Question
Explain the steps involved in a polypeptide synthesis. How are the amino acids activated during polypeptide synthesis?
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Answer
- Initiation:
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In prokaryotes, initiation requires the large and small ribosome subunits, the mRNA, initiation tRNA and three initiation factors (IFs).
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Activation of amino acid: Amino acids become activated by binding with aminoacyl tRNA synthetase enzyme in the presence of ATP.
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Aminoacid(AA) + ATP → synthetases Aminoacy lt RNA AA - AMP - Enzymecomplex + Pi
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Transfer of amino acid to tRNA: The AA - AMP - Enzyme complex formed reacts with specific tRNA to form aminoacyl-tRNA complex. AA - AMP - Enzyme complex + tRNA → AA - tRNA + AMP + Enzyme.
- The cap region of mRNA binds to the smaller subunit of ribosome.
- The ribosome has two sites, A-site and P-site.
- The smaller subunit first binds to the initiator mRNA and then binds to the larger subunit so that initiation codon (AUG) lies on the P-site.
- The initiation tRNA, i.e., methionyl tRNA then binds to the P-site.
- Elongation of polypeptide chain:
- Another charged aminoacyl tRNA complex binds to the A-site of the ribosome at the second codon.
- A peptide bond is formed between carboxyl group (-COOH) of amino acid at P-site and amino group (-NH) of amino acid at A-site by the enzyme peptidyl transferase.
- The ribosome slides over mRNA from codon to codon in the 5′ → 3′ direction.
- According to the sequence of codons, amino acids are attached to one another by peptide bonds and a polypeptide chain is formed.
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