生化课件C5-control of enzyme activity

1、Theenzymaticactivityofsomeenzymesarepreciselyregulatedinlivingorganismstomeetphysiologicalrequirements.,C5RegulationofEnzymeactivity,FeedbackregulationAllostericenzymeReversiblecovalentmodificationProteolyticactivationRegulationofenzymesynthesisandbreakdown,1.Feedbackinhibition,Inbiologicalsystemsth

2、eratesofmanyenzymesarealteredbythepresenceofeffectors(inhibitorsoractivators).Buildingupofapathwaysendproductultimatelyslowstheentirepathway.Thisiscalledfeedbackinhibitionandoftentakesplaceatthecommittedstepinthepathway.Thecommittedstepisthefirststeptoproduceanintermediatewhichisuniquetothepathwayin

3、question,andthereforenormallycommitsthemetabolitetofurthermetabolismalongthatpathway.,ThreonineIsoleucine,Threoninedehydratase,Asmanymetabolicpathwaysarebranched,feedbackinhibitionmustallowthesynthesisofoneproductofabranchedpathwaytoproceedevenwhenanotherispresentinexcess.,Sequentialfeedbackinhibiti

4、on,协同反馈抑制concertedfeedbackinhibition,累积反馈抑制cumulativefeedbackinhibition,2.1Theenzymecatalyzingthefirststepofasyntheticpathwayisoftenanallostericenzyme.Forexample,threoninedehydrataseintheIlesynthesispathway,andaspartatetranscarbamoylase(ATCase,thebestunderstoodallostericenzyme)inthepyrimidinenucleot

5、idesynthesispathway.,2.Allostericenzymes(similartohemoglobin)areregulatedbyreversible,noncovalentbindingofmodulators(oftenbeingmetabolites).,2.2Inmanyallostericenzymesthesubstratebindingsiteandthemodulatorbindingsitesareondifferentsubunits,thecatalytic(C)andregulator(R)subunits,respectively.Bindingo

6、fthepositive(stimulatory)modulator(M)toitsspecificsiteontheregulatorysubunitscommunicatedtothecatalyticsubunitthroughaconformationalchange.Thischangerendersthecatalyticsubunitactiveandcapableofbindingthesubstrate(S)withhigheraffinity.,2.3Inallostericenzymes,thebindingofasubstratemoleculetooneactives

7、iteaffectsthebindingofsubstratemoleculestootheractivesitesintheenzyme;thedifferentactivesitesaresaidtobehavecooperativelyinbindingandactingonsubstratemolecules.,2.4Allostericenzymesareoftenmulti-subunitproteins,withoneormoreactivesitesoneachsubunit.,ATCase(Aspartatetranscarbamoylase),Thisallostericr

8、egulatoryenzymehastwostackedcatalyticclusters,eachwiththreecatalyticpolypeptidechain(inshadesofblueandpurple),andthreeregulatoryclusters,eachwithtworegulatorypolypeptidechains(inredandyellow).Theregulatoryclustersformthepointsofatrianglesurroundingthecatalyticsubunits.Modulatorbindingproduceslargech

9、angesinenzymeconformationandactivity.,+ATP,+CTP,2.5AllostericenzymesdonotfollowtheMichaelis-MentenkineticsTheydoexhibitsaturationeffect,butdonotshowahyperboliccurve(usuallyasigmoidalcurve)whenplottingVoagainstS.ThesubstrateconcentrationatwhichVoishalfmaximalisreferredasK0.5(whichisnotKm!),Thecurveha

10、sasteepsectioninthemiddleofthesubstrateconcentrationrange,reflectingtherapidincreaseinenzymevelocitywhichoccurscoveranarrowrangeofsubstrateconcentrations.Thisallowsallostericenzymestobeparticularlysensitivetosmallchangesinsubstrateconcentrationwithinthephysiologicalrange.,Theenzymeactivityvarieswhen

11、theconcentrationofthemodulatorsvary,hencethesynthesispathwayisopenonlywhentheendproductislacking(closedwhentheendproductisabundant).,别构酶的主要特点：寡聚酶别构效应S形动力学曲线酶系统的第一个酶，或代谢分支酶,3.Theactivityofmanyenzymesareregulatedbyreversiblecovalentmodifications3.1Phosphorylation,themostcommonreversiblecovalentmodific

12、ation,isahighlyeffectivemeansofswitchingtheactivityoftargetenzymes.,3.2ProteinkinasescatalyzethetransferofaphosphategroupfromanATPmoleculetothesidechainsofSer,Thr,orTyrresiduesinproteins.3.3Proteinphosphatasescatalyzethehydrolysisofphosphorylgroupsattachedtoproteins,thusreversingtheeffectsofkinases.

13、,3.4Thedegreeofspecificityofproteinkinasesvaries.Somecatalyzethephosphorylationofmanydifferenttargetproteins(atsitesofconservedsequences,e.g.,proteinkinaseArecognizesaconservedsequencemadeofArg-Arg-X-Ser-ZorArg-Arg-X-Thr-Z).Somephosphorylateasingleproteinorseveralcloselyrelatedones.3.5Phosphorylatio

14、nisahighlyeffectivemeansofcontrollingtheactivityofproteinsforstructural,thermodynamic,andkineticreasons.,Regulationofglycogenphosphorylaseactivity,Glucose(red),AMP(darkblue,allostericactivator),pyridoxalphosphate(PLP,B6derivative,lightblue),andser14(yellow),Glycogenphosphorylase,磷酸化,腺苷酰化,尿苷酰化,糖基化,甲基

15、化,4.Manyenzymesareactivatedbyspecificproteolyticcleavage4.1Theseenzymesaresynthesizedasinactiveprecursorscalledzymogensorproenzyme.4.2Theyareactivatedbycleavageofoneorseveralspecificpeptidebonds.4.3Proteolyticactivation,incontrastwithallostericcontrolandreversiblecovalentmodification,canoccurjustonc

16、einthelifeoftheenzymemolecule.,4.4Thedigestiveenzymesthathydrolyzeproteinsaresynthesizedaszymogensinthestomachandpancreas.4.5Bloodclottingismediatedbyacascadeofproteolyticactivation.4.6Someproteinhormonesaresynthesizedasinactiveprecursors(e.g.,insulinisderivedfromproinsulinbyproteolyticremovalofapeptide).,5.Boththesynthesisanddegradationofthecertainimportantenzymesaretightlycontrolled5.1Amountofsomeenzymesareincreasedwhencertaininducers(oftenenzymesubstrates)arepresentinthecells.Thisisoftenseeninbacteriumcells.5.2Thepresenceoflactose乳糖inacultur