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Chemistry advanced

Peptide Bond Formation in the Ribosome

The ribosome catalyzes peptide bond formation at the peptidyl transferase center using A, P, and E sites. Kinetic proofreading ensures translational fidelity through GTP hydrolysis and monitoring mechanisms.

By Wikipept Community | 2 min read
ribosomepeptide bondpeptidyl transferasetranslationproofreading

The Peptidyl Transferase Center

The ribosome is a ribozyme, meaning the catalytic activity resides in RNA rather than protein. The peptidyl transferase center (PTC) is located in the large ribosomal subunit (50S in bacteria, 60S in eukaryotes). Here, the 23S rRNA (28S in eukaryotes) positions substrates and stabilizes the transition state for peptide bond formation.

The reaction is a nucleophilic attack: the alpha-amino group of the aminoacyl-tRNA in the A site attacks the carbonyl carbon of the peptidyl-tRNA in the P site. The resulting tetrahedral intermediate collapses, transferring the growing peptide chain to the A-site tRNA. This process is remarkably fast, occurring at rates exceeding 15 bonds per second in bacteria.

The A, P, and E Sites

The ribosome has three tRNA binding sites arranged along the mRNA:

  • A site (aminoacyl): Accepts the incoming aminoacyl-tRNA. This is where the decoding center monitors codon-anticodon complementarity.
  • P site (peptidyl): Holds the tRNA attached to the growing peptide chain. After bond formation, the deacylated tRNA remains here temporarily.
  • E site (exit): The deacylated tRNA moves here before being released from the ribosome.

During each elongation cycle, the ribosome translocates one codon along the mRNA. EF-G (eEF2 in eukaryotes) catalyzes this movement using GTP hydrolysis.

Kinetic Proofreading

Translation fidelity relies on a two-step verification process called kinetic proofreading. First, the decoding center checks codon-anticodon geometry. Correct base pairing induces a conformational change in the small subunit that promotes GTP hydrolysis by EF-Tu (eEF1A). Incorrect pairings are rejected before GTP is hydrolyzed.

Second, after GTP hydrolysis, EF-Tu releases the aminoacyl-tRNA. If the codon-anticodon match is still incorrect, the tRNA can dissociate before peptide bond formation. This kinetic window provides a second chance to reject wrong tRNAs.

The overall error rate is approximately one mistake per 10,000 amino acids incorporated, combining both steps.

Mnemonic: “APE Walks the mRNA”

Remember the site order with “APE Walks”: A (Aminoacyl) then P (Peptidyl) then E (Exit), walking along the mRNA in that direction. The peptide grows from N-terminus to C-terminus as the ribosome moves 5 prime to 3 prime.

Practical Insight

Antibiotics like chloramphenicol and linezolid bind the PTC directly, blocking peptide bond formation. Macrolides like erythromycin block the exit tunnel, preventing the growing peptide from emerging. Understanding the PTC explains why these antibiotics are selectively toxic to bacterial ribosomes.