2 Important Steps that are involved in the Initiation of Polypeptide Chain Synthesis

This involves two steps viz., preparation of the site of protein synthesis and initiation of the polypeptide chain.

(i) Preparation of the site of synthesis:

Ribosomes in cytoplasm are the actual sites of protein synthesis. These are usually separated into their subunits (50s and 30s), when they are not participating in protein synthesis.

The mRNA usually attaches to the 30s subunit through its 5′ end carrying the AUG codon. In the process of attachment certain substances called initiation factors (IF) help.

These factors are of three types in prokaryotes (IF1, IF2, IF3) and are of six types in eukaryotes (eIF2, eIF2al, eIF2a2, eIF2a3, eIFa4 and eIF3).

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The attachment of the mRNA to the 30s submits requires the help of IF3 and Mg⁺⁺. The IF3 probably unwinds the rRNA (ribosomal RNA), so that it can bind to mRNA. After the attachment of mRNA, an mRNA – 30s complex is formed.

The starting amino acid in a polypeptide is methionine (met) in the case of eukaryotes and N-formly methionine (f-met) in the case of prokaryotes.

The amino acid (met or f-met) tRNA complex attaches to the initiation codon (AUG) on the mRNA 30-s complex through its anticodon (UAC). This forms the 30s initiation complex and requires the participation of IF2, IF3 and GTP (energy donor).

During the formation of the initiation complex the 50s subunit combines with the 30s subunit to form the 70s unit. This union is facilitated by the participation of IF1. With the formation of the 70s ribosome) the initiation of the polypeptide chain gegins.

(ii) Initiation of the polypeptide chain:

Each ribosome has two cavities – P site (polypeptide site) and Asite (aminoacyl sites). AA TRNA complex occupies the a site, while the met – tRNA (tRNA with the initiator aminoacid – methionine) will be at the P site.

This occupation requires a molecule of GTP and the participation of IF,. The energy of GTP is utilized for the formation of the peptide linkage.

Peptide linkage occurs when the carboxyl (COOH) group of peptidyl tRNA (met. tRNA) present at the P site reacts with the amino group (NH₂) of AA tRNA at the A site. The enzyme peptidyl transfers catalyses the formation of peptide bond. Later the AA+tRNA moves from A site to the P site in the presence of GTP and 1F.