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

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ATP

O P O

O

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CH2 O

H

C

H

CH

C

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H

C

N

CHCN

CC

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NH2

O– P

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OH � H� � –O P

O

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P O

O

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HO

CH2 O

H

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H

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H

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N

CHCN

CC

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NH2

O

sugar (ribose)

nucleotidebase (adenine)

H2O

C03-F13-OB12USB

Crowle Art Group

Deborah Crowle

2nd pass

Biology 12

FN

CO

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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

�

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(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

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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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