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.
- 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.
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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).