B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 28
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A substance that can lower theactivation energy of a reaction is termed a catalyst; catalysts increase the rate ofchemical reactions because they allow a much larger proportion of the randomcollisions with surrounding molecules to kick the substrates over the energy barrier, as illustrated in Figure 2–22. Enzymes are among the most effective catalystsYdenzyme lowersactivationenergy forcatalyzedreactionY XbreactantXXc(A)cproductuncatalyzedreaction pathway(B)productenzyme-catalyzedreaction pathwayFigure 2–21 The important principleof activation energy. (A) Compound Y(a reactant) is in a relatively stable state,and energy is required to convert it tocompound X (a product), even though X isat a lower overall energy level than Y.
Thisconversion will not take place, therefore,unless compound Y can acquire enoughactivation energy (energy a minus energyb) from its surroundings to undergo thereaction that converts it into compound X.This energy may be provided by means ofan unusually energetic collision with othermolecules. For the reverse reaction,X → Y, the activation energy will bemuch larger (energy a minus energy c);this reaction will therefore occur muchmore rarely. Activation energies arealways positive; note, however, that thetotal energy change for the energeticallyfavorable reaction Y → X is energy cminus energy b, a negative number.(B) Energy barriers for specific reactionscan be lowered by catalysts, as indicatedby the line marked d. Enzymes areparticularly effective catalysts because theygreatly reduce the activation energy for thereactions they perform.Chapter 2: Cell Chemistry and Bioenergeticsenergy requiredto undergothe enzyme-catalyzedchemical reactionnumber of molecules58energy neededto undergo anuncatalyzedchemical reactionenergy per moleculemolecules withaverage energyknown: some are capable of speeding up reactions by factors of 1014 or more.Enzymes thereby allow reactions that would not otherwise occur to proceed rapMBoC6 e3.13/2.22idly at normal temperatures.Enzymes Can Drive Substrate Molecules Along Specific ReactionPathwaysAn enzyme cannot change the equilibrium point for a reaction.
The reason is simple: when an enzyme (or any catalyst) lowers the activation energy for the reactionY → X, of necessity it also lowers the activation energy for the reaction X → Y byexactly the same amount (see Figure 2–21). The forward and backward reactionswill therefore be accelerated by the same factor by an enzyme, and the equilibrium point for the reaction will be unchanged (Figure 2–23). Thus no matter howmuch an enzyme speeds up a reaction, it cannot change its direction.Despite the above limitation, enzymes steer all of the reactions in cells throughspecific reaction paths.
This is because enzymes are both highly selective andvery precise, usually catalyzing only one particular reaction. In other words, eachenzyme selectively lowers the activation energy of only one of the several possiblechemical reactions that its bound substrate molecules could undergo. In this way,sets of enzymes can direct each of the many different molecules in a cell along aparticular reaction pathway (Figure 2–24).The success of living organisms is attributable to a cell’s ability to makeenzymes of many types, each with precisely specified properties.
Each enzymeX(A)YUNCATALYZED REACTIONAT EQUILIBRIUMX(B)YENZYME-CATALYZED REACTIONAT EQUILIBRIUMFigure 2–23 Enzymes cannot change the equilibrium point for reactions. Enzymes, like allcatalysts, speed up the forward and backward rates of a reaction by the same factor. Therefore, forboth the catalyzed and the uncatalyzed reactions shown here, the number of molecules undergoingthe transition X → Y is equal to the number of molecules undergoing the transition Y → X when theratio of Y molecules to X molecules is 3 to 1. In other words, the two reactions reach equilibrium atexactly the same point.Figure 2–22 Lowering the activationenergy greatly increases the probabilityof a reaction.
At any given instant, apopulation of identical substrate moleculeswill have a range of energies, distributed asshown on the graph. The varying energiescome from collisions with surroundingmolecules, which make the substratemolecules jiggle, vibrate, and spin. For amolecule to undergo a chemical reaction,the energy of the molecule must exceedthe activation-energy barrier for thatreaction (dashed lines).
For most biologicalreactions, this almost never happenswithout enzyme catalysis. Even withenzyme catalysis, the substrate moleculesmust experience a particularly energeticcollision to react (red shaded area). Raisingthe temperature will also increase thenumber of molecules with sufficient energyto overcome the activation energy neededfor a reaction; but in marked contrast toenzyme catalysis, this effect is nonselective,speeding up all reactions (Movie 2.2).CATALYSIS AND THE USE OF ENERGY BY CELLS59energyFigure 2–24 Directing substrate molecules through a specific reactionpathway by enzyme catalysis.
A substrate molecule in a cell (green ball)is converted into a different molecule (blue ball) by means of a series ofenzyme-catalyzed reactions. As indicated (yellow box), several reactionsare energetically favorable at each step, but only one is catalyzed by eachenzyme. Sets of enzymes thereby determine the exact reaction pathway thatis followed by each molecule inside the cell.has a unique shape containing an active site, a pocket or groove in the enzymeinto which only particular substrates will fit (Figure 2–25). Like all other catalysts,enzyme molecules themselves remain unchanged after participating in a reactionand therefore can function over and over again. In Chapter 3, we discuss furtherhow enzymes work.How Enzymes Find Their Substrates: The Enormous Rapidity ofMolecular MotionsAn enzyme will often catalyze the reaction of thousands of substrate moleculesevery second.
This means that it must be able to bind a new substrate moleculein a fraction of a millisecond. But both enzymes and their substrates are presentin relatively small numbers in a cell. How do they find each other so fast? Rapidbinding is possible because the motions caused by heat energy are enormouslyfast at the molecular level. These molecular motions can be classified broadly intothree kinds: (1) the movement of a molecule from one place to another (translational motion), (2) the rapid back-and-forth movement of covalently linked atomswith respect to one another (vibrations), and (3) rotations.
All of these motionshelp to bring the surfaces of interacting molecules together.The rates of molecular motions can be measured by a variety of spectroscopictechniques. A large globular protein is constantly tumbling, rotating about its axisabout a million times per second. Molecules are also in constant translationalmotion, which causes them to explore the space inside the cell very efficiently bywandering through it—a process called diffusion. In this way, every molecule ina cell collides with a huge number of other molecules each second.
As the molecules in a liquid collide and bounce off one another, an individual moleculemoves first one way and then another, its path constituting a random walk (Figure2–26). In such a walk, the average net distance that each molecule travels (as the“crow flies”) from its starting point is proportional to the square root of the timeinvolved: that is, if it takes a molecule 1 second on average to travel 1 μm, it takes4 seconds to travel 2 μm, 100 seconds to travel 10 μm, and so on.The inside of a cell is very crowded (Figure 2–27). Nevertheless, experimentsin which fluorescent dyes and other labeled molecules are injected into cells showthat small organic molecules diffuse through the watery gel of the cytosol nearlyenzymeenzymeactive sitemolecule A(substrate)CATALYSISenzyme–substratecomplexenzyme–productcomplexmolecule B(product)Figure 2–25 How enzymes work.
Each enzyme has an active site to which one or more substratemolecules bind, forming an enzyme–substrate complex. A reaction occurs at the active site,producing an enzyme–product complex. The product is then released, allowing the enzyme to bindfurther substrate molecules.MBoC6 m2.46c/2.1860Chapter 2: Cell Chemistry and Bioenergeticsas rapidly as they do through water.
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