Pathways of Intracellular Signal Transduction文档格式.docx

Pathways of Intracellular Signal Transduction文档格式.docx

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ThecAMPPathway:

SecondMessengersandProteinPhosphorylation

Intracellularsignalingwasfirstelucidatedbystudiesoftheactionofhormonessuchasepinephrine,whichsignalsthebreakdownofglycogentoglucoseinanticipationofmuscularactivity.In1958,EarlSutherlanddiscoveredthattheactionofepinephrinewasmediatedbyanincreaseintheintracellularconcentrationofcyclicAMP（cAMP）,leadingtotheconceptthatcAMPisasecondmessengerinhormonalsignaling（thefirstmessengerbeingthehormoneitself）.CyclicAMPisformedfromATPbytheactionofadenylylcyclaseanddegradedtoAMPbycAMPphosphodiesterase（Figure13.18）.Asdiscussedearlier,theepinephrinereceptoriscoupledtoadenylylcyclaseviaaGproteinthatstimulatesenzymaticactivity,therebyincreasingtheintracellularconcentrationofcAMP（seeFigure13.11）.

Figure13.18

SynthesisanddegradationofcAMP.CyclicAMPissynthesizedfromATPbyadenylylcyclaseanddegradedtoAMPbycAMPphosphodiesterase.

HowdoescAMPthensignalthebreakdownofglycogen?

ThisandmostothereffectsofcAMPinanimalcellsaremediatedbytheactionofcAMP-dependentproteinkinase,orproteinkinaseA,anenzymediscoveredbyDonalWalshandEdKrebsin1968.TheinactiveformofproteinkinaseAisatetramerconsistingoftwocatalyticandtworegulatorysubunits（Figure13.19）.CyclicAMPbindstotheregulatorysubunits,leadingtotheirdissociationfromthecatalyticsubunits.Thefreecatalyticsubunitsarethenenzymaticallyactiveandabletophosphorylateserineresiduesontheirtargetproteins.

Figure13.19

RegulationofproteinkinaseA.TheinactiveformofproteinkinaseAconsistsoftworegulatory（R）andtwocatalytic（C）subunits.BindingofcAMPtotheregulatorysubunitsinducesaconformationalchangethatleadstodissociationofthecatalytic（more...）

Intheregulationofglycogenmetabolism,proteinkinaseAphosphorylatestwokeytargetenzymes（Figure13.20）.Thefirstisanotherproteinkinase,phosphorylasekinase,whichisphosphorylatedandactivatedbyproteinkinaseA.Phosphorylasekinaseinturnphosphorylatesandactivatesglycogenphosphorylase,whichcatalyzesthebreakdownofglycogentoglucose-1-phosphate.Inaddition,proteinkinaseAphosphorylatestheenzymeglycogensynthase,whichcatalyzesglycogensynthesis.Inthiscase,however,phosphorylationinhibitsenzymaticactivity.ElevationofcAMPandactivationofproteinkinaseAthusblocksfurtherglycogensynthesisatthesametimeasitstimulatesglycogenbreakdown.

Figure13.20

RegulationofglycogenmetabolismbyproteinkinaseA.ProteinkinaseAphosphorylatesbothglycogensynthaseandphosphorylasekinase.Glycogensynthase（whichcatalyzesglycogensynthesis）isinhibitedbythisphosphorylation,whereasphosphorylase（more...）

Thechainofreactionsleadingfromtheepinephrinereceptortoglycogenphosphorylaseprovidesagoodillustrationofsignalamplificationduringintracellularsignaltransduction.Eachmoleculeofepinephrineactivatesonlyasinglereceptor.However,eachreceptormayactivateuptoahundredmoleculesofGs.EachmoleculeofGsthenstimulatestheenzymaticactivityofadenylylcyclase,whichcancatalyzethesynthesisofmanymoleculesofcAMP.SignalamplificationcontinuesaseachmoleculeofproteinkinaseAphosphorylatesmanymoleculesofphosphorylasekinase,whichinturnphosphorylatemanymoleculesofglycogenphosphorylase.Hormonebindingtoasmallnumberofreceptorsthusleadstoactivationofamuchlargernumberofintracellulartargetenzymes.

Inmanyanimalcells,increasesincAMPactivatethetranscriptionofspecifictargetgenesthatcontainaregulatorysequencecalledthecAMPresponseelement,orCRE（Figure13.21）.Inthiscase,thesignaliscarriedfromthecytoplasmtothenucleusbythecatalyticsubunitofproteinkinaseA,whichisabletoenterthenucleusfollowingitsreleasefromtheregulatorysubunit.Withinthenucleus,proteinkinaseAphosphorylatesatranscriptionfactorcalledCREB（forCRE-bindingprotein）,leadingtotheactivationofcAMP-induciblegenes.SuchregulationofgeneexpressionbycAMPplaysimportantrolesincontrollingtheproliferation,survival,anddifferentiationofawidevarietyofanimalcells.

Figure13.21

CyclicAMP-induciblegeneexpression.ThefreecatalyticsubunitofproteinkinaseAtranslocatestothenucleusandphosphorylatesthetranscriptionfactorCREB（CRE-bindingprotein）,leadingtoexpressionofcAMP-induciblegenes.

Itisimportanttorecognizethatproteinkinases,suchasproteinkinaseA,donotfunctioninisolationwithinthecell.Tothecontrary,proteinphosphorylationisrapidlyreversedbytheactionofproteinphosphatases.Someproteinphosphatasesaretransmembranereceptors,asdiscussedintheprecedingsection.Anumberofothersarecytosolicenzymesthatremovephosphategroupsfromeitherphosphorylatedtyrosineorserine/threonineresiduesintheirsubstrateproteins.Theseproteinphosphatasesservetoterminatetheresponsesinitiatedbyreceptoractivationofproteinkinases.Forexample,theserineresiduesofproteinsthatarephosphorylatedbyproteinkinaseAareusuallydephosphorylatedbytheactionofaphosphatasecalledproteinphosphatase1（Figure13.22）.ThelevelsofphosphorylationofproteinkinaseAsubstrates（suchasphosphorylasekinaseandCREB）arethusdeterminedbyabalancebetweentheintracellularactivitiesofproteinkinaseAandproteinphosphatases.

Figure13.22

RegulationofproteinphosphorylationbyproteinkinaseAandproteinphosphatase1.ThephosphorylationoftargetproteinsbyproteinkinaseAisreversedbytheactionofproteinphosphatase1.

AlthoughmosteffectsofcAMParemediatedbyproteinkinaseA,cAMPcanalsodirectlyregulateionchannels,independentofproteinphosphorylation.CyclicAMPfunctionsinthiswayasasecondmessengerinvolvedinsensingsmells.ManyoftheodorantreceptorsinsensoryneuronsinthenoseareGprotein-coupledreceptorsthatstimulateadenylylcyclase,leadingtoanincreaseinintracellularcAMP.RatherthanstimulatingproteinkinaseA,cAMPinthissystemdirectlyopensNa+channelsintheplasmamembrane,leadingtomembranedepolarizationandinitiationofanerveimpulse.

CyclicGMP

CyclicGMP（cGMP）isalsoanimportantsecondmessengerinanimalcells,althoughitsrolesarenotasclearlyunderstoodasthoseofcAMP.CyclicGMPisformedfromGTPbyguanylylcyclasesanddegradedtoGMPbyaphosphodiesterase.Asdiscussedearlierinthischapter,differenttypesofguanylylcyclasesareactivatedbybothnitricoxideandpeptideligands.StimulationoftheseguanylylcyclasesleadstoelevatedlevelsofcGMP,whichthenmediatebiologicalresponses,suchasbloodvesseldilation.TheactionofcGMPisfrequentlymediatedbyactivationofacGMP-dependentproteinkinase,althoughcGMPcanalsoacttoregulateothertargets,includingionchannels.

Thebest-characterizedroleofcGMPisinthevertebrateeye,whereitservesasthesecondmessengerresponsibleforconvertingthevisualsignalsreceivedaslighttonerveimpulses.ThephotoreceptorinrodcellsoftheretinaisaGprotein-coupledreceptorcalledrhodopsin（Figure13.23）.Rhodopsinisactivatedasaresultoftheabsorptionoflightbytheassociatedsmallmolecule11-cis-retinal,whichthenisomerizestoall-trans-retinal,inducingaconformationalchangeintherhodopsinprotein.RhodopsinthenactivatestheGproteintransducin,andtheαsubunitoftransducinstimulatestheactivityofcGMPphosphodiesterase,leadingtoadecreaseintheintracellularlevelofcGMP.ThischangeincGMPlevelinretinalrodcellsistranslatedtoanerveimpulsebyadirecteffectofcGMPonionchannelsintheplasmamembrane,similartotheactionofcAMPinsensingsmells.

Figure13.23

RoleofcGMPinphotoreception.AbsorptionoflightbyretinalactivatestheGprotein-coupledreceptorrhodopsin.TheαsubunitoftransducinthenstimulatescGMPphosphodiesterase,leadingtoadecreaseinintracellularlevelsofcGMP.

PhospholipidsandCa2+

Oneofthemostwidespreadpathwaysofintracellularsignalingisbasedontheuseofsecondmessengersderivedfromthemembranephospholipidphosphatidylinositol4,5-bisphosphate（PIP2）.PIP2isaminorcomponentoftheplasmamembrane,localizedtotheinnerleafletofthephospholipidbilayer（seeFigure12.2）.AvarietyofhormonesandgrowthfactorsstimulatethehydrolysisofPIP2byphospholipaseC—areactionthatproducestwodistinctsecondmessengers,diacylglycerolandinositol1,4,5-trisphosphate（IP3）（Figure13.24）.DiacylglycerolandIP3stimulatedistinctdownstreamsignalingpathways（proteinkinaseCandCa2+mobilization,respectively）,soPIP2hydrolysistriggersatwo-armedcascadeofintracellularsignaling.

Figure13.24

HydrolysisofPIP2.PhospholipaseC（PLC）catalyzesthehydrolysisofph