Protein synthesis
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Protein synthesis. Types and site of protein synthesis: 1. body proteins: antibodies, plasma proteins, contractile proteins in the muscles , all enzymes and some hormones. Mammary gland = milk proteins, muscles makes contractile proteins etc. Requirements for protein synthesis. Amino acids:

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Protein synthesis

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Protein synthesis

Protein synthesis

  • Types and site of protein synthesis:

  • 1. body proteins: antibodies, plasma proteins, contractile proteins in the muscles , all enzymes and some hormones.

  • Mammary gland = milk proteins, muscles makes contractile proteins etc.


Requirements for protein synthesis

Requirements for protein synthesis

  • Amino acids:

  • tRNA

  • mRNA

  • Aminoacyl –t RNA synthetase enzyme

  • Ribosome

  • Protein factors

  • High energy compounds :ATP and GTP


Steps

Steps

  • Initiation

  • 1. it requires mRNA molecule is selected for translation by ribosomes. Once the mRNA binds to the ribosome ,the later reads the genetic code on the mRNA and translation begins.


Requirements for initiation process

Requirements for initiation process

  • tRNA

  • Ribosomes

  • mRNA

  • Amino acids

  • GTP and ATP

  • At least 10 eukaryotes initiation factors (eIFs)


Initiation can be divided into 4

Initiation can be divided into 4

  • Ribosomal dissociation :dissociation of the complete ribosome 80S into its 40S and 60S.

  • Step is carried out by two initiation factor (eIF-3 and eIF1A) which binds to 40 S subunit ,dissociating it from 80S and prevent its reassociation.


Formation of 43s preinitiation complex

Formation of 43S preinitiation complex

  • Complex is formed by a ternary complex consisting of met-tRNAs,GTP and eIF-2 to the 40S ribosome:

  • 1. first: binding of GTP with eIF-2.This binary complex then binds to met –tRNA ;to form a ternary complex.

  • 2. ternary complex then binds to the 40S ribosomal subunits to form 43S preinitiation complex.


Formation of 43s initiation complex

Formation of 43S initiation complex

  • Complex is formed by binding of mRNA to the 43S preinitiation complex.

  • 1. initiation factor four subunit (eIF4)interact with mRNA cap.

  • 2.mRNA cap-together with eIF4-facilitate the binding of mRNA to the 43S preinitiation complex.

  • 3.ATP hydrolysis occurs during binding of mRNA.

  • 4. recognition of initiation codon: AUG at untranslated region of mRNA.The scanning mechanism starts by binding of 43S preinitiation complex with mRNA neaqr its 5’end.Then it moves 3’end until it meets first AUG codon.ATP hydrolysis is essential for scanning process.


Formation of 80s initiation complex

Formation of 80S initiation complex

  • Complex is formed by binding of the 48S initiation complex with 60S ribosomal subunit.

  • 1. binding involves the hydrolysis of the GTP bound to eIF2 by eIF-5,with release of initiation factors IA and 3.These factors are then recycles.

  • 2. complete ribosome 80S contains two sites for tRNAmolecules.These are P site (peptide) site which is occupied now by met-tRNA and A (amino) site which is free and ready to receive the subsequent aminoacyltRNA.


Elongation

Elongation

  • 1. elongation of the polypeptide chain involves the addition of amino acids to the carboxyl end of the growing polypeptide chain.

  • 2. during elongation the ribosome moves from the 5’ end to the 3’ end of the mRNA.


Requirements for elongation

Requirements for elongation

  • A. 80S initiation complex

  • B. aminoacyl –tRNA’s

  • C.GTP

  • D. Eukaryotic elongation factors,eIF-1 alpha and eIF-2

  • Met –tRNA is present in P site(from initiation)


Elongation can be divided into 3 stages

Elongation can be divided into 3 stages.

  • Binding of amino acyltRNA to the A site

  • A. entry of a new aminoacyl –tRNA to the empty A site on the ribosome requires proper codonrecognition.This depends on the complementary bases of both codon and anticodon.

  • At first eEF-a forms a complex with GTP and the entering aminoacyl-tRNA.


Protein synthesis

Cont.

  • This complex allows the aminoacyl –tRNA to enter the A site with the release of eEF-1GTP and GDP and phosphate .These factors are often are then recycled.

  • Peptide bond formation:

  • The carboxyl group of the amnioacyl-tRNA in P site transferred to and bind with the amino group of the new aminoacyl–t RNA in the A site .This reaction is catalyzed by peptidyltransferase present in 60S subunits.


Translocation

Translocation

  • 1. after the peptide bond has been formed ,the ribosome moves three nucleotides towards the 3’ end mRNA.

  • 2. This process is known as translocation and it requires translocation enzyme ,eEF-2 and GTP.


Protein synthesis

Cont.

  • 3. movement of ribosome ,the following events occurs:

  • 1. release of uncharged tRNA from P site of ribosome.

  • 2. tansfer of the newly formed peptidlytRNA from A site to occupy P site.

  • A site becomes free,thus it can be occupied by another new aminoacyltRNA according to the codon-anticodonrecognition.Then process will repeated.


Protein synthesis

Cont.

  • For each new peptide bond formed ,four high energy phosphate bonds are cleaved.

  • The charging of the tRNA mol. With aminoacyl group requires the hydrolysis of an ATP to an AMP +ppi .Thus equivalent to the hydrolysis of 2ATP’s to 2 ADP +ppi .

  • The entry of aminoacyltRNA into the A site leads to the hydrolysis of an GTP to GDP + pi


Protein synthesis

Cont.

  • The translocation of the newly formed peptidyltRNA in the site A site into P site by eEF-2 ,similarly to the hydrolysis of one GTP to GDP +ppi.


Termination

termination

  • 1.It is the final stop which occurs after multiple cycles of elongation and formation of protein mol.

  • 2. it occurs when ribosome moves to bring one of the three termination (nonsense) codons into A site. These are UAA,UAG and UGA.

  • Releasing factors (eRF) 1,2 and 3 which are present in A site can recognize all the three termination codons.


Protein synthesis

Cont.

  • Releasing factors together with GTP and peptidyltransferase promote the hydrolysis of the bond between peptide and tRNA occupying the P site.This hydrolysis leads to :

  • 1. release of both peptide and tRNA.

  • 2. dissociation of 80S ribosome into its 40S and 60S units which are then recycled.


Polyribosomes polysomes

Polyribosomes(polysomes)

  • Because the length of the nucleotide sequence of most mRNA s, more than one ribosome can translate the same mRNA at the same time .Such a complex of one mRNA and a number of ribosomes is called a polyribosomes of polysomes.


Post translational modification

Post translational modification

  • Conversion of inactive protein to active proteins:e.gzymogens,proinsulin.

  • Glycosylation: addition of carbohydrates, glycoproteins

  • Hydroxylation: addition of OH group, hydroxyproline and hydroxy lysine.

  • Phosphorylation: additon of phosphate group,glycogensynthase .


Inhibitors

Inhibitors

  • Streptomycin. Binds to ribosomes;dissociation of mRNA from the ribosomes.

  • Tetracyclins: prevent aminoacyl t RNA asnticodon from recognizing their corresponding codons.

  • Chloramphenicol: inhibits peptidyltransferase.


Protein synthesis

Cont.

  • Puromycin: inhibits the elongation

  • Erythromycin: inhibits the translocation process.


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