Q.61 The peptidyl transferase enzyme activity is located in the
1. Variable loop of tRNA
2. Large ribosomal subunit
3. Acceptor arm of tRNA
4. Small ribosomal subunit
Peptidyl transferase activity resides in the large ribosomal subunit, functioning as a ribozyme within the 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes). This catalyzes peptide bond formation during translation.
Translation Elongation Basics
Peptidyl transferase forms peptide bonds between P-site peptidyl-tRNA and A-site aminoacyl-tRNA in the ribosome’s PTC (peptidyl transferase center).
It’s a ribozyme—no protein enzyme—RNA catalysis confirmed by crystal structures.
Small subunit decodes mRNA; large handles bond formation and tunnel for nascent chain.
Correct Answer: 2. Large ribosomal subunit
Activity localizes to domain V of large subunit rRNA, positioning A/P-site tRNA CCA ends for nucleophilic attack.
Prokaryotic 50S/ eukaryotic 60S equivalents; antibiotics like puromycin target this site.
Option Breakdown
| Option | Role in Translation | Correct? |
|---|---|---|
| 1. Variable loop of tRNA | Recognition/modification site; no catalytic activity. | No |
| 2. Large ribosomal subunit | PTC ribozyme site; peptide bond catalysis. | Yes |
| 3. Acceptor arm of tRNA | Amino acid attachment site (CCA-3’OH); substrate only. | No |
| 4. Small ribosomal subunit | mRNA/tRNA decoding via anticodon-codon pairing. | No |
Ribozyme Mechanism
rRNA A2451 (prokaryote numbering) positions substrates; 2′-OH of P-site A76 attacks A-site carbonyl.
No proteins required; proves RNA world hypothesis for ribosome origins.
