Signal Transduction-信号传导入门

SignalTransductionCopyright©1999-2008byJoyceJ.Diwan.Allrightsreserved.BiochemistryofMetabolismManyenzymesareregulatedbycovalentattachmentofphosphate,inesterlinkage,totheside-chainhydroxylgroupofaparticularaminoacidresidue(serine,threonine,ortyrosine).H3N+CCOOCHOHCH3Hthreonine(Thr)H3N+CCOOCH2OHHserine(Ser)AproteinkinasetransferstheterminalphosphateofATPtoahydroxylgrouponaprotein.AproteinphosphatasecatalyzesremovalofthePibyhydrolysis.ProteinOH+ATPProteinOPOOO+ADPPiH2OProteinKinaseProteinPhosphatasePhosphorylationmaydirectlyalteractivityofanenzyme,e.g.,bypromotingaconformationalchange.Alternatively,alteredactivitymayresultfrombindinganotherproteinthatspecificallyrecognizesaphosphorylateddomain.E.g.,14-3-3proteinsbindtodomainsthatincludephosphorylatedSerorThrinthesequenceRXXX[pS/pT]XP,whereXcanbedifferentaminoacids.Bindingto14-3-3isamechanismbywhichsomeproteins(e.g.,transcriptionfactors)mayberetainedinthecytosol,&preventedfromenteringthenucleus.Proteinkinasesandphosphatasesarethemselvesregulatedbycomplexsignalcascades.Forexample:SomeproteinkinasesareactivatedbyCa++-calmodulin.ProteinKinaseAisactivatedbycyclic-AMP(cAMP).ProteinOH+ATPProteinOPOOO+ADPPiH2OProteinKinaseProteinPhosphataseAdenylateCyclase(AdenylylCyclase)catalyzes:ATPcAMP+PPiBindingofcertainhormones(e.g.,epinephrine)totheoutersurfaceofacellactivatesAdenylateCyclasetoformcAMPwithinthecell.CyclicAMPisthusconsideredtobeasecondmessenger.NNNNNH2OOHOHHHH2CHOPOO-1'3'5'4'2'cAMPPhosphodiesteraseenzymescatalyze:cAMP+H2OAMPThephosphodiesterasethatcleavescAMPisactivatedbyphosphorylationcatalyzedbyProteinKinaseA.ThuscAMPstimulatesitsowndegradation,leadingtorapidturnoffofacAMPsignal.NNNNNH2OOHOHHHH2CHOPOO-1'3'5'4'2'cAMPProteinKinaseA(cAMP-DependentProteinKinase)transfersPifromATPtoOHofaSerorThrinaparticular5-aminoacidsequence.ProteinKinaseAintherestingstateisacomplexof:•2catalyticsubunits(C)•2regulatorysubunits(R).R2C2R2C2Eachregulatorysubunit(R)ofProteinKinaseAcontainsapseudosubstratesequence,likethesubstratedomainofatargetproteinbutwithAlasubstitutingfortheSer/Thr.Thepseudosubstratedomainof(R),whichlacksahydroxylthatcanbephosphorylated,bindstotheactivesiteof(C),blockingitsactivity.R2C2+4cAMPR2cAMP4+2CWheneach(R)binds2cAMP,aconformationalchangecauses(R)torelease(C).ThecatalyticsubunitscanthencatalyzephosphorylationofSerorThrontargetproteins.PKIs,ProteinKinaseInhibitors,modulateactivityofthecatalyticsubunits(C).ViewananimationofactivationofProteinKinaseA.RhodopsinPDB1F88GProteinSignalCascadeMostsignalmoleculestargetedtoacellbindatthecellsurfacetoreceptorsembeddedintheplasmamembrane.Onlysignalmoleculesabletocrosstheplasmamembrane(e.g.,steroidhormones)interactwithintracellularreceptors.Alargefamilyofcellsurfacereceptorshaveacommonstructuralmotif,7transmembranea-helices.Rhodopsinwasthefirstofthesetohaveits7-helixstructureconfirmedbyX-raycrystallography.Rhodopsinisunique.Itsenseslight,viaaboundchromophore,retinal.Most7-helixreceptorshavedomainsfacingtheextracellularsideoftheplasmamembranethatrecognize&bindsignalmolecules(ligands).E.g.,theb-adrenergicreceptorisactivatedbyepinephrine&norepinephrine.Crystallizationofthisreceptorwasaidedbygeneticallyengineeringinsertionofthesolubleenzymelysozymeintoacytosolicloopbetweentransmembranea-helices.b-AdrenergicReceptorPDB2RH1LysozymeinsertligandSeeananimatedimageoftheb2-adrenergicreceptorstructureinawebsiteoftheKobilkalab.Thesignalisusuallypassedfroma7-helixreceptortoanintracellularG-protein.Seven-helixreceptorsarethuscalledGPCR,orG-Protein-CoupledReceptors.Approx.800differentGPCRsareencodedinthehumangenome.G-protein-CoupledReceptorsmaydimerizeorformoligomericcomplexeswithinthemembrane.Ligandbindingmaypromoteoligomerization,whichmayinturnaffectactivityofthereceptor.VariousGPCR-interactingproteins(GIPs)modulatereceptorfunction.EffectsofGIPsmayinclude:alteredligandaffinityreceptordimerizationoroligomerizationcontrolofreceptorlocalization,includingtransfertoorremovalfromtheplasmamembranepromotingcloseassociationwithothersignalproteinsG-proteinsareheterotrimeric,with3subunitsa,b,g.AG-proteinthatactivatescyclic-AMPformationwithinacelliscalledastimulatoryG-protein,designatedGswithalphasubunitGsa.Gsisactivated,e.g.,byreceptorsforthehormonesepinephrineandglucagon.Theb-adrenergicreceptoristheGPCRforepinephrine.a&gsubunitshavecovalentlyattachedlipidanchorsthatbindaG-proteintotheplasmamembranecytosolicsurface.AdenylateCyclase(AC)isatransmembraneprotein,withcytosolicdomainsformingthecatalyticsite.AChormonesignaloutsideGPCRplasmamembraneGTPGDPATPcAMP+PPiaggacytosolGDPbbGTPTheasubunitofaG-protein(Ga)bindsGTP,&canhydrolyzeittoGDP+Pi.ThesequenceofeventsbywhichahormoneactivatescAMPsignaling:1.InitiallyGahasboundGDP,anda,b,&gsubunitsarecomplexedtogether.Gb,g,thecomplexofb&gsubunits,inhibitsGa.AChormonesignaloutsideGPCRplasmamembraneGTPGDPATPcAMP+PPiaggacytosolGDPbbGTP2.Hormon