When the ribosome is assembled, two tRNA binding sites are created; these are designated 'P' and 'A' (P stands for peptidyl, A stands for aminoacyl). (Some books also discuss an 'E' site, from which used tRNA molecules are ejected, but for the sake of simplicity, we won't consider it here.) The initiator tRNA is in the P site, and the A site will be filled by the tRNA with the anticodon that is complementary to the codon next to the start. (In this case, it is the tRNA that binds proline.) |
When the second tRNA base pairs with the appropriate codon in the mRNA, an enzyme called peptidyl transferase (one of the 31 proteins in the large ribosomal subunit) catalyzes the formation of a peptide bond between the two amino acids present (while breaking the bond between fmet and its tRNA). |
At this point, the whole ribosome shifts over one codon. This shift requires several elongation factors (not shown) and energy from the hydrolysis of GTP. The result of the shift is that the uncharged tRNA that was in the P site is ejected, and the tRNA that was in the A site is now in the P site. The A site is free to accept the tRNA molecule with the appropriate anticodon for the next codon in the mRNA. |
The next tRNA base pairs with the next codon, and peptidyl transferase catalyzes the formation of a peptide bond between the new amino acid and the growing peptide chain. |
Once again, the ribosome shifts over, so that the uncharged tRNA is expelled, and the tRNA with the peptide chain occupies the P site. (This is why this site is called the 'peptidyl' site - after the shift, it contains the tRNA with the growing peptide chain. The other site will accept a tRNA with an amino acid, hence the name 'aminoacyl' site.) The process of shifting and peptide bond formation continues over and over until a termination codon is encountered. The elongation process is fairly rapid, with prokaryotic ribosomes able to add 15 amino acids to the growing polypeptide every second. The process is also relatively error-free. Only one mistake is made every 10,000 amino acids. For large proteins of 1000 amino acids, that would mean one wrong amino acid in every 10 polypeptides. |