271211 - Protein Synthesis (Translasi) - Bu Lely

7/28/2019 271211 - Protein Synthesis (Translasi) - Bu Lely

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PROTEIN SYNTHESIS(TRANSLATION)

C-4 Tutorial Group


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The Characteristics of Codon

Degenerate multiple codons mustdecode the same amino acidUnambiguous given a specificcodon, only a single amino acid isindicatedNon-overlapping the process of protein synthesis does not involve anyoverlap of codons


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The Characteristics of Codon

Not punctuated the message is read ina continuing sequence of nucleotide tripletsuntil a translation stop codon is reached

Universal The frequency of use of eachamino acid codon varies considerablybetween species and among differenttissues within a species


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Components of Translation

Amino acid all amino acid must bepresent at the time of proteinsynthesistRNA attach to specific amino acid,contains anticodonAminoacyl-tRNA synthase attachthe amino acids to their corresponding tRNAs


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mRNA as a template for thesynthesis of the polypeptide chainRibosom

rRNA structure of base-pairingRibosomal protein interact with other component of translation system

A-binding site aminoacyl tRNAP-binding site peptidyl tRNAE-site exit of the ribosome


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Protein factors as catalytic factors/ enzymes

ATP & GTP source of energy


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Steps in Protein Synthesis

Initiation

Elongation

Termination

Postranslation Modification


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Initiation

The binding of GTP by eIF-2 binarycomplex

Binary complex then binds to met-tRNA ternary complexTernary complex binds to the 40Sribosomal subunit the 43S preinitiation

complexThe 43S preinitiation complex stabilized byassociation with eIF-3 and eIF-1A.


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Initiation

The association of mRNA with the 43Spreinitiation complex + ATP hydrolysis 48S initiation complex60S ribosomal subunit + 48S initiationcomplex + hydrolysis of the GTP bound toeIF-2 by eIF-5 = release of the initiationfactors bound to the 48S initiation complex

40S and 60S subunits to form the 80Sribosome

At this point, the met-tRNA i is on the P siteof the ribosome, ready for the elongationcycle to commence.


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Elongation

Binding of aminoacyl-tRNA to the Asiteaminoacyl-tRNA carries out anucleophilic attack on the esterifiedcarboxyl group of the peptidyl-tRNAoccupying the P site bypeptidyltransferase


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Elongation

elongation factor 2 (EF2) binds to anddisplaces the peptidyl tRNA from the A siteto the P site. In turn, the deacylated tRNA

is on the E site,from which it leaves theribosomethe energy requirements for the formationof one peptide bond = hydrolysis of two

ATP molecules to ADP and of two GTPmolecules to GDP = hydrolysis of four high-energy phosphate bonds.


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Termination

The elongation will stop terminatingcodon of mRNA if stop codons (UAA,UAG, UGA) appears in the A siteThe releasing complex (Releasingfactor RF1 + releasing factor RF3 +GTP) hydrolysis peptide bonds andrelease protein and dissociate the80S into 40S and 60S


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The Regulation of eIF-4EControls the Rate of InitiationThe 4F complex is particularly important in

controlling the rate of protein translation.4F is a complex consisting of 4E, which binds tothe m7G cap structure at the 5 end of the mRNA,

and 4G, which serves as a scaffolding protein.In addition to binding 4E, 4G binds to eIF-3, whichlinks the complex to the 40S ribosomal subunit.It also binds 4A and 4B, the ATPase-helicasecomplex that helps unwind the RNA.4E is responsible for recognition of the mRNA capstructure, which is a rate-limiting step intranslation.


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REGULATION ONTRANSLATIONAL STAGE

In eukaryotes, eukaryotic initiationfactor 2 (eIF2) is the center of regulation mechanism as eIF2 isneeded in initiation stage of proteinsynthesis.eIF2 can be inhibited byphosphorylation.


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REGULATION ON POST-TRANSLATIONAL STAGE

Post-translational modification isdefined as protein structure alterationafter being released by ribosome.

Modification takes place inendoplasmic reticulum and Golgiapparatus.

This modification can be classifiedinto two groups:Chemical modification.Protein folding.


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Posttranslational Modification

The result of translation proproteins require modifications before attainingbiologic activity

Kind of posttranslationalremoval amino acid residues byaminopeptidasesacetylationphosphorylationmethylationubiquitinylationglycosylation

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271211 - Protein Synthesis (Translasi) - Bu Lely