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Each enzyme binds tightly to one or moremolecules, called substrates, and holds them in a way that greatly reduces theactivation energy of a particular chemical reaction that the bound substratescan undergo. A substance that can lower the activation energy of a reaction isCATALYSISANDTHEUsEOF ENERGYBYCELLS73e n z y m er o w e r sactivatione n e r g yf o rcatalyzedreactionY+XIIooco6(A)ff'113'J""'1"*"v(B):lzJffic;€attalf:iy (areactant)Figure2-44Theimportantprinciple(A)compoundof activationenergy.isin arelativelystablestate,andenergyisrequiredto convertit to compoundX (aproduct),eventhoughX isat a loweroverallenergylevelthanY Thisconversionwillnottakeplace,therefore,unlesscompoundY canacquireenoughactivationenergy(energya minusenergyb)fromitssurroundingsto undergothereactionthatconvertsit intocompoundX.Thisenergymaybeprovidedby meansof anunusuallyenergeticcollisionwithothermolecules.Forthereversereaction,X -+ Y theactivationenergywill be muchlarger(energya minusenergyc);thisreactionwillthereforeoccurmuchmorerarely.Activationpositive;energiesarealwaysnote,however,thatthetotalenergychangefortheenergeticallyfavorablereactionY -+ X isenergyc minus(B)Energyenergyb,a negativenumber.barriersfor specificreactionscanbe loweredbycatalysts,asindicatedbythelinemarkedd.
Enzymesareparticularlyeffectivecatalystsbecausetheygreatlyreducetheactivationenergyforthereactionstheyperform.termed a catalyst; catalystsincreasethe rate of chemical reactions becausetheyallow a much larger proportion of the random collisions with surroundingmolecules to kick the substratesover the energy barrier, as illustrated in Figure2-45.Enzymes are among the most effective catalystsknown, capable of speeding up reactions by factors of 101aor more.
They thereby allow reactions thatwould not otherwise occur to proceed rapidly at normal temperatures.Enzymes are also highly selective.Each enzyme usually catalyzesonly oneparticular reaction: in other words, it selectivelylowers the activation energy ofonly one of the several possible chemical reactions that its bound substratemolecules could undergo. In this way, enzymes direct each of the many differentmolecules in a cell along specific reaction pathways (Figure 2-46).The success of living organisms is attributable to a cell's ability to makeenzymes of many types, each with precisely specified properties.
Each enzymehas a unique shape containing an actiue site, a pocket or groove in the enzymeinto which only particular substrates will fit (Figure z-42). Like all other catalysts, enzyme molecules themselves remain unchanged after participating in areaction and therefore can function over and over again. In chapter 3, we discuss further how enzymes work.Im a n y m o l e c u l e sh a v ee n o u g he n e r g yt o u n d e r g othe enzyme-catalyzedc h e m i c arl e a c t i o nIE.=>396oa l m o s tn o m o l e c u l e sh a v et h e v e r y h i g he n e r g yn e e d e dt ou n d e r g oa nuncatalyzedF>o!b9m o l e c u l ews itha v e r a g ee n e r g ychemical reaction-.5e n e r g yp e r m o l e c u l e+activatione n e r g yf o rcatalyzedreactionactivatione n e r g yf o ru ncatalyzedreactronFigure2-45 Loweringthe activationenergygreatlyincreasesthe probabilityof reaction.At any giveninstant,apopulationof identicalsubstratemoleculeswill havea rangeof energies,distributedas shownon the graph.Thevaryingenergiescomefrom collisionswith surroundingmolecules,which makejiggle,vibrate,the substratemoleculesand spin.Fora moleculeto undergoachemicalreaction,the energyof themoleculemustexceedthe activationenergybarrierfor that reaction;for mostbiologicalreactions,this almostneverhappenswithoutenzymecatalysis.Evenwith enzymecatalysis,the substratemoleculesmustexperiencea particularlyenergeticcollisionto react(redshadedareo).Raisingthe temperaturecan alsoincreasethe numberof moleculeswithsufficientenergyto overcometheactivationenergyneededfor a reaction;but in contrastto enzymecatalysis,thiseffectis nonselective,speedingup allreactions.74Chapter2: CellChemistryand Biosynthesis'i.o(odryriverbedl a k ew i t hn$,&l)\\(*- .U'\flowings t r e amWAVESau n c a t ay z e d r e a c t i o n - w a v e sn o t a r g ee n o u g ht o s u r m o u n bt arrlercatalyzedreaction-waves often surmount barrier(A)tt2^I)acouncatalyzed(B)e n z y m ec a t a l y s i so f r e a c t i o n1How EnzymesFindTheirSubstrates:TheEnormousRapidityof MolecularMotionsAn enzyme will often catalyzethe 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 enzl'rnesand their substratesare 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 broadlyinto three kinds: (1) the movement of a molecule from one place to another(translational motion), (2) the rapid back-and-forth movement of covalentlylinked atoms with respect to one another (vibrations), and (3) rotations. All ofthese motions help to bring the surfacesof interacting molecules together.The rates of molecular motions can be measured by a variety of spectroscopic techniques.A large globular protein is constantly tumbling, rotating aboutits axis about a million times per second.
Molecules are also in constant translational motion, which causesthem to explore the space inside the cell very efficiently by wandering through it-a process called diffusion. In this way, everymolecule in a cell collides with a huge number of other molecules each second.As the molecules in a liquid collide and bounce off one another, an individualmolecule moves first one way and then another, its path constituting a randomwalk (Figure 2-48). In such a walk, the average net distance that each moleculetravels (asthe crow flies) from its starting point is proportional to the square rootof the time involved: that is, if it takes a molecule I second on averageto travel1 pm, it takes 4 secondsto travel 2 pm, 100 secondsto travel 10 pm, and so on.The inside of a cell is very crowded (Figure 2-49).
Nevertheless,experimentsin which fluorescent dyes and other labeled molecules are injected into cellsactivesitem o l e c u l eA(substrate)enzymesubstratecomolexenzymeproductcomolexmolecule B(product)Figure2-46 Floatingball analogiesforenzyme catalysis.<TAAA>(A)A barrierdam is loweredto representenzymecatalysis.The greenball representsapotentialreactant(compoundY) that isbouncingup and down in energyleveldue to constantencounterswith waves(ananalogyfor the thermalbombardmentof the reactantmoleculewith the surroundingwatermolecules).Whenthe barrier(activationenergy)isloweredsignificantly,it allowsthefavorablemovementof theenergeticallyball(thereactant)downhill.(B)Thefourwallsof the box reoresentthe activationenergybarriersfor four differentchemicalreactionsthat areall energeticallyfavorable,in the sensethat the productsareat lowerenergylevelsthan thereactants.ln the left-handbox, none ofthesereactionsoccursbecauseeventhelargestwavesarenot largeenoughtosurmountany of the energybarriers.Inthe right-handbox, enzymecatalysislowersthe activationenergyfor reactionnumber1 only;now the jostlingof thewavesallowspassageofthe reactantmoleculeoverthis energybanier,inducingreaction1.(C)A branchingriverwith a set of barrierdams(yellowboxes)servesto illustratehow a seriesofenzyme-catalyzedreactionsdeterminesthe exactreactionpathwayfollowedbyeachmoleculeinsidethe cell.Figure2-47 How enzymeswork.
Eachenzymehasan activesiteto whichoneor more substrotemoleculesbind,formingan enzyme-substratecomplex.A reactionoccursat the activesite,producingan enzyme-productcomplex.fhe productis then released,allowingtheenzymeto bind furthersubstratemnlarrrla<75CATALYSISAND THEUSEOF ENERGYBYCELLSshow that small organic molecules diffuse through the watery gel of the cytosolnearly as rapidly as they do through water. A small organic molecule, for example, takes only about one-fifth of a second on average to diffuse a distance of10 pm. Diffusion is therefore an efficient way for small molecules to move thelimited distances in the cell (a tlpical animal cell is 15 pm in diameter).Since enzymes move more slowly than substrates in cells, we can think ofthem as sitting still.
The rate of encounter of each enzyme molecule with its substrate will depend on the concentration of the substrate molecule. For example,some abundant substratesare present at a concentration of 0.5 mM. Since purewater is 55.5 M, there is only about one such substrate molecule in the cell forevery 10swater molecules. Nevertheless,the active site on an enzyrne moleculethat binds this substrate will be bombarded by about 500,000random collisionswith the substrate molecule per second.
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