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In effect, thereaction Y -+ Z will then act as a "siphon' to drive the conversion of all ofmolecule X to molecule Y and thence to molecule Z (Figure 2-54) . For example,10s104103102lor11010-210-31o-41o-s-7.1(-29.7)-s.7 (-23.8)-4.3(-18.0)- 2 . 8( - 1 1 . 7 )-1.4(-s.e)0 (0)'r.4(s.9)2 . 8( 1 1 . 7 )4.3(18.0)s.7(23.8)7.1(2s.7)V a l u eosf t h e e q u i l l b r i ucmo n s t a n tfor the simplewerecalculatedY = X usingthechemlcalreactionequationglvenin the textTheAG"givenhereis in kilocaloriespermoleat 37"C,with kilojoules(1 kilocaloriepermolein parenthesess )si se q u atl o 4 l 8 4 k i l o j o u l e Ain the text,AG'representsexplainedunderdifferencethe free-energy(whereal1conditionsstandardarepresenlar acomponents'lof 0 mole/liter)concentrationFromthistable,we seethat if thereischangefree-energystandarda favorable(AG")of -a 3 kca/mole(-l B0 kJlmole)forY + X,therewill be 1000the transitlonin stateX thantimesmoremolecuJes(K= 1000),in stateY at equilibriumchapter2:cellchemistryandBiosynthesis78XYXYUNCATALYZEDREACTIONENZYME-CATALYZEDREACTIONseveral of the reactions in the long pathway that converts sugars into CO2 andH2O would be energetically unfavorable if considered on their or,rm.But thepathway neverthelessproceeds becausethe total AG for the seriesof sequentialreactions has a large negative value.But forming a sequential pathway is not adequate for many purposes.

Oftenthe desired pathway is simply X -+ Y without further conversion of Y to someother product. Fortunately, there are other more general ways of using enzymesto couple reactions together. How these work is the topic we discuss next.Figure2-53 Enzymescannotchangethe equilibriumpoint for reactions,Enzymes,likeall catalysts,speedup theforwardand backwardratesof a reactionby the samefactor.Therefore,for boththe catalyzedand the uncatalyzedreactionsshownhere,the numberofmoleculesundergoingthe transitionX -+ Y is eoualto the numberofmoleculesundergoingthe transitionY -+ X when the ratioof Y moleculesto Xmoleculesis 3.5to 1. In otherwords,thetwo reactionsreacheouilibriumatexactlythe samepoint.ActivatedCarrierMoleculesAre Essentialfor BiosynthesisThe energy released by the oxidation of food molecules must be stored temporarily before it can be channeled into the construction of the many othermolecules needed by the cell. In most cases,the energy is stored as chemicalbond energy in a small set of activated "carrier molecules,"which contain one ormore energy-rich covalent bonds.

These molecules diffuse rapidly throughoutthe cell and thereby carry their bond energy from sites of energy generation to thesites where energy is used for bioslnthesis and other cell activities (Figure 2-55).The activated carriers store energy in an easily exchangeable form, either asa readily transferable chemical group or as high-energy electrons, and they canserve a dual role as a source of both energy and chemical groups in biosyntheticreactions. For historical reasons,these molecules are also sometimes referred toas coenzymes.The most important of the activated carrier molecules are ATPand two molecules that are closely related to each other, NADH and NADPHas we discuss in detail shortly.

We shall see that cells use activated carriermolecules like money to pay for reactions that otherwise could not take place.ze q u i l i b r i u mp o i n t f o rX * Y r e a c t i o na l o n ee q u i l i b r i u mp o i n t f o rY*Z reactionalone(c)<--Xze q u i l i b r i u mp o i n t f o r s e q u e n t i arl e a c t i o n sX + Y + ZFigure2-54 How an energeticallyunfavorablereactioncan be driven by asecond,following reaction.(A)Atequilibrium,therearetwiceas manyX moleculesasY molecules,becauseX isof lowerenergythanY.(B)At equilibrium,thereare25 timesmoreZ moleculesthanY molecules,becauseZ is of much lowerenergythanY.(C)lf the reactionsin (A)and (B)arecoupled,nearlyall of the Xmoleculeswill be convertedto Zmolecules.as shown.CATALYSISANDTHEUSEOF ENERGYBYCELLS79e n e r g e t i c Ial yu n f a v o r ab lereacilono x i d i z e df o o dmoleculeFigure2-55 Energytransferand the roleof activatedcarriersin metabolism.Byactivatedservingas energyshuttles,carriermoleculesperformtheirfunctionthat linkthe breakdownasgo-betweensand the releaseofof food moleculesenergy (catabolism)to the energyof smalland largerequiringbiosynthesisorganicmolecules(anabolism).moleculeavailable in cellCATABOLISMANABOLISMTheFormationof an ActivatedCarrierls Coupledto anEnergetically FavorableReaction\Alhen a fuel molecule such as glucose is oxidized in a cell, enzyme-catalyzedreactions ensure that a large part ofthe free energy that is releasedby oxidationis captured in a chemically useful form, rather than being releasedas heat.

Thisis achieved by means of a coupled reaction, in which an energetically favorablereaction drives an energetically unfavorable one that produces an activated carrier molecule or some other useful energy store. Coupling mechanisms requireenzymes and are fundamental to all the energy transactions of the cell.The nature of a coupled reaction is illustrated by a mechanical analogy inFigure 2-56, in which an energetically favorable chemical reaction is representedby rocks falling from a cliff. The energy of falling rocks would normally be entirelywasted in the form of heat generated by friction when the rocks hit the ground(seethe falling brick diagram in Figure 2-39). By careful design, however, part ofthis energy could be used instead to drive a paddle wheel that lifts a bucket ofwater (Figure 2-568). Because the rocks can now reach the ground only aftermoving the paddle wheel, we say that the energeticallyfavorable reaction of rockfalling has been directly coupledto the energetically unfavorable reaction of lifting the bucket of water.

Note that because part of the energy is used to do workin (B), the rocks hit the ground with less velocity than in (A), and correspondingly less energy is dissipated as heat.Similar processesoccur in cells, where enzymes play the role of the paddlewheel in our analogy.

By mechanisms that will be discussed later in this chapter, they couple an energetically favorable reaction, such as the oxidation offoodstuffs, to an energetically unfavorable reaction, such as the generation ofFigure2-56 A mechanicalmodelillustratingthe principleof coupledThe spontaneouschemicalreactions.reactionshownin (A)couldserveas ananalogyfor the directoxidationofglucoseto CO2and HzO,whichproducesheatonly.In (B)the samereactionisthis secondcoupledto a secondreaction;ofto the synthesisreactionis analogousTheenergyactivatedcarriermolecules.producedin (B)is in a more usefulformthan in (A)and can be usedto driveavarietyof otherwiseenergetically(C).reactionsunfavorable(c)l-Ior-^\\f;""K--k i n e t i ce n e r g yo f f a l l i n g r o c k sl st r a n s f o r m e di n t o h e a t e n e r g yo n l yp a r t o f t h e k i n e t i ce n e r g yi s u s e dt o l i f ta b u c k e to f w a t e r ,a n d a c o r r e s p o n d i n g l ys m a l l e ra m o u n t i s t r a n s { o r m e di n t o h e a tt h e p o t e n t i a lk i n e t i ce n e r g ys t o r e di nthe raisedbucket of water can beu s e dt o d r i v e h y d r a u l i cm a c h i n e tsh a tcarry out a variety of useful tasks80Chapter2: CellChemistryand Biosynthesisp h o s p h o a n h y d r i dbeo n d sJ.Ao- o- ortl-o-P-o-P-o-P-o-cr,,ililil1ooolo-H* +- O - PI- O H+o-[-o-r-o-:H,ooolADPI n o r g a nt cphosphate (P;)an activated carrier molecule.

As a result, the amount of heat releasedby the oxidation reaction is reduced by exactly the amount of energy that is stored in theenergy-rich covalent bonds of the activated carrier molecule. The activated carrier molecule in turn picks up a packet of energy of a size sufficient to power achemical reaction elsewherein the cell.Figure2-57 The hydrolysisof ATPtoADPand inorganicphosphate,Thetwoin ATPare heldtooutermostphosphatesthe restof the moleculeby high-energyphosphoanhydridebondsand arereadilyAs indicated,watercan betransferred.addedto ATPto form ADPand inorganicphosphate(Pi).Thishydrolysisof theterminalphosphateof ATPyieldsbetween11 and 13 kcal/moleof usableenergy,dependingon the intracellularconditions.The largenegativeAG of thisreactionarisesfrom severalfactors.Releaseof the terminalphosphategroupremovesan unfavorablereoulsionbetweenadjacentnegativecharges;inaddition,the inorganicphosphateion (Pi)releasedis stabilizedby resonanceandby favorablehydrogen-bondformationwith water.ATPls the MostWidelyUsedActivatedCarrierMoleculeThe most important and versatile of the activated carriers in cells is ATP (adenosine triphosphate).

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