U01.01.018 Protein synthesis

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Learning Objectives

  • Master the three stages of translation: Initiation, Elongation, and Termination.
  • Distinguish between Prokaryotic (70S) and Eukaryotic (80S) ribosomal subunits.
  • Identify the energy requirements (ATP vs. GTP) and ribosomal sites (A-P-E).
  • Understand Post-translational modifications, including trimming and covalent alterations.

1. Translation Initiation

Initiation involves the assembly of the ribosomal subunits around the mRNA and the initiator tRNA.

  • Eukaryotes: eIFs identify the 5′ cap. The 40S subunit + 60S subunit assemble to form the 80S ribosome (Mnemonic: Even numbers).
  • Prokaryotes: The Shine-Dalgarno sequence aligns the 30S subunit with the start codon via the 16S rRNA. 30S + 50S = 70S (Mnemonic: Prime numbers).
  • Energy: Requires GTP for assembly.

 

2. Elongation: “Going APE”

Proteins are synthesized from the N-terminus to the C-terminus. The ribosome moves along the mRNA in the 5′ → 3′ direction.

Site Function
A site Incoming Aminoacyl-tRNA binds here (except the first methionine).
P site Accommodates the growing Peptide chain.
E site Holds the Empty tRNA as it Exits.
  • Mechanism: rRNA (a ribozyme) catalyzes peptide bond formation. The ribosome then advances 3 nucleotides (Translocation).
  • Energy Mnemonic:
    • ATP: tRNA Activation (charging).
    • GTP: tRNA Gripping and Going places (translocation).

3. Termination

Occurs when a stop codon is reached. Release factors (eRFs) recognize the codon and utilize GTP to release the completed polypeptide.

 

Activity

4. Post-translational Modifications

Trimming

Removal of N- or C-terminal propeptides to transform an inactive zymogen into a mature protein (e.g., Trypsinogen → Trypsin).

Covalent Alterations

  • Phosphorylation, Glycosylation, Methylation, Acetylation.
  • Hydroxylation: Essential for collagen (requires Vitamin C).
  • Ubiquitination: “Tags” a protein for destruction by the proteasome.

Chaperones

Intracellular proteins (e.g., Heat Shock Proteins) that facilitate and maintain correct protein folding.

 

Activity