工业催化原理-英文课件chapter22

§2.3EnzymecatalysisThestudyofthekineticsofenzymecatalyzedreactionisstillinitsinfancy.Verylittleisknownaboutthemechanism.Mainfeaturesspecificity.Forexample,UreasewillonlycatalyzethehydrolysisofureainaverynarrowpHandtemperatureregion.highefficiencymildreactioncondition:roomtemperature60C,nearneutralconditionpH7HighefficiencydecompositionofH2O2at22CcatalystkL/molsEaKJ/molnone10-773.2Fe2+56.042.3catalase3.51077.1Structureofenzymesproteinssuchaspolymerof-aminoacidNH2CHR-CO-(NH-CRH-CO-)nNH-CRH-COOH2.3.1ReactionkineticsofenzymecatalysisInfluenceofenzymeandsubstrateconcentration:Atlowsubstrateconcentration,thereactionsnormallyshowfirstorderwithrespecttosubstrate.Whileathighsubstrateconcentrationtherateoftentendstoalimitingvaluewhichisindependentofsubstrateconcentration–zero-thorderkinetics.2.3.1ReactionkineticsofenzymecatalysisMichaelismechanismk1k2E+SESE+Pk-1usingsteadystateapproximationtocomplex[ES]d[ES]/dt=k1[E][S]-k-1[ES]-k2[ES]=0[ES]+[E]=[E]o[ES]=k1[E]o[S]/(k-1+k2+k1[S])Thenwehavethereactionrate:r=d[P]/dt=k2[ES]=k2k1[E]o[S]/(k-1+k2+k1[S])=k2[E]o[S]/(km+[S])herekm=(k-1+k2)/k1iscalledasMichaelisconstant2.3.1Reactionkineticsofenzymecatalysisr=k2[E]o[S]/(km+[S])Atlowsubstrateconcentration[S]kmr=(k2[E]o/km)[S]firstorderkineticsathighsubstrateconcentration[S]kmr=rmax=k2[E]ozerothorderkinetics2.3.1ReactionkineticsofenzymecatalysisPhysicalmeaningofMichaelisconstantr=rmax[S]/(km+[S])when[S]=kmgetr=rmax/2whenreactionratereachesahalfofmaximum,theconcentrationofsubstrateequalsMichaelisconstant.reactionratervssubstrateconcentration[s]rmaxrmax/2km[S]2.3.1ReactionkineticsofenzymecatalysisEvaluationofkmandk2byreciprocalformr=rmax[S]/(km+[S])1/r=km/rmax1/[S]+1/rmaxLineweaver-Burkplot:1/rvs1/[S]km=slope/intercept,k2[E]o=1/interceptalsotheinterceptwithXaxisis-1/kmLineweaver-Burkplot1/rslope=Km/rmax1/rmax1/[S]1/Km1/r=km/rmax1/[S]+1/rmax2.3.2InhibitedenzymecatalyzedreactionTheexistenceofsomeimpuritycaninhibitcatalyticactivityanddecreasethereactionrate.Competitiveinhibition:theinhibitorandsubstratecompetitivelycombinewithenzymeincatalyzedreaction.2.3.2InhibitedenzymecatalyzedreactionMechanismKSk2E+SESE+PKIE+IEIwhereKS=[E][S]/[ES]kmKI=[E][I]/[EI]areequilibriumconstantsand[E]o=[E]+[ES]+[EI]Sameasaboveusingsteadystateapproximationtocomplex[ES],thenwehaver=rmax[S]/(KS(1+[I]/KI)+[S])1/r=KS(1+[I]/KI)/rmax·1/[S]+1/rmax2.3.2InhibitedenzymecatalyzedreactionDifferenceoftherateequationsNormalenzymereactionInhibitedenzymereaction]s[1rkr1r1]s[k]s[rrmaxmmaxmmax]s[1r)K]I[1(Kr1r1]s[)K]I[1(K]s[rrmaxISmaxISmax2.3.2InhibitedenzymecatalyzedreactionSincekm=(k-1+k2)/k1KS=[E][S]/[ES]=k-1/k1kmInLineweaver-Burkplot,fornormalreactionSlope=km/rmaxforinhibitedreactionSlope=KS(1+[I]/KI)/rmaxkmslopeisincreasedapproximately(1+[I]/KI)timesLineweaver-Burkplot1/rslope=Ks(1+[I]/KI)/rmaxslope=Km/rmax1/rmax1/[S]1/Km2.3.2InhibitedenzymecatalyzedreactionTheinhibitioneffectshaveotherkindsofformssuchasnon-competitiveinhibitionandanti-competitiveinhibition.Non-competitiveinhibition:theinhibitorcancombinewiththeintermediateESaswellascombinewithenzyme.Anti-competitiveinhibition:theinhibitorcanonlycombinewiththeintermediateESbutwithoutenzyme.2.3.2InhibitedenzymecatalyzedreactionNormalenzymereactionk1k2E+SESE+Pk-1InhibitedreactionKSk2E+SESE+PKIE+IEI2.3.2InhibitedenzymecatalyzedreactionAnti-competitiveinhibitionKSk2E+SESE+PKIES+IESINon-competitiveInhibitionKSk2E+SESE+PKIE+IEIKIES+IESIKSEI+SEI2.3.3InfluenceofpHvalueGenerally,thereexistsanoptimumpHvaluewhichiscorrespondingtothehighestreactionrate.Veryoften,theoptimumvalueispH7(neutral).Thebasicideaisthattheactivecentersoftheenzyme,especiallythesecontainamino-acidstructures,canexistinthreeformsofionization.2.3.3InfluenceofpHvalueForexample:NH3+---COOHNH3+---COO-NH2---COOprotonatedformneutralformdeprotonatedformacidiccasebasiccaseEH2EHE2.3.3InfluenceofpHvaluethethreeformsofenzymeareinequilibriumandeachofthemcanreactwiththesubstrate.itispostulatedthatonlytheneutralformEHScanyieldproducts.Intheacidic(orbasic)condition,theequilibriummovestowardstothecomplexEH2S(orES)whichyieldslittleproducts.ThiscanqualitativelyexplainwhythereactionrateisoptimumatpHvaluearound7.2.3.3InfluenceofpHvalueK1K2EH2EHEK3k-1||k1K4EH2SEHSESk2||EH+product2.3.3InfluenceoftemperatureThereisnosimpledependenceofthereactionrateontemperatureforenzymecatalysis.Followingaresomegeneralphenomena:Enzymecatalyzedreactionusuallyoccuratatemperaturebelow60C.Andtheactivationenergyisrelativelylow,lessthan40KJ/molingeneral,comparedwithothertypecatalyzedreactions.2.3.3InfluenceoftemperatureIfaenzymeisheatedatover60Cforonlyashortperiod,thepropertiesofenzymewillalterandsometimethisprocesswouldbeirreversible.Thischangeistermedasdenaturationwhichcouldcausetheactivitydecreasingmarkedly.2.3.3InfluenceoftemperatureWhentemperatureincrease,therateconstantkwhichdependsontheactivationenergygoesupwhereasthedenaturationeffectisuptoo.Thisdual-foldeffectsmakesthe