Protein Synthesis (Translation)

Protein Synthesis

Protein synthesis (Translation) occurs in the cytoplasm with the help of ribosomes.
  • Proteins are polymers of amino acids.
  • Peptide bonds join adjacent amino acids.
  • The amino group of one amino acid and the carboxyl group of adjacent amino acid take part in peptide bond formation.
  • mRNA carries the information for arrangement of amino acids in a polypeptide chain.

Activation of amino acids/ charging of tRNA:

Amino-acyl tRNA synthetase and ATP, activate amino acids.It reacts with its specific tRNA to form amino-acyl tRNA.

  • An ester bond is formed between the –COOH group of amino acid and –OH group of terminal adenine at 3′end of tRNA.
     AA + ATP → AA–AMP (amino-acyl adenylate).
     AA–AMP + tRNA → AA– tRNA (amino-acyl tRNA).

Translation occurs in 3 steps:

1)Initiation:

  • Initiating amino acid is Methionine. It is formylated methionine (fMet) in prokaryotes
  • Initiating codon is AUG.
  • Initiating methionine has an initiator tRNA molecule.
  • Initiation factors; IF-1, IF-2, IF-3 help in initiation.
  • Initiating aminoacyl t RNA → fMet-tRNAfmet.

  1. With the help of IF-3, mRNA binds to the 30S subunit of the ribosome. 16S rRNA of the 30S subunit of ribosome binds to Shine-Dalgarno sequence in the UTRs (untranslated regions) at the 5′ end of mRNA.initiation
  2. IF-2 along with GTP helps in the binding of fMet-tRNAfmet to the initiation codon on mRNA.initiation
  3. IF-1 helps in the binding of the 50S subunit of the ribosome to the 30S subunit.initiation3
      50S subunit of the ribosome has 2 sites:
  • A-site for incoming aminoacyl-tRNA.
  • P-site for growing peptide chain.
  • An E-site for empty tRNA as it exits.

The initiator, amino-acyl tRNA binds to the P-site and the A-site is free for the next aminoacyl-tRNA.

complete initiation

2)Elongation:

  • Elongation always occurs in 5′→3′ direction.
  • It occurs with the help of Elongation factorsEF-T, EF-G.
Elongation occurs in 3 steps:
  1. By attaching the next aminoacyl-tRNA to A-site. It occurs with the help of EF-T and GTP.elongation1
  2.  -NH2 group of amino acid present at the A-site and -COOH group of amino acid present at P-site form the peptide bond.The amino acid at P-site is displaced from its tRNA.Peptidyl Transferase (a Ribozyme)  forms the peptide bond.elongation2
  1. Translocation: the shifting of the ribosome to the next codon on mRNA takes place with the help of EF-G and GTP.

The 3 steps of elongation are repeated to form the complete polypeptide chain.

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elongation

3)Termination:

Termination codon comes to lie on the A-site.

termination 1

  • Release Factors; RF-1, RF-2 recognize the termination codons.

                ◊RF-1 recognizes UAA and UAG.

                ◊RF-2 recognizes UAA and UGA.

  • The nascent polypeptide chain is released by RF, by breaking the ester bond between the last amino acid and its tRNA. It uses GTP and peptidyl transferase.dissociation

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