Food Chemistry and Fish in Nutrition ( 2+1)

8.3.1.2 Elongation

Elongation, a cyclic process, involves several steps catalyzed by proteins called elongation factors (eEF). These steps are (1) binding of aminoacyl - tRNA to the A site, (2) peptide bond formation, and (3) translocation.

In the complete 80S ribosome formed during the process of initiation, the A site is free. The binding of the proper aminoacyl-tRNA in the A site requires proper codon recognition Elongation factor eEF-1a forms a complex with GTP and the entering aminoacyl-tRNA. This complex then allows the aminoacyl-tRNA to enter the A site with the release of eEF-1a-GDP and phosphate.

Peptide Bond Formation: The a-amino group of the new aminoacyl-tRNA in the A site caries out a nucleophileic attack on the esterfied carboxyl group of the peptidyl-tRNA occupying the P site. This reaction is catalyszed by a protein component, peptidyltransferase, of the 60S ribosomal sub-unit. This enzymatic activity may be performed by some RNA component o the ribosome-perhaps the 23S rRNA.

Translocation: Upon removal of the peptidyl moiety from the tRNA in the P site, the discharged tRNA quickly dissociates from the P site. Elongation factor 2(3EF-2) and GTP are responsible for the translocation of the newly formed peptidyl tRNA at the A site into the empty P site, The GTP required for eEF-2 is hydrolyzed to GDP and phosphate during the translocation process. The translocation of the newly formed peptidyl-tRNA and its corresponding codon into the P site then frees the A site for another cycle of aminoacyl-tRNA codon recognition and elongation.