Enzyme kinetics_3

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1、CHAPTER10EnzymeKineticsOneofthemostfascinatingareasofstudyinchemicalkineticsisenzymecatalysis.Thephenomenonofenzymecatalysisusuallyresultsinaverylargeincreaseinreac-tionrate(ontheorderof106to1018)andhighspecificity.Byspecificity,wemeanthatanenzymemoleculeiscapableofselectivelycatal

2、yzingcertainreactants,calledsubstrates,whilediscriminatingagainstothermolecules.Thischapterpresentsthebasicmathematicaltreatmentofenzymekineticsanddiscussesthetopicsofenzymeinhibition,allosterism,andthee¤ectofpHonenzymekinetics.10.1GeneralPrinciplesofCatalysisAcatalystisasubstanc

3、ethatincreasestherateofareactionwithoutitselfbeingconsumedbytheprocess.Areactioninwhichacatalystisinvolvediscalledacata-lyzedreaction,andtheprocessiscalledcatalysis.Instudyingcatalysis,keepinmindthefollowingcharacteristics:1.AcatalystlowerstheGibbsenergyofactivationbyprovidingadi

4、¤erentmechanismforthereaction(Figure10.1).Thismechanismenhancestherateanditappliestoboththeforwardandthereversedirectionsofthereaction.∆G∆GReactantsReactantsGibbsenergy∆rG°Gibbsenergy∆rG°ProductsProductsReactioncoordinateReactioncoordinate(a)(b)Figure10.1Gibbsenergychangefor(a)an

5、uncatalyzedreactionand(b)acatalyzedreaction.Acatalyzedreactionmustinvolvetheformationofatleastoneintermediate(betweenthereactantandthecatalyst).TheDGisthesameinbothcases.363r364Chapter10:EnzymeKinetics2.Acatalystformsanintermediatewiththereactant(s)intheinitialstepofthemechanism

6、andisreleasedintheproduct-formingstep.Thecatalystdoesnotappearintheoverallreaction.3.Regardlessofthemechanismandtheenergeticsofareaction,acatalystcannota¤ecttheenthalpiesorGibbsenergiesofthereactantsandproducts.Thus,catalystsincreasetherateofapproachtoequilibrium,butcannotalterth

7、ethermodynamicequilibriumconstant.Humanshaveusedcatalystsforthousandsofyearsinfoodpreparationandwinemaking.Industrially,hundredsofbillionsofdollarsworthofchemicalsareproducedannuallywiththeaidofcatalysts.Therearethreetypesofcatalysis:heterogeneous,homogeneous,andenzymatic.Inahete

8、rogeneouslycatalyzedreaction,thereactant