The polypeptide chain is transferred to aminoacyl-tRNA

KEY TERMS:

• Peptidyl transferase is the activity of the ribosomal 50S subunit that synthesizes a peptide bond when an amino acid is added to a growing polypeptide chain. The actual catalytic activity is a property of the rRNA.
• Puromycin is an antibiotic that terminates protein synthesis by mimicking a tRNA and becoming linked to the nascent protein chain.

KEY CONCEPTS:

• The 50S subunit has peptidyl transferase activity.
• 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 is elongated 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 6.26. The activity responsible for synthesis of the peptide bond is called peptidyl transferase.

The nature of the transfer reaction is revealed by the ability of the antibiotic puromycin to inhibit protein synthesis. Puromycin resembles an amino acid attached to the terminal adenosine of tRNA. Figure 6.27 shows that puromycin has an N instead of the O that joins an amino acid to tRNA. The antibiotic is treated by the ribosome as though it were an incoming aminoacyl-tRNA. Then the polypeptide attached to peptidyl-tRNA is transferred to the NH₂ group of the puromycin.

Because the puromycin moiety is not anchored to the A site of the ribosome, the polypeptidyl-puromycin adduct is released from the ribosome in the form of polypeptidyl-puromycin. This premature termination of protein synthesis is responsible for the lethal action of the antibiotic.

Peptidyl transferase 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. The aminoacyl end then swings into a location close to the end of the peptidyl-tRNA. This site has a peptidyl transferase activity that essentially ensures a rapid transfer of the peptide chain to the aminoacyl-tRNA . 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 (see 6.19 23S rRNA has peptidyl transferase activity).