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3.2ATP: Energy Currency of the CellThousandsof reactions takeplace in livingcells.Manyreactions require theaddi-tionofenergyfortheassemblyofcomplexmoleculesfromsimplereactants.ThesereactionsincludeDNAsynthesis,proteinsynthesis,andtheconstructionofcellwallsandothercellularstructures.Othercell-drivenactions—suchasmusclecontractionsinanimals, themotionofflagella inbacteria(Figure 1),andthemovementofsapwithinatree—alsorequireasupplyofenergy.
Since somanycellular functionsrequireenergy,cellsneedaconstant supplyofenergy.EventhoughthespeciesonEarthareverydiverse,allcellsineveryorganismuse the same energy carrier for almost all of their energy-driven actions. Thisenergy comes in the form of a compound called adenosine triphosphate, or ATP.ATPdirectlysuppliestheenergythatpowersnearlyeverycellularfunction,anditisconsideredtheuniversalenergy“currency.”ThetypesofworkthatarecarriedoutbyATPincludemechanical,transport,andchemicalwork(Table 1).
Table 1 Types of Work Performed by ATP
Mechanical work Transport work Chemical work
• beating of cilia or movement of flagella
• contraction of muscle fibres
• movement of chromosomes during mitosis/meiosis
• process of pumping substances across membranes against their concentration gradient
• process of supplying chemical potential energy for non-spontaneous, endergonic reactions, including protein synthesis and DNA replication
Figure 1 A bacterium with flagella
ATP Hydrolysis and Free energyAdenosine triphosphate (ATP) consists of three parts: a nitrogenous base calledadenine,whichislinkedtoafive-carbonsugarcalledribose,whichinturnislinkedto a chain of three phosphate groups (Figure 2, next page). ATP contains largeamountsoffreeenergy.Theenergyofthemoleculeishighbecauseofitsthreenega-tively charged phosphate groups. The phosphate groups crowd together, and theircloseproximitycreatesamutualrepulsionoftheirelectrons.Themutualrepulsioncontributestotheweaknessofthebondholdingthegroupstogether.ThebondsofATPareeasilybrokenbyacatalyzedreactionwithwater—aprocesscalledhydrolysis(Figure2).Thehydrolysisreactionresultsinthebreakingoffoftheend(orterminal)phosphategroupandtheformationoftwoproducts—adenosinediphosphate(ADP)andaninorganicphosphate(Pi).Inaddition,anH1ionisreleasedintothesolution.Asbondsinthesenewproductsform,freeenergyisreleased.
ATP 1 H2O S ADP 1 Pi∆G5 –30.5kJ/mol
Note that the H+ ion is not normally shown in the chemical equation, since it isunderstoodtobeassociatedwiththeformationofPi.
Recall, from Section 3.1, that during a chemical reaction, bonds in the reactantsbreakandnewbondsintheproductsform.DuringATPhydrolysis,bondsformwhenanew—OHgroupattachestothephosphorusatomofthephosphategroupandwhenanelectronattachestotheoxygenthatremainsontheADPmolecule.EnergyisalsoreleasedastheH+ioninteractswithwatermolecules.Thebondrearrangementsandthechangeinentropyresult inanoverallfreeenergychangeof–30.5kJ/mol.WhenATPsplitsintoADPandPiwithinacell,thephosphategroup,ratherthanremainingfree insolution,oftenbecomesattachedtoanothermolecule,whichresults inadif-ferentbondingarrangement.
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ATP and energy CouplingInaprocesscalledenergycoupling(Section3.1),ATPcanbemovedintoclosecon-tactwitha reactantmoleculeofanendergonic reaction.Then,during thereaction,theterminalphosphategroupbreaksawayfromtheATPandtransferstothereactantmolecule.Attachingaphosphategrouptoanotherorganicmoleculeisaprocesscalledphosphorylation.Itresultsinthemoleculegainingfreeenergyandbecomingmorereac-tive.EnergycouplingrequiresanenzymetobringtheATPmoleculeclosetothereactantmoleculeoftheendergonicreaction.TherearespecificsitesontheenzymethatbindboththeATPmoleculeandthereactantmolecule.Inthisway,thetwomoleculesarebroughtclosetooneanother,andthetransferofthephosphategrouptakesplace.
Mostoftheworkcarriedoutinacellisdependentonphosphorylationforenergy.Anexampleofenergycouplingthatiscommontomostcellsisthereactioninwhichammonia, NH3, is added to glutamic acid (Figure 3(a), next page). The resultingproductof thisreactionisglutamine,which isanaminoacid.Thereactioncanbewrittenasfollows:
glutamicacid 1 ammonia S glutamine 1 H2O∆G5 114.2kJ/molTheglutaminethatisproducedtakespartintheassemblyofproteinsduringpro-
tein synthesis.Thepositivevalueof∆G shows that the reaction isendergonicandcannot proceed spontaneously. Therefore, the coupling of this reaction with ATPhydrolysis gives it the necessary energy to proceed. As a first step, the phosphategroup is removed from the ATP and transferred to the glutamic acid molecule,formingglutamylphosphate(Figure 3(b),nextpage):
glutamicacid 1 ATP S glutamylphosphate 1 ADP∆G , 0Thechangeinfreeenergy,∆G,isnegativeforthisreaction.Thismeansthatthereac-tionisexergonicandcanproceedspontaneously.Inthesecondstepofthereaction,glutamylphosphatereactswithammonia:
glutamylphosphate 1 ammonia S glutamine 1 Pi(inorganicphosphate)∆G , 0
phosphorylation the transfer of a phosphate group, usually from ATP, to another molecule
P i ADP
energy
O– P
O
O–
O P
O
O–
ATP
O P O
O
O–
HO
CH2 O
H
C
H
CH
C
OH
H
C
N
CHCN
CC
N
HC
N
NH2
O– P
O
O–
OH � H� � –O P
O
O–
P O
O
O–
HO
CH2 O
H
C
H
CH
C
OH
H
C
N
CHCN
CC
N
HC
N
NH2
O
sugar (ribose)
nucleotidebase (adenine)
H2O
C03-F13-OB12USB
Crowle Art Group
Deborah Crowle
2nd pass
Biology 12
FN
CO
Pass
Approved
Not Approved
three phosphate groups
Figure 2 ATP releases large amounts of free energy during a hydrolysis reaction as new bonds form in the products. The reaction results in the addition of a new OH group to a released phosphate as well as the addition of an electron to the terminal oxygen on the ADP and the release of an H+ ion into solution.
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Thesecondstepof thisreactionalsohasanegative∆Gvalue,so italsoproceedsspontaneously. The overall ∆G value for the two-step reaction is 216.3 kJ/mol(Figure 3(c)).Thisexamplerepresentsatwo-stependergonicreactionthatiscoupledto ATP hydrolysis, but there are hundreds, thousands, or even millions of stepsinvolvedintheassemblyofhugeproteinmoleculesorthesynthesisofanentireDNAgenome.Theprocessisanalogoustobuildingahugewall,onebrickatatime.Thisstep-by-stepapproachiswhatenablescells toperformthese largetasks.ATPmol-eculesprovideenergyeverystepoftheway.
Figure 3 Energy coupling using ATP: (a) Ammonia is added to glutamic acid to form glutamine. This is a non-spontaneous, endergonic reaction. (b) In the presence of ATP, a phosphate is transferred to glutamic acid. This forms glutamyl phosphate, which spontaneously reacts with ammonia to form glutamine. (c) ATP provides the free energy for the overall reaction and allows it to occur spontaneously.
�G � �14.2 kJ/mol
�G � �14.2 kJ/mol
�G � �30.5 kJ/mol
net �G � �16.3 kJ/mol
glutamicacid glutamine
glutamyl phosphate
ammonia
(c) free energy change with ATP
�
� �
��
(b) with ATP hydrolysis
(a) without ATP
Glu � NH3 Glu � NH2
ATP
Glu
Glu
Glu
GluADP �G � 01
2
ADP � Pi
NH2
NH3
NH3
ATP
Glu
Glu
NH2
�G � 0
P
P i
P
regeneration of ATPForcellstokeepfunctioning,theymustregenerateATPmolecules.Intheprevioussection,youlearnedhowthehydrolysisofATPintoADPandinorganicphosphate,Pi,isanexergonicreactionthatcanbecoupledwithendergonicreactionstodrivethemforward spontaneously. ATP coupling reactions occur continuously in living cellsand,consequently,anenormousnumberofATPreactionsarerequired.
CellsgenerateATPbycombiningADPwithPi.IfATPhydrolysisisanexergonicreaction,thenthereverseprocess,ATPsynthesisfromADPandPi,isanendergonicreaction.Therefore,ATPsynthesisrequirestheadditionoffreeenergy.TheenergyneededtodriveATPsynthesisusuallycomesfromtheexergonicbreakdownofcom-plex molecules containing an abundance of free energy. These complex moleculesareinthefoodsweeat:carbohydrates,fats,andproteins.Allofthesemoleculesaresourcesoffreeenergy.InChapter5,youwilllearnthatlightenergyisalsoaprimarysourceofenergyusedtodriveATPsynthesis.Atleast10milliontimeseverysecond,ATPmoleculesarehydrolyzedandresynthesizedinatypicalcell,illustratingthatthis
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ATP is the universal energy CurrencyThereisverylittleATPinourdiet,andyetwerequireitasaconstantenergysourcewithinourcells.ATPhydrolysisreleasesenergyobtainedfromthefoodsweeat,andthenmoreenergyfromfoodisusedtoreassembletheATP.IfcellsconstantlyneedtouseenergyfromfoodtoreassembleATP,thenwhydocellsuseATPastheirenergycurrencytobeginwith?Itwouldseemthatcellscouldjustusethefoodmoleculesdirectlyassourcesofenergy.CellsuseATPasanimmediatesourceofenergybecauseithasspecificpropertiesthatareimportantforthebiochemicalreactionsthatallowpropercellfunctioning.
ATPprovidesamanageableamountofenergy,andcouplesinverysimilarwaystothousandsofdifferentreactionsinourcells.ThisabilitytocoupletosomanydifferentendergonicreactionsgivesATPits“universal”characteristic.ThewidespreaduseofATPinalllivingthingsisanevolvedcharacteristic.Manyothermoleculesareenergyrich,buttheyvaryinsizeandshape,intheamountofenergytheyrelease,andinthetypesofreactionstowhichtheycancouple.Also,theavailabilityofthesemoleculesisnotalwaysreliableorpredictable.Ifacertainreactionrequiredtheuseofapar-ticularfoodmolecule—forexample,glucose—asanenergysourcetodriveacoupledreaction,andthecelldidnothaveanyglucose,itcouldnotpowerthegivenreaction,eveniftherewereotherenergy-richmoleculesinthecell.TheabilitytoassembleATPusingtheenergyfromavarietyoffoodmoleculesensuresthatallvitalreactionsinthecellcanbeperformed.Complexfoodmoleculesalsorequirenumerousreactionstoreleasetheirenergy,butATPcanbecreatedandaccessedimmediately.TheonlyrequirementisthatatleastoneofthesefoodsourcesisavailableforgeneratingATP.AlthoughATPistheenergycurrencyofcellsandistheimmediatesourceofenergytodrivecellularprocesses,it isnottheonlyenergycarrierincells.Thereareotherphosphatecarriers,suchasguanosinetriphosphate(GTP),thatareusedspecificallyascarriersofhigh-energyelectrons.
Inthissection,youlearnedthatcellscanuseATPasasourceofenergytodriveendergonicreactions.Thereare,however,otherfactorsthatinfluenceacell’sabilitytoperformendergonicandevenexergonicreactions.Criticalamongthesefactorsisthe need to overcome the activation energy requirements for a particular reaction(Section3.1).Iftheactivationenergyrequirementisveryhighbecausethebondsinthereactantsareverystrong,areactionwillnotstartevenifitisexergonicoverall.Asyouwilllearninthenextsection,someproteinscalledenzymesplayanimportantroleinloweringtheactivationenergyforcertainreactions.
Figure 4 The ATP cycle couples reactions that release free energy (exergonic) to reactions that require free energy (endergonic).
energy
Exergonic reactions supply energyfor endergonic reaction producing ATP.
Exergonic reaction hydrolyzing ATP providesenergy for endergonic reactions in the cell.
energy
ADP�
ATP/ADPcycle
H2O H2O
P i
ATP
ATP cycle the cyclic and ongoing breakdown and re-synthesis of ATP
cycleoperatesatanastonishingrate.Infact,ifATPwerenotformedinthecellfromADPandPi,theaveragehumanwouldneedabout75kgofATPperday.Thecon-tinuedbreakdownandresynthesisofATPisaprocesscalledtheATP cycle(Figure 4).
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Summary
• ATPsuppliesenergydirectlytochemicalreactionsinallcells.Itistheuniversalenergycurrencyinlivingorganisms.
• ATPhydrolysisresultsintheformationofADPandPi,andreleasesalargeamountoffreeenergyintheprocess.
• ATPhydrolysiscanbecoupledtoendergonicreactionsinacelltopowerhundredsofreactions.Thisprocessiscalledenergycoupling.
• Phosphorylation,theprocessofattachingaphosphategrouptoanotherorganicmolecule,causesthemoleculetogainfreeenergyandbecomemorereactive.
• ATPisregeneratedfromADPandPiduringpartoftheATPcycle.
Review3.2
Questions
1. ProvideanexampleofauseofATPenergyforeachofthefollowingtasks:(a) chemicalwork(b) mechanicalwork(c) transportwork K/u
2. WhatdoestheuniversalityofATPinalllivingspeciessuggestabouttherelationshipofspeciestooneanother? A
3. (a) DescribethestructureofanATPmolecule.(b) HowdoesthestructureoftheATPmolecule
relatetothelargeamountsoffreeenergyitcontains? K/u
4. DescribetheprocessofATPhydrolysis. K/u
5. DuringthehydrolysisofATP,energyisreleased,butmostofthemolecule,theADPportion,remainsintact.Howdoesthiscomparetowhathappenstoamoleculeofglucosewhenitisusedasanenergysource? T/i
6. Withaclassmate,explaintherelationshipbetweentheATPcycleandthecouplingofexergonicandendergonicreactions. K/u C
7. Whichofthefollowinggroupsofmoleculeshasmoreoverallfreeenergy?Explainyouranswer. K/u
• Group1:glutamicacid,NH3,andATP• Group2:glutamine,ADP,andPi
8. Whyisitnecessaryforcellsto“recycle”ADPandPiratherthanjustreleasethemaswasteproducts? K/u
9. UseaflowchartoranothergraphicorganizertoexplaintheATPcycle. K/u C
10. Describeaphosphorylationreaction. K/u
11. Examinethefollowingsetofreactions: T/i
(i) A 1 B S C ∆G 514.4kJ/mol(ii) D S E 1 F ∆G 523.0kJ/mol(iii)M 1 N S P 1 R ∆G 526.2kJ/mol(iv)S 1 T S V ∆G 5 12.1kJ/mol
(a)Whichofthesereactionsarespontaneous?(b)Listallcombinationsoftworeactionsthat
wouldresultinanoverallspontaneousreaction.12. ExplainwhyitisadvantageousforcellstouseATP
asanenergysourceratherthanusingglucose,lipids,orproteindirectly. K/u
13. Anunusualformofbiologicalworkistheproductionoflight.ConductresearchonlinetodeterminehowfirefliesuseATPenergytoproducetheirwell-knownflashesoflight(Figure 5).Howefficientisthisprocess? T/i
WEB LINK
Figure 5
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