生物化學Proteinsynthesis課件

ProteinsynthesisChapter13ReversetranscriptiontranslationThenucleotidesequenceofmRNAistranslatedintoasequenceofaminoacidsbyageneticcode(遺傳密碼)fromits5’endto3’end.whatthetranslationneeds

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

tRNA(carrier,interpreter)13.1ThegeneticcodeThegeneticcodeistherulesthatspecifyhowthenucleotidesequenceofanmRNAistranslatedintotheaminoacidsequenceofapolypeptide.Thenucleotidesequenceisreadastripletcodon(三聯(lián)子密碼).遺傳密碼mRNA分子中所儲存的遺傳信息是由其分子的四種堿基以特定順序排列成的三個一組的三聯(lián)體代表的，每種氨基酸都有其特定的三聯(lián)體密碼。Thatmeansthreenucleotidesproduceoneaminoacid(43).Featuresofgeneticcodethegeneticcodeisuniversal.〔通用性〕itisalmostthesameinallorganisms.thegeneticcodeisunambiguous.(不重復)eachcodoncorrespondstoonlyoneaminoacid.

thegeneticcodeisdegenerate(簡并).

inmostcases,oneaminoacidiscodedbyseveralcodons,whicharecalledsynonymouscodons(同義密碼子).

thegeneticcodeiswobbling(擺動)

eachcodonisrecognizedbyatripletofbases,calledananticodon(反義密碼子)

inRNA.Eachcodonbasepairswithitscomplementaryanticodonbase.However,thepairingofthethirdbaseofacodonislessstringentthanforthefirsttwobases.

13.2TransferRNA

thesecondarystructureoftRNAiscloverleaf(三葉草形).Itcontainsfivearms:acceptorstem(受體臂),Darm(雙氫尿嘧啶臂),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〔反密碼子－密碼子〕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起始復合物)Thelargeribosomalsubunitbindstoforma70sinitiationcomplex(70S起始復合物)withthehydrolysisofGTPElongationofpeptidechainAminoacyl-tRNAbinding:thecorrespondingaminacyl-tRNAforthesecondcodonbindstotheAsite〔進位〕〔GTPhydrolysis〕Peptidebondformation:thecarboxylendoftheaminoacidboundtothetRNAinthePsitebecomesjoinedbyapeptidebondtotheaminogroupoftheaminoacidlinkedtothetRNAintheAsite，catalyzedbypeptidyltransferase(轉肽酶)Translocation(移位):ribosomemovesalongthemRNA(5’to3’)bythreenucleotidessothatthedipeptidyl-tRNAintheAsitemovestothePsitetoplacethenextcodonintheAsite(GTPhydrolysis)Aftertranslocation,theAsiteisemptyandreadytoreceivethenextaminoacyl-tRNA.Termination(合成終止)TerminationcodonsappearattheAsite(UAG,UAA,UGA)Releasefactors(終止因子)bindstotheribosome.Peptidyltransferaseistransformedtobecomeahydrolase(水解酶)sothatthepolypeptideistransferredtoawatermolecule,effectivelycleavingthebondbetweenthepolypeptideandtRNAinthePsite.Thenthepolypeptideleavestheribosome,flollowedbymRNAandfreetRNA.Theribosomedissociatesinto30Sand50Ssubunitsreadytostarttranslationafresh.(1ATP)EnergyyieldAminoacidactivation:2A