Ribosomal Translation

Ribosomal Translation Mechanisms

Summary

Ribosomal translation synthesizes proteins by decoding mRNA into polypeptide chains through initiation, elongation, and termination phases. The ribosome functions as a ribozyme, with rRNA catalyzing peptide bond formation.

Key Points

  • 1Three phases: initiation, elongation, termination
  • 2Ribosome is a ribozyme—RNA catalyzes peptide bond formation
  • 3GTP hydrolysis drives the process forward
  • 4Error rate approximately 1 in 10,000

Ribosomal translation is the universal process by which genetic information is converted into the primary structure of proteins.

Overview

Translation decodes the nucleotide sequence of mRNA into the amino acid sequence of proteins. This process occurs on the ribosome, a massive ribonucleoprotein complex.

The Three Phases

Initiation

The assembly of the translation machinery:

  • Small ribosomal subunit binds mRNA
  • Initiator tRNA (carrying Met/fMet) recognizes start codon (AUG)
  • Large ribosomal subunit joins
  • Initiation factors guide the process (eIF in eukaryotes, IF in bacteria)
  • GTP hydrolysis provides energy

    • Elongation

    The repetitive cycle of chain extension:

    Step 1: Decoding (Codon Recognition)

  • Aminoacyl-tRNA enters the A site
  • Correct codon-anticodon pairing triggers GTP hydrolysis by EF-Tu/eEF1A
  • Proofreading ensures fidelity

    • Step 2: Peptidyl Transfer

  • The growing peptide chain is transferred from P-site tRNA to A-site amino acid

    • - Catalyzed by the peptidyl transferase center (PTC) of the ribosome

  • Forms a new peptide bond

    • Step 3: Translocation

  • The ribosome moves one codon along mRNA
  • Catalyzed by EF-G/eEF2 with GTP hydrolysis
  • A-site tRNA moves to P site; P-site tRNA to E site
  • E-site tRNA exits

    • Termination

    When a stop codon (UAA, UAG, UGA) reaches the A site:

  • Release factors (RF) recognize stop codons
  • RF triggers hydrolysis of the peptidyl-tRNA bond
  • Newly synthesized protein is released
  • Ribosome subunits dissociate

    • The Ribosome as a Ribozyme

    A crucial insight: the ribosome is fundamentally an RNA enzyme:

    - The 23S rRNA (bacteria) / 28S rRNA (eukaryotes) catalyzes peptide bond formation

  • Ribosomal proteins are structural, not catalytic
  • This supports the RNA World hypothesis
  • Explains conservation of rRNA across all life

    • Fidelity Mechanisms

    Translation achieves remarkable accuracy (error rate ~10⁻⁴):

    - Initial selection: Correct codon-anticodon pairing

    - Kinetic proofreading: GTP hydrolysis provides checkpoints

    - Accommodation: Only correct tRNAs are accommodated into the A site

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