第8章生化 (刘建华).ppt

第8章生化 (刘建华).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