细胞生物学2010课件chapter

Chapter4Biomembranes:TheirStructure,ChemistryandFunctions

Learningobjectives:AbriefhistoryofstudiesonthestructrureoftheplasmamembraneModelofmembranestructure:anexperimentalperspectiveThechemicalcompositionofmembranesCharacteristicsofbiomembraneAnoverviewofthefunctionsofbiomembranes

AbriefhistoryofstudiesonthestructrureoftheplasmicmembraneA.Conception:Plasmamembrane(cellmembrane),Intracellularmembrane,Biomembrane.B.ThehistoryofstudyOverton(1890s):

LipidnatureofPM;Toanswerthequestionthathowmanylipidlayerswereinmembrane,in1925GorterandGrendelextractedthelipidsfromaknownnumberoferythrocytesandspreadthelipidfilmonawatersurface.Theareaoflipidfilmonthewaterwasabouttwice(1.8-2.2)theestimatedtotalsurfaceareaoftheerythrocytes,sotheyconcludedthattheerythrocyteplasmamembraneconsistedofnotonebuttwolayersoflipids.

GortterandGrendel(1925):

ThebasisofmembranestructureisalipidbilayerCellphysiologists(1920sand1930s):ThedecreaseinsurfacetensionsofPMmightbeexplainedbytheproteins.H.DavsonandJ.Danielli(1935):

“sandwichmodel”

Membranesalsocontainproteins.

Ifthemembranesonlyconsistofpurelipids,itcouldnotexplainallthepropertiesofmembranes.Forexample,sugars,ions,andotherhydrophilicsolutesmoveintoandoutofcellsmuchmorereadilythancouldbeexplainedbythepermeabilityofpurelipidbilayers.Toexplainsuchdifferences,DavsonandDanielliinvokedthepresenceofproteinsinmembranesin1935.

Theplasmamembraneiscomposedofalipidbilayerthatislinedonbothitsinnerandoutersurfacebyalayerofglobularproteins;inadditionto,thepresenceofprotein-linedporesforpolarsolutesandionstoenterandexitthecell.J.D.Robertson(1959):TheTEMshowing:thetrilaminarappearanceofPM;

Unitmembranemodel;S.J.SingerandG.Nicolson(1972):

fluid-mosaicmodel;K.Simonsetal(1997):

lipidraftsmodel;FunctionalraftsinCellmembranes.Nature387:569-5722023/3/57脂筏（lipidraft）Lipidraftsaremembranemicrodomainsenrichedinsaturatedphospholipidsandcholesterol.质膜上富含鞘磷脂和胆固醇的微结构域。约70nm，动态结构，位于质膜的外小页。具有较长的饱和脂肪酸链，分子间的作用力较强，所以这些区域结构致密，介于无序液体与液晶之间，称为有序液体（Liquid-ordered）。在低温下这些区域能抵抗非离子去垢剂的抽提，所以又称为抗去垢剂膜（detergent-resistantmembranes，DRMs）。像一个蛋白质停泊的平台，与膜的信号转导、蛋白质分选均有密切的关系。2023/3/58

脂筏中的胆固醇就像胶水一样，它对具有饱和脂肪酸链的鞘磷脂亲和力很高，而对不饱和脂肪酸链的亲和力低;用甲基-β-环糊精（methyl-β-cyclodextrin）去除胆固醇，抗去垢剂的蛋白就变得易于提取。膜中的鞘磷脂主要位于外小页，而且大部分都参与形成脂筏。脂筏的面积可能占膜表面积的一半以上。脂筏的大小是可以调节的，小的独立脂筏可能在保持信号蛋白呈关闭状态方面具有重要作用;当必要时，这些小的脂筏聚集成大一个大的平台，在那里信号分子（如受体）将和它们的配件相遇，启动信号传递途径

2.SingerandNicolson’sModelofmembranestructure:Thefluid-mosaicmodelisthe“centraldogma”ofmembranebiology.Thecorelipidbilayerexistsinafluidstate,capableofdynamicmovement.Membraneproteinsformamosaicofparticlespenetratingthelipidtovaryingdegrees.TheFluidMosaicModel,proposedin1972bySingerandNicolson,hadtwokeyfeatures,bothimpliedinitsname.3.ThechemicalcompositionofmembranesA.MembraneLipids:TheFluidPartoftheModelPhospholipids:PhosphoglycerideandsphingolipidsGlycolipids

Sterols(isonlyfoundinanimals)Membranelipidsareamphipathic.Therearethreemajorclassesoflipids:Threekindsofmovementof

MembranelipidsThreekindsofmovement:Rotationaboutitslongaxis;Lateraldiffusionbyexchangingplaces;Transversediffusion,or“flip-flop”fromonemonolayertotheother.Flippasescatalyzetheflip-flop.TheeffectsoffattyacidcompositiononmembranefluidityThelengthofthefattyacidThedegreeofunsaturationoftheirsidechainsThetemperature.TheeffectsofsterolsonmembranefluidityLiposomeandapplicationStudyonnature;genetransfer;asacarrier.2023/3/522Thephysicalstateofthelipidofamembrane◆相变温度（transitiontemperature）

在生理条件下,膜脂多呈拟液态（liquid-likestate）。温度下降至某点,则变为晶态（frozencrystallinegel）。一定温度下,晶态又可熔解再变成液晶态，这种临界温度称为相变温度。◆相变(phasetransition)

在不同温度下发生的膜脂状态的改变称为相变。不同的膜脂由于成分不同而各有其相变温度。2023/3/523

Factorsofinfluencebilayerfluidity

◆脂肪酸链的影响

膜脂肪酸链对流动性的影响主要是不饱和程度和链的长短：■Thegreaterthedegreeofunsaturationofthefattyacidsofthebilayer,thelowerthetemperaturebeforethebilayergels.

■Theshorterthefattyacylchainsofaphospholipid,theloweritsmeltingtemperature.

2023/3/524◆胆固醇的影响■cholesterolmoleculesareorientedwiththeirsmallhydrophilichydroxylgrouptowardthemembranesurfaceandtheremainderofthemoleculeembeddedinthelipidbilayer.■Thehydrophobicringsofacholesterolmoleculeareflatandrigid,andtheyinterferewiththemovementsofthefattyacidtailsofthephospholipids■在相变温度以上,它可使磷脂分子的脂酰链末端的运动减小,即限制膜的流动性。■在相变温度以下,可增加脂类分子脂酰链的运动,这样可以增强膜的流动性。2023/3/525胆固醇对流动性的影响?2023/3/526◆卵磷脂/鞘磷脂比值的影响■哺乳动物膜中,卵磷脂(phosphatidycholine)和鞘磷脂(sphingomyelin)的含量约占整个膜脂的50%;■卵磷脂所含的脂肪酸链的不饱和程度高,链较短,相变温度低,因此卵磷脂含量高,流动性大;■而鞘磷脂的脂肪酸链的饱和程度高,相变温度也高,因此,鞘磷脂的含量高,流动性低。2023/3/527◆影响膜蛋白移动的因素■整合蛋白相互间的影响■膜骨架的影响■细胞外基质的影响■相邻细胞的影响■细胞外配体、抗体、及药物大分子的影响B.MembraneProteins:The“Mosaic”PartoftheModelMembranescontainintegral,peripheral,andlipid-anchoredproteins:Rolled-upsheethelixAmphipathichelix

lipid-anchoredproteinsNoncovalentinteractionsIntegralproteinsareamphipathic,withhydrophobicdomainsanchoringtheminthebilayerandhydrophilicregionsformingfunctionaldomainsoutsideofthebilayer.Channelproteinshavehydrophiliccoresthatformaqueouschannelsinthemembrane-spanningregion.Peripheralproteinsareattachedtothemembranebyweakbondsandareeasilysolubilizad.Lipid-attachoredmembraneproteinsaredistinguishedbyboththetypesoflipidanchorandtheirorientation.ProteinscanbeseperatedbySDS-polyacrylamidegelelectrophoresisTheorientationofintegralproteinscanbedeterminedusingnonpenetratingagentsthatlabeltheproteins.Identificationoftransmembranedomainscanbepredictedfromtheaminoacidsequenceusingahydropathyplot.Membranedomainsandpolarity.Protein/lipidratiosvaryconsiderablyamongdifferentmembranetypes.Lipidandproteincomponentsofmembranesareboundtogetherbynon-covalentforces.Detergent---smallamphipathicmoleculesSDS:

CH3-(CH2)11-OSO3-Na+CH3CH3

CH3

–C

–CH2

–C

–(O-CH2-CH2)10-OH

CH3CH3TritonX-100:Integralproteinsareembeddedinthemembrane;theirremovalrequiresdetergents.C.MembraneCarbohydratesMembranecontaincarbohydratesconvalentlylinkedtolipidsandproteinsontheextracellularsurfaceofthebilayer.Glycoproteinshaveshort,branchedcarbohydratesforinteractionswithothercellsandstructuresoutsidethecell.Glycolipidshavelargercarbohydratechainsthatmaybecell-to-cellrecognitionsites.4.CharacteristicsofbiomembraneA.DynamicnatureofbiomembraneFluidityofmembranelipid.Itgivemembranestheabilitytofuse,formnetworks,andseparatecharge;Motilityofmembraneprotein.ThelateraldiffusionofmembranelipidscandemonstratedexperimentallybyatechniquecalledFluorescenceRecoveryAfterPhotobleaching(FRAP).

B.Theasymmetryofbiomembranes—ThefoundationofmembranefunctionTheasymmetryofmembranelipidsandglycolipids:Theinnerandoutermembraneleafletswereshowntohavedifferentlipidcompositions.

Lipldasymmentrygivesthemembraneleafletsdifferentphysicalandchemicalpropertiesappropriateforthedifferentinteractionsoccurringatthetwomembranefaces.TheasymmetricdistributionofPLinHETheasymmetryofmembraneproteinandglycoprotein:Integralproteinsattachtothebilayerasymmetrically,givingthemembraneadistinct“sidedness”.ThemembranecarbohydratesonlydistributingonESface.

IntegralproteinshaveorientationwithinMembranes.

Thedistributionofintegralproteinscanbeanalyzedbyfreeze-fractureandfreeze-etchingtechniques.C.TheinhomogeneityofmembranesLipidcompositioncaninfluencetheactivityofmembraneproteinsanddeterminethephysicalstateofthemembrane.BiomembranehaveagglomerationModelofLipidraftinTGN

5.AnOverviewofmembranefunctions

1.Definetheboundariesofthecellanditsorganelles.2.Svideforandregulatetransportprocesses.4.containthe