Biology
KEY CONCEPTS:
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.
- Bypassing Involves Ribosome Movement
Certain sequences trigger a bypass event, when a ribosome stops translation, slides along mRNA with peptidyl-tRNA remaining in the P site, and then resumes translation (see Figure 7.30). This is a rather rare phenomenon, with only ~3 authenticated examples...
- 23s Rrna Has Peptidyl Transferase Activity
KEY CONCEPTS:Peptidyl transferase activity resides exclusively in the 23S rRNA. The sites involved in the functions of 23S rRNA are less well identified than those of 16S rRNA, but the same general pattern is observed: bases at certain positions...
- Translocation Moves The Ribosome
KEY TERMS:Translocation is the movement of the ribosome one codon along mRNA after the addition of each amino acid to the polypeptide chain. KEY CONCEPTS: Ribosomal translocation moves the mRNA through the ribosome by 3 bases. Translocation moves deacylated...
- Elongation Factor Tu Loads Aminoacyl-trna Into The A Site
KEY TERMS:Elongation factors (EF in prokaryotes, eEF in eukaryotes) are proteins that associate with ribosomes cyclically, during addition of each amino acid to the polypeptide chain. EF-Tu is the elongation factor that binds aminoacyl-tRNA and places...
- Protein Synthesis Occurs By Initiation, Elongation, And Termination
KEY TERMS:The A site of the ribosome is the site that an aminoacyl-tRNA enters to base pair with the codon. The P site of the ribosome is the site that is occupied by peptidyl-tRNA, the tRNA carrying the nascent polypeptide chain, still paired with the...
Biology
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.
- 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).
- Bypassing Involves Ribosome Movement
Certain sequences trigger a bypass event, when a ribosome stops translation, slides along mRNA with peptidyl-tRNA remaining in the P site, and then resumes translation (see Figure 7.30). This is a rather rare phenomenon, with only ~3 authenticated examples...
- 23s Rrna Has Peptidyl Transferase Activity
KEY CONCEPTS:Peptidyl transferase activity resides exclusively in the 23S rRNA. The sites involved in the functions of 23S rRNA are less well identified than those of 16S rRNA, but the same general pattern is observed: bases at certain positions...
- Translocation Moves The Ribosome
KEY TERMS:Translocation is the movement of the ribosome one codon along mRNA after the addition of each amino acid to the polypeptide chain. KEY CONCEPTS: Ribosomal translocation moves the mRNA through the ribosome by 3 bases. Translocation moves deacylated...
- Elongation Factor Tu Loads Aminoacyl-trna Into The A Site
KEY TERMS:Elongation factors (EF in prokaryotes, eEF in eukaryotes) are proteins that associate with ribosomes cyclically, during addition of each amino acid to the polypeptide chain. EF-Tu is the elongation factor that binds aminoacyl-tRNA and places...
- Protein Synthesis Occurs By Initiation, Elongation, And Termination
KEY TERMS:The A site of the ribosome is the site that an aminoacyl-tRNA enters to base pair with the codon. The P site of the ribosome is the site that is occupied by peptidyl-tRNA, the tRNA carrying the nascent polypeptide chain, still paired with the...