The Polypeptide Chain Is Transferred to Aminoacyl-tRNA

KEY CONCEPTS
- The 50S subunit has peptidyl transferase activity, as provided by an rRNA ribozyme.
- The nascent polypeptide chain is transferred from peptidyl-tRNA in the P site to aminoacyl-tRNA in the A site.
- Peptide bond synthesis generates deacylated tRNA in the P site and peptidyl-tRNA in the A site.
The ribosome remains in place while the polypeptide chain iselongated by transferring the polypeptide attached to the tRNA in the P site to the aminoacyl-tRNA in the A site. The reaction is shown in Figure 1 . The component responsible for synthesis of the peptide bond is called peptidyl transferase. It is a function of the large (50S or 60S) ribosomal subunit. The reaction is triggered when EF-Tu releases the aminoacyl end of its tRNA, which then swings into a location close to the end of the peptidyltRNA.
This site has a peptidyl transferase activity that essentially ensures a rapid transfer of the peptide chain to the aminoacyltRNA. Both rRNA and 50S subunit proteins are necessary for this activity, but the actual act of catalysis is a property of the ribosomal RNA of the 50S subunit .

FIGURE 1. Peptide bond formation takes place by a reaction between the polypeptide of peptidyl-tRNA in the P site and the amino acid of aminoacyl-tRNA in the A site.
The nature of the transfer reaction is revealed by the ability of the antibiotic puromycin to inhibit translation. Puromycin resembles an amino acid attached to the terminal adenosine of tRNA. Figure 2 shows that puromycin has a nitrogen instead of the oxygen that joins an amino acid to a tRNA. The antibiotic is treated by the ribosome as though it were an incoming aminoacyl-tRNA, after which the polypeptide attached to peptidyl-tRNA is transferred to the –NH group 2 of the puromycin.

FIGURE 2.Puromycin mimics aminoacyl-tRNA because it resembles an aromatic amino acid linked to a sugar-base moiety.
The puromycin moiety is not anchored to the A site of the ribosome; as a result, the polypeptidyl-puromycin adduct is released from the ribosome in the form of polypeptidyl-puromycin. This premature termination of translation is responsible for the lethal action of the antibiotic.