物质的跨膜运输

Chapter5 LearningObjectives 1 Principlesofmembranetransport 2 Passivetransportandactivetransport 3 Twomainclassesofmembranetransportproteins CarriersandChannels 4 Theiontransportsystems 5 EndocytosisandPhagocytosis cellularuptakeofmacromoleculesandparticles A TheMovementofSubstancesAcrossCellMembranes Amotorneuroncellbodyinthespinalcord A Manythousandsofnerveterminalssynapseonthecellbodyanddendrites Thesedeliversignalsfromotherpartsoftheorganismtocontrolthefiringofactionpotentialsalongthesingleaxonofthislargecell B Micrographshowinganervecellbodyanditsdendritesstainedwithafluorescentantibodythatrecognizesacytoskeletalprotein green Thousandsofaxonterminals red fromothernervecells notvisible makesynapsesonthecellbodyanddendrites theyarestainedwithafluorescentantibodythatrecognizesaproteininsynapticvesicles 1 Principlesofmembranetransport Theplasmamembraneisaselectivelypermeablebarrier Itallowsforseparationandexchangeofmaterialsacrosstheplasmamembrane Figure11 1Therelativepermeabilityofasyntheticlipidbilayertodifferentclassesofmolecules Thesmallerthemoleculeand moreimportant thefewerhydrogenbondsitmakeswithwater themorerapidlythemoleculediffusesacrossthebilayer B Theprotein freelipidbilayersarehighlyimpermeabletoions Ifunchargedsolutesaresmallenough theycanmovedowntheirconcentrationgradientsdirectlyacrossthelipidbilayerbysimplediffusion Mostsolutescancrossthemembraneonlyifthereisamembranetransportproteintotransferthem Passivetransport inthesamedirectionasaconcentrationgradient Activetransport ismediatedbycarrierproteins againstaconcentrationgradient requireaninputofenergy Diffusionofsmallmoleculesacrossphospholipidbilayers Figure11 2Permeabilitycoefficients cm sec forthepassageofvariousmoleculesthroughsyntheticlipidbilayers Therateofflowofasoluteacrossthebilayerisdirectlyproportionaltothedifferenceinitsconcentrationonthetwosidesofthemembrane Multiplyingthisconcentrationdifference inmol cm3 bythepermeabilitycoefficient cm sec givestheflowofsoluteinmolespersecondpersquarecentimeterofmembrane Aconcentrationdifferenceoftryptophanof10 4mol cm3 10 4 10 3L 0 1M forexample wouldcauseaflowof10 4mol cm3x10 7cm sec 10 11mol secthrough1cm2ofmembrane or6x104molecules secthrough1microns2ofmembrane C Theenergeticsofsolutemovement Diffusionisthespontaneousmovementofmaterialfromaregionofhighconcentrationtoaregionoflowconcentration Thefree energychangeduringdiffusionofnonelectrolytesdependsontheconcentrationgrdient Thefree energychangeduringdiffusionofelectrolytesdependsontheelectrochemicalgrdient D Transportprocesseswithinaneukaryoticcell 2 Passivetransportandactivetransport A Comparisonoftwoclassesoftransport Figure11 7Kineticsofsimplediffusioncomparedtocarrier mediateddiffusion Whereastherateoftheformerisalwaysproportionaltothesoluteconcentration therateofthelatterreachesamaximum Vmax whenthecarrierproteinissaturated Thesoluteconcentrationwhentransportisathalfitsmaximalvalueapproximatesthebindingconstant KM ofthecarrierforthesoluteandisanalogoustotheKMofanenzymeforitssubstrate Thegraphappliestoacarriertransportingasinglesolute thekineticsofcoupledtransportoftwoormoresolutes seetext aremorecomplexbutshowbasicallysimilarphenomena B Twoclassesofmembranetransportproteins Carrierproteinsareresponsibleforboththepassiveandtheactivetransport Channelproteinsareonlyresponsibleforpassivetransport Figure11 8Threetypesofcarrier mediatedtransport Theschematicdiagramshowscarrierproteinsfunctioningasuniports symports andantiports Carrierproteinsbindoneormoresolutemoleculesononesideofthemembraneandthenundergoaconformationalchangethattransferthesolutetotheothersideofthemembrane Thecarrierprotein theGlucosetransporter GluT1 intheerythrocytePM alterconformationtofacilitatethetransportofglucose Facilitatediffusion Protein mediatedmovement movementdownthegradient Mostofthechannelproteinsareionchannels includingthreetypes withionchannelsthattheycanbeopenedandclosed Figure11 36 Amodelforthestructureoftheacetylcholinereceptor Fivehomologoussubunits a a b g d combinetoformatransmembraneaqueouspore Theporeislinedbyaringoffivetransmembraneahelices onecontributedbyeachsubunit Initsclosedconformation theporeisthoughttobeoccludedbythehydrophobicsidechainsoffiveleucines onefromeachahelix whichformagatenearthemiddleofthelipidbilayer Thenegativelychargedsidechainsateitherendoftheporeensurethatonlypositivelychargedionspassthroughthechannel Bothoftheasubunitscontainanacetylcholine bindingsite whenacetylcholinebindstobothsites thechannelundergoesaconformationalchangethatopensthegate possiblybycausingtheleucinestomoveoutward 电压门控离子通道 铰链细胞失水 应力激活的离子通道 2X1013N 0 04nm 3 Activetransport Carrierprotein mediatedmovementupthegradient A Thisprocessdiffersfromfacilitateddiffusionintwocrucialaspects Activetransportmaintainsthegradientsforpotassium sodium calcium andotherionsacrossthecellmembrane Alwaysmovessolutesupaconcentrationorelectrochemicalgradient ActivetransportcouplesthemovementofsubstancesagainstgradientstoATPhydrolysis i eAlwaysrequirestheinputofenergy B Cellscarryoutactivetransportinthreemainways Coupletheuphilltransportofonesoluteacrossmembranetothedownhilltransportofanother CoupleuphilltransporttothehydrolysisofATP Mainlyinbacteria coupleuphilltransporttoaninputofenergyfromlight C DirectactivetransportdependsonfourtypesoftransportATPases ThefourclassesofATP poweredtransportproteins P typestandsforphosphorylation ABC ATP bindingCassette superfamily bacteria humans Twotransmembrane T domainsandtwocytosolicATP binding A domains TheNa K ATPase AcouplingactivetransporttoATPhydrolysis TheNa K ATPaserequiresK outside Na andATPinside andisinhibitedbyouabain TheratioofNa K pumpedis3 2foreachATPhydrolyzed TheNa K ATPaseisaP typepump ThisATPaseseruentiallyphosphorylatesanddephosphory latesitselfduringthepumpingcycle TheNa K ATPaseisfoundonlyinaniimals TheactivetransportofNa K ATPaseisusedtomaintainselectrochemicaliongradients andtherebymaintainscell sexcitability TheNa K pumoisrequiredtomaintainosmoticbalanceandstabilizecellvolume ThebiologicalfunctionsofNa K pump formingaphosphorylatedproteinintermediate AModelMechanismfortheNa K ATPase OtherP typepunps includingH andCa ATPases andH K ATPases PlantcellshaveaH transportingplasmamembranepump Thisprotonpumpplaysakeyroleinthesecondarytransportofsolutes inthecontrolofcytosolicpH andpossiblyincontrolofcellgrowthbymeansofacidificationoftheplantcellwall Ca2 pump Ca2 ATPasepresentinboththeplasmamembraneandthemembranesoftheER Itcontain10transmembrane helices ThisCa2 pumpfunctionstoactivelytransportCa2 outofthecytosolintoeithertheextracellularspaceorthelumenoftheER H K ATPases epithelialliningofthestomach whichsecretesasolutionofconcentratedacid upto0 16NHCl intothestomachchamber TheV typepump utilizetheenergyofATPwithoutformingaphosphorylatedproteinintermediate Vacuolar V type pumpactivelytransportH acrossthemembranesofcytoplasmicorganellesandvacuoles Theyprecentinlysosomes secretorygranules andplantcellvacuoles havealsobeenfoundintheplasmamembranesofavarietyofcells kidneytubules 4 IndirectactivetransportisdrivenbyIongradients Cotransport A Sugars aminoacids andotherorganicmoleculesintocells Theinwardtransportofsuchmoleculesuptheirconcentrationgradientsisoftencoupledto anddrivenby theconcomitantinwardmovementoftheseionsdowntheirelectrochemicalgradients Animalcells Sodiumions Na K ATPase Plant fungi bacterium Protons H ATPase Gradientscreatedbyactiveionpumpingstoreenergythatcanbecoupledtoothertransportprocesses Thedifferencebetweenanimalandplantcellstoabsorbnutrients B Cotransport Symportandantiport Na linkedsymportersimportaminoacidsandglucoseintomanyanimalcells Na linkedantiporterexportsCa fromcardiacmusclecells Medicine OuabainanddigoxinincreasetheforceofheartmusclecontractionbyinhibitingtheNa K ATPase FewerCa ionsareexported 5 Endocytosis Largemoleculesenterintocells A Endocytosisimportsextracellularmoleculesdissolvedorsuspendedinfluidbyformingvesiclesfromtheplasmamembrane Bulk phaseendocytosisdoesnotrequiresurfacemembranerecognition Itisthenonspecificuptakeofextracellularfluids Receptor mediatedendocytosis RME followsthebindingofsubstancestomembranereceptors B Phagocytosis Theuptakeoflargeparticles Including macromolecules celldebris evenmicroorganismsandothercells PhagocytosisisusuallyrestrictedtospecializedcellscalledPhagocytes Phagocytosisisinitiatedbycellularcontactwithanappropriatetarget PhagocytosismaybestimulatedbytheopsoninsPhagocytosisisdrivenbycontractileactivitiesofMF C Receptor mediatedendocytosis Structureofaclathrin coatedvesicle Modelfortheformationofaclathrin coatedpitandtheselectiveincorporationofintegralmembraneproteinsintoclathrin coatedvesicles Theendocyticpathwayisdividedintotheearlyendosomesandlateendosomespathway Materialsintheearlyendosomesaresorted Integralmembraneproteinsareshippedbacktothemembrane Otherdissolvedmaterialsandboundligands Multivesicularbody MTmediatedtransport thelateendosomes Moleculesthatreachthelateendosomesaremovedtolysosomes 6 ExocytosisConstitutiveexocytosispathwayRegulatedexocytosispathway 7 MembranePotentialsandNerveImpulses K gradientsmaintainedbytheNa K ATPaseareresponsiblefortherestingmembranepotential Restingstate AllNa andK channelsclosed Depolarizingphase Na channelsopen triggeringanactionpotential Repolarizingphase Na channelsinactivated K channelsopen Hyperpolarizingphase K channelsremainopen Na channelsinactivated B Theactionpotential Thechangesinionchannelsandmembranepotential Thesequenceofeventsduringsynaptictransmission Excitablemembranesexhibit all or none behavior Propagationofactionpotentialsasanimpulse Chapter5 B CellSignaling LearningObjectives 1 Someofthebasiccharacteristicsofcellsignaling2 Thetypesofsignalmolecules receptors molecularswitchesandeffectors 3 Thedifferentsignaltransductionpathways 4 Theconvergence divergence andcrosstalkingbetweendifferentsignalingpathways 1 Overviewofcellsignaling A Someofthebasiccharacteristicsofcellsignaling Cellmustrespondappropriatelytoexternalstimulitosurvive Cellsrespondtostimuliviacellsignaling Recognitionofthestimulusbyaspecificplasmamembranereceptor Transferofasignalacrosstheplasmamembrane Transmissionofthesignaltoeffectormoleculeswithinthecell whichcausesachangeincellularactivities Cessationofthecellularresponseduetoinactivationofthesignalmolecule Signaltransductionpathwaysconsistofaseriesofsteps signalmagnification Eachcellisprogrammedtorespondtospecificcombinationsofexreaceluularsignalmolecules Differentcellscanresponddifferentlytothesameextracellularsignalmolecule Figure15 9Thesamesignalingmoleculecaninducedifferentresponsesindifferenttargetcells Insomecasesthisisbecausethesignalingmoleculebindstodifferentreceptorproteins asillustratedin A and B Inothercasesthesignalingmoleculebindstoidenticalreceptorproteinsthatactivatedifferentresponsepathwaysindifferentcells asillustratedin B and C Inallofthecasesshownthesignalingmoleculeisacetylcholine D Acellcanremembertheeffectofsomesignals afterthesignalhasdisappeared Ca2 ProteinkinaseactivitedbyCa2 tophosphorylateitselfandotherproteins theautophosphorylationkeepsthekinaseactivelongafterCa2 levelsreturntonormal providingamemorytraceoftheinitialsignal Transientextracellularsignalsofteninducemuchlonger termchangesincellsduringthedevelopmentofamulticellularorganism Theyusuallydependonself activatingmemorymechanismsthatoperatefurtherdownstreaminasignalingpathway atthelevelofgenetranscription B Theformsofcellcommunication Differenttypesofchemicalsignalscanbereceivedbycells Gapjunction C SignalMoleculesandReceptors signalmolecules Lipid solublehormones Water solublehormones nitricoxide NO andcarbonmonoxide CO ascellularmessengers Receptorsincludethreeclasses glycoproteins D TwotypesofintracellularsignalingproteinsthatactasMolecularSwitches Phosphorylationanddephosphorylationviaproteinkinasesandphosphatases Therebystimulatingorinhibitingtheactivities GAPsinactivateG protein GEFsactivatesG protein GDIs guaninenucleotide dissociationinhibitors maintaintheG proteininactive 2 Signaltransdutionmediatedbythereceptorswithincells A Somesmallhydrophobichormones steroidhormones whosereceprorsareintracellulargeneregulatoryproteins Figure15 13Earlyprimaryresponse A anddelayedsecondaryresponse B thatresultfromtheactivationofanintracellularreceptorprotein Theresponsetoasteroidhormoneisillustrated butthesameprinciplesapplyforallligandsthatactivatethisfamilyofreceptorproteins Someoftheprimary responseproteinsturnonsecondary responsegenes whereasothersturnofftheprimary responsegenes Theactualnumberofprimary andsecondary responsegenesisgreaterthanshown Asexpected drugsthatinhibitproteinsynthesissuppressthetranscriptionofsecondary responsegenesbutnotprimary responsegenes B NitricoxidecouplesGprotein linkedreceptorstimulationinendothelialcellstorelaxationofsmoothmusclecellsinbloodvessels Ithasbeenknownformanyyearsthatacetylcholinedilatebloodvesselsbycausingtheirsmoothmusclestorelax In1980 Furchgottconcludedthatbloodvesselsaredilatedbecausetheendothelialcellsproduceasignalmoleculethatmakessmoothmusclecellsrelax In1986workbyFurchgottandparallelworkbyLouisIgnarroidentifiedNOasthesignalthatcauserelaxationofthevascularsmoothmuscle 1998 ReceivedNobelPrize TheactionofNitricoxideonbloodvessels Themechanismbywhichacetylcholinestimulationoftheendothelialcellsleadstosmoothmusclerelaxationalsoexplainsthemechanismofactionofthechemicalnitroglycerin Thedrugsildenafil soldunderthetradenameViagra isaninhibitorofacyclicGMP specificphosphodiesterasethatnormallycatalyzesthebreakdownofcyclicGMP Thecarbonmonoxide CO actsasacellularmessengertostimulatetheproductionofcGMPbystimulatingG cyclase 3 Signaltransductionmediatedbythereceptorsonthecellsurface A MediatedbytheIon LinkedReceptorswhichconvertchemicalsignalsintoelectricalones 4or6 helixtransmembranereceptor B SignaltransductionmediatedbyGprotein linkedreceptors ThestructureofGprotein linkedreceptors Seven helixtransmembrane C terminal Ser andThr rich thesitesofphosphorylationmakeforthedesensitizationofGPLR Figure15 18TwomajorpathwaysbywhichG protein linkedcell surfacereceptorsgeneratesmallintracellularmediators Inbothcasesthebindingofanextracellularligandalterstheconformationofthecytoplasmicdomainofthereceptor causingittobindtoaGproteinthatactivates orinactivates aplasmamembraneenzyme InthecyclicAMP cAMP pathwaytheenzymedirectlyproducescyclicAMP IntheCa2 pathwaytheenzymeproducesasolublemediator inositoltrisphosphate thatreleasesCa2 fromtheendoplasmicreticulum ThestructureandactivationofGproteins Figure15 23AcurrentmodelofhowGscouplesreceptoractivationtoadenylylcyclaseactivation Aslongastheextracellularsignalingligandremainsbound thereceptorproteincancontinuetoactivatemoleculesofGsprotein therebyamplifyingtheresponse Moreimportant analphascanremainactiveandcontinuetostimulateacyclasemoleculeformanysecondsafterthesignalingliganddissociatesfromthereceptor providingevengreateramplification C CyclicAMPsignalingpathway G proteinactivationandinactivationcycle TheactivationofproteinkinaseAbycyclicAMPs Secondmessengers cAMP aneffector amplifytheresponsetoasingleextracellularligandbycAMPtotriggerareactioncascade ThecascadestartswiththebindingofcAMPtocAMP dependentproteinkinaseA PKAinhibitsglycogensynthaseandactivatesphosphorylasekinase DoubleMessengersystem Figure15 32TwointracellularpathwaysbywhichactivatedC kinasecanactivatethetranscriptionofspecificgenes Inone redarrows C kinaseactivatesaphosphorylationcascadethatleadstothephosphorylationofapivotalproteinkinasecalledMAP kinase whichinturnphosphorylatesandactivatesthegeneregulatoryproteinElk 1 Elk 1isboundtoashortDNAsequenceinassociationwithanotherDNA bindingprotein Intheotherpathway greenarrows C kinaseactivationleadstothephosphorylationofIk B whichreleasesthegeneregulatoryproteinNF kBsothatitcanmigrateintothenucleusandactivatethetranscriptionofspecificgenes Themechanismsthatshutoffasignalareasimportantasthemechanismsthatturniton Choleraiscausedbyabacteriumthatmultipliesintheintestine whereitproducesaproteincalledcholeratoxin Thisentersthecellsliningtheintestineandmodifiesthe subunitofGproteinsothatitcannolongerhydrolyzeitsboundGTP Thealtered subunitthusremainsintheactivestateindefinitely continuingtotransmitasignaltoitstargetproteins OutflowofNa andwaterintothegut D ThepathwaythroughphospholipaseCresultsinariseinintracellularCa Cytolasmiccalciumlevelsaredeterminedbyeventswithinamembrane IP3 Ca2 pathwayandDG PKCpathway ElevationofcytosolicCa2 viatheIPsignalingpathway Signals GPLR GP PLC IP3andDAG twinsignals IP3 IP3receptor Ca2 channel locatedatthesurfaceofsER ElevationofcytosolicCa2 DAG activatesPKC tophosphoralateSerandThrontargetproteins Calciumbindstocalcium bindingproteins CaM whichaffectsotherproteins Structureofcalmodulin acytosolicproteinof148aminoacidsthatbindCa2 ions Regulatingcalciumconcentrationsinplantcells Cytosoliccalciumchangesinresponsetoseveralstimuli includinglight pressure gravity andhormones Calciumsignalingaidsindecreasingturgorpressureinguardcells Ca2 CaMdep proteinkinase CaM kinase mediatemanyoftheactionsofCa2 inanimalcells Thefunctionsofincreasethelevelsofcytosoliccalcium CaM start upembryodevelopmentafterthefecundation excitatingcontractofmusclecells excitatingsecretionofendocrineandnervecells G protein linkedreceptordesensitizationdependsonreceptorphosphorylationbyPKA PKC CaMK2orG protein linkedreceptorkinases GRKs Thetargetcellscanbecomedesensitizedtoasignalmoleculebyfiveways Threegeneralwaysofthedesensitization 1 Receptorinactivationbyalteration 2 Receptorsequestrationbyinternalization 3 Receptordown regulationbydestroyinginLs Sequestration down regulation inactivation inactivation inhibitoryprotein E Receptortyrosinekinase RTK andRTK Rassignalingpathway RTKarethesecondmajortypeofcell surfacereceptors SignalingligandsofRTKs 1 Nervegrowthfactor NGF 2 Platelet derivedgrowthfactor PDGF 3 Fibroblastgrowthfactor FGF 4 Epidermalgrowthfactor EGF 5 insulinandinsulin likeGF IGF 1 6 ephrins Eph 7 vascularendothelialfactor VEGF Sixclassesofenzyme linkedreceptorshavethusfarbeenidentified Receptortyrosinekinase RTK Tyrosine kinase associatedreceptor Recep