CoA(thecisisomercanalsobeconverted,buttotheDisomer).,.,The2nddehydrogenationiscatalyzedbyL-b-hydroxylacyl-CoAdehydrogenase,convertingL-b-hydroxylacyl-CoAtob-ketoacyl-CoA(notactontheDisomer),withelectronscollectedbyNAD+.,.,Theacyl-CoAacetyltransferase(orcommonlycalledthiolase)catalyzesthenucleophilicattackofCoAtothecarbonylcarbon,cleavingb-ketoacyl-CoAbetweentheaandbcarbon(thiolysis),generatingtwoacyl-CoAmoleculeswithoneenteringthecitricacidcycleandtheotherreenteringtheboxidationpathway.Thefirstthreereactionsoftheboxidationusedthesamereactionstrategyasthelastthreereactionsinthecitricacidcycle,allinvolvingtheoxidationofahighlyreducedcarbon(from-CH2-to-CHO-).,.,.,Aconservedreactionsequencetoinduceacarbonylfunctiononthecarbonbtoacarboxylgroup,.,Stagesoffattyacidoxidation,.,Palmitol-CoA+7CoA+7FAD+7NAD+7H2O8acetyl-CoA+7FADH2+7NADH+7H+,Thecompleteoxidizationofeach16-carbonpalmitate(toH2OandCO2)yields106ATP(32ATPperglucose,bothhavingabout60%ofactualenergyrecovery).,.,Palmitoyl-CoA+23O2+108Pi+108ADPCoA+108ATP+16CO2+23H2O,*,.,Hibernatinggrizzlybearsusebodyfatastheirsolefuel,.,9.Oxidationofunsaturatedfattyacidsrequiresoneortwoauxiliaryenzymes,anisomeraseandareductase,Theisomeraseconvertsacis-3doublebondtoatrans-2doublebond.Thereductase(2,4-dienoyl-CoAreductase)convertsatrans-2,cis-4structuretoatrans-3structure,whichwillbefurtherconvertedtoatrans-2structurebytheisomerase.NADPHisneededforthereduction(fromtwodoublebondstoone).,.,Oxidationofamonounsaturatedfattyacid:theenoyl-CoAisomerasehelpstorepositionthedoublebond,.,Bothanisomeraseandareductaseareneededforoxidizingapolyunsaturatedfattyacid.,.,10.Propionyl-CoAgeneratedfromodd-numberfattyacids(andthreeaminoacids)isconvertedtosuccinyl-CoA,Odd-numberfattyacids,commonlyfoundinthelipidsofsomeplantsandmarineorganisms,willalsobeoxidizedviatheboxidationpathway,butgeneratingapropionyl-CoAatthelaststepofthiolysis.Propionyl-CoAisconvertedtosuccinyl-CoAviaanunusualenzymaticpathwayofthreereactions.,.,Propionyl-CoAcanbeconvertedtosuccinyl-CoAviaacarboxylation,anepimerizationandanintramoleculargroupshifting,.,Propionyl-CoAisfirstcarboxylatedattheacarbontoformD-methylmalonyl-CoA,catalyzedbypropionyl-CoAcarboxylase.TheD-methylmalonyl-CoAisthenepimerizedtotheL-isomerbytheactionofanepimerase.TheL-methylmalonyl-CoAisthenconvertedtosuccinyl-CoAbyanintramolecularrearrangementviafreeradicalintermediates,withthecatalysisofmethylmalonyl-CoAmutase,using5-deoxyadenosylcobalamin(orcoenzymeB12)asacoenzyme.,.,CoenzymeB12enzymesactinintramolecularrearrangements,methylation,andreductionofribonucleotidestodeoxynucleotides.Perniciousanemia(恶性贫血)iscausedbyafailuretoabsorbvitaminB12,whichmayinturnbecausedbyadeficiencyoftheintrinsicfactor,a59kDglycoproteinneededforvitaminB12absorption.VitaminB12issynthesizedbyafewintestinalbacteria(notplants).,.,3-DstructureofcoenzymeB12andpenicillin,.,11.Therateofb-oxidationinmitochondriaislimitedbytheenteringrateoffattyacyl-CoA,Fattyacyl-CoAinthecytosolcanentermitochondriaforoxidativedegradationorbeconvertedtotriacylglycerolinthecytosolwhenexcessglucoseispresentthatcannotbeoxidizedorstoredasglycogenandisconvertedintofattyacidsforstorage.ThekeyregulatoryproteiniscarnitineacyltransferaseI,whichisinhibitedbymalonyl-CoA,thefirstintermediateforfattyacidsynthesisfromacetyl-CoA.,.,Coordinatedregulationoffattyacidsynthesisandbreakdown,.,12.Fattyacidoxidationalsooccursinperoxisomes(glyoxysomes),Peroxisomeistheprincipleorganelleinwhichfattyacidsareoxidizedinplantcells.Alsoviathefour-stepboxidationpathway,buttheelectronscollectedbyFADaredirectlypassedtoO2,producingtheharmfulH2O2,whichisimmediatelyconvertedtoH2OandO2.Energyreleasedisdissipatedasheat,notconservedinATP.,.,Comparisonofb-oxidationinmitochondrionandinperoxisome/glyoxysome,.,Theacetyl-CoAproducedinanimalperoxisomesistransportedintocytosol,whereitisusedinthesynthesisofcholesterolandothermetabolites.Theacetyl-CoAproducedinplantperoxisomes/glyoxysomes(especiallyingerminatingseeds)isconvertedtosuccinateviatheglyoxylatecycle,andthentoglucoseviagluconeogenesis.,.,Triacylglycerolsasglucosesourceinseeds,.,Theb-oxidationenzymesinmitochondriaandperoxisomesareorganizeddifferently:beingseparateenzymesinmitochondria(asingram-positivebacteria)andonecomplexinperoxisomes(asingram-negativebacteria),.,Boundtotheinnermitochondrialmembrane,.,13.Acetyl-CoAinlivercanbeconvertedtoketonebodieswhencarbohydratesupplyisnotoptimal,Underfastingordiabeticconditions,oxaloacetateconcentrationinhepatocytewillbelow:therateofglycolysisislow(thusthesupplyofprecursorsforreplenishingoxaloacetateiscutoff)andoxaloacetateissiphonedoffintogluconeogenesis(tomaintainbloodglucoselevel).Theacetyl-CoAgeneratedfromactivefattyacidoxidationcannotbeoxidizedviathecitricacidcycleandwillbeconvertedtoacetoacetate,b-hydroxylbutyrate,andacetone(i.e.,theketonebodies,酮体)inmitochondriaforexporttoothertissues.,.,Ketonebodyformationandexportfromtheliver,.,.,Formationofketonebodiesfromacetyl-CoA,.,14.Ketonebodiesareconvertedbacktoacetyl-CoAinextrahepatictissues,D-b-hydroxylbutyrateisreoxidizedtoacetoacetate(catalyzedbythedehydrogenase).Acetoacetateisthenconvertedtoacetoacetyl-CoAusingsuccinyl-CoA(catalyzedbyb-ketoacyl-CoAtransferase).Acetoacetyl-CoAiscleavedtotwoacetyl-CoA(againcatalyzedbythiolase).Heart,skeletalmuscleandtherenalcortexuseacetoacetateinpreferencetoglucose.Thebrainadaptstotheutilizationofacetoacetateduringstarvationanddiabetes.,.,b-Hydroxybutyratecanbeusedasafuelbyextrahepatictissues,Notpresentinliver,.,Overproductionofketonebodiesinuncontrolleddiabetesorseverelyreducedcalorieintakecanleadtoacidosisandketosis.,.,Keywords,Activationandtransportoffattyacidsb-oxidation-reactionandregulationKe