1.1.2POST-TRANSLATIONALMODIFICATIONS:TERMINALGROUPSMultifariouscovalentstructuralmodificationsofaminoacidresiduestake ceinvivosubsequenttotheirincorporationintonascentpolypeptidechainsduringtranslationontheribosome.TerminalAminoThefateoffirstaminoacidofnascentIneukaryotes: theN-formylgroupisgenerallyveryrapidlyremoved,subsequentcleavageofthemethionylresidueoccursinthemajorityofcases.Theeffectivenessofthemethionylaminopeptidaseatthiscleavageisdependentupontheidentityofthesecondresidue.Inyeast(Saccharomycescerevisiae):themethionineiscomple removedifthepenultimateresiduehasaradiusofgyrationof0.129nmorless(Gly,Ala,Ser,Cys,Thr,Pro,Val).Infungalandm lianmitochondria:cleavageoftheinitiatormethioninedoesnotoccur,althoughinatleastone RNAproteinitdoesInengineeringbacteria:overexpressionofproteinsinEscherichiacolismidtechnologycanleadtounnaturalretentionofthemethionylTheN-acetylgroupisacommonconstituentofInEhrlichascitecells:Itwassuggestedthatabout80%ofthesolubleproteinsinEhrlichascitescellsareN-Inhighereukaryotes：therewasnoevidencethatthesecellswereunusualintheircontentofN-acetylgroups,andthispropertymaybetypicalofhighereukaryotes.Inlowereukaryotes:theproportionofN-acetylatedproteinsislower,butstillsubstantial.N-acetylationisgenerallyacotranslationalevent,occurringwhenthenascentpeptidechainisabout20-50residueslong.DifferencesinthefrequencyofacetylationofdifferentN-terminalresidues,intheorderAla,Ser>Met,Gly,Asp>Asn,lle,Thr,Val>otherresidues.ProteinsfromhighereukaryotesweremorelikelytobeacetylatedthosefrombacteriaorSomeeukaryoticproteinsexpressedinE.coliarepartiallyN-acetylatedMet-AspandMet-GlusequencesattheN-terminiofmutantiso-l-cytochromecexpressedinSaccharomycescerevisiaewerecomple acetylated,themajorityofN-acetylatedmethionineresiduesinproteinsarefollowedbyasparticacidorglutamicacidThefunctionofprotein todegradationandubiquitin-dependent2).Acetylationofpeptidehormonesderivedfrom grosslyaffectstheirbiologicalactivityandstability3).AcetylationoftheN-terminusof isrequiredhigh-affinitybindingtoFatty1.Amino-terminalglycineresiduesmanyproteinsbearfattyacylgroups,mostcommonlymyristoyl肉豆蔻酰，十四酰(C14:0),andothervariantfattyacylgroup.(figure)Myristoylappearancechance:ApartfromtheobligatoryN-terminalglycineresidue,thesubstratespecificityofproteinN-myristoyltransferasesshowsapreferenceforserineatposition5andforsmallneutralresiduesatposition2Function:Thehydrophobicmyristoylgroupinfluencestheassociationofproteinswithmembranesandbindingbetweenproteins,suchas assembly,andconfersstructuralstabilityoncAMP-dependentproteinkinase2.AsecondtypeofN-terminalfattyacylationisthelinkage,mainlyofpalmitoyl棕櫚酰groupsbutalsoofotherfattyacylgroups,totheaminogroupoftheS-residuesinmurein胞壁質(zhì)lipoproteinofE. andmembranepenicillinasesofGram-positivebacteriaAwidespreadmodification,thepyroglutamyl(pyrrolidone吡咯烷酮carboxylylor5-carboxypyrrolidin吡咯棱啉-2-one)groupisformedbycyclizationofN-terminalglutamineresidueswithlossofammonia.Thiscanoccurinvitro,withoutenzymiccatalysis,althoughitislikelythatinvivothereiscatalysis.OtherAcyl（1）N-formylmethioninestructureinnascentprokaryoticproteins,whichisrarelyretainedinmatureproteins,honeybeemelittinhasN-formylglycine（2）.The-ketoacylgrouppyruvoyl 酰基isfoundinseveralenzymesofaminoacidmetabolism,includinghistidinedecarboxylaseofLactobacillus30a,phosphatidyl磷脂酰-serinedecarboxylaseofE.coli,prolineredu eofC.Sticklandiiandadenosyl-methioninedecarboxylasefromE.（3).Therelateda-ketobutyryl ispresentinsheepliverserine-dehydrataseandurocanase尿ofPseudomonasfunction:Thesederivatives,formedbydeaminationofserineorthreoninc,respectively,attheN-terminalsoftheproteinprecursorsoftheenzymes,functioninawaysimilartothatofpyridoxylcoenzyme.（4）D-GlucuronicacidislinkedthroughanamidegrouptotheN-terminusofcutinasefromFusariumsolanipis鐮刀霉.（5）Aminoacyla.Anonribosomal,buttRNA-dependent,extractofE.colicatalyzedtheincorporationofleucine,phenylalanine,andtryptophanintoprotein;mostoftheleucinewasfoundtobeTheenzymeresponsiblehasbeenfurthercharacterizedandtransfersleucine,Phenylalanine,ormethioninetoproteinsubstrateswithN-terminalarginineorlysineresidues.b.Ineukaryotesthereisanarginyl-tRNA-proteintransferaseactivity.TheadditionofN-terminalargininetoproteinswithacidicN-terminalresiduessensitizesthemtodegradationbytheubiquitin-dependentpathwayGlycationoftheaminogroupofVal-lofthechainofhumanhemoglobinhasbeenextensivelystudied.1-Deoxy,l-(N-valine)-fructose,formedbyAmadorirearrangementoftheinitialSchiffbaseispresentintheminorhemoglobinspeciesA1c.OtherproteinN-terminalgroupsmaybesimilarlyglycated.AslowconversionoftheAmadoriproducttofluorescent,cross-linkedstructuresknownasadvancedglycosylationendproducts(AGEs)takes ceinvivo. Alkylationoftheamino-terminalgroupisrarerthanacylation,butseveraltypesofproteinhaveN-methylatedamino-terminalresidues.EcoliribosomalproteinL11hasN-Me3AlaL16hasN-Me-Met,L33hasN-Me-Ala,andIF3hasN-Me-Met. N-Me-pheispresentattheN-terminusofseveralbacterialpilins菌毛素e.gthatofPseudomonasaeruginosaKandN-Me-MetinE.coliCheZHistone2BfrominvertebratesMe2ProinthestarfishAsieriarubensMeProinDrosophilaandMe3AIainTetrahymena四膜蟲Crithidiaoncopelti?cytochromec557Me2ProandsomemyosinlightchainsMeAlaareexamplesofeukaryoticproteinswithmethylatedamino-terminals.IntheseproteinstheN-terminalsequencesaresimilar,(Me3AlaorMe1-2Pro)-Pro-Lys.Terminala-CarboxylThepresenceofaC-terminalamidegroupinproteinsappearstobeessentiallylimitedtosmallpeptidehormonesandinsecttoxins(Mainsetal.,1983)someexamplesare激素,calcitoninCT降鈣素,MSH,gastrin胃泌激素andmelittin蜂毒素.Theamidegroupisgenerallyessentialforfullbiologicalactivityofthesepeptides.ThepeptidehormonesarederivedfromprecursorproteinsviaoxidationofintermediateswithC-terminalglycineresidues,resultinginretentionoftheglycinenitrogenatomwithintheamidegroupMethylMethylestersC-terminalS-cysteineresidues(seeThioetherLinkages,below)arefoundinavarietyofeukaryoticproteins.AsecondtypeofC-terminalmethylation,ofaleucineresidue,hasbeenfoundina36-kDapolypeptideofbovinebrain.AminoAcid-TubulinisreversiblymodifiedinvivobytheadditionoftyrosineinapeptidebondtotheC-terminus.Thefunctionofthismodificationisstillunclear.Inadditiontotwoglutamicresidueside-chaincarboxylgroups,theC-terminalcarboxylgroupofLys-213ofratliverhistoneHIacceptsADP-riboseinanester(O-glycosidic)linkage.Furtheratta residuesthroughribosyl-ribosyllinkagesyieldspoly(ADP-ribosyl)ThismodificationisreversibleinGlycolipidAnchorAsubstantialnumberofproteinsareattachedtomembranesviaglycosylphosphatidylinositolGPI糖基磷脂酰肌醇structuresofthegeneralformshowninFigure5.Structure:Variationsofthisgeneralstructureincludetheadditionofabranchingglycanmoiety,whichmaybearadditionalethanolaminephosphatetoacoremannose甘露糖,andpalmitoyl棕櫚tothe2or3-positionofthemyoinositolBiosyntheticpathway:ThebiosyntheticpathwayinvolvestransferoftheGPIstructuretothenewC-terminusreleasedfollowingproteolysisbetweentwosmallresiduesofaprecursorproteinthatbearsahydrophobictail.TheC-terminalglycineresidueofubiquitin,asmallproteinfoundinalleukaryotes,islinked,followingactivationtoubiquiti