生物化學(xué)-生化課件chapter

Chapter7

PortraitofanAllostericProteinThetransitionfromanaerobic厭氧toaerobic需氧lifewasamajorstepinevolutionbecauseituncoveredarichreservoirofenergy.Eighteentimesasmuchenergyisextractedfromglucoseinthepresenceofoxygenasinitsabsence.由厭氧呼吸到需氧呼吸,生命體從葡萄糖汲取能量增加到18倍(15倍).Vertebrateshaveevolvedtwoprincipalmechanismsforsupplyingtheircellswithacontinuousandadequateflowofoxygen.1.acirculatorysystemthatactivelydeliversoxygentocells.循環(huán)系統(tǒng)2.theuseofoxygen-carryingmoleculestoethelimitationimposedbythelowsolubilityofoxygeninwater.載氧分子(氧在水中溶解度低)Theoxygencarriersinvertebratesaretheproteinshemoglobin(Hb)andmyoglobin(Mb).Hb,whichiscontainedinredbloodcells,servesastheoxygencarrierinbloodandalsoplaysavitalroleinthetransportofcarbondioxideandhydrogenion.Mb,whichislocatedinmuscle,providesareservesupplyofoxygenandfacilitatethemovementofoxygenwithinmuscle.肌紅蛋白可從血紅蛋白接受分子氧并儲(chǔ)藏在肌肉細(xì)胞中,當(dāng)氧供應(yīng)受限制時(shí)釋放給細(xì)胞色素氧化酶.也可促成分子氧直接從肌細(xì)胞表面轉(zhuǎn)移給線粒體.MbandHbillustratemanyimportantprinciplesofproteinconformation,dynamics,andfunction.Theirthree-dimensionalstructures,knowninatomicdetail,revealmuchabouthowproteinsfold,bindothermoleculesandintegrateinformation.折疊,結(jié)合其它分子,整合信息ThebindingofO2byHbisregulatedbyH+,CO2,andorganicphosphates(2,3-BPG).Theseregulatorsgreatlyaffecttheoxygen-bindingpropertiesofHbbybindingtositesontheproteinfarfromwhereO2isbound.H+,CO2,

2,3-BPG的結(jié)合位點(diǎn)與氧結(jié)合位點(diǎn)相距甚遠(yuǎn),但可(通過(guò)變構(gòu)作用)調(diào)控氧氣與血紅蛋白的結(jié)合Indeed,interactionsbetweenspatiallydistinctsites,termedallostericinteractions變構(gòu)作用,occurinmanyproteins.Allostericeffectsplayacriticalroleincontrollingandintegratingmoleculareventsinbiologicalsystems.Hbisthebest-understoodallostericprotein.thediscoveryofmutantHbfirstrevealedthatdiseasecanarisefromachangeofasingleaminoacidinaprotein.TheconceptofmoleculardiseasecamefromstudiesofamutantHb.Hbhasalsobeenarichsourceofinsightintothemolecularbasisofevolution.OxygenBindstoaHeme血紅素ProstheticGroupThecapacityofMborHbtobindoxygendependsonthepresenceofanonpolypeptideunit,namely,ahemegroup.ItgivesMbandHbtheirdistinctivecolor(紅色含鐵載氧色素).Manyproteinsrequiretightlybound,specificnonpolypeptideunitsfortheirbiologicalactivities.Suchaunitiscalledaprostheticgroup輔基.Aproteinwithoutitscharacteristicprostheticgroupistermedanapoprotein脫輔蛋白.脫輔蛋白+輔基=全蛋白Thehemeconsistsofanorganicpartandanironatom.Theorganicpart,protoporphyrin原卟啉,ismadeupoffourpyrrolerings吡咯環(huán).血紅素=原卟啉(及側(cè)鏈)+鐵原子原卟啉=四個(gè)吡咯環(huán)+連接它們的甲叉橋Thefourpyrrolesarelinkedbymethanebridges甲叉橋toformatetrapyrrolering.Fourmethyl甲基,twovinyl乙烯,andtwopropionate丙酸sidechainsareattached.Theironatominhemebindstothefournitrogensinthecenteroftheprotoporphyrinring.Theironcanformtwoadditionalbonds,oneoneithersideofthehemeplane.Theseironatomscanbeintheferrous(+2)ortheferric(+3)oxidationstate,andthecorrespondingformsofHbarecalledferrohemoglobin亞鐵血紅蛋白andferrihemoglobin高鐵血紅蛋白(alsocalledmethemoglobin).Onlyferrohemoglobin,the+2oxidationstate,canbindoxygen.ThesamenomenclatureappliestoMb.只有兩價(jià)態(tài)的亞鐵血紅蛋白才能結(jié)合氧

MyoglobinHasaCompactStructureandaHighContentofαhelicesTheelucidationofthethree-dimensionalstructureofMbandHbarelandmarksinbiochemistry.x-raycrystallography結(jié)晶學(xué)canrevealthestructureofmoleculesaslargeasproteins.KendrewchoseMbforx-rayanalysisbecauseitisrelativelysmall,easilypreparedinquantity,andreadilycrystallized.肌紅蛋白分子量小,易大量制備,易結(jié)晶IthadtheadditionaladvantageofbeingcloselyrelatedtoHb.與血紅蛋白十分相似Mbfromtheskeletalmuscleofthespermwhale抹香鯨wasselectedbecauseitisstableandformsexcellentcrystals.Theskeletalmuscleofdivingmammals,suchaswhales,seals,andporpoises海豚,isparticularlyrichinMb,whichservesasastoreofoxygenduringadive.水生哺乳動(dòng)物用肌紅蛋白作為潛水時(shí)氧氣儲(chǔ)存In1957,Kendrewandhiscolleaguessawwhatnoonehaseverseenbefore:athree-dimensionalpictureofaproteinmoleculeinallitscomplexity.Thestructurewasmoreintricatethanhadbeenimagined.Itsmeaningwasrevealedtwoyearslater,whenahigh-resolutionimagewasobtained.Awealthofstructuraldetailemerged.Thepositionsof1200ofthe1260nonhydrogenatomswereclearlydefinedtoaprecisionofbetterthan0.3?.Thecourseofthemainchainandthepositionofthehemegroupareshown.SomeimportantfeaturesofMbareMbisextremelycompact.極為緊密About75%ofthemainchainisinanα-helicalconformation.8majorhelicalsegments,5nonhelicalsegmentsliebetweenhelices,twoothernonhelicalregions.富含α-螺旋(共8個(gè),即ABCDEFGH),5個(gè)非螺旋片段居螺旋之間(BC,DE間無(wú)連接片段),另2個(gè)非螺旋片段分別在肌紅蛋白的氨基端和羧基端Fourofthehelicesareterminatedbyaprolineresidue,whosefive-memberedringdoesnotfitwithinastraightstretchofαhelix.Themain-chainpeptidegroupsareplanar,andthecarbonylgroupofeachistranstotheNH.主鏈肽基在一個(gè)平面上,其中C=O和NH為反式關(guān)系.Theinsideandoutsidearewelldefined.Thereislittleemptyspaceinside.

內(nèi)部幾乎無(wú)空隙

AHindered阻隔的HemeEnvironmentisEssentialforReversibleOxygenation氧合

ThehemegroupislocatedinacreviceintheMbmolecule.Thehighlypolarpropionatesidechainsofthehemeareonthesurface.AtphysiologicalPH,thesecarboxylicacidgroupsareionized.(血紅素的)離子化的丙酸側(cè)鏈處在蛋白表面Therestofthehemeisinsidethemolecule,whereitissurroundedbynonpolarresiduesexceptfortwohistidines.血紅素其它部分處于非極性殘基包圍之中(除兩個(gè)組氨酸外)TheironatomofthehemeisdirectlybondedtooneoftheseHis.ThisHis,whichoccupiesthefifthcoordinationpositioniscalledtheproximalhistidine鄰接(近側(cè))組氨酸(F8).Theironatomisabout0.3?outoftheplaneoftheporphyrin卟啉,onthesamesideasHisF8.Theoxygen-bindingsiteisontheothersideofthehemeplane,atthesixthcoordinationposition配位位置.AsecondHisresidue(E7),termedthedistal遠(yuǎn)側(cè)histidine,isnearthehemebutnotbondedtoit.TheconformationsofthethreephysiologicallypertinentformsofMb—deoxymyoglobin,oxymyoglobin,andferrimyoglobin—areverysimilarexceptatthesixthcoordinationposition..TheaxisoftheboundO2isatanangletotheiron-oxygenbond.Theoxygen-bindingsitecomprisesonlyasmallfractionofthevolumeoftheMbmolecule.WhyisthepolypeptideportionofMbneededforoxygentransportandstorage?氧結(jié)合部位只占蛋白分子的極小部分,肌紅蛋白的其它部分起阻隔鐵—氧—鐵“三明治”形成(“三明治”內(nèi)的鐵原子被氧化為3價(jià))的作用.Theanswerliesintheoxygen-bindingpropertiesofanisolatedhemegroup.Inwater,afreeferroushemegroupcanbindoxygen,butitdoessoforonlyafleetingmoment稍縱即逝.ThereasonisthatO2veryrapidlyoxidizedtheferroushemetoferricheme,whichcannotbindoxygen.(heme-O2–hemesandwich)Inmyoglobin,thehemegroupismuchlesssusceptibletooxidationbecausetwomyoglobinmoleculescannotreadilyassociatetoformaheme-O2-hemecomplex.TheformationofthissandwichisstericallyhinderedbythedistalHisandotherresiduessurroundingthesixthcoordinationsite.Thestrongestevidencefortheimportanceofstericfactorsindeterminingtherateofoxidationofhemecomesfromstudiesofsyntheticmodelcompounds(picket-fence柵欄ironporphyrincomplexes).ThesecompoundshaveaprotectiveenclosureforbindingO2ononesideoftheporphyrinring,whereastheothersideisleftunhinderedsothatitcanbindabase.Infact,whenthebaseissubstitutedimidazole咪唑(likeHis),theoxygenaffinityofthepicket-fencecompoundislikethatofMb.Furthermore,thepicketfencestabilizestheferrousformofthisironporphyrinandthusenablesittoreversiblybindoxygenforlongperiods.人工合成柵欄鐵卟啉化合物具有與Mb相似的氧親和力,(柵欄)使得此鐵卟啉穩(wěn)定在二價(jià)態(tài),并能長(zhǎng)時(shí)間地克逆結(jié)合(釋放)氧.與游離血紅素不同.Thecriticaldifferencebetweenthismodelcompoundandfreehemeisthepresenceofthepicketfence,whichblockstheformationofthesandwichdimmer二聚體.(鐵-氧—鐵-氧)CarbonMonoxideBindingisDiminishedbythePresenceoftheDistalHistidineCarbonmonoxideisapoisonbecauseitcombineswithferromyoglobinandferrohemoglobinandtherebyblocksoxygentransport.AnisolatedhemeinsolutiontypicallybindsCOsome25,000timesasstronglyasO2.However,thebindingaffinityofMbandHbforCOisonlyabout200timesasgreatasforO2.血(肌)紅蛋白中的血紅素結(jié)合一氧化碳的親和力比它的游離狀態(tài)低得多Howdotheseproteinssuppresstheinnatepreferenceofhemeforcarbonmonoxide?Theanswercomesfromx-raycrystallographicandinfrared紅外spectroscopicstudiesofcomplexesofCOandO2withisolatedironporphyrins.theproteinforcesCOtobindatanangleratherthaninline.ThisbentgeometryintheglobinsweakenstheinteractionofCOwiththeheme.ThedecreasedaffinityofMbandHbforCOisbiologicallyimportant.Carbonmonoxidewasapotentialhazardlongbeforetheemergenceofindustrializedsocieties.Thelevelofendogenouslyformed內(nèi)源carbonmonoxideissuchthatabout1%ofthesitesinMbandHbareblockedbyCO,atolerabledegreeofinhibition.However,endogenouslyproducedCOwouldcausemassivepoisoningiftheaffinityoftheseproteinsforCOwerelikethatofisolatedironporphyrins.ThischallengewassolvedbytheevolutionofhemeproteinsthatdiscriminatebetweenO2andCObystericallyimposingabentandhenceweakermodeofbindingforCO.Thus,Mbhascreatedaspecialmicroenvironmentthatconfersdistinctivepropertiesonitsprostheticgroup.Ingeneral,thefunctionofaprostheticgroupismodulatedbyitspolypeptideenvironment.多肽微環(huán)境賦予其輔基獨(dú)特的性質(zhì)和功能Forexample,thesamehemegrouphasquiteadifferentfunctionincytochromec,aproteinintheterminaloxidationchaininthemitochondriaofallaerobicorganisms.Incytochromec細(xì)胞色素c,thehemeisareversiblecarrierofelectronsratherthanofoxygen.Hemehasyetanotherfunctionintheenzymecatalase過(guò)氧化氫酶,whereitcatalyzestheconversionofhydrogenperoxideintowaterandoxygen.

不同蛋白含有相同的輔基(血紅素)→不同的多肽微環(huán)境(蛋白構(gòu)象)使相同的輔基具有不同的功能→(血紅蛋白輸氧,細(xì)胞色素c電子載體,過(guò)氧化氫酶催化過(guò)氧化氫轉(zhuǎn)變成水和氧)TheCentralExonofMyoglobinEncodesaFunctionalHeme-BindingUnit

mosteukaryoticgenesaremosaicsofexons(codingsequences)andintrons(noncodinginterveningsequences).exonsencodediscretestructuralandfunctionalunitsofproteins.分離的外顯子編碼各自(所對(duì)應(yīng))的結(jié)構(gòu)域或功能域Myoglobinillustratesthisgeneralprinciplenicely.ThegeneforMbconsistsofthreeexons:nearlytheentireheme-bindingsiteisspecifiedbythecentralexon.Theregionsencodedbytheothertwoexonsmakeveryfewcontactswiththeheme.Digestionofapomyoglobinwithclostripain梭菌蛋白酶,anarginine-specificprotease,yieldsapolypeptidecontainingresidues32to139,whichcorrespondstothecentralexonandpartoftheC-terminalone.Thisfragmentbindsheme.Furthermore,thiscomplex,calledmini-myoglobin,bindsO2andCOreversibly.Indeed,theratesofassociationanddissociationarenearlythesameasthoseoftheintactprotein.Thesefindingsindicatethattheconformationofmini-myoglobinisverysimilartothatofnativemyoglobin.Becausetheamino-terminalexonandthelast14residuesoftheC-terminalexonarenotessentialforreversibleoxygenationitisevidentthatthecentralexoncontainsmuchoftheinformationforbindinghemeandmaintainingthenativefoldofanoxygen-bindingprotein.中央外顯子含有結(jié)合血紅素和保持(此氧結(jié)合)蛋白天然構(gòu)象的信息

Theexon-intronorganizationoftheconstituentchainsofHbisverysimilartothatofMb.AminoacidsequencecomparisonssuggestthatthegenesforMbandHbdivergedsome700millionyearsago.Thus,thecentralexonoftheseoxygencarriersisanancientpieceofDNAthatencodedafunctionalheme-bindingmoduleeonsago.HemoglobinConsistsofFourPolypeptideChainsVertebrateHb,theoxygentransporterinerythrocytes,consistoffourpolypeptidechains,twoofonekindandtwoofanother.Thefourchainsareheldtogetherbynoncovalentattractions.Eachcontainsahemegroupandasingleoxygen-bindingsite.HbA,theprincipalHbinadults,consistsoftwoalpha(α)chainsandtwobeta(β)chains.(α2β2)AdultsalsohaveaminorHb(~2%ofthetotalHb)calledHbA2,whichcontainsdelta(δ)chainsinplaceoftheβchainsofHbA(α2δ2).EmbryosandfetuseshavedistinctiveHb.Shortlyafterconception妊娠,embryossynthesizezeta(ζ)chains(whichareα-likechains)andepsilon(ε)chains(whichareβ-like).Inthecourseofdevelopment,ζisreplacedbyα,andεisreplacedbygamma(γ),andthenbyβ.ζ→α

ε→γ→βThemajorHbduringthelattertwo-thirdsoffetallife,HbF,hasthesubunitcompositionα2γ2.Theαandζchainscontain141residues;theβ,γ,andδchainscontain146residues.WhydoesHbconsistofmultiplepolypeptides,andwhydotheydiffer?SubunitinteractionsareattheheartofHb’scapacitytotransportO2,CO2,andH+inaphysiologicallyresponsiveway.

正常成人HbA(α2β2),少數(shù)成人HbA2(α2δ2),胎兒(三個(gè)月前)

ζ2ε2,胎兒(三至九個(gè)月)HbF(α2γ2),出生后α2β2.X-RayAnalysisofHemoglobin:aLaborofLove心甘情愿的工作(樂(lè)此不疲)overaQuarterCenturyTheHbmoleculeisnearlyspherical,withadiameterof55?.Thefourchainsarepackedtogetherinatetrahedralarray.Thehemegroupsarelocatedincrevicesneartheexteriorofthemolecule,oneineachsubunit.Thefouroxygen-bindingsitesarefarapart;thedistancebetweenthetwoclosestironatomsis25?.Eachαchainisincontactwithbothβchains.Incontrast,therearefewinteractionsbetweenthetwoαchainsorbetweenthetwoβchains.

HemoglobinSubunitsCloselyResembleMyoglobininThree-DimensionalStructureThethree-dimensionalstructuresofMbandtheαandβchainsofhumanHbarestrikinglysimilar.

TheeighthelicesineachchainofHbarevirtuallysuperposable可疊合onthoseofMb.Thiscloseresemblanceinthefoldingoftheirmainchainswasunexpectedbecausetheiraminoacidsequencesareratherdifferent.Infact,thesethreechainsareidenticalatonly24of141positions.Hence,quitedifferentaminoacidsequencescanspecifyverysimilarthree-dimensionalstructures.Itisevidentthatthethree-dimensionalformofspermwhaleMbandoftheαandβchainsofhumanHbhasbroadbiologicalsignificance.Infact,thismotif模體,calledtheglobinfold珠蛋白折疊,iscommontoallknownvertebrateMb.theintricatefoldingofthepolypeptidechain,firstdiscoveredinMb,isnature’sfundamentaldesignforanoxygencarrier:itplacesthehemeinanenvironmentthatenablesittocarryoxygenreversibly.ThegenesforMbandforα,β,andotherchainsofHbarevariationsonafundamentaltheme.Thisfamilyofgenesalmostcertainlyarosebygeneduplicationanddiversification多樣化.TheaminoacidsequencesofHbfrommorethan60species(rangingfromlampreyeels七鰓鰻tohumans)areknown.Acomparisonofthesesequencesshowsconsiderablevariabilityatmostpositions.However,ninepositionshavethesameresidueinmostspeciesstudiedthusfar.固定不變的(九個(gè))殘基都有特定的重要功能ThesehighlyconservedresiduesareespeciallyimportantforthefunctionoftheHbmolecule.Forexample,theinvariantF8histidineisdirectlybondedtothehemeiron.Severalofthemdirectlyaffecttheoxygen-bindingsite.Anotherinvariantresidue,tyrosineHC2(H螺旋羧基端的非螺旋區(qū)),stabilizesthestructurebyformingahydrogenbindbetweentheHandFhelices.GlycineB6isinvariantbecauseofitssmallsize:asidechainlargerthanahydrogenatomwouldnotallowtheBandEhelicestoapproacheachotherascloselyastheydo.ProlineC2maybeessentialbecauseitdefinesoneendoftheChelix.TheaminoacidresiduesintheinteriorofHbvaryconsiderably.However,thechangeisalwaysofonenonpolarresidueforanother(asfromalaninetoisoleucine).Hb球狀分子內(nèi)部殘基(組成)變異很大,但都是非極性殘基之間的替代Thus,thestrikingnonpolarcharacteroftheinteriorofthemoleculeisconserved.Thenonpolarcoreisimportantinbindingthehemegroupandinstabilizingthethree-dimensionalstructureofeachsubunit.Incontrast,surfaceresiduesarehighlyvariable.Indeed,fewareconsistentlypositivelyornegativelycharged.Itmightbethoughtthatprolineresidueswouldbepreservedbecauseoftheirroleashelixbreakers.However,thisisnotso.Onlyoneprolineisinvariant,yetthelengthsanddirectionsofthehelicesinallglobinsareverysimilar.Obviously,thereareotherwaysofterminatingorbendingαhelices.Forexample,anαhelixcanbedestabilizedbyhydrogenbindingbetweentheOHgroupofaserineorthreonineresidueandamain-chaincarbonylgroup.AllosterricInteractionsEnableHemoglobintoCoordinatelyTransportO2,CO2,andH+TheαandβsubunitsofHbhavethesamestructuraldesignasMb.However,newpropertiesofprofoundbiologicalimportanceemergewhendifferentsubunitscametogethertoformatetramer.不同亞單位組成的四聚體結(jié)構(gòu)使Hb具有一些Mb所沒(méi)有的,具重要生物學(xué)意義的特性HbisamuchmoreintricateandsentientmoleculethanisMb.前者復(fù)雜敏感得多HbtransportsH+andCO2inadditiontoO2.Hb能運(yùn)送H+andCO2

Furthermore,theoxygen-bindingpropertiesofHbareregulatedbyinteractionsbetweenseparate,nonadjacentsites.Hb的相距甚遠(yuǎn)的(四個(gè)氧結(jié)合)位點(diǎn)間的相互作用能調(diào)控它的氧結(jié)合性質(zhì)Hbisanallostericprotein,whereasMbisnot.Thisdifferenceisexpressedinthreeways:ThebindingofO2toHbenhancesthebindingofadditionalO2tothesameHbmolecule.Inotherwords,O2bindscooperativelytoHb.Incontrast,thebindingofO2toMbisnotcooperative.Hb與氧氣的結(jié)合是協(xié)同的.Mb不是.TheaffinityofHbforoxygendependsonpH,whereasthatofMbisindependentofpH.TheCO2moleculealsoaffectstheoxygen-bindingcharacteristicsofHb.BothH+andCO2promotethereleaseofboundO2.Reciprocally,O2promotesthereleaseofboundH+andCO2.pH和CO2影響Hb與氧氣的結(jié)合.theoxygenaffinityofHbisfurtherregulatedbyorganicphosphatessuchas2,3-bisphosphoglycerate(BPG).TheresultisthatHbhasaloweraffinityforoxygenthanMb.

2,3-二磷酸甘油酸也影響Hb與氧氣的結(jié)合.Hb(比Mb)與氧氣的親和力較低(特別在氧分壓較低時(shí)).OxygenBindsCooperativelytoHbThesaturation飽和度Yisdefinedasthefractionaloccupancyofalltheoxygen-bindingsitesinasolution.ThevalueofYcanrangefrom0(allsitesempty)to1(allsitesfilled).AplotofYversuspO2,thepartialpressureofoxygen,iscalledanoxygendissociationcurve.TheoxygendissociationcurvesofMbandHbdifferintwoways:First,foranygivenpO2YishigherforMbthanforHb.ThismeansthatMbhasahigheraffinityforoxygenthandoesHb.在氧解離(結(jié)合)曲線中,給定橫坐標(biāo)時(shí),Mb的Y值大,即Mb具較高的氧親和力

OxygenaffinitycanbecharacterizedbyaquantityP50,whichisthepartialpressureofoxygenatwhich50%ofsitesarefilled.ForMb,P50

1torr;forHb,P50

26torrs.TheseconddifferenceisthattheoxygendissociationcurveofMbishyperbolic雙曲線,whereasthatofHbissigmoidal

S型.ForMbY=pO2/(pO2+P50)(hyperbola)(P158)TheHillCoefficientisaMeasureofCooperativity協(xié)同性thecurveobtainedfromtheoxygen-bindingdataforHbagreeswiththeequationderivedforthehypotheticalequilibrium希爾系數(shù):希爾圖中(當(dāng)Y=0.5時(shí))的斜率Hb(O2)n＝Hb+nO2

→logY/1―Y=nlogpO2―nlogP50Aplotoflog[Y/(1-Y)]versuslogpO2,calledaHillplot,approximatesastraightline.Itsslopenatthemidpointofthebinging(Y=0.5)iscalledtheHillcoefficient.Thevalueofnincreaseswiththedegreeofcooperativity;themaximumpossiblevalueofnisequaltothenumberofbindingsites.MbgivesaHillplotwithn=1.0,whichmeansthatO2moleculesbindindependentlyofeachother,asindicatedinequation1.Mbn=1.0,即不同分子結(jié)合氧相互獨(dú)立Incontrast,theHillcoefficientof2.8forHbindicatesthatthebindingofoxygeninHbiscooperative.Bindingatonehemefacilitatesthebindingofoxygenattheotherhemeonthesametetramer.Hbn=2.8,一個(gè)血紅素結(jié)合(釋放)氧會(huì)促進(jìn)同一四聚體中其它的血紅素氧的結(jié)合(釋放).Conversely,theunloadingofoxygenatonehemefacilitatestheunloadingofoxygenattheothers.Inotherwords,thehemegroupsofaHbmoleculecommunicatewitheachother.ThemechanismofcooperativebindingofoxygenbyHb,sometimescalledHbinteraction.同一血紅蛋白分子中的血紅素之間是相互聯(lián)系的TheCooperativeBindingofOxygenMakesHbaMoreEfficientOxygenTransporterWhatisthebiologicalsignificanceofthecooperativebindingofoxygenbyHb?TheoxygensaturationofHbchangesmorerapidlywithchangeinthepartialpressureofO2thanitwouldiftheoxygen-bindingsiteswereindependentofeachother.Hb的氧飽和度隨氧分壓變化的速率比假定的(氧結(jié)合位點(diǎn)相互獨(dú)立)情況快得多Assumethatthealveolar肺泡pO2is100torrs,andthatthepO2inthecapillaryofanactivemuscleis20torrs.LetP50=26torrs,andtaken=2.8.ThenYinthealveolarcapillarieswillbe0.98,andYinthemusclecapillarieswillbe0.32.TheoxygendeliveredwillbeproportionaltothedifferenceinY,whichis0.66.ΔY是血紅蛋白氧運(yùn)載能力的量度LetusnowmakethesamecalculationforahypotheticaloxygencarrierforwhichP50isalso26torrs,butinwhichthebindingofoxygenisnotcooperative(n=1).ThenY(alveoli)=0.79,andY(muscle)=0.43,andsothedifferenceinYisequalto0.36.n=1時(shí)ΔY=0.36,n=2.8時(shí)ΔY=0.66Thus,thecooperativebindingofoxygenbyHbenablesittodeliver1.83timesasmuchoxygenundertypicalphysiologicalconditionsasitwouldifthesiteswereindependent.H+andCO2PromotetheReleaseof

O2:theBohrEffectMbshowsnochangeinoxygenbindingoverabroadrangeofpH,nordoesCO2haveanappreciableeffect.InHb,however,acidityenhancesthereleaseofoxygen.

IncreasingtheconcentrationofCO2(atconstantpH)alsolowerstheoxygenaffinity.Inrapidlymetabolizingtissue,suchascontractingmuscle,muchCO2andacidareproduced.ThepresenceofhigherlevelsofCO2andH+inthecapillariesofsuchmetabolicallyactivetissuepromotesthereleaseofO2fromoxyhemoglobin.Thereciprocaleffectoccursinthealveolarcapillariesofthelungs.The[O2]↑thereunloadsH+andCO2fromHb.TheselinkagesbetweenthebindingofO2,H+,andCO2areknownastheBohreffect.玻爾效應(yīng):血紅蛋白的氧親和力隨pH(和CO2分壓)的變化而變化.BPGLowerstheOxygenAffinityofHbTheoxygenaffinityofHbwithinredcellsislowerthanthatofHbinfreesolution.2,3-BPGbindstoHbandhasalargeeffectonitsaffinityforoxygen.Thishighlyanionic陰離子organicphosphateispresentinhumanredcellsataboutthesamemolarconcentrationasHb.IntheabsenceofBPG,theP50oftheHbis1torr,likethatofMb.Initspresence,P50es26torrs.Thus,BPGlowerstheoxygenaffinityofHbbyafactorof26,whichisessentialinenablingHbtounloadoxygenintissuecapillaries.BPGdiminishestheoxygenaffinityofHbbybindingtodeoxyhemoglobinbutnottotheoxygenatedform.紅細(xì)胞內(nèi)存在2,3-BPG,在低(高)氧分壓的條件下(通過(guò)與脫氧Hb的結(jié)合),大大降低Hb的氧親和力(不結(jié)合,無(wú)作用).FetalHbhasaHigherOxygenAffinitythanMaternalHbFetuseshavetheirownkindofHb,calledHbF(α2γ2),whichdiffersfromadultHbA(α2β2).AnimportantpropertyofHbFisthatithasahigheroxygenaffinityunderphysiologicalconditionsthandoesHbA.胎兒HbF的氧親和力高于成人HbA(的氧親和力).因?yàn)樗c2,3-BPG的結(jié)合較HbA弱(這一結(jié)合會(huì)大大降低蛋白的氧親和力).ThehigheroxygenaffinityofHbFoptimizesthetransferofoxygenfromthematernaltothefetalcirculation.這有利于氧氣由母體(循環(huán)系統(tǒng))向胎兒(循環(huán)系統(tǒng))的傳送(通過(guò)胎盤(pán)循環(huán)).HbFisoxygenatedattheexpenseofHbAontheothersideoftheplacentalcirculation.Thehigheroxygenaffinityoffetalbloodwasknownformanyyears,butanunderstandingofitsbasiscouldcomeonlyafterthediscoveryofBPG.HbFbindsBPGlessstronglythandoesHbAandconsequentlyhasahigheroxygenaffinity.IntheabsenceofHbBPG,theoxygenaffinityofHbFisactuallylowerthanthatofHbA.Weseehereaclear-cutbiologicaladvantageofhavingdistinctformsofaprotein,calledisoformsorisotypes同工型,indifferenttissues.TheQuaternaryStructureofHbChangesMarkedlyonOxygenationHbcanbedissociatedintoconstituentchains.ThepropertiesoftheisolatedαchainareverymuchlikethoseofMb.Theαchainbyitselfhasahighoxygenaffinity,ahyperbolicoxygendissociationcurve,andoxygen-bindingcharacteristicsthatareinsensitivetopH,CO2concentration,andBPGlevel.α,β4,Mb之間的氧結(jié)合性質(zhì)非常相似.Theisolatedβchainsreadilyassociatetoformatetramer(β4).LiketheαchainandMb,β4lackstheallostericpropertiesofHbandhasahighoxygenaffinity.Inshort,theallostericpropertiesofHbarisefrominteractionsbetweenitssubunits.ThefunctionalunitofHbisatetramerconsistingoftwokindsofpolypeptidechains.

Hb的變構(gòu)性質(zhì)來(lái)自其亞基之間的相互作用.它的功能單位是兩種不同肽鏈組成的四聚體.crystalsofdeoxyhemoglobinshatteredwhentheywereexposedtooxygen.Deoxymyoglobincrystals,incontrast,canbindandreleaseoxygenwithoutlosingtheirform.脫氧Hb的晶體在遇氧時(shí)破碎,而脫氧肌紅蛋白的晶體在結(jié)合和釋放氧氣時(shí)晶形不變,說(shuō)明Hb結(jié)合氧后發(fā)生了重大的構(gòu)象的改變.TheshatteringofHbcrystalssuggestedthattheproteinundergoesamajorconformationalchangeonbindingO2.oxy-anddeoxyhemoglobindiffermarkedlyinquaternarystructure.Theoxygenatedmoleculeismorecompact.Thechangesinthecontactsbetweentheαandβchainsareofspecialinterest.應(yīng)特別關(guān)注α和β鏈之間的界面(在氧合時(shí)四級(jí)結(jié)構(gòu)的)改變

Inthetransitionfromoxy-todeoxyemoglobin,largestructuralchangestakeplaceattwoofthefourcontactregions(theα1β2contactandtheidenticalα2β1contact)butnotattheothers(theα1β1contactandtheidenticalα2β2contact).Infact,theα1β2contactregionisdesignedtoactasaswitchbetweentwoalternativestructures.Thetwoformsofthisdovetailed帶鳩尾槽的interfacearestabilizedbydifferentsetsofhydrogenbonds.Thisinterfaceiscloselyconnectedtothehemegroups,andsostructuralchangesinitaffectthehemes.Reciprocally,structuralchangesatthehemesaffectthisinterface.(亞基之間的)界面結(jié)構(gòu)改變和血紅素(與氧結(jié)合的狀態(tài))相互影響Mostresiduesinitarethesameinallspeciesbecauseofitskeyroleinmediatingallostericinteractions.構(gòu)成不同物種的(亞基之間的)界面的大多數(shù)