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:
- 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.
- 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.
- IF-2 along with GTP helps in the binding of fMet-tRNAfmet to the initiation codon on mRNA.
- IF-1 helps in the binding of the 50S subunit of the ribosome to the 30S subunit.
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.
- Elongation always occurs in 5′→3′ direction.
- It occurs with the help of Elongation factors; EF-T, EF-G.
Elongation occurs in 3 steps:
- By attaching the next aminoacyl-tRNA to A-site. It occurs with the help of EF-T and GTP.
- -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.
- 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.
Termination codon comes to lie on the A-site.
- 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.