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1 Chapter8.Energyandenergytransformations
Thechapter8outlineconsistsofthematerialthatwetalkedaboutduringthelecturesand
thematerialthatwasleftout.Thematerialthatwasleftoutispresentedheresoyouhad
thoroughcomprehensionofthematerialthatwedidtalkabout.Thematerialthatwasleft
outwillnotbeexplicitlytested.Therewego.
- Energyisabilityofthesystemtodowork.Theunitforenergyisthesameasthe
unitofwork–thejoule(J),oracalorie(Cal).
- Potentialandkineticenergy
- Conceptofaclosedandanopensystem:
o Athermodynamicallyclosedsystemisonethatcanexchangeheatandwork
withitssurroundings,butnotmatter.
o Athermodynamicallyopensystemcanexchangebothenergyandmatter.
o Spaceshipisaclosesystem
o Organismsareopensystems
o Livingcellsareopensystems
- Twolawsofthermodynamics
o Energyofuniverseisaconstant=aprincipleofconservationofenergy;
energycanbetransformedortransferredbutitcannotbecreatedor
destroyed.
o Energytransformationsareimperfect,someenergyisalwayslostduring
transformations;
o Thislossofenergyduringtransformationsexhibitsintheincreaseof
entropyandenthalpy.
o Entropyisanenergystoredinvariousmotionsofatomsandmoleculesin
thesystem.ThetermforEntropyisS.TheunitforentropyisJoulesper
temperatureinKelvin(J/K).
o Enthalpyisheatevolvedorabsorbedduringenergytransformation.
o Entropyoftheuniverseincreasesasaresultofenergytransformations.The
termforEnthalpyisH.TheunitforEnthalpyisjoule(J).
o Spontaneousprocessoccurswithoutanyinputofenergy(howeverlongit
takes);thusitisaprocess,orareaction,thatincreasestheentropyofthe
universe.
o Spontaneousprocessesareexothermicmeaningthattheenergyisreleased
inthecourseoftheiroccurrence.
o Foranon-spontaneousprocesstooccursomeenergyneedstobe
expended.
o Non-spontaneousprocessesareendothermicmeaningthattheenergy
needstobespentfortheseprocessestooccur.
- Gibbsfreeenergyisafunctionthatdescribesastateofasystemandrelatesto
itsabilitytodowork.
o Gibbsfreeenergyfunctionunitesentropyandenthalpy.
o Itisallenergythatisavailabletodowork,thus“freeenergy”
equals“availableenergy.”
o ThetermforGibbsfreeenergyisG.
o TheformulaforGibbsenergyisG=H-T*SwhereTistemperatureinKelvin.
2 -
-
o Youcanseefromthisformulathatalossoftheenergyintheformofhear
meansareductionintheabilityofthesystemtodowork.
o TheprocessesinthesystemthataredescribedbytheGibbsfreeenergyare
reversible.Thedirectionoftheprocess(forwardorreverse)isdetermined
bythechangeintheGibbsfreeenergyofthereaction.
o TheGibbsenergyisminimizedwhenasystemreacheschemicalequilibrium
atconstantpressureandtemperature.
Biologicalsystemsarehighlyorganized;cellscreateorderedstructuresfromthe
lessorderedmaterials;energyisspenttodothat.
o Complexlivingsystemsincreasetheirownorderbydecreasingentropyof
thesurrounding/universe.
o Inclosedsystems,reactionseventuallyreachequilibrium,noworkwhenis
possible,thusadeathwilloccur.
o Cellsareopensystemsthatarenotinequilibrium,buttheyareexperiencing
aconstantflowofmaterialsinandout.
o Adefiningfeatureoflifeisthatcellsconstantlyconductmetabolismthatis
neverinequilibrium.
o Metabolismcanbedefinedasaperpetualdisequilibrium.
AmetabolicreactionrepresentsasystemthatcanbedescribedintermsofGibbs
freeenergy.Changesintheenergyofasystemaredescribedintermsofchanges
inGibbsenergy.ThechangeinGibbsenergyofthesystemcanbepositiveor
negative.Forachemicalreaction,includingmetabolicreactions,thesignofthe
changeinGibbsfreeenergydetermineswhetherareactionisspontaneousornonspontaneous.
o ChangesinGibbsfreeenergyofthesystemaredescribedintermsof
correspondingchangesinentropyandenthalpy.
o AtermforachangeinGibbsfreeenergyisΔG.TheunitofΔGisCal/Mol
o AtermforchangeinenthalpyisΔS.
o AtermforchangeinentropyisΔH.
o AformulaforachangeinGibbsfreeenergyis
ΔG=ΔH-T*ΔS
o ThechangeoftheGibbsfreeenergyofthereactionisanultimate
determinantofthedirectionofthereaction.
o IfΔGforreactioniszero,thesystemisinequilibrium.
o IfΔGforreactionisnegative,thereactionwillgoforwardspontaneously.
Suchreactionissaidtobeexergonic.
o IfΔGforreactionispositive,thereactionwillgobackwards.Forsuch
reactiontogoforwardsomeenergywillneedtobespent.Thereactionfor
whichΔGispositiveissaidtobeendergonic.
o AnexergonicreactionevenifithasverynegativeΔGstillrequiresan
activationenergytobespentforthereactiontogetstarted.
o Inlivingcells,activationenergyisprovidedbytheenzymes.
o Anendergonicreactionrequiresnotonlytheactivationenergytobespent,
butalsotheadditionalenergyexpendituretoenablecreatingproduct
chemicalspeciespossessingtheGibbsfreeenergythatishigherthanthatof
theoriginalreactants.
3 o Thus,todeterminethecourseofreaction,oneneedstobeabletocalculatea
changeinGibbsfreeenergy.
o Cellsperformbothexergonicandendergonicreactionsusingenzymes.You
canseefromtwodiagramsabovethatenergyrequiredtopowerendergonic
reactionismuchhighercomparedtotheenergythatisrequiredtopoweran
exergonicreaction.
o Becausethecourseofreactionisalsodeterminedbytheells
o OnlytheprocesseswithnegativeΔGarespontaneous.
o Spontaneousprocessescanbeharnessedtoperformwork.
-
Metabolicreactionsinlivingsystemsareperformedbytheenzymes.
o Enzymesfunctionascatalyststhatfacilitatetransformationsofreactants
intotheproducts.
o IfGibbsfreeenergyfortheproductsislowerthanitisforthereactants,then
ΔGisnegativeandthereactionisspontaneous.However,itmighttake
millionsofyearsfortheproductstoform.Sothecellsuseenzymestospeed
upthereactions.
o Enzymesspeedupthereactionsbycreatingintermediateproducts,socalled
transitionstateintermediates.
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o TransitionstateintermediatespossessGibbsfreeenergythatishigherthan
thatofthereactants,thereforesomeenergyneedstobespentforthe
catalyzedreactiontogoforward.
o Enzymescoupleexergonic
o Incellsthisadditionalenergyusually,butnotalways,comesinformofATP.
o TotalΔGforanenzymaticreactionisasumofΔG0thatisaconstant
determinedforthestandardenzymaticconditions(astandardtemperature,
pH,pressure,andconcentrationofproductsandreactants),andasecond
termthatdependsonanaturallogarithmoftheconcentrationsof
productsandreactants.Thatrelationshipimpliesthatcellcanperform
highlythermodynamicallyunfavorablereactionsnotonlybyinvestingsome
energybutalsobyincreasingtheconcentrationofthereactants.
Metabolismincludecatabolismandanabolism.
o Aseriesofmetabolicreactionsconstituteametabolicpathway.
o Metabolicpathwayscanbecatabolicoranabolic.
o Acatabolicpathwayincellreleasesfreeenergyinaseriesofreactions.This
energyisharvestedandthenusedtodoworkinacell.
o Acatabolicpathwayisexothermic.
o Ananabolicpathwayinacelltransformsexistingmoleculesandcreatesnew
molecules.Itusesenergyreleasedduringcatabolicreactionstodothis
work.
o Ananabolicpathwayisendothermic.
o Aworkdonebyacellincludes:
§ MakingmacromoleculessuchasDNAandproteins.
§ Movingmoleculesaround.
§ Conductingactionpotentialsinnervoussystem.
§ Celldivision.
§ Cellgrowth.
§ Secretionofproductsmadeincell.
o Enzymescouplecatabolicandmetabolicreactionstodowork.
Chapter9.Bioenergetics-theenergeticsoflivingsystems.
- Theenergeticsoflivingsystemsincludestransformationofoutside-systemenergy
(suchassunlight,chemicalreactions,orheat)intotheenergyofchemicalbondsby
theautotrophs,followedbytheutilizationofthisenergyofchemicalbondsbythe
heterotrophs.
o Autotrophsincludephotoautotrophsandchemoautotrophs.
o Photoautotrophsaregreenplantsandalgaethatutilizesunlight.
o Chemoautotrophsareprokaryotesbacteriaandarchaeathatobtaintheir
energybyoxidationofinorganiccompounds.
o Heterotrophsobtainenergybyoxidationoforganic(carbon-containing)
compoundscreatedbyautotrophs.
- Oxidationislossofelectron(s)byachemicalspecies(suchasanatomoranion).
Againofelectron(s)byachemicalspeciesiscalledareduction.
o Oxidationandreductionreactionsarealwayscoupledtogether.Theyare
twosidesofthesamecoin:thereisnooxidationwithoutreduction,andvise
versa.ThisiswhythesekindofchemicaltransformationsarecalledRedOx
reactions.
5 o Aspeciesthatwasoxidizedservedasanelectrondonor.
o Aspeciesthatwasreducedservedasanelectronacceptor.
o Theconceptofelectrondonor
andelectronacceptordoesnot
requireawholeelectrontobe
transferred.Rather,itisa
differenceinrelative
electronegativitythatdefines
whetheramemberofapairisa
donororanacceptor.
o Electronegativityisameasure
ofthetendencyofanatomto
attractabondingpairof
electrons.
o Electronegativityispossiblebecausetheelectronpairssharedbetween
twoatomsarenotnecessarilysharedequally,forexampleinCl2the
sharedelectronpairsissharedequallywhileinNaClthe3selectronis
strippedfromtheNaatomandisincorporatedintotheelectronicstructure
oftheClatom.
o Aspeciesthatwasoxidizedoftencanbeoxidizedfurther.
o Aspeciesthatwasreducedoftencanbereducedfurther.
o Substancesvaryintheirtendencytobecomeoxidizedorreducedi.e.to
donateoracceptelectrons.
o Ameasureofatendencyofachemicalspeciestoacquireelectronsand
therebybereducedisthereduction(orredox)potentialofasubstance.
Theunitofredoxpotentialismillivolts(mV).Oxygenhasveryhighredox
potential.
o Thehighertheelectronegativitythehighertheredoxpotentialofchemical
species.
o Manyspeciescanactasbothelectrondonorsorelectronacceptors
dependingontheenvironmentandothersubstancestheyreactwith.
o Electronsflowfromdonorswithlow(ormorenegative)redoxpotentialto
acceptorswithhigh(ormorepositive)redoxpotential.
o Becauseonlyareductionhalfofaredoxreactionisconsideredinaredox
potentialconcept,onlyaredoxhalfofacompleteredoxreactionistaken
intoaccountwhencomparingredoxpotentialofchemicalspecies.
o Aredoxhalf-reactionisonewhichshowseitherreductionORoxidation,but
notboth.
o Considerareaction2Mg(s)+O2(g)→2MgO(s)
Sincethereare2Mgonleftside,atotalof4electronsarelostaccordingto
thefollowingoxidationhalfreaction:
2Mg(s)→2Mg2++4e−
Ontheotherhand,O2wasreduced:itsoxidationstategoesfrom0to-2.
Thus,areductionhalf-reactioncanbewrittenfortheO2asitgains4
electrons:
O2(g)+4e−→2O2−
Theoverallreactionisthesumofbothhalf-reactions:
2Mg(s)+O2(g)+4e−→2Mg2++2O2−+4e−
6 However,onlythereductionhalfreactionO2(g)+4e−→2O2−
isconsideredwhencomparingoxygenredoxpotentialwiththatofanother
chemicalspecies.Thehalf-reactionapproachallowstorankchemical
speciesbytheirredoxpotential.Youcanseeinthetablebelowthatoxygen
isoxidizedinthereactionthatleadstothereductionofCrO2toCrmetal.
o Chemoautotrophsuseelectroncarrierstotransferelectronsfroma
reductanttoanacceptorwithamorepositive(higher)reductionpotential,
andtheytherebyallowthereleaseoffreeenergy,whichisoftenusedinthe
formationofATP.
o TheElectrontowerorganizedthereduction
potentialsofhalfreactionsinawaythatallows
visualizingasequenceofmetabolicreactions
leadingtosequentialreleaseoftheenergy
storedinthechemicalbondsofglucose.
o Considerburningglucoseinoxygen–this
processisveryfastandreleasesalotofheat.
Inlivingcells,thesameprocessishappening
veryslowlyutilizingametabolicpathway
calledcellularrespiration.
o Theenergeticsoflivingsystemsisbasedona
step-wiseoxidation(akindofthestaircasetowalkdownthetower)of
organiccompounds,mainlyglucose.
o AconceptofElectronTowerhelpstovisualizethatthegreaterthefallof
electronsthemorepotentialenergythatcanbeharvestedinthebalanced
reaction.
7 Electron
donors
Electron
acceptors
-Thereactionformulaforglucosecombustionreactionis
C6H12O6+6O2--->6CO2+6H2O+energy-thesameasaforthecellular
respirationreaction.Duringthecellularrespiration,thefuel(glucose)isoxidized,
andtheoxygenisreduced.
o GlucoseoxidationishighlyexergonicreactionwithΔG-686Kcal/Mol.
o Whyitisthatbeingsuchanexergonicreaction,glucosecombustiondoes
happenspontaneouslyinthepresenceofoxygen?
o Asmentionedabove,theactivationenergyisneeded.
- Whyitisthatthelivingcellsarecapableofreleasingenergyslowlyinmultiple
smallsteps?
8 o Becauselivingcellshaveavarietyofelectroncarriers,andeachisusedin
particulartypesofredoxreactions;theparticularcarrierusedinanygiven
reactionwilldependonthenatureandlocationofthereaction.
o Electroncarriersaremoleculesthatarejustbelowtheglucoseinthe
electrontowerandincludeNAD+,NADP+,andFAD.
o Duringvariousstepsofglucoseoxidation,NAD+,NADP+,andFADare
reducedtoNADH,NADPH,andFADH2.
o Theenergystoredinelectroncarriersisstoredtobeharnessedlaterinthe
formofATPthatservesasanenergycurrencyinlivingcells.ATPpossesses
high-energyphosphatebondsthatreleaseenergypertheirhydrolysis.
-
Cellularrespirationincludesthreesteps:
o Glycolysis
o Krebscycle
o Oxidativephosphorylation
9 - Glycolysisisthefirststepinglucosecatabolism.
o Glycolysisisauniversalpathway;itpresentinallorganisms:fromyeastto
mammals.
o Ineukaryotes,glycolysistakesplaceinthecytosol.
o Glycolysisisanaerobic;itdoesnotrequireoxygen.
o Netreactionofglycolysisis:
Glucose+2NAD++2Pi+2ADP=2pyruvate+2ATP+2NADH+2H2O
-ATPIsInitiallyRequired(Investmentstage,ATPisconsumedinsteps1and3):
o Reaction1:Phosphorylationofglucosetoglucose-6phosphate.
o Thisreactionrequiresenergyandsoitiscoupledtothe
hydrolysisofATPtoADPandPi.
o Enzymehexokinasephosphorylatestheglucoseandthusglucosebecomes
highlycharged.Chargedglucoseisunabletocrossthecellmembrane
anymore.Thus,onceglucoseentersthecell,itgetsphosphorylated
andtrapped.
o Thisstepisirreversible.Glucosetransporterstransportonlyfreeglucose,
notphosphorylatedglucose.
o Reaction2:Isomerizationofglucose-6-phosphatetofructose6-phosphate.
o Thisisareversiblereaction.Thefructose-6-phosphateisquickly
consumedandtheforwardreactionisfavored.
o Reaction3:isanotherkinasereaction.Phosphorylationofthehydroxyl
grouponC1formingfructose-1,6-bisphosphate.
o Enzyme:phosphofructokinase.Thisallostericenzymeregulatesthepace
ofglycolysis.
o ReactioniscoupledtothehydrolysisofanATPtoADPandPi.
o Thisisthesecondirreversiblereactionoftheglycolyticpathway.
o Reaction4:fructose-1,6-bisphosphateissplitinto23-carbonmolecules,
DHAPandGAP.
o Reaction5:DHAPandGAPareisomersofeachotherandcanreadilyinterconvertbytheactionoftheenzymetriose-phosphateisomerase.
o GAPisasubstrateforthenextstepinglycolysissoalloftheDHAPis
eventuallydepleted.So,2moleculesofGAPareformedfromeach
moleculeofglucose
o Uptothisstep,2moleculesofATPwererequiredforeachmoleculeof
glucosebeingoxidized
- ATPisProducedinthepayoffstageofglycolysis(steps7and10)
o Theremainingstepsreleaseenoughenergytoshiftthebalancesheettothe
positiveside.Thispartoftheglycolyticpathwayiscalledasthepayoffor
harveststage.
o Sincethereare2GAPmoleculesgeneratedfromeachglucose,eachofthe
remainingreactionsoccurtwiceforeachglucosemoleculebeingoxidized.
o Reaction6:GAPisdehydrogenatedbytheenzymeGAPDH.Intheprocess,
NAD+isreducedtoNADH+H+fromNAD.TheproductisBPG.
o Reaction7:BPGhasahigh-energybondatC1.Thishigh-energybondis
hydrolyzedandtheenergyreleasedisusedtogenerateATPfromADP.
Product:3-phosphoglycerate.
o Reaction8:ThephosphategroupisshiftedfromC3toC2toform2-
phosphoglycerate.Enzyme:phosphoglyceratemutase.
o Reaction9:Dehydration;awatermoleculeisremovedtoform
phosphoenolpyruvate.
10 o Inglycolysis,theATPisproducedbythesubstratelevelphosphorylation
thatisaccomplishedbythetransferofaphosphategroupfromasubstrate
totheADP.MechanismofATPproductionisdifferentduringtheoxidative
phosphorylationwhereitisproducedbytheactionofenzymeATP
synthase.
o Reaction10:Enolphosphatehasahighenergybond.Itishydrolyzedto
formpyruvatewiththesynthesisofATP.Thisirreversiblereactionis
catalyzedbytheenzymepyruvatekinase.
-Fateofpyruvatedependsontheavailabilityofoxygen.
o Duringtheglycolysis,glucoseissplitintotwo3-carbonmoleculesof
pyruvate.
o NADHisformedfromNAD+duringglycolysis.
o Theredoxbalanceofthecellhastobemaintainedforfurthercyclesof
glycolysistocontinue.
o Therefore,NAD+shouldberegenerated.
o NAD+canberegeneratedbyoneofthefollowingreactions/pathways:
§ Pyruvateisconvertedtolactate
§ Pyruvateisconvertedtoethanol
§ InthepresenceofO2,NAD+isregeneratedbyETC.Pyruvateis
convertedtoacetylCoAwhichentersTCAcycleandgetscompletely
oxidizedtoCO2.
- LactateFermentationinmuscles
o LactateisformedintheactivemuscletoregenerateNAD+fromNADHso
thatglycolysiscancontinue.
o Inhighlyactivemuscle,thereisanaerobicglycolysisbecausethesupplyof
O2cannotkeepupwiththedemandforATP,lactatebuildsupcausinga
dropinpHwhichinactivatesglycolyticenzymes;thesymptomsbeingpain
andfatigueofthemuscle.
o ThemusclecannotspareNAD+forre-conversionoflactatebackto
pyruvate.
o Lactateistransportedtotheliverwhereitcanbereconvertedtopyruvate
bythereversereactionandpyruvatethenconvertedtoglucosethroughthe
gluconeogenesis.
o Theglucoseissuppliedbythelivertovarioustissuesincludingmuscle.
o Thisinter-organcooperationduringhighmuscularactivityiscalledasthe
Coricycle.
-SubstratecycleorFutilecycle
o Inasubstratecycle,thereisexpenseofATPwithoutacoupledbiosynthetic
reaction,thus,itisalsocalledasafutilecycle
o Exampleincludestep2ofglycolysis
• F-6-P+ATP(PFK)F-1,6-BP+ADPF-1,6-BP+H2OFBPaseF-6-P+Pi
• Net:ATP+H2OADP+Pi+energy(heat)
o Thisreactionsisutilizedinbrownfatofmammalstomaintainbody
temperature.
-Citricacidcycle;AKAKrebscycle;AKATCAcycle.
- Ifoxygenisavailable,thenextstep(aftertheglycolysisiscompleted)isthe
formationofacetylcoenzymeA(acetylCoA)whichistheinitiatorofthecitricacid
cycleandlinkscitricacidcycletotheglycolysis.
11 -
-
InthepresenceofO2thepyruvateCH3COCOO−istransportedintothe
mitochondrionaidedbyaspecialtransportprotein.Pyruvateisacharged
moleculeandcannotfreelycrossthemembrane.
OneenzymePDHcatalyzes3steps:(1)theCO2isremoved,(2)NAD+isreducedto
NADH,and(3)theremainingacetateislinkedwithCoenzymeAtoformAcetylCoA
CoAisaderivativeofavitaminB5whosedeficiencycausesBeriberidisease.
TheAcetylCoApossessesaveryhighenergybond(indicatedbyawavyline)that
poisestheAcetylCoAatthetopoftheenergyhillandreadytogodownhillinthe
subsequentreactionsofgradualstep-wiseoxidationthattheKrebscycleis.
Itisacyclicpathwaythattakesplaceinsidethemitochondrialmatrix.All
intermediatesarederivativesofcitricacid,atricarboxylicacid.
Totalof8steps.Allexcept3arereversibl.e
AcetylCoA,a2carbonmoleculeentersthecyclebycondensationwiththe4-
carbonoxaloacetate.EachofthetwocarbonsofacetylCoAareoxidizedand
removedasCO2intwoseparatereactions.Oxaloacetateisregenerated.
3moleculesofNAD+arereducedto3moleculesofNADH+H+
onemoleculeofFADisreducedtoFADH2
onemoleculeofGDPisphosphorylatedtoGTP.Thisiscalledsubstratelevel
phosphorylation(asopposedtooxidativephosphorylation).
o OxidationofpyruvatetoacetylCoAiscatalyzedbythepyruvate
dehydrogenasecomplexinthemitochondria
o Mitochondriaconsistofinnerandoutermembranesandthematrix
o EnzymesofthePDHcomplexandtheTCAcycle(exceptsuccinate
dehydrogenase)areinthematrix
o Aspecialtransporterpresentintheinnermitochondrialmembraneallows
entryofamoleculeofpyruvateinexchangeforahydroxideion
o ThefirstreactionisanactivationofpyruvatebytheactionofthePyruvate
Dehydrogenase(PDH)complex.
o InPDHreactiononecarbonofpyruvateisoxidizedtoCO2andoneNADHis
formed.
12 -
o ThePDHstepisirreversible;asaresult,animalsarenotabletosynthesize
glucosefromacetylCoA(fat)
Overallreaction:AcetylCoA+3NAD++FAD+GDP+Pi+2H2O2CO2+CoASH+
3NADH+2H++FADH2+GTP
TheTCAcycleisregulatedatthe3irreversiblestepsconductedbytheenzymes
citratesynthase,isocitratedehydrogenaseandα-ketoglutaratedehydrogenase.
-AnabolicroleofTCAcycle.
- IntermediatesoftheTCAcycleserveasprecursorsforbiosynthesisofvarious
biomolecules
- Manyaminoacidsaresynthesizedstartingwithtransaminationofα-ketoglutarate
oroxaloacetate
- PorphyrinsandhemearesynthesizedfromsuccinylCoA
- Oxaloacetateisalsotheprecursorofpurinesandpyrimidines
- Fattyacidsandsterolsaresynthesizedfromcitrate
-Oxidativephosphorylation.
- TheNADHandFADH2,formedduringglycolysis,β-oxidationandtheTCAcycle,
giveuptheirelectronstoreducemolecularO2toH2O.
- Electrontransferoccursthroughaseriesofproteinelectroncarriers,thefinal
acceptorbeingO2;thepathwayiscalledastheelectrontransportchain,ETC.
- ETCtakesplaceininnermitochondrialmembranewherealloftheelectroncarriers
arepresent.
13 -
ThefunctionofETCistofacilitatethecontrolledreleaseoffreeenergythatwas
storedinreducedcofactorsduringcatabolism.
Energyisreleasedwhenelectronsaretransportedfromhigherenergy
NADH/FADH2tolowerenergyO2
ThisenergyisusedtophosphorylateADP.
ThiscouplingofATPsynthesistoNADH/FADH2oxidationiscalledoxidative
phosphorylation.
Oxidativephosphorylationisresponsiblefor90%oftotalATPsynthesisinthecell.
-TheChemiosmoticTheory
- Explainsthemechanismofoxidativephosphorylation.
- WhenelectronsaretransportedalongthecomponentsoftheETC,the
accompanyingprotonsarereleased.
- PartofthefreeenergyharvestedduringtheETCisusedtopumpprotonsoutofthe
mitochondrialmatrix.
- TheresultingunevendistributionofprotonsgeneratesapHgradientandacharge
gradientacrosstheinnermitochondrialmembrane.
- Theelectrochemicalpotentialenergygeneratedbythesegradientsiscalledas
ProtonMotiveForce.
- ThereturnofprotonstothemitochondrialmatrixiscoupledtoATPsynthesis.
- ProtonsreturnthroughtheanintegralmembraneproteinATPsynthasethat
harnessestheenergyoftheprotonstosynthesizeATPfromtheADTandinorganic
phosphate.
- ATPsynthaseisamultiplesubunitcomplexthatbindsADPandinorganic
phosphateandconvertsthemtoATP
- AsignificantamountofenergyisuseduptopumpH+outofthemitochondria.Only
athirdisusedforATPsynthesis.
-MitochondriaareBiochemicalHubs
- ThemitochondrialmatrixcontainsenzymesofPDH,TCAcycle,β-oxidationand
aminoacidoxidation.
- Mitochondrialmatrixisenclosedbytwomembranes.
- ComponentsoftheETCarelocatedontheinnermembrane;thefoldedcristae
providealargesurfacearea.
- Theinnermembraneishighlyimpermeableandrequiresspecifictransporters.
- Transportersspecificforpyruvate,fattyacids,aminoacids,ATP/ADP,phosphate
andprotonsarefoundintheinnermembrane.
- Theoutermembraneispermeabletosmallmoleculesandionsduetothespecial
transmembraneproteinsthatformchannelsintheoutermembrane.
- Thereareproteinsspecializedtoinhibitoxidativephosphorylationbyuncoupling
ETCfromoxidativephosphorylation.Uncouplerscarryprotonsacrossthe
mitochondrialmembranemakingit‘leaky’forH+.ThepHandelectricalgradientis
notgeneratedandATPisnotsynthesized.
- Inthepresenceofanuncouplingagent,energyreleasedviatheETCisconverted
intoheat.
- Thismechanismisusedbyhibernatinganimalstostaywarminthewinter,since
theydon’tneedATPforanabolicprocesseswhiletheyareresting.
- Weusethismechanismtogenerateheatinourbrownfat.
14 -TransportofNADHintomitochondria
- GlycolyticpathwayresultsinthereductionofNAD+toNADHinthecytosol
- NADHisoxidizedtoNAD+bytheETCinthemitochondria
- ThemitochondrialmembraneisimpermeabletoNADH,thus,atransportsystem
wouldberequiredtoallowentrytoNADHintothemitochondrialmatrix
- InsteadofNADHmoleculedirectlyenteringthemitochondria,thereareelectron
shuttlesystemsthatacceptelectronsfromcytosolicNADH,entermitochondria,
andgiveuptheelectronstoelectronacceptorsinthemitochondrialmatrix
-CompleteOxidationofglucose
- Completeoxidationofglucoseinvolvesthefollowingpathwaysandnetreactions:
- Glycolysis:glucose+2ADP+2Pi+2NAD+2pyruvate+2ATP+2NADH+2H++
2H2O
- PDHcomplex:2pyruvate+2CoA+2NAD+2acetylCoA+2CO2+2NADH
- TCAcycle:2acetylCoA+6NAD++2FAD+2GDP+2Pi+4H2O4CO2+6NADH+
4H++2FADH2+2GTP+2CoA
- Overalloxidation:glucose+2ADP+2GDP+4Pi+8NAD++2FAD+2H2O6CO2+
2ATP+2GTP+8NADH+6H++2FADH2