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食品專業(yè)英語LESSON4AminoAcidsAndProteinsProteinsaremoleculesofgreatsize,complexity,anddiversity.Theyarethesourceofdietaryaminoacids,bothessentialandnonessential,thatareusedforgrowth,maintenance,andthegeneralwell-beingofman.Thesemacromolecules,characterizedbytheirnitrogencontents,areinvolvedinmanyvitalprocessesintricatelyassociatedwithalllivingmatter.Inmammalsandmanyinternalorgansarelargelycomposedofproteins.Mineralmatterofboneisheldtogetherbycollagenousprotein.Skin,theprotectivecoveringofthebody,oftenaccountsforabout10%ofthetotalbodyprotein.Someproteinfunctionasbiocatalysts(enzymesandhormones)toregulatechemicalreactionswithinthebody.Fundamentallifeprocess,suchasgrowth,digestionandmetabolism,excretion,conversionofchemicalenergyintomechanicalwork,etc,arecontrolledbyenzymesandhormones.BloodplasmaproteinsandhemoglobinregulatetheosmoticpressureandPHofcertainbodyfluids.Proteinsarenecessaryforimmunologyreactions.Antibodies,modifiedplasmaglobulinproteins,defendagainsttheinvasionofforeignsubstancesofmicroorganismsthatcancausevariousdiseases,foodallergiesresultwhencertainingestedproteinscauseanapparentmodificationinthedefensemechanism.Thisleadstoavarietyofpainful,andoccasionallydrastic,conditionsincertainindividuals.Foodshortagesexistinmanyareasoftheworld,andtheyarelikelytobecomemoreacuteandwidespreadastheworld’spopulationincreases.providing

1adequatesuppliesofproteinposesamuchgreaterproblemthanprovidingadequatesuppliesofeithercarbohydrateorfat.Proteinsnotonlyaremorecostlytoproducethanfatsorcarbohydratesbutthedailyproteinrequirementperkilogramofbodyweightremainsconstantthroughoutadultlife,whereastherequirementsforfatsandcarbohydratesgenerallydecreasewithage.Asbrieflydescribedabove,proteinshavediversebiologicalfunctions,structures,andproperties.Manyproteinsaresusceptibletoalterationbyanumberofrathersubtlechangesintheimmediateenvironment.Maximumknowledgeofthecomposition,structure,andchemicalpropertiesoftherawmaterials,especiallyproteins,isrequiredifcontemporaryandfutureprocessingoffoodsistobestmeettheneedsofmankind.Aconsiderableamountofinformationisalreadyavailable,althoughmuchofithasbeencollectedbybiochemistsusingaspecificfoodcomponentasamodelsystem,AminoAcidsAminoacidsarethe“buildingblocks”ofproteins.Therefore,tounderstandthepropertiesof

2proteins,adiscussionofthestructuresandpropertiesofaminoacidsisrequired.Aminoacidsarechemicalcompounds,whichcontainbothbasicaminogroupsandacidiccarboxylgroups.Aminoacidsfoundinproteinshaveboththeaminoandcarboxylgroupsonthea-carbonatom;a-aminoacidshavethefollowinggeneralstructure:AtneutralpHvaluesinaqueoussolutionsboththeaminoandthecarboxylgroupsareionized.Thecarboxylgrouplosesaprotonandobtainsanegativecharge,whiletheaminogroupgainsaprotonandhenceacquiresapositivecharge.Asaconsequence,aminoacidspossessdipolarcharacteristics.Thedipolar,orzwitterions,formofaminoacidshasthefollowinggeneralstructure:Severalpropertiesofaminoacidsprovideevidenceforthisstructure:theyaremoresolubleinwaterthaninlesspolarsolvents;whenpresentincrystallineformtheymeltordecomposeatrelativelyhightemperatures(generallyabove200):andtheyexhibitlargedipolemomentsandlargedielectricconstantsinneuralaqueoussolutions.TheRgroupsorsidechains,ofaminoacidsandproteins.thesesidechainsmaybeclassifiedintofourgroups.Aminoacidswithpolar-uncharged(hydrophilic)rgroupscanhydrogenbondwithwaterandaregenerallysolubleinaqueoussolutions.Thehydroxylsofserine,heroine,andtyrosine;thesulfhydrylofthinlyofcysteine,andtheamidesofasparaginesandglutaminearethefunctional

3moietiespresentinrgroupsoftheclassofaminoacids.Twoofthese,thetoilofcysteineandthehydroxyloftyrosine,areslightlyionizedatPG7andcanloseaprotonmuchmorereadilythanothersinthisclass.Theamidesofasparaginesandglutaminearereadilyhydrolyzedbyacidorbasetoasparticandglutamicacids,respectively.Aminoacidswithnonpolar(hydrophobic)rgroupsarelesssolubleinaqueoussolventsthanaminoacidswithpolarunchargedrgroups.Fiveaminoacidswithhydrocarbonsidechainsdecreaseinpolarityasthelengthofthesidechainisincreased.Theuniquestructureofpraline(anditshydoxylatedderivative,hydroxyproline)causesthisaminoacidtoplayauniqueroleinproteinstructure.Theaminoacidswithpositivelycharged(basic)rgroupsatph6-7arelysine;argiinehasapositivelychargedquanidinogroup.Atph7.010%oftheimidazolegroupsofhistidinemoleculesareprorogated,butmorethan50%carrypositiveatph6.0.Thedicarboxylicaminoacids,asparicglutamic,possessnetnegativechargesntheneutralphrange.Animportantartificialmeal-flavoringfoodadditiveisthemonosodiumsaltofglutamicacid.PeptidesWhentheaminogroupofoneaminoacidreactswiththecarboxylgroupofanotheraminoacid,

4apeptidebondisformedandamoleculeofwaterisreleased.ThiscanbondjoinsaminoacidstogethertoformproteinsThepeptidebondisslightlyshorterthanottersinglec-nbonds.Thisindicatesthatthepeptidebondhassomecharacteristicsofadoublebond,becauseofresonancestabilizationwiththecarbony1oxygen.Thusgroupadjacenttothepeptidebondcannotrotatefreely,thisrigidityofthepeptidebondholdsthesixatomsinasingleplane.theamino(_NH_)groupdoesnotionizebetweenphoand14duetothedouble-bondpropertiesofthepeptidebond.Inaddition,rgroupsonaminoacidresidues,becauseofstarchhindrance,forceoxygenandhydrogenofthepeptidebondtoexistonatransconfiguration.Therefore,thebackboneofpeptidesandproteinshasfreerotationintwoofthethreebondsbetweenaminoacids.Ifafewaminoacidsarejoinedtogetherbypeptidebondsthecompoundiscalleda”mostnaturalpeptidesareformedbythepartialhydrolyticofproteins;however,afewpeptidesareimportantmetabolites.Ansetimeandcarnosinearetwoderivativesofhistaminethatarefoundinmusclespfanimals.Thebiochemicalfunctionofthesepeptidesisnotunderstood.Glutathioneoccursinmammalianblood,yeast,andespeciallyintissuesofrapidlydividingcells.Itisthoughttofunctioninoxidativemetabolismanddetoxification.

5Duirngoxidation,twomoletculesofglutathiunejoinvinadisulfidebridge(-S-S)betweentwocysteineisnotfoundinproteins.Otherpeptidesfunctinoasantibodiesandhormones.Oxytocinandhormones.Oxytocinandvasopressinareexamplesofpeptidehormones.ProteinstructureProteinsperformawidevarietyofbiologicalfunctionsandsincetheyarecomposedofhundredsofaminoacids,theirstructuresaremuchmerecomplexthanthoseofpeptides.Enzymesareglobularproteinsproducedinlivingmatterforthespecialpurposeofcatalyzingvitalchemicalreactionsthatotherwisedonotoccurunderphysiologicalconditions.Hemoglobinandmyoglobinarehemo-containingproteinsthattransportoxygenandcarbondioxideinthebloodandmuscles.Themajormuscleproteins,actinandmyosin,convertchemicalenergytomechanicalwork,whileproteinsintendons(collagenandelastim)bindmusclestobones,skin,hairyfingernails,andtoenailsarepertinaciousprotectivesubstance.Thefoodscientistisconcernedaboutproteinsinfoodssinceknowledgeofproteinstructureandbehaviorallowshimtomoreablymanipulatefoodsforthebenefitmankind.Nearlyaninfinitenumberofproteinscouldbesynthesizedfromthe21naturaloccurringaminoacids.However,ithasbeenestimatedthatonlyabout2000differentproteinsexistinnature.

6Thenumberisgreaterthanthisifoneconsiderstheslightvariationsfoundinproteinsfromdifferentspecies.Thelinearsequenceofaminoacidsinproteinisreferredtoast“primarystructure“.Inafewproteinstheprimarystructurehasbeendeterminedandoneprotein(ribonuclease)hasbeensynthesizedinthelaboratory.Itistheuniquesequenceofaminoacidsthatimpartsmanyofthefundamentalpropertiestodifferentproteinandtertiarystructures.Iftheproteincontainsaconsiderablenumberofaminoacidswithhydrophobicgroups,itssolubilityinaqueoussolventsisprobablelessthanthatofproteinscontainingaminoacidswithmanyhydrophilicgroups.Iftheprimarystructureoftheproteinwerenotfolded,proteinmoleculeswouldbeexcessivelylongandthin.Aproteinhavingamolecularweightof13,000wouldbe448athick.Thisstructureallowsexcessiveinteractionwithothersubstances,anditisnotfoundinnatureThethree-dimensionalmannerinwhichrelativelyclosemembersoftheproteinchainarearrangedisreferredtoas”secondarystructure.”examplesorsecondarystructurearethea-helixofwool,thepleated-sheetconfigurationofsilk,andthecollagenhelix.Thenativestructureofaproteinisthatstructurewhichpossessesthelowestfeasiblefreeenergy.Therefore,thestructureofaproteinisnotrandombutsomewhatordered.whentherestrictionsofthepeptidebondaresuperimposedonapolyaminoacidchainofaglobularprotein,aright

7handedcoil,the∝-helix,appearstobeoneofthemostorderedandstablestructuresfeasible.the∝-helixcontains3.6aminoacidresiduesperturnloftheproteinbackbone,withthergroupsoftheaminoacidsextendingoutwardfromtheaxisofthehelicalstructure,hydrogenbondingcanoccurbetweenthenitrogenofonepeptidebondandtheoxygenofanotherpeptidebondfourresiduesalongtheproteinchain,thehydrogenbondsarenearlyparalleltotheaxisofthehelix,lendingstrengthtothehelicalstructure,sincethisarrangementallowseachpeptidebondtoformahydrogenbond,thestabilityofthestructuregreatlyenhanced.Thecoilofthehelixissufficientlycompactandstablesthatevensubstanceswithstrongtendenciestoparticipateinhydrogenbonding,suchaswater,cannotenterthecore.Asecondarysaturationfoundinmanyfibrousproteinsistheβ-pleatedsheetconfiguration.Inthisconfigurationthepeptidebackboneformsazigzagpattern,withthergroupsoftheaminoacidsextendingaliveandbelowthepeptidechain.Sinceallpeptidebondsareavailableforhydrogenbonding,thisconfigurationallowsmaximumcross-linkingbetweenadjacentpolypeptidechainsandthusgoodstability.Bothparallel-pleatedsheet,wherethepolypeptidechainsruninoppositedirections,arepossible.Wheregroupsarebulkyorhavelittlecharges,theinteractionsofthergroupsdonotallowthepleated-sheetconfigurationtoexist.silkandinsectfibersarethebestexamplesoftheβ-sheet,althoughfeathersofbirdscontainacomplicatedformoftheseconfiguration.Anothertypeofsecondarystructureoffibrousproteinsisthecollagenhelix.collagenisthemost

8abundantproteininhighervertebrates,accountingforone-thirdofthetotalbodyprotein,collagenresistsstretching,isthemajorcomponentoftendons,andcontainsone-thirdglycineandone-fourthprolineorhydroxyprolinetherigidrgroups,andthelackofhydrogenbondingbypeptidelinkagesinvolvingprolineandhydroxyproline,preventsformationofan∝-helicalstructureandforcesthecollagenpolypeptidechainintoanoddkinked-typehelix.Peptidebondscomposedofglycineforminterchainhydrogenbondswithtwoothercollagenpolypeptidechains,andthisresultsinastabletriplehelix.Thistriple-helicalstructureiscalled“tropocollagen”andithasamolecularweightof3000,000Daltons.Themannerin,whichlargeportionsofitproteinchainarearrangedisreferredtoastertiarystructure.Thisinvolvesfoldingofregularuntsofthesecondarystructureaswellasthestructuringofareasofthepeptidechainthataredevoidofsecondarystructure.forexample,someproteinscontainareaswhere∝-helicalstructureexistsandotherareaswherethisstructurecannotform.dependingontheaminoacidsequence,thelengthofthe∝-helicalportionsareheldtogetherbyhydrogenbondsformedbetweenrgroups,bysaltlinkages,byhydrophobicinteractions,andbycovalentdisulfide(-s-s-0linkages.Thestructuresdiscussedsofarhaveinvolvedonlyasinglepeptidechain.Thestructureformedwhenindividual(subunit)polypeptidechainsinteracttoformanativeproteinmoleculeisreferredtoas“quaternarystructure”.Thebondingmechanismsthatholdproteinchainstogetheraregenerallythesameasthoseinvolvedintertiarystructure,withthepossibleexceptionthatdisulfidebondsdonotassistinmaitainingthequaternarystructuresofproteins

9第四課氨基酸和蛋白質(zhì)蛋白質(zhì)錯(cuò)綜復(fù)雜、多種多樣的大分子物質(zhì),是食物必須氨基酸和非必須氨基酸的來源。人體利用這些氨基酸以滿足生長(zhǎng)發(fā)育、修復(fù)組織和維持正常健康生活的要求。這些大分子以含氮為其特征，參與了許多與各種有生命物質(zhì)有復(fù)雜聯(lián)系的生命過程。在包括人類在內(nèi)的哺乳動(dòng)物中，蛋白質(zhì)起著機(jī)體改造成分的作用，肌肉和許多體內(nèi)器官主要由蛋白質(zhì)構(gòu)成。骨骼中的礦物質(zhì)靠膠原蛋白得以保持在一起。機(jī)體的保護(hù)層—皮膚中的蛋白質(zhì)通常占機(jī)體蛋白質(zhì)總量的10%的左右。有些蛋白質(zhì)有生物催化劑（酶和激素）的作用，以調(diào)節(jié)體內(nèi)的化學(xué)反應(yīng)?；镜纳^程如生長(zhǎng)、消化、代謝、排泄、化學(xué)能轉(zhuǎn)變成機(jī)械功等等都受酶和激素的控制。某些體液的滲透壓和pH值受制于血漿蛋白和血紅蛋白。蛋白質(zhì)對(duì)免疫反應(yīng)是必不可少的?？贵w（改性的血漿球蛋白能引起疾病的外來雜質(zhì)和微生物的入侵。當(dāng)某些攝入的蛋白質(zhì)使防御機(jī)制產(chǎn)生明顯的變化時(shí)，便發(fā)生人體的生物過敏。這就導(dǎo)致某些個(gè)體身上出現(xiàn)各種各樣的疾病，且有時(shí)是急劇的病情。食物短缺現(xiàn)象在世界許多地區(qū)存在。隨著人口的增加，這個(gè)問題很可能愈來愈尖銳、愈普遍。而蛋白質(zhì)供應(yīng)不足問題遠(yuǎn)比碳水化合物或脂肪供應(yīng)不足更為嚴(yán)重。蛋白質(zhì)不僅它的產(chǎn)出費(fèi)用要比碳水化合物或脂肪的產(chǎn)出費(fèi)用為高，而且每千克每天所需的蛋白質(zhì)量造整個(gè)成年

10期是恒定的，而每天所需的脂肪和碳水化合物量一般都隨著年齡的增長(zhǎng)而逐漸減少。正如上面簡(jiǎn)述的一樣，蛋白質(zhì)有多種不同的結(jié)構(gòu)、性質(zhì)和生理功能。許多蛋白質(zhì)容易受周圍環(huán)境的一系列微妙變化的影響而發(fā)生變化。要想使現(xiàn)在和將來的食品加工能理想的滿足人類的需要，就必須徹底了解原料特別是蛋白質(zhì)的組成結(jié)構(gòu)和化學(xué)性質(zhì)。目前，已經(jīng)有這方面的大量資料可供利用，不過其中大部分是生物化學(xué)家利用某一特定食物成分作為模擬物系加以收集的。氨基酸氨基酸是蛋白質(zhì)的“結(jié)構(gòu)單元”。因此，要了解蛋白質(zhì)的性質(zhì)，舊需要討論氨基酸的結(jié)構(gòu)和性質(zhì)。氨基酸是既含氨基又含酸性羧基的化合物。蛋白質(zhì)中的氨基酸在α-碳原子上同時(shí)有氨基和羧基。α-氨基酸具有如下的一般結(jié)構(gòu)：在中性pH水溶液中，氨基和羧基都呈離子狀態(tài)。羧基失去一個(gè)質(zhì)子而帶負(fù)電荷，同時(shí)氨基得到一個(gè)質(zhì)子而帶正電荷。結(jié)果氨基酸便具有偶極的特性。氨基酸的這種偶極形式（即兩性形式）有如下的一般結(jié)構(gòu)：氨基酸有好幾種性質(zhì)都反映了這種結(jié)構(gòu)，這些性質(zhì)是：它們易溶于水而不易溶于極性很小的溶劑：當(dāng)以晶體形式存在時(shí)，它們要在較高溫度（一般在200℃以上）下熔化或分解；它們?cè)谥行匀芤悍N顯示出很大的偶極矩和介電常數(shù)。

11氨基酸的側(cè)鏈R基團(tuán)對(duì)氨基酸和蛋白質(zhì)的化學(xué)性質(zhì)產(chǎn)生重大的影響。這些側(cè)鏈可以分為四類。帶有極性非荷電的（親水的）R基團(tuán)的氨基酸能與水形成氫鍵，通常能溶于水溶液。絲氨酸、蘇氨酸和酪氨酸的羥基，半胱氨酸的硫氫基（即硫醇）以及天冬酰胺和谷氨酰胺的酰胺基時(shí)出現(xiàn)在這類氨基酸R基團(tuán)中的功能部分，其中半胱氨酸的硫羥基和酪氨酸的羥基在pH7時(shí)能輕度離子化，因而比這類中其它氨基酸更容易失去質(zhì)子。天冬酰胺和谷氨酰胺的酰胺基容易被酸和堿水解，分別形成天冬氨酸和谷氨酸。帶有非極性（疏水的）R集團(tuán)的氨基酸在水溶液中的溶解性比帶有極性非荷電的R基團(tuán)的氨基酸要小得多。帶有烴側(cè)鏈的五種氨基酸，其側(cè)鏈隨側(cè)鏈長(zhǎng)度增加而降低。脯氨酸（以及其烴基衍生物羥脯氨酸）的獨(dú)特結(jié)構(gòu)使這種氨基酸在蛋白質(zhì)結(jié)構(gòu)中有獨(dú)特的地位。pH6～7時(shí)帶正電荷（堿性的）R基團(tuán)的氨基酸有賴氨酸、精氨酸和組氨酸。賴氨酸帶正電的原因主要在于氨基，而精氨酸則具有帶正電荷的胍基。pH7時(shí)組氨酸分子中的咪唑基有10%質(zhì)子化，但在pH6時(shí)則有50%以上帶正電荷。二羥基氨基酸（天冬氨酸和谷氨酸）在中性pH范圍內(nèi)帶凈負(fù)電荷，谷氨酸的一鈉鹽是一種重要的膳食調(diào)味用的人造食品添加劑。肽

12當(dāng)一個(gè)氨基酸分子的氨基與另一個(gè)氨基酸分子的羧基起反應(yīng)時(shí),便形成一個(gè)肽鍵，同時(shí)釋放出一分子水。這種C-N鍵把眾多的氨基酸連接在一起形成蛋白質(zhì)。這種肽鍵比其它簡(jiǎn)單的C-N鍵略短,這說明由于肽鍵與羰基氧的共振穩(wěn)定作用，肽鍵具有了一定的雙鍵特性。這樣，緊鄰肽鍵的基團(tuán)就不能自由轉(zhuǎn)動(dòng)了。肽鍵的這種剛性使六個(gè)原子保持在一個(gè)平面上。H由于肽鍵的雙鍵性質(zhì)，亞氨基（-NH-)在pH0～14之間均不能離子化。此外，由于立體位阻現(xiàn)象，氨基酸殘基上的R基團(tuán)迫使肽鍵上的氧原子和氫原子只能以反式構(gòu)型存在。因此，多肽和蛋白質(zhì)的主鏈只可能在氨基酸之間的三個(gè)鍵中的兩個(gè)鍵上作自由轉(zhuǎn)動(dòng)。如果少數(shù)幾個(gè)氨基酸以肽鍵連接起來，這樣的化合物就稱為“肽”大多數(shù)天然肽是由蛋白質(zhì)部分水解形成的?？墒?，有少數(shù)肽則是重要的代謝產(chǎn)物。鵝肌肽和肌肽是動(dòng)物肌肉中組氨酸的兩種衍生物，這些肽生物化學(xué)功能目前還不清楚。谷胱甘肽存在于哺乳動(dòng)物血液、酵母之中，特別是快速分解的細(xì)胞質(zhì)組織中。一般認(rèn)為，這種肽具有參與氧化代謝和解毒作用的功能。氧化過程中，兩分子谷胱甘肽通過兩個(gè)半胱氨酸殘基之間的二硫鍵-S-S-連接起來。在蛋白質(zhì)中未曾發(fā)現(xiàn)谷氨酸的γ-羰基與半胱氨酸連成的肽鍵。此外，還有具抗體和激素功能的肽。催產(chǎn)素和抗利尿素就是肽激素的例子。

13蛋白質(zhì)的結(jié)構(gòu)蛋白質(zhì)執(zhí)行的多種多樣的生物功能，而且由于它是數(shù)百個(gè)氨基酸組成的，故其結(jié)構(gòu)遠(yuǎn)比肽為復(fù)雜。酶是生物中產(chǎn)生的球狀蛋白質(zhì)，目的是專門催化某些生物化學(xué)反應(yīng)，不然的話，這些化學(xué)反應(yīng)在生理?xiàng)l件下是不會(huì)發(fā)生的。血紅蛋白和肌紅蛋白是輸送血液和肌肉中氧和二氧化碳的含血紅素的蛋白質(zhì)。重要的肌肉蛋白—肌動(dòng)蛋白和肌球蛋白把化學(xué)能轉(zhuǎn)變成機(jī)械能；而腱中的蛋白質(zhì)（膠原蛋白和彈性蛋白）則是將肌肉粘連在骨骼上。皮膚、毛發(fā)、指（趾）甲是蛋白質(zhì)類的保護(hù)物質(zhì)。食品科學(xué)家之所以關(guān)注食物蛋白質(zhì)，是因?yàn)檎莆樟说鞍踪|(zhì)結(jié)構(gòu)和功能方面的知識(shí)，才能更好的加工和處理食品，造福于人類。用21種天然存在的氨基酸幾乎可以合成無數(shù)的蛋白質(zhì)。可是，據(jù)估計(jì)自然界只存在約2000種不同的蛋白質(zhì)，如果考慮不同物種蛋白質(zhì)存在的微小差異，則蛋白質(zhì)數(shù)目就超過此數(shù)。蛋白質(zhì)分子內(nèi)氨基酸的直線排列次序被看作是蛋白質(zhì)的一級(jí)結(jié)構(gòu)。少數(shù)蛋白質(zhì)的一級(jí)結(jié)構(gòu)已經(jīng)被確定。而且已在實(shí)驗(yàn)室里合成了其中一種蛋白質(zhì)（核糖核酸）。正是這種氨基酸的獨(dú)特排列順序賦予不同蛋白質(zhì)以許多基本特性，且在很大程度上決定了它們的二級(jí)和三級(jí)結(jié)構(gòu)。如果蛋白質(zhì)中含有大量帶疏水基團(tuán)的氨基酸，則它在水溶劑中的溶解性就可能比帶許多親水基團(tuán)的氨基酸的蛋白質(zhì)差。

14如果蛋白質(zhì)的一級(jí)結(jié)構(gòu)不是折疊的，那么蛋白質(zhì)分子就會(huì)很長(zhǎng)很細(xì)。分子量為13,000蛋白質(zhì)分子就應(yīng)有448納米長(zhǎng)和3.7納米粗這種結(jié)構(gòu)將使它們可能與其它物質(zhì)發(fā)生過度的相互反應(yīng)，然而這樣的結(jié)構(gòu)在自然界還未發(fā)現(xiàn)。蛋白質(zhì)鏈中相互靠近的鏈節(jié)與鏈節(jié)之間的三維排列方式即為蛋白質(zhì)的“二級(jí)結(jié)構(gòu)”。二級(jí)結(jié)構(gòu)的具體例子有羊毛蛋白的α-螺旋結(jié)構(gòu)，蠶絲蛋白的折疊片結(jié)構(gòu)和膠原蛋白的螺旋結(jié)構(gòu)。蛋白質(zhì)的自然結(jié)構(gòu)是含有最低可能自由能的結(jié)構(gòu)。因此，蛋白質(zhì)的結(jié)構(gòu)不是任意的，而是有一定規(guī)則的。當(dāng)球狀蛋白質(zhì)的聚氨基酸鏈上受到肽鍵約束時(shí)，右螺旋（即α-螺旋）看來是最有規(guī)則、最為穩(wěn)定的合理結(jié)構(gòu)之一。每一圈α-螺旋的蛋白質(zhì)主鏈上含有3.6個(gè)氨基酸殘基，而氨基酸的R基團(tuán)則從螺旋結(jié)構(gòu)的軸線向外伸出，一個(gè)肽鍵的氮能夠與另一個(gè)沿蛋白質(zhì)主鏈相距四個(gè)氨基酸殘基處的氧形成氫鍵。此氫鍵差不多與α-螺旋軸線平行，賦予螺旋結(jié)構(gòu)以強(qiáng)度。由于這樣的排列能使每一個(gè)肽鍵都能形成氫鍵，因此大大加強(qiáng)了結(jié)構(gòu)的穩(wěn)定性。螺旋圈是非常緊密和堅(jiān)固的，所以即使象水那樣的有強(qiáng)烈參與形成氫鍵趨勢(shì)的物質(zhì)，也不能進(jìn)入螺旋中央部分。出現(xiàn)在許多纖維狀蛋白質(zhì)中的二級(jí)結(jié)構(gòu)是β-折疊片結(jié)構(gòu)。在這構(gòu)型中，肽主鏈呈鋸齒形，其氨基酸的R基團(tuán)向肽鏈的上方和下方伸展。由于所有肽鍵都可供氫鍵形成，故這種構(gòu)型能夠使相鄰的多肽鏈之間充分形成交聯(lián)，從而具有良好的穩(wěn)定性。有兩種折疊片，即相鄰多肽鏈走向相同的平行折疊片和走向相反的反平行折疊片，均有可能存在。如果多肽鏈中R基團(tuán)過大，或帶有同種電荷，則R基團(tuán)間的相互作用使β-折疊片不可能形成。蠶絲和昆蟲纖維蛋白是β-折疊結(jié)構(gòu)的最好例子，而鳥類羽毛中所含的是這種構(gòu)型的復(fù)雜形式。

15纖維蛋白的另一種二極結(jié)構(gòu)是膠原螺旋。膠原螺旋是高等脊椎動(dòng)物中最豐富的一種蛋白質(zhì)，占動(dòng)物體蛋白總量的1/3。膠原蛋白中含有1\3的甘氨酸和1\4脯氨酸或羥脯氨酸，能抵抗拉伸,是腱的主要組分。由于R基團(tuán)的剛性以及脯氨酸、羥脯氨酸參與的肽式鍵合不能形成氫鍵的原因，所以α-螺旋結(jié)構(gòu)無法形成，迫使膠原多肽鏈變成一種零散結(jié)節(jié)式的螺旋體，膠原多肽鏈中由甘氨酸構(gòu)成的肽鍵與另兩條多肽鏈形成了鍵間氫鍵，產(chǎn)生了一種穩(wěn)定的三股螺旋。此三股螺旋結(jié)構(gòu)稱為“原膠原”，其分子量為30萬道爾頓。蛋白質(zhì)鏈中大鏈段的排列方式稱為蛋白質(zhì)的“三級(jí)結(jié)構(gòu)”。它包括二級(jí)結(jié)構(gòu)常規(guī)單元的折疊以及無二級(jí)結(jié)構(gòu)肽鏈若干區(qū)域的結(jié)構(gòu)化。例如，某些蛋白質(zhì)中包含了有α-螺旋結(jié)構(gòu)存在的區(qū)域和另外不能形成這種結(jié)構(gòu)的區(qū)域。根據(jù)氨基酸順序的不同，此α-螺旋段的長(zhǎng)度也不同，并賦予獨(dú)特的三級(jí)結(jié)構(gòu)，這些折疊部分是靠R基團(tuán)之間所形成的氫鍵，靠鹽鍵、疏水相互作用以及共價(jià)二硫鍵（-S-S-）而結(jié)合在一起的。至此所討論的結(jié)構(gòu)僅涉及單個(gè)肽鏈的結(jié)構(gòu)。各個(gè)（亞單位）多肽鏈相互作用變成天然蛋白質(zhì)分子時(shí)所形成的結(jié)構(gòu)即為蛋白質(zhì)的“四級(jí)結(jié)構(gòu)”。使蛋白質(zhì)鍵結(jié)合在一起的鍵合機(jī)制通常與三級(jí)結(jié)構(gòu)中所述的相同，可能的例外情況是雙硫鍵不參與蛋白質(zhì)四級(jí)結(jié)構(gòu)的保持。專業(yè)英語詞匯

16intricatea.復(fù)雜的,錯(cuò)綜的,纏結(jié)的,難懂的collagenousa.膠原的globulin球蛋白plasma血漿，原生質(zhì)immunological免疫的hemoglobin血紅蛋白basicamino堿性的氨基acidiccarboxyl酸性的羧基aqueous①水的②含水的③水成的

17proton質(zhì)子,氕核dipolar偶極的,兩極的zwitterion兩性離子crystalline①結(jié)晶的,晶狀②清澈的hydrophilic親水的serine絲氨酸,羥基丙氨酸t(yī)hreonine羥基丁氨酸,蘇氨酸t(yī)yrosine酪氨酸,3-對(duì)羥苯基丙氨酸sulfhydryl氫硫的～enzyme硫化氫解酶～group巰基cysteine半胱氨酸,巰基丙氨酸cystine胱氨酸,雙巰丙氨酸

18amide①酰胺②氨化物asparagine天門冬酰胺glutamine谷氨酰胺asparticacid天門冬氨酸,丁氨二酸glutamicacid谷氨酸proline脯氨酸,氮戊環(huán)-[2]-基羧酸lysine賴氨酸arginine精氨酸histidine組氨酸,咪唑丙氨酸quanidino胍imidazoln.咪唑;1,3-二氮雜茂

19resonancen.①回聲,反響②[物]共振,共鳴;諧振③[醫(yī)]叩響imino亞氨sterica.空間的,位的anserine鵝肌肽carnosine肌肽glutathione谷胱甘肽peptide肽,縮氨酸oxytocinn.(垂體)后葉催產(chǎn)素vasopressinn.后葉加(血)壓素,加壓素carbonyl羰基,碳酰hemoglobin血紅蛋白hemo-表示“血”

20myoglobin肌紅蛋白actin肌動(dòng)蛋白myosin肌球蛋白tendon腱,筋根collagen膠原,膠原蛋白elastin彈性蛋白ribonuclease核糖核酸酶hydrophobic疏水的restrictionn.限制,限定,約束vertebraten.脊椎動(dòng)物a.①有椎骨的,有脊椎的,脊椎動(dòng)物的②(作品等)結(jié)構(gòu)嚴(yán)密的kinkn.①(繩索,頭發(fā)的)細(xì)結(jié),絞纏②奇想,怪念頭,乖僻③(奇特的)妙法④(頸背等處的)

21肌肉痙攣,抽筋⑤[美](結(jié)構(gòu)或設(shè)計(jì)等的)缺陷vt.使紐結(jié),使絞纏vi紐結(jié),打結(jié)glycine甘氨酸,氨基醋酸t(yī)ropocollagen原膠原dalton道爾頓devoida.缺乏,沒有(of)covalent共價(jià)～bond共價(jià)鍵quaternarya.①四個(gè)一組的,四部組成的,第四的②四元的,四價(jià)的,季的③[地]第四紀(jì)的n.①四,四個(gè)一組,第四組中的組成部分②[數(shù)]四進(jìn)制③[地]第四紀(jì)zigzag①之字形，Z字形，鋸齒形；②之字形的線條（或道路、壕溝、裝飾等）；③蜿蜒曲折，盤旋彎曲專業(yè)英語總結(jié)

22EnglishKnowledgepoint:1.beof+名詞，相當(dāng)于形容詞。F：Proteinsaremoleculesofgreatsize,complexity,anddiversity,proteinsaremoleculesofgreatsize,complexity,anddiversity.roteinsaremoleculesofgreatsize,complexity,anddiversity,proteinsaremoleculesofgreatsize,complexity,anddiversity.2.atPH7,atneutralPH(用介詞at)

23SOMEGOODSENTENCE:1.Aminoacidsarethe“buildingblocks”ofproteins.Skintheprotectivecoveringofthebody,oftenaccountsforabout10%ofthetotalbodyprotein.2.Skintheprotectivecoveringofthebody,oftenaccountsforabout10%ofthetotalbodyprotein.3.Manyproteinsaresusceptibletoalterationbyanumberofrathersubtlechangesintheimmediateenvironment.

244.Aconsiderableamountofinformationisalreadyavailable,althoughmuchofithasbeencollectedbybiochemistsusingaspecificfoodcomponentasamodelsystem.5.Severalpropertiesofaminoacidsprovideevidenceforthisstructure.6.Theaminoisresponsibleforthepositivechargeoflysine.whileargininehasapositivelychargedquanidinogroup.EnglishKnowledgepoint:1.beof+名詞，相當(dāng)于形容詞。F：Proteinsaremoleculesofgreatsize,complexity,anddiversity,proteinsaremoleculesofgreatsize,complexity,anddiversity.roteinsaremoleculesofgreatsize,complexity,anddiversity,proteinsaremoleculesofgreatsize,complexity,anddiversity.2.atPH7,atneutralPH(用介詞at)專業(yè)英語難點(diǎn)SOMEGOODSENTENCE:1.Aminoacidsarethe“buildingblocks”ofproteins.Skintheprotectivecoveringofthebody,oftenaccountsforabout10%ofthetotalbodyprotein.

252.Skintheprotectivecoveringofthebody,oftenaccountsforabout10%ofthetotalbodyprotein.3.Manyproteinsaresusceptibletoalterationbyanumberofrathersubtlechangesintheimmediateenvironment.4.Aconsiderableamountofinformationisalreadyavailable,althoughmuchofithasbeencollectedbybiochemistsusingaspecificfoodcomponentasamodelsystem.5.Severalpropertiesofaminoacidsprovideevidenceforthisstructure.6.Theaminoisresponsibleforthepositivechargeoflysine.whileargininehasapositivelychargedquanidinogroup.