discussed in detail about the protein synthesis and steps involved in translation in prokaryotes and eukaryotes, and post translational changes

1. TRANSLATION
The geneticinformationstoredinDNA ispassedontoRNA (throughtranscription) andultimately
expressedinthe language of proteins.The biosynthesisof aproteinora polypeptideinalivingcell is
referredtoas translation.
Variability of cellsintranslation:Erythrocyteslackthe machineryof translationandtherefore cannot
synthesisproteins.The normal livercellsare veryrichinthe proteinbiosyntheticmachinery-livermaybe
regardedas the proteinfactoryinthe humanbody.
Codonanticodonrecognition:The codonof the mRNA isrecognizedbythe anticodonof tRNA.Theypair
inantiparallel direction(5’→3’of mRNA with3’→5’of tRNA).The complementarybase pairingoccurs
betweenthe firsttwobasesof codonandthe last two
basesof anticodon.The anticodonregionof the tRNA
consistsof sevennucleotidesanditrecognizesthe
three
letter
codonin
mRNA:
Wobble Hypothesis:
Put forthbyCrick.Phenomenoninwhichasingle tRNA can
recognize more thanone codon.The thirdbase (3’-base) in
the codon oftenfailstorecognise the specific
complementarybase inthe anticodon(5’-base)Wobblingis
due to the difference inspatialarrangementof the 5’-endof the anticodon

2. Mutationsand geneticcode:
Sickle cell anaemiaisdue toa single base alteration(CTC→CACinDNA andGAG→GUG in RNA).The
resultisglutamate atthe 6th
positionof the β chainof haemoglobinisreplacedbyvaline.Thishappens
due to the alterationof the codonGUG of mRNA codesfor valine insteadof glutamate (codedbyGAGin
normal people)
Proteinbiosynthesis
Proteinsynthesisinvolvesthe translationof nucleotidebase sequence of mRNA intothe language of
aminoacidsequence.Itisdividedintofivestages
1) Requirementof the components
2) Activationof the aminoacids
3) Proteinsynthesisproper
4) Chaperonsandproteinfolding
5) Post-translational modifications
1)REQUIREMENT OF THE COMPONENTS
1) Aminoacids:20 aminoacidpresentinmanproteinstructure 10of themissynthesizedbyman.10
essential aminoacidhave tobe providedthroughdiet.Deficiencyinthe dietarysupplyof anyone
of the essential aminoacidsstopsthe translation.Inprokaryotesthere isnorequirementof
aminoacidssince all the 20 are synthesizedfromthe inorganiccomponents.
2) Ribosomes:Functionallyactive ribosomesare the centresorfactoriesforproteinsynthesis.
Ribosomesare the huge complex structures(70Sforprokaryotesand80 S for eukaryotes)of
proteinsandribosomal RNAs.Eachribosome consistsof twosubunits-one bigandone small.The
functional ribosomesconsistsof twosites-A site andPsite.Eachsite coversboththe subunits.A
site isfor binding of aminoacyl tRNA.Psite isforbindingpeptidyltRNA.A siteisthe acceptorsite
and P site isthe donorsite.Ineukaryotes,there is
anothersite calledexitsite.
The ribosomesare locatedinthe cytosomal fraction
of the cell.Theyare foundinassociationwithrough
endoplasmicreticulam(RER) toformclustersRER
ribosomes,whereproteinsynthesisoccurs.Poly
ribosome (Polysome) isusedwhenseveral
ribosomessimultaneouslytranslateonasingle
mRNA

3. 3)MessengerRNA:The informationrequiredforthe synthesisof agivenproteinispresentonthe
mRNA.The DNA haspassedthe geneticinformationinthe formof codons to mRNA to translate
intoproteinsequence.
4)TranferRNA (tRNA):Theycarrythe aminoacidsandhandthemoverto the growingpeptide
chain.The aminoacidiscovalentlyboundtotRNA atthe 3’ end.EachtRNA has a three nucleotide
base sequence-the anticodon whichisresponsible torecognizethe codon(complementary
bases) of mRNA forproteinsynthesis.
5)Energysources:ATPandGTP are requiredforthe energyinproteinsynthesis.Reactionsinvolve
the breakdownof ATPor GTP respectivelytoAMPand GMP withthe liberationof
pyrophosphate
6)Proteinfactors:Proteinsare neededforthe initiation,elongationandterminationof protein
synthesis.
2)ACTIVATIONOF AMINOACIDS
A groupof enzymesare namelyaminoacyl tRNA
synthetasesare requiredforthisprocess.The aminoacid
isfirstattachedto the enzyme utilizingATPtoform
enzyme-AMP-aminoacidcomplex.The aminoacidisthen
transferredtothe 3’ end of the tRNA to form aminoacyl
tRNA.
3)PROTEIN SYNTHESIS PROPER
The proteinsynthesisoccursonthe ribosomes (rather
polyribosomes).ThemRNA isreadinthe 5’→3’ direction
and the polypeptide synthesisproceedsfromN terminal
endto C-terminal end.TranslationisdirectionalandcollinearwithmRNA.
The prokaryoticmRNAsare polycistronic-asingle mRNA hasmanycodingregionsthatcode fordifferent
polypeptide.EukaryoticmRNA ismonocistronic-itcodesforasingle polypeptide.InProkaryotes,
translationcommensesbeforethe transcriptionof the gene iscompleted.Ineukarypticorganisms-
transcription occursin the nucleusandtranslationtakesplace inthe cytosol.Ineukaryotesprimary
transcript(hnRNA) formedfromDNA hasto undergoseveral modifications.
INITIATION OFTRANSLATION:Translationineukaryotesiscomplex andinvolvesteneukaryoticinitiation
factors (eIFs).eIFscontainmultiple(3-8) subunits.The processof translationinitiationcanbe dividedinto
foursteps
a) Ribosomal dissociation.
b) Formationof 43S preinitiationcomplex.

4. c) Formationof 48 S initiationcomplex.
d) Formationof 80 S initiationcomplex.
a) Ribosomal dissociation:The 80Sribosome dissociatestoform40 S and 60S subunits.Initiation
factor eIF-3and eIF-IA bindtothe newlyformed40Ssubunit.Itblockthe reassociationwith60S
subunit.eIF-3isknownasanti-associationfactor.
b) Formationof 43S preinitiationcomplex:A ternarycomplex containingmet-tRNAandeIF-2bound
to GTP attachesto 40S ribosomal unittoform 43S preinitiationcomplex.elF-3andeIF-1A
stabilizesthiscomplex.
c) Formationof 48S initiationcomplex:Thebindingof mRNA to43S preinitiationcomplexresultsin
the formationof 48S initiationcomplex.Thisinvolvesthe certaininteractionsbetweensome of
the eIFsand activationof mRNA.eIF-4Fcomplexisformedbythe associationof eIF-4G,eIF-4A
witheIF-4E.eIF-4F(referredtoascap bindingprotein) bindstothe capof mRNA.TheneIF-4A and
eIF-4Bbindto mRNA.ThismRNA isthentransferredto43Scomplex.Associationof 43S
preinitiationcomplex withmRNA requiresenergy-ATP→48initiationcomplex will be formed
 Recognitionof initiationcodon:The ribosomal initiationcomplexscansthe mRNA forthe
identificationof appropriate initiationcodon.5’AUGisthe initiationcodonandits
recognitionisfacilitatedbyaspecificsequence of nucleotidessorroundingit.Thismarker
sequence forthe identificationof AUGiscalledas Kozakconsensussequenses.In

5. prokaryotesthe recognitionsequence of initiationcodonisreferredtoasShine Dalgarno
sequence.
d) Formationof 80S initiationcomplex:48Sinitiationcomplex bindsto60S ribosomal subunitto
form80 S initiationcomplex.The bindinginvolvesthe hydrolysisof GTP(boundtoeIF-2).Thisstep
isfacilitatedbythe involvementof eIF-5.Asthe 80S complex isformed,the initiationfactors
boundto 48S initiationcomplexare releasedandrecycled.The activationof eIF-2requireseIF-2B
(alsocalledguanosinenucleotide exchange factor) andGTP.The activatedeIF-2(ie boundto
GTP) requireseIF-2Ctoformthe ternarycomplex.
 Regulationof initiation:eIF-4Fcontrolsthe initiationandthusthe translationprocess.eIF-4E
isprimarilyresponsibleforthe recognitionof mRNA cap.eIFalsocontrolsprotein
biosynthesis.
Initiationof translationinprokaryotes
Lesscomplicated.30ssubunitisboundto initiationfactor3(IF-3) Andit isthenattachedto ternary
complex of IF-2,formyl met-tRNA andGTP.InitiationfactorIF-1alsoparticipatesinthe formationof
preinitiationcomplex.Recognitionof codonAUG isdone throughShine-Dalgarnosequence.A 50S
ribosome unitisnowbound withthe 30S unitto produce 70S initiationcomplex inprokaryotes
ELONGATION OFTRANSLATION
Dividedintothree steps
i) Bindingof aminoacyl t-RNA toA site ii) Peptide bondFormation
iii) Translocation

6. Elongation and termination

7. i) Bindingof aminoacyl-tRNA toA site:80S
initiationcomplex containsmet-tRNA in
the P Site and A site isfree.Another
aminoacyl tRNA isplacedinthe A site.This
requirespropercodonrecognitiononthe
mRNA and involvementof elongation
factor 1 α (EF-1 α) and supplyof energyby
GTP.Afteraminoacyl tRNA isplacedinthe
A site,EF-1α and GDP are recycledto
bringanotheraminoacyl tRNA
ii) Peptide bondFormation:The enzyme
peptidyltransferase catalysesthe
formationof peptide bond.The activityof
thisenzyme lieson28S RNA of 60S
ribosomal subunit.Therefore rRNA (not
protein) referredtoasribozyme thatcatalysesthe peptide bondformation.Asthe aminoacidin
the aminoacyl-tRNA isactivated,noadditionalenergyisrequiredforpeptide bond
formation.Resultof peptide bondformationisthe attachmentof the growingpeptide chainto
the tRNA in the A site.
iii) Translocation:The ribosomesmovestothe nextcodonof the mRNA (towards3’ end) –
translocation.Thisinvolvesthe movementof growingpeptidechainfromA site to Psite.
TranslocationrequiresEF-2andGTP.GTP getshydrolysedandsuppliesenergytomove
mRNA.EF-2andGTP complex recyclesfortranslocation.The deacylated tRNAmovesintoEsite
fromwhere itleavesthe ribosome.
Incorporationofaminoacids:Aboutsix aminoacidspersecondare incorporatedduringthe course of
elongationof translationineukaryotes
TERMINATION OF TRANSLATION
The stop or terminationsignals(UAA,UAGand UCA) terminatesthe
growingpolypeptide.Theterminationcodons whichactas stop
signalsdonot have specifictRNAstobind.Asthe terminationcodon
occupiesthe ribosomal A site,the releasefactornamelyeRF
recognizesthe stopsignal. eRF-GTPcomplex,inassociationwiththe
enzyme peptidyl transferase,cleavesthe peptide bondbetweenthe
polypeptide andthe tRNA occupyingP site.Inthisreactionawater
molecule,insteadof anaminoacidisadded.Thishydrolysisreleases
the proteinandtRNA from the P site.The 80Sribosome dissociates
to form40S and 60S subunitsare recycledandmRNA isalsoreleased

8. Inhibitorsofprotein synthesis:Tetracycline,Puromycin ,Chloramphenical,Erythromycin,Diptheria
toxin
4)CHAPERONS
Theyare heatshock proteinswhichassistcertainproteinstoattainthe correctconfirmation. Some
proteinscanspontaneouslygeneratethe correctfunctionallyactive conformationegdenatured
pancreaticribonuclease .Theyreversiblybindtohydrophobicregionsof unfoldedproteinsandfolding
intermediates.Theycanstabilizeintermediates,prevent formationof incorrectintermediatesandalso
preventundesirableinteractionswithotherproteins
TYPES OF CHAPERONS:
• Hsp 70 system-theycanbindindividuallytothe substrate (protein) andhelpinthe correct
formationof proteinfolding
• Chaperonin system-large oligomericassemblywhichformsastructure intowhichthe folded
proteinare inserted.
• Hsp 60 and HSp 10
PROTEIN MISFOLDINGAND DISEASES:
• Cysticfibrosis-autosomal recessive diseases
• Prions(proteinousinfectiousagents)-cellularaccumulationof aggregatesof misfoldedprotein
or theirpartiallydegradedproductswhichmayevencause neurological diseases
6)POST TRANSLATIONAL MODIFICATIONS