Myoglobincontainsasinglepolypeptidechainof153aminoacidresiduesofknownsequenceandasingleironprotoporphyrin,orheme,group.Thesamehemegroupisfoundinhemoglobin,theoxygen-bindingproteinoferythrocytes,andisresponsibleforthedeepred-browncolorofbothmyoglobinandhemoglobin.Myoglobinisparticularlyabundantinthemusclesofdivingmammalssuchasthewhale,seal,andporpoise,whosemusclesaresorichinthisproteinthattheyarebrown.Storageanddistributionofoxygenbymusclemyoglobinpermittheseanimalstoremainsubmergedforlongperiodsoftime.Thefirstbreakthroughinunderstandingthethree-dimensionalstructureofaglobularproteincamefromx-raydiffractionstudiesofmyoglobincarriedoutbyJohnKendrewandhiscolleaguesinthe1950s.Myoglobinisarelativelysmall(Mr16,700),oxygen-bindingproteinofmusclecells.Itfunctionsbothtostoreoxygenandtofacilitateoxygendiffusioninrapidlycontractingmuscletissue.七、球狀蛋白質與三級結構1962年諾貝爾化學獎

(一)球狀蛋白質〔結構域〕的分類:1.反平行α螺旋蚯蚓肌紅蛋白子宮珠蛋白流感病毒血凝素煙草花葉病毒蛋白Severalexamplesofantiparallelα-proteins.2.α,β-結構丙糖磷酸異構酶〔側〕丙糖磷酸異構酶〔頂〕丙酮酸激酶Parallelβ-arrayproteins—theeight-strandedβ-barrelsoftriosephosphateisomerase(a,sideview,andb,topview)and(c)pyruvatekinase.Severaltypicaldoublywoundparallelβ-sheetproteins.3.全β-結構大豆胰蛋白酶抑制劑玉紅氧還蛋白木瓜蛋白酶結構域2Examplesofantiparallelβ-sheetstructuresinproteins.伴刀豆凝集素晶體蛋白Examplesoftheso-calledGreekkeyantiparallelβ-barrelstructureinproteins.鏈霉屬枯草桿菌蛋白酶谷胱甘肽復原酶結構域3甘油醛-3-磷酸脫氫酶結構域2Sheetstructuresformedfromantiparallelarrangementsofβ-strands.(a)Streptomycessubtilisininhibitor,(b)glutathionereductasedomain3,and(c)theseconddomainofglyceraldehyde-3-phosphatedehydrogenaserepresentminimalantiparallelβ–sheetdomainstructures.Ineachofthesecases,anantiparallelβ-sheetislargelyexposedtosolventononefaceandcoveredbyhelicesandrandomcoilsontheotherface.4.富含金屬或二硫鍵的蛋白質胰島素磷脂酶A2高氧還勢鐵蛋白鐵氧還蛋白花菜蛋白(二)球狀蛋白質三維結構的特征1.含多種二級結構元件;2.具有明顯的折疊層次;3.球蛋白分子是緊密近似球狀的實體;4.疏水側鏈在分子內部,親水側鏈在分子外表;5.分子外表有空穴,是結合底物,效應物等配體,行使生物學功能的活性部位,空穴通常是一個疏水的區(qū)域。八、膜蛋白的結構

1.具有單個跨膜肽段的膜蛋白2.具有7個跨膜肽段的膜蛋白3.β桶型膜蛋白-膜孔蛋白(二)脂錨定膜蛋白皰癥性口炎糖蛋白甲狀腺球蛋白乙酰膽堿酯酶糖脂A*九、蛋白質折疊和結構預測(一)蛋白質的變性1.變性蛋白質的特點:

空間結構破壞，一級結構完好；活性喪失；溶解度降低；易分解。2.引起變性的因素:

加熱、輻射、高壓等物理因素；有機溶劑、酸、堿、尿素等化學因素。3.變性的協(xié)同性:

變性發(fā)生在較窄的范圍內。4.復性:

一定條件下可以恢復活性。Proteindenaturation.Resultsareshownforproteinsdenaturedbytwodifferentenvironmentalchanges.Ineachcase,thetransitionfromthefoldedtounfoldedstateisfairlyabrupt,suggestingcooperativityintheunfoldingprocess.(a)Thermaldenaturationofhorseapomyoglobin(myoglobinwithoutthehemeprostheticgroup)andribonucleaseA.Themidpointofthetemperaturerangeoverwhichdenaturationoccursiscalledthemeltingtemperature,orTm.Thedenaturationofapomyoglobinwasmonitoredbycirculardichroism,atechniquethatmeasurestheamountofhelicalstructureinamacromolecule.DenaturationofribonucleaseAwastrackedbymonitoringchangesintheintrinsicfluorescenceoftheprotein,whichisaffectedbychangesintheenvironmentofTrpresidues.(b)Denaturationofdisulfide-intactribonucleaseAbyguanidinehydrochloride(GdnHCl),monitoredbycirculardichroism.7

5

3

1種可能連接方式中的1種核糖核酸酶的復性(二)氨基酸序列規(guī)定蛋白質的三維結構(三)蛋白質折疊的熱力學(四)蛋白質折疊的動力學

1.折疊不是隨機搜索的過程。

2.折疊時先形成局部二級結構，再形成結構域,熔球態(tài)，最后形成完整的三級結構。

3.折疊時二硫鍵異構酶和肽基脯氨酸異構酶起重要作用。

4.分子伴侶在折疊時有重要作用，主要是防止肽鏈的錯誤折疊.。Amodelforthestepsinvolvedinthefoldingofglobularproteins.Chaperoneproteinsmayassistintheinitiationofthefoldingprocess.(五)蛋白質結構的預測1.二級結構的預測參照表5-6,按Pi=Ai(被計算的氨基酸殘基處于某一二級結構的份數(shù)〕/Ti(肽鏈中的各種氨基酸殘基處于某一二級結構的份數(shù)〕的計算值預測,Pi大于1表示該氨基酸殘基傾向于形成所預測的二級結構，Pi小于1表示該氨基酸殘基傾向于形成其他構象。相鄰6個殘基至少4個傾向于形成螺旋，即為螺旋核，由此延伸到4個殘基的pα平均值小于1，螺旋內不可出現(xiàn)Pro。相鄰6個殘基至少3個傾向于形成折疊片，即為折疊核，由此延伸到4個殘基的Pβ平均值小于1。Theseresiduesareclassifiedasstronghelixformers(Hα),helixformers(hα),weakhelixformers(Iα),indifferenthelixformers(iα),helixbreakers(bα),andstronghelixbreakers(Bα).SimilarclasseswereestablishedbyChouandFasmanforβ-sheet–formingability.Suchalgorithmsareonlymodestlysuccessfulinpredictingtheoccurrenceofhelicesandsheetsinproteins.Themolecularmotionsofthepeptideandtheeffectsofthewatermoleculesweretakenintoaccountinmappingthemostlikelypathstothefinalstructureamongthecountlessalternatives.Thesimulatedfoldingtookplaceinatheoreticaltimespanof1ms;however,thecalculationrequiredhalfabillionintegrationstepsontwoCraysupercomputers,eachrunningfortwomonths.Asimulatedfoldingpathway.Thefoldingpathwayofa36-residuesegmentoftheproteinvillin(anactin-bindingproteinfoundprincipallyinthemicrovilliliningtheintestine)wassimulatedbycomputer.Theprocessstartedwiththerandomlycoiledpeptideand3,000surroundingwatermoleculesinavirtual“waterbox.〞2.三級結構的預測和計算機模擬

原理復雜，已由一些成功的例證。AcomparisonofthestructuresoffourproteindomainsandpredictionsofthesestructuresbytheprogramLINUSbyRoseandSrinivasan.十、亞基締合和四級結構

(一)有關四級結構的一些概念亞基(單體)，同多聚，雜多聚(二)四級締合的驅動力主要有范氏作用力，氫鍵，離子鍵和疏水作用力，其中疏水作用力最為重要，二硫鍵對于穩(wěn)定四級結構有重要意義。(三)亞基相互作用的方式

亞基之間的分布有各種各樣的對稱關系。Quaternarystructureofdeoxyhemoglobin.X-raydiffractionanalysisofdeoxyhemoglobin(hemoglobinwithoutoxygenmoleculesboundtothehemegroups)showshowthefourpolypeptidesubunitsarepackedtogether.(a)Aribbonrepresentation.(b)Aspace-fillingmodel.Theα

subunitsareshowningrayandlightblue;theβ

subunitsinpinkanddarkblue.Notethatthehemegroups(red)arerelativelyfarapart.Thequaternarystructureofliveralcoholdehydrogenase.Withineachsubunitisasix-strandedparallelsheet.Betweenthetwosubunitsisatwo-strandedantiparallelsheet.ThepointinthecenterisaC2symmetryaxis.Isologousandheterologousassociationsbetweenproteinsubunits.(a)Anisologousinteractionbetweentwosubunitswithatwofoldaxisofsymmetryperpendiculartotheplaneofthepage.(b)Aheterologousinteractionthatcouldleadtotheformationofalongpolymer.亞基之間的同多聚和雜多聚(c)Aheterologousinteractionleadingtoaclosedstructure—atetramer.(d)Atetramerformedbytwosetsofisologousinteractions.Thepolypeptidebackboneoftheprealbumindimer.Themonomersassociateinamannerthatcontinuestheβ-sheets.AtetramerisformedbyisologousinteractionsbetweenthesidechainsextendingoutwardfromsheetD′A′G′H′HGADinbothdimers,whichpacktogethernearlyatrightanglestooneanother.Severalpossiblesymmetricarraysofidenticalproteinsubunits,including(a)cyclicsymmetry,(b)dihedralsymmetry,and(c)cubicsymmetry,includingexamplesoftetrahedral(T),octahedral(O),andicosahedral(I)symmetry.Rotationalsymmetryinproteins.(a)Incyclicsymmetry,subunitsarerelatedbyrotationaboutasinglen-foldaxis,wherenisthenumberofsubunitssorelated.Theaxesareshownasblacklines;thenumbersarevaluesofn.OnlytwoofmanypossibleCnarrangementsareshown.(b)Indihedralsymmetry,allsubunitscanberelatedbyrotationaboutoneorbothoftwoaxes,oneofwhichistwofold.D2symmetryismostcommon.(c)Icosahedralsymmetry.Relatingall20triangularfacesofanicosahedronrequiresrotationaboutoneormoreofthreeseparaterotationalaxes:twofold,threefold,andfivefold.Anend-onviewofeachoftheseaxesisshownattheright.Theoligomericstatesofhemerythrininvariousmarineworms.(a)ThehemerythrininThermistezostericolacrystallizedasamonomer;(b)theoctamerichemerythrincrystallizedfromPhascolopsisgouldii;(c)thetrimerichemerythrincrystallizedfromSiphonosomacollectedinmangroveswampsinFiji.Viralcapsids.(a)Poliovirus.Thecoatproteinsofpoliovirusassembleintoanicosahedron300?indiameter.Icosahedralsymmetryisatypeof