Question

How is the factor-binding center important for translation elongation and termination?

Answer 1

Ribosome elongates the polypeptide chain through the addition of the amino acids. The amino acid sequence is determined by the order of the codons in the specific mRNA. Elongation is a cyclic process which involves the elongation factors. It is divided into three steps:-

1. Binding of aminoacyl- tRNA to A site:-
The 80S initiation complex contains met- tRNAi in the P site and the A site is free. Another aminoacyl- tRNA is placed in the A site. This requires proper codon recognition on the mRNA and the involvement of elongation factor 1a (EF-1a), and supply of energy by GTP. The elongation factor and GTP are recycled to bring another aminoacyl-tRNA.

2. Peptide bond formation:-

The enzyme peptidyltransferase catalyses the formation of peptide bonds which lies on 28S RNA of 60S ribosomal subunit and is also called as ribozyme. No additional energy is needed for peptide bond formation as aminoacyl-tRNA is already active. The net result of peptide bond formation is the attachment of the growing peptide chain to the tRNA in the A site.

3. Translocation:-

Translocation involves the movement of the ribosomes to the next codon of mRNA. It involves the growing peptide chain from A site to P site. Translocation involves elongation factor-2 (EF-2) and GTP. GTP gets hydrolysed and supplies energy to move mRNA. Elongation factor-2 and GTP both are recycled.

This was the steps explained for the eukaryotic cell. In prokaryotic cells, the elongation factor are EF-Tu and EF-Ts.

Termination of Translation:-

Termination of translation are done by stop codons which are UAA, UAG and UCA. They terminates the growing polypeptide. The termination codons does not have any specific tRNA's to bind and as they are located on ribosomal A site, the release factor eRF recognises the stop signal. eRF- GTP complex, with enzyme peptidyltransferase, cleaves the peptide bond and a water molecule is added. The hydrolysis releases the protein and tRNA from the P site.