Electron Transport Reaction in Mitochondria

Thebodygetsenergythroughtheoxidationoffoodsuchasglucoseandfattyacids.Thechemicalenergycontainedinthesefoodsisextractedandconverteduntilitreachesacommonform,thehigh-energyphosphatebondsofATP.ThehydrolysisofATPishighlyfavorableandiscoupledtoavarietyofenergeticallyunfavorableprocessestodrivethemforward.HowistheenergyofglucosecapturedandconvertedtomakeATP?Mostoftheenergyofglucoseorfattyacidsisextractedthroughoxidationtoproducethereducedhigh-energyelectroncarriersNADHandFADH2.Fromthere,theenergyistransferrednexttotheelectrontransportsystemassociatedwiththemitochondrialinnermembrane.Thischainincludesaseriesofproteincomplexesandnon-membranecofactorsthattransfertheelectronsfromNADHandFADH2inaseriesofredoxreactionsfromcarriertocarrier.Oxygenisthefinalelectronacceptorattheendofthechain,resultingintheproductionofwater.Theoxygenwebreath,andwhichistransportedbyhemoglobininthebloodtoallofthetissues,servesthispurposeandallowselectrontransporttooccur.Astheelectronspassthroughthechain,theytransfertheirenergytothecomplexes,whichusetheenergytopumpprotonsoutofthemitochondrialmatrix,creatingaprotongradientacrosstheinnermitochondrialmembrane.Thechemicalenergythatstartedwithglucose,andwastransferredtoNADHandFADH2,isthenconvertedtotheenergyofaconcentrationgradient.Theinnermitochondrialmembraneisimpermeabletoprotonsonitsown,sotheenergyoftheprotongradientisstable,waitingtoberecaptured.TheenergyisrecapturedbyATPsynthaseintheinnermitochondrialmembrane.Thisenzymeallowsprotonstoflowbackdowntheirconcentrationgradientacrossthemembrane,andintheprocessusestheenergyofthegradienttodriveATPsynthesis.Themovementoftheelectronsthroughelectrontransport,theprotongradientandATPsynthesisareallcoupledprocessesthatrequireeachothertooccur.ThecelldoesnotstoreenergyasATP,butonlyhasenoughATPonhandforitsimmediateenergyneeds.Ifelectrontransportceasesorisinhibited,thenATPsynthesisalsorapidlyhalts.ThisregulationensuresthatATPproductioncloselymatchestheneedsofthecell.GlycolysisandtheKrebscyclearealsocloselylinkedtotheenergyneedsofthecell.TheabundanceofATP,NADHandpathwayintermediatesregulateskeystepsinthesepathwayssothatareactivatedwhenenergyisrequiredtofeedtheelectrontransportsystemandtheyareinhibitedwhennotneededtosavemetabolicenergy.Ifoxygenisabsent,electrontransportandtheKrebscyclerapidlyhalt,leavingglycolysisandfermentationasthemainmeansofenergyproduction.Duringaerobicexercise,therapidconsumptionofATPleadstouseoftheprotongradienttomakemoreATP,increasedelectrontransporttoregeneratetheprotongradient,increasedoxygenconsumption,andincreasedactivityoftheKrebscycleandglycolysistosupplyhighenergyelectronstodriveelectrontransport.UncouplingagentsallowprotonstoflowacrossthemitochondrialmembranewithoutproducingATP.Thechemicalcompounddinitrophenol(DNP),forexample,cantransportprotonstoflowacrosstheinnermitochondrialmembranewithoutATPsynthase.Inthepresenceofdinitrophenol,energyisconsumedtopumpprotonsoutofmitochondria,butthisenergyisnotrecapturedinchemicalforminATP.Instead,thisenergyisreleasedasheat.Dinitrophenolwasonceusedasdietremedytoloseweightwithoutexerciseordiet,butthiscompoundisametabolicpoisonandresultedindeathsinwhenpurposefullygiventohumans.Proteinsalsocanactasuncouplingagentsinthemitochondria.Amitochondrialuncouplingproteinisfoundinbrownadiposetissue.AnincreaseintheactivityofuncouplingproteinsincreasesheatproductionbyallowingprotonstoflowdowntheirgradientwithoutmakingATPandmayserveasprotectionagainstcold,aswellasapotentialmeansofobesitycontrol.

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