1ChapterIII

ProteinStructureandFunction2(I)Secondary

Structure：

Thelocalspatialarrangementofaminoacidresiduesthatarenearbyinthesequence,thatis,therelativepositionsofbackboneatomsofacertainpeptidesegment.Thesidechainsarenotconsidered.

Forms:α

helix,β

pleatedsheet,β

turn,π

helix,

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.

Adjacentchainsinabsheetcanruninoppositedirections(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.5.RandomCoil16Somesecondarystructureunitsarecloseenoughinspacetoformregularsupersecondarystructureunits,suchasααα，βββ，βαβ，alsonamedasmotif（模體).Intermediatelevelbetweensecondaryandtertiarystructures6.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（結(jié)構(gòu)域）21NADHPyruvateLactateDehydrogenaseNterminalCterminal

EGFReceptorIntracellularproteinkinasedomainisregulatedviathebindingofthepeptidehormoneEGFtoitsextracellulardomain.2223*Features：

Tothesinglepeptideprotein,tertiarystructureisthehighestlevelofstructure.

Formhydrophilicsurfaceandhydrophobicinnercore.*MajorBond：HydrophobicInteraction24Myoglobin(肌紅蛋白)25Quaternarystructuredefinesthepolypeptidecompositionofaproteinforanoligomericprotein,andthespatialrelationshipsbetweenitssubunits.Subunit（亞基）

Eachpolypeptidechaininsuchaproteiniscalledasubunit.

Subunitisinactivewhenitexistsalone.(III)

Quaternary

Structure26Features:QuarternarystructureariseswhentwoormorepolypeptidesjointoformaproteinSubunitisinactivewhenitexistsonitsown.Subunitsarelinkedbysecondarybonds（H-bonds,ionicinteractions,andhydrophobicinteractions）Ifthepeptidechainsarelinkedbycovalentbonds(disulfidebond)，itisnotbelongtoquaternarystructure.Polypeptidechainscanbeindimer,trimer..,aswellashomo-orhetero-form.Forexample,hemoglobiniscomposedof4polypeptidechainsLinktovideo27NH3＋

COO－

β2

NH3＋

COO－

α2

COO－NH3＋

β1COO－NH3＋

α1AspHisArgAspLysLysAspArgHisAsp94146141126404012614114694IonicForcesinHemoglobinQuaternaryStructureofHemoglobin282930

PrimaryStructure：Peptidebond、Disulfidebond

SecondaryStructure：Hydrogenbond

TertiaryStructure：Hydrophobicinteraction

QuaternaryStructure：Ionicbond

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

35DisulfidebridgeStrongcovalentbondsbetweensulfuratomsintheaminoacidcysteine36Thefoldingofmanyproteinsisprotectedbychaperoninproteinsthatshieldoutbadinfluences.

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

分子病相應(yīng)蛋白質(zhì)分子的異?；蛉笔х牋罴?xì)胞貧血血紅蛋白家族性高膽固醇血癥低密度脂蛋白受體原發(fā)性痛風(fēng)病磷酸核糖焦磷酸酶白化病酪氨酸酶血友病A與B凝血因子Ⅷ與Ⅸ重度聯(lián)合免疫缺陷癥(SCID)腺苷脫氨酶苯丙酮酸尿癥苯丙氨酸羥化酶蠶豆病6-磷酸葡萄糖脫氫酶頑固性佝僂病25-羥維生素D31-羥化酶Lesch-Nyhan(自毀臉容)次黃嘌呤-鳥嘌呤磷酸綜合征核糖轉(zhuǎn)移酶MolecularDisease分子病Inheriteddiseasesinwhichthemanifestationsareduetoalterationsinproteinprimarystructureandfunction.TheAAvariationisduetothegenemutation.442.Proteinshavingsimilaraminoacidsequencesdemonstratethefunctionalsimilarity.

*InsulinA8A9A10A30HumanThrSerIleThrBovineAlaSerValAlaSwineAlaSerIleAlaOvineAlaGlyValAla45ACTH

(促腎上腺皮質(zhì)激素)andMSH

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

Comparisonoftheirprimarystructurecanhelptounderstandtheevolutionaryrelationshipbetweendifferentspecies.OrganismswhicharecloserintheprocessofspeciesevolutionwillhavemoresimilarprimarystructureofcytochromeC.47(II)SpatialstructureandfunctionProteinswillexperiencemultipleprocessestobecomecorrectlyfolded,thatis,havingacorrectstructure.Theincorrectproteinstructuremayleadtofunctionalternationordiseases.Aparticularspatialstructureofaproteinisstronglycorrelatedwithitsspecificbiologicalfunctions.481.Amphipathicαhelix492IonChannelHydrophobicaminoacidHydrophobicaminoacidHydrophilicaminoacidCellMembrane2.Thestructuralpropertiesofsilkareduetobeta

pleatedsheets.Thepresenceofsomanyhydrogenbondsmakeseachsilkfiberstrongerthansteel.5051Aneffectthatariseswhenthebindingofaneffectormoleculeatthepolymericprotein'sallostericsiteregulatesproteinactivityasaresultofconformationalchanges.1)Onlyproteinswhichhavequaternarystructurepossessthisproperty.2)Allostericagentsaresmallphysiologicalmolecules,suchasO2、ATP3)Allostericsiteisasiteotherthantheprotein'sactivesite.4)Slightlychangeconformationcanincreaseordecreaseproteinactivitysubsequently.

3.Allosteric

effect（變構(gòu)效應(yīng))5253Theironatommovesintotheplaneofthehemeonoxygenation.HistidineF8anditsassociatedresiduesarepulledalongwiththeironatom.54TheAllostericEffectofHemoglobins55SchematicDiagramofAllostericEffectofHemoglobin56573.蛋白質(zhì)構(gòu)象改變可導(dǎo)致構(gòu)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,瘋牛病)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（蛋白質(zhì)的變性）1.DefinitionTheprocessinwhichaprotein