生物化学Proteinsynthesis课件

ProteinsynthesisChapter13ReversetranscriptiontranslationThenucleotidesequenceofmRNAistranslatedintoasequenceofaminoacidsbyageneticcode(遗传密码)fromits5’endto3’end.whatthetranslationneeds

mRNA(template)aminoacids(materials)ribosome(factory,rRNAandprotein)ATPandGTP(energy)

tRNA(carrier,interpreter)13.1ThegeneticcodeThegeneticcodeistherulesthatspecifyhowthenucleotidesequenceofanmRNAistranslatedintotheaminoacidsequenceofapolypeptide.Thenucleotidesequenceisreadastripletcodon(三联子密码).遗传密码mRNA分子中所储存的遗传信息是由其分子的四种碱基以特定顺序排列成的三个一组的三联体代表的，每种氨基酸都有其特定的三联体密码。Thatmeansthreenucleotidesproduceoneaminoacid(43).Featuresofgeneticcodethegeneticcodeisuniversal.〔通用性〕itisalmostthesameinallorganisms.thegeneticcodeisunambiguous.(不重复)eachcodoncorrespondstoonlyoneaminoacid.

thegeneticcodeisdegenerate(简并).

inmostcases,oneaminoacidiscodedbyseveralcodons,whicharecalledsynonymouscodons(同义密码子).

thegeneticcodeiswobbling(摆动)

eachcodonisrecognizedbyatripletofbases,calledananticodon(反义密码子)

inRNA.Eachcodonbasepairswithitscomplementaryanticodonbase.However,thepairingofthethirdbaseofacodonislessstringentthanforthefirsttwobases.

13.2TransferRNA

thesecondarystructureoftRNAiscloverleaf(三叶草形).Itcontainsfivearms:acceptorstem(受体臂),Darm(双氢尿嘧啶臂),TψCarm(假尿嘧啶臂),anticodonarmandextraarm.thetertiarystructureoftRNAisaninverse“L〞form.ThefunctionoftRNAEachtRNAbecomescovalentlybondedtoaspecificaminoacidtoformaminoacyl-tRNA(氨酰tRNA)whichrecognizesthecorrespondingcodoninmRNAandensuresthatthecorrectaminoacidisaddedtothegrowingpolypeptidechain.13.3RibosomesRibosomesarelargeribonucleoprotein(核蛋白)particlesthatcontainmoreRNAthanproteinanddissociateintolargeandsmallsubunits.13.4ProcessoftranslationDuringtranslationthemRNAisreadfrom5’endto3’end,andproteinissynthesizedinanNterminaltoCterminaldirection.PrincipleforDNAreplication:base-pairingPrincipleforproteinsynthesis:anticodon-codonbase-pairing〔反密码子－密码子〕Processoftranslation

aminoacidactivation——synthesisofaminoacyl-tRNA(氨酰tRNA)

initiationoftranslation

elongationoftranslation

terminationoftranslationAminoacid+tRNA+ATP

Aminoacyl-tRNA+AMP+PPiaminoacyl-tRNAsynthetase(氨酰tRNA合成酶)EachtRNAcarriesonlyasingleaminoacid,andaseparateaminoacyl-tRNAsynthetaseexistsforeveryaminoacid.

InprokaryotestheinitialaminoacidisfMet(甲酰甲硫氨酸)，andthecorrespondinginitialaminocyl-tRNAisfMet-tRNAfMet；

Ineukaryotes,theinitialaminoacidisMet，thecorrespondinginitialaminoacyl-tRNAisMet-tRNAMet.InitiationstepP位A位Thesmallsubunit(30S)bindstothemRNATheinitiatortRNAchargedwithN-formylmethionine(fMet-tRNA)bindstoformthe30Sinitiationcomplex(30S起始复合物)Thelargeribosomalsubunitbindstoforma70sinitiationcomplex(70S起始复合物)withthehydrolysisofGTPElongationofpeptidechainAminoacyl-tRNAbinding:thecorrespondingaminacyl-tRNAforthesecondcodonbindstotheAsite〔进位〕〔GTPhydrolysis〕Peptidebondformation:thecarboxylendoftheaminoacidboundtothetRNAinthePsitebecomesjoinedbyapeptidebondtotheaminogroupoftheaminoacidlinkedtothetRNAintheAsite，catalyzedbypeptidyltransferase(转肽酶)Translocation(移位):ribosomemovesalongthemRNA(5’to3’)bythreenucleotidessothatthedipeptidyl-tRNAintheAsitemovestothePsitetoplacethenextcodonintheAsite(GTPhydrolysis)Aftertranslocation,theAsiteisemptyandreadytoreceivethenextaminoacyl-tRNA.Termination(合成终止)TerminationcodonsappearattheAsite(UAG,UAA,UGA)Releasefactors(终止因子)bindstotheribosome.Peptidyltransferaseistransformedtobecomeahydrolase(水解酶)sothatthepolypeptideistransferredtoawatermolecule,effectivelycleavingthebondbetweenthepolypeptideandtRNAinthePsite.Thenthepolypeptideleavestheribosome,flollowedbymRNAandfreetRNA.Theribosomedissociatesinto30Sand50Ssubunitsreadytostarttranslationafresh.(1ATP)EnergyyieldAminoacidactivation:2A