细胞生物学 第3讲 蛋白质化学特性

ChapterIII

ProteinStructureandFunction2(I)Secondary

Structure：

Thelocalspatialarrangementofaminoacidresiduesthatarenearbyinthesequence,thatis,therelativepositionsofbackboneatomsofacertainpeptidesegment.Thesidechainsarenotconsidered.

Forms:α

helix,β

pleatedsheet,β

turn,randomcoilMajorBond：Hydrogenbond

I.ConformationofProtein3Right-handedhelix3.6aminoacidresiduesperturnofhelixThepitchofthehelixis0.54nm,diameteris0.23nmTheN-Hofeverypeptidebondishydrogen-bondedtotheC=Oofneighboringpeptidebondlocatedfourpeptidebondsawayinthesamechain,including13atoms

,soalsoknownas3.613helix.AllthemainchainC=OandNHarehydrogenbonded.LinusPauling

1.α-helix(1)4Thealphahelixisacoiledsecondarystructureduetoahydrogenbondeveryfourthaminoacid56Directionofhydrogenbondsareparalleltotheverticalaxisofhelix.Thestabilityofanα-helixarisesprimarilyfromhydrogenbonds.Thesidechainsareontheoutsideofthehelix,notdirectlyparticipateintheformationofhelix.α-helixisthemoststablesecondaryconformation1.-helix(2)：78Adjacentpeptideunitformazigzagorpleatedpattern,

theintersectionangleis110。.β-Sheetsarestabilizedbyhydrogenbondingbetweenpolypeptidestrands.

Thedirectionofhydrogenbondsareverticaltothepeptidestrands.

Adjacentchainsinaβ-sheetcanruninoppositedirections(antiparallelbsheet)orinthesamedirection(parallelbsheet).Thesidechainsofadjacentaminoacidspointinoppositedirections2.β-pleatedsheetstructure91011Peptidechainarisesatight180°turn.

Aβ-turninvolvesfouraminoacidresidues,thefirstresiduesishydrogenbondedtothefourth.Prolineisoftenpresentinβ-turnOftenlieontheglobulinsurfaceandservekeybiologicrole.

3.β-turn1213Left-handedhelix，4.4aminoacidresiduesperturn.Hydrogenbondsstabilizetheπ-helix，everyhydrogenbondacross18atoms,soalsonamedas4.418

helix.Oftenfoundincollagen.Tripleleft-handedhelixestwisttoformright-handedsuperhelixandturntocollagenousfibers.4.π-helix1415Generalnameofaseriesofunorderedconformationinprotein.

Importantstructuralandfunctionalsegmentsofprotein.4.RandomCoil16Somesecondarystructureunitsarecloseenoughinspacetoformregularsupersecondarystructureunits,suchasααα，βββ，βαβ.Supersecondaryunitsthathavespecificfunctionarenamedasmotif（模体).Intermediatelevelbetweensecondaryandtertiarystructures5.Supersecondary

StructureααofCytochromeCΒαβofPCNAΒβofplasminogen17MotifinCalcium-bindingProteinZincFinger

Sidechainscandisruptorinducetheformationofsecondarystructure

Shape:Prohavingarigidring(–helixdisrupter)Size:–helixand-sheetneedsAAsofsmallsidechain.Leu,Ile,Trp,andAsn,havingbulkysides,hardtoformα–helixandβ-sheet)Charge:ToomanychargedAAsinashortregionofonepeptideishardtoform–helix.1819*Definition:Theentirethree-dimensionalconformationofapolypeptidechain.Itreferstothespatialarrangementofaminoacidresiduesthatarefarapartinthesequenceandtothepatternofdisulfidebonds.Itindicates,inthree-dimensionalspace,howsecondarystructuralassembletoformdomainsandhowthesedomainsrelatespatiallytooneanother.(II)Tertiary

Structure20Singleorseveralsupersecondarystructureunitsgatherandfoldindependentlyintoacompact,stablestructure,termeddomain.Domainisthefunctionelementofprotein.Thedifferentdomainsofaproteinareoftenassociatedwithdifferentfunctions.Domainisthepartialfoldingregionattheleveloftertiarystructure.

Motifisitssubunit.Everydomainisencodedbyoneexon.Domain（结构域）21NADHPyruvateLactateDehydrogenaseNterminalCterminal

EGFReceptorIntracellularproteinkinasedomainisregulatedviathebindingofthepeptidehormoneEGFtoitsextracellulardomain.2223*Features：

Tothesinglepeptideprotein,tertiarystructureisthehighestlevelofstructure.

Formhydrophilicsurfaceandhydrophobicinnercore.*MajorBond：HydrophobicInteraction24Myoglobin(肌红蛋白)25Quaternarystructuredefinesthepolypeptidecompositionofaproteinforanoligomericprotein,andthespatialrelationshipsbetweenitssubunits.Subunit（亚基）

Eachpolypeptidechaininanoligomericproteiniscalledasubunit.

Subunitisinactivewhenitexistsalone.(III)

Quaternary

Structure26Features:QuarternarystructureariseswhentwoormorepolypeptidesjointoformaproteinSubunitisinactivewhenitexistsonitsown.Subunitsarelinkedbysecondarybonds（H-bonds,ionicinteractions,andhydrophobicinteractions）Ifthepeptidechainsarelinkedbycovalentbonds(disulfidebond)，itisnotbelongtoquaternarystructure.Polypeptidechainscanbeindimer,trimer..,aswellashomo-orhetero-form.Forexample,hemoglobiniscomposedof4polypeptidechains27NH3＋

COO－

β2

NH3＋

COO－

α2

COO－NH3＋

β1COO－NH3＋

α1AspHisArgAspLysLysAspArgHisAsp94146141126404012614114694IonicForcesinHemoglobin282930

PrimaryStructure：Peptidebond、Disulfidebond

SecondaryStructure：Hydrogenbond

TertiaryStructure：Hydrophobicinteraction

QuaternaryStructure：Ionicbond

(IV)Non-covalentBondsstabilizeProteinStructure31HydrogenbondAhydrogenatomissharedbytwootheratoms.H-donor:theatomtowhichHatomismoretightlyattached,andtheotherisH-acceptor.32HydrophobicinteractionNonpolarmoleculestendtoclustertogetherinwater,thatis,aminoacidswithnonpolarsidechainsclusterinthecoreoftheprotein,outofcontactwithwater33Achargedgroupisabletoattractanothergroupofoppositecharges.Ionicinteraction34Theattractionbetweenapairofatomsincreasesastheycomecloser,untiltheyarerepelledbyvanderWaalscontactdistance.vanderWaalsforce

35DisulfidebridgeStrongcovalentbondsbetweensulfuratomsintheaminoacidcysteine36Thefoldingofmanyproteinsisprotectedbychaperoninthatshieldoutbadinfluences.

Chaperon37Post-translationalModification3839II.Structure-FunctionRelationshipofProteinsSequenceofDNA

AminoacidsequenceofproteinConformationofProteinFunctionofProteinPrimarystructureisbasis,Conformationisthekeyfactor.401.ThealternationofkeyAAsinaproteinwillcausethelossofitsbiologicalfunctions

Sicklecellanemia

Abnormalhemoglobin,

developbecauseofasingleaminoacidsubstitution.Thisisthefirstcaseofmoleculardiseaseidentifiedinhistory(I)PrimaryStructureandFunction41Oxygen-carryingcapacityofHbSdrop.

Theabnormalredcellsarethin,elongated,sickle-shaped.Sicklingdecreasesthecellsflexibilityandcauseshemolysis.HbAβ

Val-His-Leu-Thr-Pro-Glu-Glu-Lys…HbSβ

Val-His-Leu-Thr-Pro-Val-Glu-Lys…4243

分子病相应蛋白质分子的异常或缺失镰状细胞贫血血红蛋白家族性高胆固醇血症低密度脂蛋白受体原发性痛风病磷酸核糖焦磷酸酶白化病酪氨酸酶血友病A与B凝血因子Ⅷ与Ⅸ重度联合免疫缺陷症(SCID)腺苷脱氨酶苯丙酮酸尿症苯丙氨酸羟化酶蚕豆病6-磷酸葡萄糖脱氢酶顽固性佝偻病25-羟维生素D31-羟化酶Lesch-Nyhan(自毁脸容)次黄嘌呤-鸟嘌呤磷酸综合征核糖转移酶MolecularDisease分子病Inheriteddiseasesinwhichthemanifestationsareduetoalterationsinproteinprimarystructureandfunction.TheAAvariationisduetothegenemutation.442.Proteinshavingsimilaraminoacidsequencesdemonstratethefunctionalsimilarity.

*InsulinA8A9A10A30HumanThrSerIle

ThrBovineAlaSerVal

AlaSwineAlaSerIle

AlaOvineAlaGlyVal

Ala45ACTH

(促肾上腺皮质激素)andMSH

(促黑激素)haveasamepeptidesegment,soACTHalsohasthefunctionofpromotingmelanogenesis.46CytochromeCisaproteinwhichcanbefoundinallaerobicorganisms.

Comparisonoftheirprimarystructurecanhelptounderstandtheevolutionaryrelationshipbetweendifferentspecies.OrganismswhicharecloserintheprocessofspeciesevolutionwillhavemoresimilarprimarystructureofcytochromeC.47(II)SpatialstructureandfunctionProteinswillexperiencemultipleprocessestobecomecorrectlyfolded,thatis,havingacorrectstructure.Theincorrectproteinstructuremayleadtofunctionalternationordiseases.Aparticularspatialstructureofaproteinisstronglycorrelatedwithitsspecificbiologicalfunctions.481.Amphipathicαhelix492IonChannelHydrophobicaminoacidHydrophobicaminoacidHydrophilicaminoacidCellMembrane2.Thestructuralpropertiesofsilkareduetobetapleatedsheets.Thepresenceofsomanyhydrogenbondsmakeseachsilkfiberstrongerthansteel.505051Theregulationofaproteincausedbybindinganeffectormoleculeatanallostericsiteanditssubsequentconformationchange.1)Onlypolymericproteinspossessthisproperty.2)Allostericagentsaresmallphysiologicalmolecules,suchasO2、ATP3)Allostericsiteisasiteotherthantheprotein'sactivesite.4)Slightlychangeconformationcanincreaseordecreaseproteinactivitysubsequently.

3.Allosteric

effect（变构效应)5253Theironatommovesintotheplaneofthehemeonoxygenation.HistidineF8anditsassociatedresiduesarepulledalongwiththeironatom.54TheAllostericEffectofHemoglobins55SchematicDiagramofAllostericEffectofHemoglobin56573.蛋白质构象改变可导致构象病ProteinConformationalDisorders：Aclassofdiseasesinwhichcertainproteinsfailtofoldintotheirnormalconformationandlosetheirnormalfunction,therebydisruptthefunctionofcells,tissuesandorgansofthebody.58PathogenicMechanism：Somemisfoldingproteinsaggregateandformanti-proteaseamyloidosis,andtherebycausedisease.Diseases:Alzheimer'sdisease,Parkinson'sdisease,priondisease,type2diabetes,amyloidosis59BSEisatransmissible,inheritableneurodegenerativediseasesinmammalscausedbyprionprotein

(PrP,朊病毒蛋白).NormalPrPisrichinα-helix，termedPrPc.PrPcisanormalconstituentofbraintissueinallmammals.AbnormalPrPisrichinβpleatedsheet,termedPrPsc.AnumberofPrPscaggregateextracellularlywithinthecentralnervoussystemtoformplaquesknownasamyloid,whichdestroybraintissuesbyconvertingthemtoaspongyappearancedisruptandleadtobraindamageanddeath.Bovinespongiformencephalopathy(BSE,疯牛病)60NH3+NH3+NH2COOHCOO-COO-PositiveIon

Zwitterion

NegativeIon（pH<PI)(pH=PI)(pH>PI)

PPPIII.

PhysicochemicalPropertiesofProtein(I)Ampholyteofprotein1.+H++H++OH－+OH―612.pIofProtein：thepHatwhichaparticularmoleculecarriesnonetelectricalcharge.3.ThechargeofproteinisrelatedtosurroundingspH.

pH<PIpositiveion

pH>PInegativeion

pH=PIelectricneutrality4.pIofmostproteinis~5.0,andnegativelychargedinbodyfluid(pH7.4)pI>7.4:basicproteins:protamine,histonepI<7.4:acidicproteins:pepsin62SerumProteinElectrophoretogram_+2163(II)MacromoleculeProperties1.StabilityofHydrophiliccolloidisdueto:⑴HydrationShell⑵ElectricRepulsion

2.Dialysis

3.UltracentrifugationMW:10,000～1,000,000Diameter:1~100nm,intherangeofcolloid(III)UVabsorptionmax：280nm（Tyr，Trp）64+++++++PositiveChargedProtein－－－－－－－－NegativeChargedProteinProteininpIHydrationShell++++++++PositiveChargedProtein－－－－－－－－NegativeChargedProteinUnstableProteinacidbaseacidbaseacidbaseDehydrationDehydrationDehydrationPrecipitationofProtein65

(IV)Denaturation（蛋白质的变性）1.DefinitionTheprocessinwhichaproteinlosesits