Post on 18-Feb-2020
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Biochemistry
5.3)FatMetabolism
Prof.Dr.KlausHeese
5. Bio-Energetics & ATP Introduction
basic physiological background info
Lipids Metabolism
DefinitionofLipids
• Substancesindairyfoods• Knownasfatsandoils• Hydrophobicandsolubleinethanollikealcohol,petroleumandchloroform
• Classifiedinto2generaltypes– Fats andwaxes(canbehydrolyzedwithesterlinkages)
– cholesterol andsteroids (cannotbehydrolyzed)
TypeofLipids
• Saturatedfat– Animaloillikemeat,milk,butter– Vegetableoillikecoconutandpalmkerneloil
• Polyunsaturatedfat– Plantsourcelikesafflower,corn,cottonseed,sunfloweroilandsoybeanoil
• Monounsaturatedfat– Plantandanimalproductlikeoliveoil,canolaoil,avocadoandpeanutoil
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LipidsFunctions• Excellentenergyreserves• Structureofcellmembranes(neuron,glia,heart)• Organpadding• Bodythermalinsulation• Essentialfattyacids(EFA)• Hormonesynthesis• Fatsolublevitaminabsorption
Cholesterol
• Plantandanimalfoodcontainsterolsbutonlyanimalfoodcontainscholesterol
• Why?Cholesterol ismadeintheliverandplantsdonothavealiver
• Cholesterol isneededtomakebile,sexhormones,steroidsandvitaminD.
• Itistheconstituentofcellmembranestructure• Dietaryrecommendation- <300mg/d• Sources– eggyolks,liver,shellfish,organfoods
Lipoproteins• LowDensityLipoproteins(LDL)– ismadebytheliverandis
comprisedofcholesterolthatisdeliveredtothecellsinthe
body
– HighlevelsofLDLisstronglycorrelatedwithheartdisease
• HighDensityLipoproteins(HDL)- madebytheliverandpicks
upcholesterolfromthecellsforrecyclingorexcretion
– HighlevelsofHDLisinverselycorrelatedwithheart
disease
– Itisprotective
BloodlevelsforLipids
• TotalCholesterol:–<200mg/dl=desirable–200-239mg/dl=borderlinehyperlipidemia–>240mg/dl=hyperlipidemia
• LDL <130mg/dlisfavorable• HDL >35mg/dlisfavorable
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FatMetabolismI’mnotfat,I’ve justgotalotofpotentialenergy!
LipidMetabolism LipidMetabolism• Digestion- HydrolysisReaction
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FattyAcidMetabolism• Whyarefattyacidsimportanttocells?
– fuelmolecules• storedastriacylglycerols
– buildingblocks• phospholipids• glycolipids
– precursorsofhormonesandothermessengers
– usedtotargetproteinstomembranesites Triglyceridesareahighlyconcentratedstore
ofenergy9kcal/gvs 4kcal/gforglycogenGlycogenisalsohighlyhydrated,2gH2O/gglycogen
- fattyacidportionishighlyreduced
FattyAcidMetabolism
• Whydotriacylglycerols storelargeamountsofenergy?– fattyacidportionishighlyreduced– nonpolarmoleculesarestoredinanhydrousform
• Wherearetriacylglycerols stored?– Adipocytes,--->breakdowntoglycerolandfattyacids
FattyAcidMetabolism
• Whatisneededfortriacylglycerolbreakdown?– bilesalts
• madeinliver,storedingallbladder• E.g.:Glycocholate (Glycocholic acid,orcholylglycine,isacrystallinebileacidinvolvedintheemulsificationoffats.Itoccursasasodiumsaltinthebileofmammals.Itisaconjugateofcholic acidwithglycine.Itsanioniscalledglycocholate)
– lipases• pancreas• hydrolyzeesterbond
Dietarytriacylglycerolsmustbebrokendownbeforebeingabsorbedbytheintestines.
Bilesalts,whichactasdetergents,areusedtosolublizethetriacylglycerols
PancreaticLipases
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• Pancreaticlipaseshydrolyzetheesterbondsofthetriacylglycerolswhileinthemicelles.
PancreaticLipases
Fatty acids and monoacylglcerols are absorbed across the plasma membrane of intestinal epithelial cells.
– Intheintestinalmucosalcells,thefattyacidsandmonoacylglycerides areresynthesizedintotriacylglycerides andpackagedintochylomicrons(particlesconsistingoftriacylglycerols andprotein,apolipoproteins).Chylomicronsandlympharedumpedviathethoracicductintotheleftsubclavian vein
Chylomicrons
ChylomicronsChylomicrons and lymph are dumped via the thoracic duct into the left subclavian vein.
• Threestagesofprocessing
– Triglycerols aredegradedtofattyacidsandglycerolintheadiposetissueandtransportedtoothertissues.
– Fattyacidsareactivatedandtransportedintothemitochondria.
– Fattyacidsarebrokendownintotwo-carbonacetyl–CoAunitsandfedintothecitricacidcycle(Kreb’scycle).
UtilizationofFattyAcidsasFuel
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FattyAcidMetabolism
• Howarefattyacidsmadeavailabletoperipheraltissuesasanenergysource?– hormonestriggerlipolysisinadiposetissue
• epinephrine,glucagon,ACTH• insulininhibitslipolysis
– releasedfattyacidsinsolubleinplasma• mustbeattachedtoserumalbuminfortransport
• Intheadiposetissue,lipasesareactivatedbyhormonesignaledphosphorylation
BreakdownofTriacylglycerols
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• Thelipasesbreakthetriacylglycerolsdowntofattyacidsandglycerol– Thefattyacidsaretransportredinthebloodbyserumalbumin
BreakdownofTriacylglycerols FattyAcidMetabolism
• Whathappenstotheglycerolreleased?– convertedtoglyceraldehyde-3-PO4
• Glycolysis(seesteps4-6)• gluconeogenesis
Theglycerolisabsorbedbytheliverandconvertedtoglycolyticintermediates.
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LipidMetabolism ActivationofFattyAcids• What must happen to fatty acids for them to be oxidized?
– activated– transported into mitochondria– Acyl CoA synthetase reaction occurs at the mitochondrial
membrane.
FattyAcidDegradation
• What is the role of carnitine in fatty acid oxidation?
• carnitine carries long-chain activated fatty acids into the mitochondrial matrix
• Carnitinecarrieslong-chainactivatedfattyacidsintothemitochondrialmatrix
TransportintoMitochondrialMatrix
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• Whatisthereactionsequencefortheoxidationoffattyacids?– firststepisanoxidation
• acylCoAdehydrogenase
- Thepathwayiscalledtheβ-oxidationpathway.- Likesuccinyl dehydrogenase,thisreactionusesFADandislinkedtoComplex2oftheelectrontransportchain.
FattyAcidOxidation
b a
1.
• Eachroundinfattyaciddegradationinvolvesfourreactions– 2nd step:HydrationtoL–3–Hydroxylacyl CoA
Fattyacidoxidation
- Thepathwayiscalledtheβ-oxidationpathway.- ThehydrationproducesonlytheL–isomer.
2.
• Eachroundinfattyaciddegradationinvolvesfourreactions– 3rd step:Oxidationto3–Ketoacyl CoAbyaL-3-hydroxyacylCoAdehydrogenase
Fattyacidoxidation
3.
• Eachroundinfattyaciddegradationinvolvesfourreactions– 4th step:Thiolysis toproduceAcetyl–CoA
• cleavageof3-ketoacylCoAbythiol groupofCoA– acylCoAshortenedby2carbons– acetylCoAformed
Fattyacidoxidation
entersKrebscycle
--->ATPgeneration
4.
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The acetyl-CoA produced by mitochondrial beta-oxidation of fatty acids enters the Kreb's cycle to produce energy, but that is not the only fate of acetyl-CoA. In liver mitochondria, some acetyl-CoA is converted to acetoacetate, beta-hydroxybutyrate, and acetone, collectively called ketone bodies. Ketone bodies are transported to other tissues such as brain, muscle or heart where they are converted back to acetyl-CoA to serve as an energy source. The brain normally uses only glucose for energy, but during starvation ketone bodies can become the main energy source for the brain. In the metabolic condition called ketosis, ketone bodies are produced faster than they are consumed by tissues and the smell of acetone can be detected on a person's breath. The smell of acetone is one indication that a person may have diabetes. The consumption of high-fat/low carbohydrate diets has been used as a weight loss program by many, intentionally inducing ketosis to consume fat stores, but these ketogenic diets can cause unwanted side effects related to increased urea production resulting from protein intake and risk of heart disease from increased cholesterol and fat intake.
Acetyl-CoA alternativepathway AcetylCoA
• Eachroundinfattyaciddegradationinvolvesfourreactions– Theprocessrepeatsitself
Fattyacidoxidation• Eachroundinfattyaciddegradationinvolvesfourreactions
Fattyacidoxidation
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2
3
4
activation/transportintomitochondria
entersKrebscycle--->ATPgeneration
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FattyAcidDegradation- ATPYield• Whataretheproductsoffattyaciddegradation?
– ForaC16fattyacid• 8acetylCoA• 7FADH2
• 7NADH+7H+
– Howmuchenergydoesthisgenerate?• 7x1.5ATP=10.5• 7x2.5ATP=17.5• 8x10ATP=80• Total=108ATP– 2ATP(activation)=106ATP
BetaOxidation
BetaOxidation BetaOxidation
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• Unsaturatedfattyacids(monounsaturated)
SpecialCases
Unsaturatedfattyacidsrequireadditionalstepsfordegradation
isomerizationshiftspositionandconfigurationofadoublebond
reductionneededtoremovedoublebondinwrongposition
goto2nd stepoffattyacidbetaoxidationpathway
• Unsaturatedfattyacids(polyunsaturated)
SpecialCases
goto2nd stepoffattyacidbetaoxidationpathway
Odd-Chain• How is the oxidation of odd-chain fatty
acids different from even-chain ones?– in final round of degradation products
are acetyl CoA and proprionyl CoA– proprionyl CoA is converted to succinyl
CoA
- Proprionyl CoA is carboxylated to give D-methylmalonyl CoA
catalyzed by proprionyl CoA carboxylaseuses biotin
• D-methylmalonyl CoA is racemizedto L form– methylmalonyl CoA mutase
• uses a derivative of vitamin B12
• Inlaststepa5-deoxyadenosylfreeradicalremovesaHatomtoaidinrearrangementofL-methylmalonyl CoAtosuccinyl CoA
enterKreb’s cycle
FattyAcidDegradation
• Where,inadditiontothemitochondriadoesfattyacidoxidationtakeplace?– peroxisomes
• Howisthisdifferentfromb oxidation?– infirststepelectronsaretransferredtoO2 Redox-ReactioncombinedwithO2
BioFuelCelloptions
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KetoneBodies
• Useoffattyacidsinthecitricacid(Kreb’s)cyclerequirescarbohydrates(CHO)fortheproductionofoxaloacetate(OAA)– ‘fatburnsinCHOflame’.
• Duringstarvationordiabetes,OAAisusedtomakeglucose
– Fattyacidsarethenusedtomakeketonebodies(acetoacetateandD–3–hydroxybutarate)
The acetyl-CoA produced by mitochondrial beta-oxidation of fatty acids enters the Kreb's cycle to produce energy, but that is not the only fate of acetyl-CoA. In liver mitochondria, some acetyl-CoA is converted to acetoacetate, beta-hydroxybutyrate, and acetone, collectively called ketone bodies. Ketone bodies are transported to other tissues such as brain, muscle or heart where they are converted back to acetyl-CoA to serve as an energy source. The brain normally uses only glucose for energy, but during starvation ketone bodies can become the main energy source for the brain. In the metabolic condition called ketosis, ketone bodies are produced faster than they are consumed by tissues and the smell of acetone can be detected on a person's breath. The smell of acetone is one indication that a person may have diabetes. The consumption of high-fat/low carbohydrate diets has been used as a weight loss program by many, intentionally inducing ketosis to consume fat stores, but these ketogenic diets can cause unwanted side effects related to increased urea production resulting from protein intake and risk of heart disease from increased cholesterol and fat intake.
Acetyl-CoA alternativepathway
‘Ketone-bodies’ as energy substrates for the brain
(3-HB)
[AcAc)](3-HB)1) 2) 3)
Ketone-bodies [2)&3)] oxidized in neurons, astrocytes and oligodendrocytes; but free fatty acids exclusively in astrocytes [ 1)-3) in astrocytes]
1)-3) Products from lipid metabolism: maternal milk, starvation and diabetes
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KetoneBodies• Ketonebodies,acetoacetateand3–hydroxybutarate (orb-
hydroxybutyrate)andacetone)areformedfromAcetyl–CoA• whenfatsarerapidlybrokendown
KetoneBodiesasFuelSource• Theliveristhemajorsourceof
ketonebodies.
– Itistransportedinthebloodtoothertissues
• Acetoacetateinthetissues
– AcetoacetateisfirstactivatedtoacetoacetatebytransferringtheCoASHfromsuccinyl–CoA.
– ItisthensplitintotwoAcetyl–CoAbyathiolase reaction
- Ketone bodies are a water soluble equivalent of Acetyl-CoA units.- Some tissues, such as cardiac muscle or kidney cortex, preferentially use ketone bodies.- The brain can use ketone bodies under conditions of long term starvation.- Because of their acidity, high concentrations of ketone bodies can be detrimental, high levels of acetoacetate decreases lipolysis
KetoneBodies KetoneBodies
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BloodGlucoseandGlucosuria KetoneBodies
DiabetesandKetoneBodies
• Whenthereisnotenoughinsulininthebloodanditmustbreakdownfatforitsenergy.
• Ketonesbuildupinthebloodandthenspilloverintotheurinesothatthebodycangetridofthem.Acetonecanbeexhaledthroughthelungs.Thisgivesthebreathafruityodor.Ketonesthatbuildupinthebodyforalongtimeleadtoseriousillnessandcoma.(Diabeticketoacidosis)
ComparisonofFatMetabolismpergramandCHOpergram
• Fatsprovideabout9kilocaloriespergramandcarbohydratesprovideabout4kilocaloriespergram.
• Usingnutritionalunits,thatis9Calories/gramforfatsand4Calories/gramforcarbohydrates.
• CHOprovideenergymorequickly• Fatsaregoodfuelforenduranceevents,butnotsprints
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• Eventhoughthecitricacidcycleintermediateoxaloacetatecanbeusedtosynthesizeglucose,Acetyl–CoAcannotbeusedtosynthesizeoxaloacetate.– ThetwocarbonsthatenterthecitricacidcycleasAcetyl–CoAleaveasCO2.
FattyAcidsCannotbeUsedtoSynthesizeGlucose
plantshaveenzymesassociatedwithglyoxylate cyclethatallowacetylCoAtoformoxaloacetate
• Fattyacidaresynthesizedanddegradedbydifferentpathways.– Synthesistakesplaceinthecytosol(notmitochondria).– Intermediatesareattachedtotheacylcarrierprotein(ACP).
– Inhigherorganisms,theactivesitesforthesynthesisreactionsareallonthesamepolypeptide.
– Theactivateddonorinthesynthesisismalonyl–ACP.– FattyacidreductionusesNADPH+H+(asopposedtoNAD+
andFAD).– ElongationstopsatC16 (palmitic acid)
FattyAcidSynthesis
• Formationofmalonyl–CoAisthecommittedstepinfattyacidsynthesis.
FormationofMalonyl CoenzymeA
Biotin,Mn2+
• Theintermediatesinfattyacidsynthesisarecovalentlylinkedtotheacylcarrierprotein(ACP)(rolesimilartocoenzymeA)
AcylCarrierProtein
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• Inbacteriatheenzymesthatareinvolvedinelongationareseparateproteins;inhigherorganismstheactivitiesallresideonthesamepolypeptide.– Tostartanelongationcycle,Acetyl–CoAandMalonyl–CoAareeachtransferredtoanacylcarrierprotein
Elongation
• Acyl-malonylACPcondensingenzymeformsAcetoacetyl-ACP.
Elongation
-Thisreactionisdrivenbythedecarboxylation.EssentiallythefreeenergyfromtheATPthatwashydrolyzedtoputtheCO2 ontotheacetyl-CoAwhenformingthemalonyl-CoAisreleasedwhentheCO2 comesoffinthecondensationreaction.
• Thenextthreereactionsaresimilartothereverseoffattyaciddegradation,except– TheNADPHisusedinsteadofNADHandFADH2– TheD–enantiomerofHydroxybutarate isformedinsteadoftheL–enantiomer
Elongation FattyAcidSynthesis• Whatarethestepsinfattyacidsynthesiscatalyzedbythefattyacidsynthasecomplex?
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• Theelongationcycleisrepeatedsixmoretimes,usingmalonyl–CoAeachtime,toproducepalmityl–ACP.
• Athioesterasethencleavesthepalmityl–CoAfromtheACP.
Elongation• Thestoichiometryofpalmitate synthesis:
– Synythesis ofpalmitate fromMalonyl–CoA
– SynthesisofMalonyl–CoAfromAcetyl–CoA
– Overallsynthesis
StoichiometryofFAsynthesis
• Acetyl–CoA is synthesized in the mitochondrial matrix, whereas fatty acids are synthesized in the cytosol; – Acetyl–CoAunitsareshuttledoutofthemitochondrialmatrixascitrate:
CitrateShuttle
- TheinnermitochondrialmembraneisimpermeabletoAcetyl-CoA- Thecitratelyase reactionrequiresanequivalentofATP.- TheshuttleallowsAcetyl-CoAtobeshuttledtothecytosol,wherefattyacidsynthesiscanoccur.- TheshuttleconsumesoneequivalentofATP.- TheshuttlealsosubstitutesanNADPHforanNADH,whichisalsoneededforsynthesis.
• ThemalatedehydrogenaseandNADP+–linkedmalateenzymereactionsofthecitrateshuttleexchangeNADHforNADPH
SourcesofNADPH
MostoftheNADPHforthesynthesisofpalmitoyl-CoAstillcomesfromthepentosephosphate pathway(6molecules)andreductionofOAAtomalatefollowedbyoxidativedecarboxylationofmalatetopyruvate(8molecules).
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In cells reducing energy power is needed and provided by the reduced form ofNADPH. The processing of glucose through the Pentose Phosphate pathway producesNADPH which is needed, e.g., for synthesis of free fatty acids from acetyl-CoA, whichare components of myelin and other neuronal structural elements
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• RegulationofAcetylcarboxylase(keyenzyme)– Global
• +insulin• - glucagon• - epinephrine
– Local• +Citrate• - Palmitoyl–CoA• - AMP
RegulationofFattyAcidSynthesis
AcetylCarboxylase(ACC)isinhibitedbyphosphorylationandallosterically activatedbybindingofcitrate
• Synthesisanddegradationarereciprocallyregulated– starvation– degradationoccursbecauseepinephrine&glucagonstimulatelipolysis
– fedstate– insulininhibitslipolysis• ACCalsoinfluencesdegradation
– malonyl CoAinhibitscarnitine acetyltransferase• limitsbetaoxidationinmitocondria
• Long-termcontrolmediatedbysythesis anddegradationofkeyenzymes– adaptivecontrol
FattyAcidMetabolismOverview
of
fatty acid synthesis
and
degradation
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LipogenesisDigestion,Mobilization,and
Transport
MaintainingBloodGlucose
Homeostasis
IntestineWhenapersoneats,bloodglucoserises.
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2
Insulinstimulatestheuptakeofglucoseintocellsandstorageasglycogenintheliverandmuscles.Insulinalsostimulatestheconversionofexcessglucoseintofatforstorage.
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7 Bloodglucosebeginstorise.
a Thestresshormoneepinephrineandotherhormonesalsobringglucoseoutofstorage.
GlucoseInsulinGlucagonGlycogen
Glucagonstimulateslivercellstobreakdownglycogenandreleaseglucoseintotheblood.a
Liver
Lowbloodglucosestimulatesthepancreastoreleaseglucagonintothebloodstream.
Asthebody'scellsuseglucose,bloodlevelsdecline.
Glucagon
Pancreas
Fatcell
Liver
Muscle
Highbloodglucosestimulatesthepancreastoreleaseinsulin.
Pancreas
Insulin