B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 30
Текст из файла (страница 30)
Whena reaction reaches equilibrium, the forwardand backward fluxes of reacting moleculesare equal and opposite.FOR THE ENERGETICALLY FAVORABLE REACTION Y → X,YXwhen X and Y are at equal concentrations, [Y] = [X], the formation of Xis energetically favored. In other words, the ΔG of Y → X is negative andthe ΔG of X → Y is positive.
But because of thermal bombardments,there will always be some X converting to Y.THUS, FOR EACH INDIVIDUAL MOLECULE,YXXYTherefore the ratio of X to Ymolecules will increase with timeconversion ofY to X willoccur often.Conversion of X to Ywill occur less oftenthan the transitionY → X, because itrequires a moreenergetic collision.EVENTUALLY, there will be a large enough excess of X over Y to justcompensate for the slow rate of X → Y, such that the number of Y moleculesbeing converted to X molecules each second is exactly equal to the numberof X molecules being converted to Y molecules each second. At this point,the reaction will be at equilibrium.YAT EQUILIBRIUM,Xthere is no net change in the ratio of Y to X, and theΔG for both forward and backward reactions is zero.The Equilibrium Constant and ∆G° Are Readily Derived fromEach OtherInspection of the above equation reveals that the ∆G equals the value of ∆G°when the concentrations of Y and X are equal. But as any favorable reaction proceeds, the concentrations of the products will increase as the concentration of thesubstrates decreases.
This change in relative concentrations will cause [X]/[Y] toMBoC6 e3.18/2.30become increasingly large, making the initially favorable ∆G less and less negative(the logarithm of a number x is positive for x > 1, negative for x < 1, and zero for x=1). Eventually, when ∆G = 0, a chemical equilibrium will be attained; here thereis no net change in free energy to drive the reaction in either direction, inasmuchas the concentration effect just balances the push given to the reaction by ∆G°.As a result, the ratio of product to substrate reaches a constant value at chemicalequilibrium (Figure 2–30).We can define the equilibrium constant, K, for the reaction Y → X as[X]K=[Y]where [X] is the concentration of the product and [Y] is the concentration of thereactant at equilibrium. Remembering that ∆G = ∆G° + RT ln [X]/[Y], and that∆G = 0 at equilibrium, we see that∆G° = –RT ln [X] = –RT ln K[Y]At 37°C, where RT = 2.58, the equilibrium equation is therefore:∆G° = –2.58 ln KCATALYSIS AND THE USE OF ENERGY BY CELLSConverting this equation from the natural logarithm (ln) to the more commonly used base 10 logarithm (log), we get∆G° = –5.94 log KThe above equation reveals how the equilibrium ratio of X to Y (expressed asthe equilibrium constant, K) depends on the intrinsic character of the molecules,(as expressed in the value of ∆G° in kilojoules per mole).
Note that for every 5.94kJ/mole difference in free energy at 37°C, the equilibrium constant changes bya factor of 10 (Table 2–2). Thus, the more energetically favorable a reaction, themore product will accumulate if the reaction proceeds to equilibrium.More generally, for a reaction that has multiple reactants and products, suchas A + B → C + D,[C][D]K=63TABLE 2–2 Relationship Betweenthe Standard Free-EnergyChange, ΔG°, and the EquilibriumConstantEquilibriumconstant[X]=K[Y]Free energy of Xminus free energyof Y[kJ/mole (kcal/mole)]105–29.7 (–7.1)104–23.8 (–5.7)103–17.8 (–4.3)The concentrations of the two reactants and the two products are multipliedbecause the rate of the forward reaction depends on the collision of A and B andthe rate of the backward reaction depends on the collision of C and D. Thus, at37°C,∆G° = –5.94 log [C][D]102–11.9 (–2.8)101–5.9 (–1.4)10–15.9 (1.4)where ∆G° is in kilojoules per mole, and [A], [B], [C], and [D] denote the concentrations of the reactants and products in moles/liter.10–211.9 (2.8)10–317.8 (4.3)The Free-Energy Changes of Coupled Reactions Are Additive10–423.8 (5.7)We have pointed out that unfavorable reactions can be coupled to favorable onesto drive the unfavorable ones forward (see Figure 2–29).
In thermodynamic terms,this is possible because the overall free-energy change for a set of coupled reactions is the sum of the free-energy changes in each of its component steps. Consider, as a simple example, two sequential reactionsX → Y and Y → Zwhose ∆G° values are +5 and –13 kJ/mole, respectively. If these two reactionsoccur sequentially, the ∆G° for the coupled reaction will be –8 kJ/mole. Thismeans that, with appropriate conditions, the unfavorable reaction X → Y can bedriven by the favorable reaction Y → Z, provided that this second reaction followsthe first.
For example, several of the reactions in the long pathway that convertssugars into CO2 and H2O have positive ∆G° values. But the pathway neverthelessproceeds because the total ∆G° for the series of sequential reactions has a largenegative value.Forming a sequential pathway is not adequate for many purposes.
Often thedesired pathway is simply X → Y, without further conversion of Y to some otherproduct. Fortunately, there are other more general ways of using enzymes to couple reactions together. These often involve the activated carrier molecules that wediscuss next.10–529.7 (7.1)[A][B][A][B]Activated Carrier Molecules Are Essential for BiosynthesisThe energy released by the oxidation of food molecules must be stored temporarily before it can be channeled into the construction of the many other moleculesneeded by the cell. In most cases, the energy is stored as chemical-bond energyin a small set of activated “carrier molecules,” which contain one or more energyrich covalent bonds. These molecules diffuse rapidly throughout the cell andthereby carry their bond energy from sites of energy generation to the sites wherethe energy will be used for biosynthesis and other cell activities (Figure 2–31).The activated carriers store energy in an easily exchangeable form, either asa readily transferable chemical group or as electrons held at a high energy level,and they can serve a dual role as a source of both energy and chemical groups inbiosynthetic reactions.
For historical reasons, these molecules are also sometimesreferred to as coenzymes. The most important of the activated carrier molecules10 (0)Values of the equilibrium constant werecalculated for the simple chemicalreaction Y ↔ X using the equationgiven in the text. The ΔG° given hereis in kilojoules per mole at 37°C, withkilocalories per mole in parentheses.One kilojoule (kJ) is equal to 0.239kilocalories (kcal) (1 kcal = 4.18 kJ). Asexplained in the text, ΔG° represents thefree-energy difference under standardconditions (where all components arepresent at a concentration of 1.0 mole/liter).
From this table, we see that ifthere is a favorable standard free-energychange (ΔG°) of –17.8 kJ/mole(–4.3 kcal/mole) for the transition Y → X,there will be 1000 times more moleculesin state X than in state Y at equilibrium(K = 1000).64Chapter 2: Cell Chemistry and BioenergeticsENERGYENERGYfoodmoleculemoleculeneeded by cellenergeticallyfavorablereactionenergeticallyunfavorablereactionFigure 2–31 Energy transfer and therole of activated carriers in metabolism.By serving as energy shuttles, activatedcarrier molecules perform their functionas go-betweens that link the breakdownof food molecules and the release ofenergy (catabolism) to the energy-requiringbiosynthesis of small and large organicmolecules (anabolism).ENERGYoxidized foodmoleculeactivated carrier moleculeCATABOLISMmoleculeavailable in cellANABOLISMare ATP and two molecules that are closely related to each other, NADH andNADPH.
Cells use such activated carrier molecules like money to pay for reactions that otherwise could not takeplace.MBoC6 m2.55/2.31The Formation of an Activated Carrier Is Coupled to anEnergetically Favorable ReactionCoupling mechanisms require enzymes and are fundamental to all the energytransactions of the cell. The nature of a coupled reaction is illustrated by amechanical analogy in Figure 2–32, in which an energetically favorable chemical reaction is represented by rocks falling from a cliff. The energy of falling rockswould normally be entirely wasted in the form of heat generated by friction whenthe rocks hit the ground (see the falling-brick diagram in Figure 2–17). By carefuldesign, however, part of this energy could be used instead to drive a paddle wheelthat lifts a bucket of water (Figure 2–32B).
Because the rocks can now reach theground only after moving the paddle wheel, we say that the energetically favorable reaction of rock falling has been directly coupled to the energetically unfavorable reaction of lifting the bucket of water. Note that because part of the energy isused to do work in Figure 2–32B, the rocks hit the ground with less velocity than inFigure 2–32A, and correspondingly less energy is dissipated as heat.Similar processes occur in cells, where enzymes play the role of the paddlewheel.
By mechanisms that we discuss later in this chapter, enzymes couple an(A)(B)Figure 2–32 A mechanical modelillustrating the principle of coupledchemical reactions. The spontaneousreaction shown in (A) could serve as ananalogy for the direct oxidation of glucoseto CO2 and H2O, which produces heatonly. In (B), the same reaction is coupledto a second reaction; this second reactionis analogous to the synthesis of activatedcarrier molecules. The energy produced in(B) is in a more useful form than in (A) andcan be used to drive a variety of otherwiseenergetically unfavorable reactions (C).(C)hydraulicmachinesheatkinetic energy of falling rocks istransformed into heat energy onlyUSEFULWORKheatpart of the kinetic energy is used to lifta bucket of water, and a correspondinglysmaller amount is transformed into heatthe potential kinetic energy stored inthe raised bucket of water can beused to drive hydraulic machines thatcarry out a variety of useful tasksCATALYSIS AND THE USE OF ENERGY BY CELLS65phosphoanhydride bondsO__O_O_ADENINEO P O P O P O CH2OOOATPRIBOSEH2OOH++__O P OHOinorganicphosphate (Pi)O+__O_ADENINEO P O P O CH2OOADPRIBOSEenergetically favorable reaction, such as the oxidation of foodstuffs, to an energetically unfavorable reaction, such as the generation of an activated carrier molecule.
In this example, the amount of heat released by the oxidation reaction isMBoC6 m2.57/2.33reduced by exactly the amount of energystored in the energy-rich covalent bondsof the activated carrier molecule. And the activated carrier molecule picks up apacket of energy of a size sufficient to power a chemical reaction elsewhere in thecell.ATP Is the Most Widely Used Activated Carrier MoleculeThe most important and versatile of the activated carriers in cells is ATP (adenosine triphosphate). Just as the energy stored in the raised bucket of water inFigure 2–32B can drive a wide variety of hydraulic machines, ATP is a convenientand versatile store, or currency, of energy used to drive a variety of chemical reactions in cells.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
