第8章生化 (刘建华)PPT文件格式下载.ppt
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Enzymes:
@#@BasicConceptsandKinetics,Enzymes,thecatalystsofbiologicalsystems,areremarkablemoleculardevicesthatdeterminethepatternsofchemicaltransformations.Theyalsomediatethetransformationofoneformofenergyintoanother.Themoststrikingcharacteristicsofenzymesaretheircatalyticpowerandspecificity.Catalysistakesplaceataparticularsiteontheenzymecalledtheactivesite.Nearlyallknownenzymesareproteins.However,proteinsdonothaveanabsolutemonopolyoncatalysis;@#@thediscoveryofcatalyticallyactiveRNAmoleculesprovidescompellingevidencethatRNAwasanearlybiocatalyst(Section2.2.2).,Proteinsarehighlyeffectivecatalystsforanenormousdiversityofchemicalreactionsbecauseoftheircapacitytospecificallybindaverywiderangeofmolecules.Byutilizingthefullrepertoire(全部)ofintermolecularforces,enzymesbringsubstratestogetherinanoptimalorientation,theprelude(前兆)tomakingandbreakingchemicalbonds.Theycatalyzereactionsbystabilizingtransitionstates,thehighest-energyspeciesinreactionpathways.Byselectivelystabilizingatransitionstate,anenzymedetermineswhichoneofseveralpotentialchemicalreactionsactuallytakesplace.,Theactivityofanenzymeisresponsiblefortheglowoftheluminescentjellyfishatleft.TheenzymeaequorincatalyzestheoxidationofacompoundbyoxygeninthepresenceofcalciumtoreleaseCO2andlight.(Left)FredBavendam/PeterArnold.,EnzymesArePowerfulandHighlySpecificCatalysts,Enzymesacceleratereactionsbyfactorsofasmuchasamillionormore.Indeed,mostreactionsinbiologicalsystemsdonottakeplaceatperceptibleratesintheabsenceofenzymes.reactions,incontrastwithuncatalyzedones.,Evenareactionassimpleasthehydrationofcarbondioxideiscatalyzedbycarbonicanhydrase(脱水酶).ThetransferofCO2fromthetissuesintothebloodandthentothealveolar(肺泡的)airwouldbelesscompleteintheabsenceofthisenzyme.Infact,carbonicanhydraseisoneofthefastestenzymesknown.Eachenzymemoleculecanhydrate106moleculesofCO2persecond.Thiscatalyzedreactionis107timesasfastastheuncatalyzedone.Enzymesarehighlyspecificbothinthereactionsthattheycatalyzeandintheirchoiceofreactants,whicharecalledsubstrates.Anenzymeusuallycatalyzesasinglechemicalreactionorasetofcloselyrelatedreactions.Sidereactionsleadingtotheformationofby-productsarerareinenzyme-catalyzed.,Proteolyticenzymescatalyzeproteolysis,thehydrolysisofapeptidebond.,Mostproteolyticenzymesalsocatalyzeadifferentbutrelatedreactioninvitronamely,thehydrolysisofanesterbond.Suchreactionsaremoreeasilymonitoredthanisproteolysisandareusefulinexperimentalinvestigationsoftheseenzymes.,Proteolyticenzymesdiffermarkedlyintheirdegreeofsubstratespecificity.Subtilisin,whichisfoundincertainbacteria,isquiteundiscriminating:
@#@itwillcleaveanypeptidebondwithlittleregardtotheidentityoftheadjacentsidechains.Trypsin,adigestiveenzyme,isquitespecificandcatalyzesthesplittingofpeptidebondsonlyonthecarboxylsideoflysineandarginineresidues.Thrombin(凝血酶),anenzymethatparticipatesinbloodclotting,isevenmorespecificthantrypsin.ItcatalyzesthehydrolysisofArg-Glybondsonly.DNApolymeraseI,atemplate-directedenzyme,isanotherhighlyspecificcatalyst.ItaddsnucleotidestoaDNAstrandthatisbeingsynthesized,inasequencedeterminedbythesequenceofnucleotidesinanotherDNAstrand.DNApolymeraseIisremarkablypreciseincarryingouttheinstructionsgivenbythetemplate.ItinsertsthewrongnucleotideintoanewDNAstrandlessthan10-6.,Figure8.1.EnzymeSpecificity.(A)Trypsincleavesonthecarboxylsideofarginineandlysineresidues,whereas(B)thrombincleavesArg-Glybondsinparticularsequencesspecifically.,Thespecificityofanenzymeisduetothepreciseinteractionofthesubstratewiththeenzyme.Thisprecisionisaresultoftheintricatethree-dimensionalstructureoftheenzymeprotein.,ManyEnzymesRequireCofactorsforActivity,Thecatalyticactivityofmanyenzymesdependsonthepresenceofsmallmoleculestermedcofactors,althoughthepreciserolevarieswiththecofactorandtheenzyme.Suchanenzymewithoutitscofactorisreferredtoasanapoenzyme(脱辅酶);@#@thecomplete,catalyticallyactiveenzymeiscalledaholoenzyme.,Cofactorscanbesubdividedintotwogroups:
@#@metalsandsmallorganicmolecules.TheenzymecarbonicanhydraserequiresZn2+foritsactivity.Glycogenphosphorylase,whichmobilizesglycogenforenergy,requiresthesmallorganicmoleculepyridoxalphosphate(PLP,磷酸吡哆醛).Cofactorsthataresmallorganicmoleculesarecalledcoenzymes.Oftenderivedfromvitamins,coenzymescanbeeithertightlyorlooselyboundtotheenzyme.Iftightlybound,theyarecalledprostheticgroups.Looselyassociatedcoenzymesaremorelikecosubstratesbecausetheybindtoand