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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

1

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.

1

2

Insulinstimulatestheuptakeofglucoseintocellsandstorageasglycogenintheliverandmuscles.Insulinalsostimulatestheconversionofexcessglucoseintofatforstorage.

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4

5

6

7 Bloodglucosebeginstorise.

a Thestresshormoneepinephrineandotherhormonesalsobringglucoseoutofstorage.

GlucoseInsulinGlucagonGlycogen

Glucagonstimulateslivercellstobreakdownglycogenandreleaseglucoseintotheblood.a

Liver

Lowbloodglucosestimulatesthepancreastoreleaseglucagonintothebloodstream.

Asthebody'scellsuseglucose,bloodlevelsdecline.

Glucagon

Pancreas

Fatcell

Liver

Muscle

Highbloodglucosestimulatesthepancreastoreleaseinsulin.

Pancreas

Insulin