Figure 5.

Schematic diagram illustrating the expanded increased affinity model. (A) When the growing polypeptide chain carries alanine, serine, or threonine (blue circle) in the penultimate position, the affinity of CHL (yellow hexagon) for the ribosome increases due to direct interactions with the side chains of these amino acids preventing accommodation of the aminoacyl moiety (red circle) of the aa-tRNA into the A site and, thereby, making peptidyl transferase reaction impossible, which results in ribosome stalling. (B) Due to the absence of the side chain, glycine residue (orange circle) can be accommodated into the PTC even in the presence of bound CHL and normally react with the P-site substrate (dashed arrow), sustaining transpeptidation reaction. Small and large ribosomal subunits are highlighted in shades of grey. A and P sites are highlighted in magenta and green, respectively. Amino acids of the growing polypeptide chain are shown by grey circles with the penultimate amino acid residue highlighted in blue.