Principles of Biochemistry

7.5.1.3 Elongation

Elongation is a cyclic process consisting of three steps catalysed by elongation factors (eEF)

1. Binding of aminoacyl tRNA to the A site

Next to the P site in an mRNA-ribosome complex (80s ribosome) is a second binding site called the A site(aminoacyl site). At this second site the next mRNA codon is exposed, and a tRNA with the appropriate anticodon binds to it. eEF-1alpha and GTP are needed.

2. Peptide formation

With amino acids in place at both the P and the A sites, the enzyme peptidyl transferase of the 60 ribosomal subunit effects the linking of the P site amino acid to the A site amino acid to form a dipeptide. This peptide bond formation leaves the tRNA at the P site empty and the tRNA at the A site bearing the dipeptide.

The empty tRNA at the P site now leaves that site and is free to pick up another molecule of its specific aminoacid. Simultaneously with the release of tRNA from the P site, the ribosome units shift along the mRNA.

3. Translocation

The shift puts the newly formed dipeptide at the P site, and the third codon of mRNA is now available, at site A, to accept a tRNA molecule whose anticodon complement this codon. The movement of a ribosome along a mRNA molecule is called translocation. Translocation is the part of translation in which a ribosome moves down a mRNA molecule three base positions (one codon) so that a new codon can occupy the ribosomal A site. The elongation factor 2 eEF-2)and GTP are responsible for this.

Now this process is repeated. The third codon, now at the A site, accepts an incoming tRNA with an accompanying amino acid; and then the entire dipeptide at the P site is transferred and bonded to the A site amino acid to give a tripeptide. The empty tRNA at the P site is released and then the ribosome shifts along the mRNA, and the process continues.