Catalysis in the ribosome: Which molecule is responsible for catalyzing the transpeptidation (peptidyl transfer) reaction during translation?

Introduction / Context:
Peptidyl transfer—the formation of a peptide bond between the growing polypeptide and the incoming amino acid—is the core chemical reaction of translation. Identifying the catalytic entity clarifies why the ribosome is considered a ribozyme.

Given Data / Assumptions:

• Process: elongation on the ribosome.
• Location: large ribosomal subunit (50S in bacteria; 60S in eukaryotes).
• We seek the catalyst of peptide-bond formation.

Concept / Approach:

The catalytic activity resides in rRNA (23S in bacteria; 28S in eukaryotes) within the peptidyl transferase center. Hence, “peptidyl transferase” describes an rRNA-based catalytic function, not a protein enzyme like DNA polymerase.

Step-by-Step Solution:

Verification / Alternative check:

Chloramphenicol inhibits the peptidyl transferase center in bacteria, stopping peptide bond formation—evidence of the site’s role. Crystallography shows rRNA nucleotides in the active center, reinforcing the ribozyme concept.

Why Other Options Are Wrong:

• RNA polymerase: transcribes RNA from DNA, not peptide bonds.
• DNA ligase / DNA polymerase: DNA replication/repair enzymes, unrelated to translation chemistry.

Common Pitfalls:

• Assuming a protein enzyme performs the catalysis; it is rRNA-based.
• Confusing transpeptidation with translocation; they are consecutive but distinct steps.

Final Answer:

Peptidyl transferase