The hydrolytic step leading to the release of a polypeptide chain from a ribosome is catalyzed by which of the following?
Wait, what catalyzes that hydrolysis? I think it's the ribosome itself. The ribosome has peptidyl transferase activity. Oh right, the peptidyl transferase center is part of the ribosome, specifically the large subunit. This enzyme catalyzes the formation of peptide bonds and also the hydrolysis to release the polypeptide when a stop codon is reached. So the answer should be the ribosomal peptidyl transferase.
Now, looking at the options. Let's say the options are A. Peptidyl transferase, B. Aminoacyl-tRNA synthetase, C. Ribonuclease, D. ATP synthase. If the correct answer is A, then the others are incorrect. B is involved in charging tRNA with amino acids, not in release. C is for degrading RNA, D is for ATP production. So the correct answer is A. The clinical pearl here is that the ribosome's own enzyme handles both peptide bond formation and termination.
**Core Concept**
The process of translation termination involves hydrolysis of the bond between the completed polypeptide chain and the tRNA in the P site of the ribosome. This reaction is catalyzed by the ribosome’s intrinsic **peptidyl transferase** activity, which functions independently of external enzymes.
**Why the Correct Answer is Right**
**Peptidyl transferase**, located in the large ribosomal subunit (e.g., 28S rRNA in eukaryotes), catalyzes two key reactions: peptide bond formation during elongation and hydrolysis of the ester bond between the polypeptide and tRNA during termination. When a stop codon is reached, release factors bind to the A site, inducing conformational changes that activate peptidyl transferase to cleave the polypeptide-tRNA bond, freeing the chain.
**Why Each Wrong Option is Incorrect**
**Option B: Aminoacyl-tRNA synthetase** – Incorrect. This enzyme charges tRNA with cognate amino acids during translation initiation, not termination.
**Option C: Ribonuclease** – Incorrect. Ribonucleases degrade RNA, not catalyze peptide bond hydrolysis.
**Option D: ATP synthase** – Incorrect. ATP synthase generates ATP via oxidative phosphorylation, unrelated to translation.
**Clinical Pearl / High-Yield Fact**
The peptidyl transferase reaction is ribozyme-mediated (catalyzed by rRNA), not protein-based. Inhibitors like chloramphenicol block this activity, halting bacterial protein synthesis. Always associate **peptidyl transferase** with both peptide bond formation *and* termination.
**Correct Answer: A. Peptidyl transferase**