Translation Elongation and Termination

Translation Elongation and Termination

Translation Elongation: Growing the Amino Acid (AA) Chain

• Definition: The process of sequential addition of amino acids to a nascent polypeptide chain, guided by the mRNA sequence.

• Energy Source: This process is primarily fueled by Guanosine Triphosphate (GTP).

• Three Key Steps:

  1. Recruitment of Charged tRNAs: Specific aminoacyl-tRNAs are brought to the ribosome's A-site.

  2. Formation of Peptide Bonds: A peptide bond is formed between the incoming amino acid and the growing polypeptide chain.

  3. Translocation Along the mRNA: The ribosome moves along the mRNA, shifting the tRNAs to the next sites.

Charged tRNAs (Aminoacyl-tRNA)

• Role: Charged tRNAs are tRNAs that have a specific amino acid covalently attached to their $3'$ end.

• Charging Enzyme: The attachment of an amino acid to its cognate tRNA is catalyzed by a specific enzyme called Aminoacyl-tRNA Synthetase.

• Charging Process:

  • Step 1: An amino acid (e.g., Phenylalanine - Phe) and ATP bind to the aminoacyl-tRNA synthetase enzyme.

  • Step 2: ATP is hydrolyzed, releasing pyrophosphate (PPi), and the amino acid is activated by forming an aminoacyl-AMP intermediate. $( ext{Amino Acid} + ext{ATP} <br>ightarrow ext{Aminoacyl-AMP} + ext{PPi})$

  • Step 3: The activated aminoacyl-AMP transfers its amino acid to the $3'$ hydroxyl group of the adenosine residue at the CCA sequence located at the $3'$ end of the tRNA. AMP is released.

  • Step 4: The newly formed charged tRNA, now called aminoacyl-tRNA, is released from the enzyme, ready to participate in translation.

• tRNA Structure:

  • Cloverleaf Model (Secondary Structure): This model illustrates the characteristic stem-loop structures of tRNA.

    • $3'$ End: Contains the CCA sequence where the amino acid attaches.

    • $5'$ End: Typically phosphorylated.

    • D Loop: Contains dihydrouridine residues.

    • T Loop (T$ext{Ψ}$C loop): Contains ribothymidine (T) and pseudouridine $( ext{Ψ} )$ residues.

    • Variable Loop: Its size varies significantly among different tRNAs.

    • Anticodon Loop: Contains the anticodon sequence.

  • Anticodon: A three-nucleotide sequence in the anticodon loop that base-pairs with a complementary codon on the mRNA, ensuring the correct amino acid is incorporated.

  • Tertiary Structure: In three dimensions, the tRNA folds into a compact L-shaped structure.

Elongation Factors (EFs)

These protein factors assist with the recruitment of tRNAs and the translocation of the ribosome during elongation.

• In Bacteria:

  • EFTu (Elongation Factor thermo unstable): Responsible for transporting the incoming charged tRNA (aminoacyl-tRNA) to the A-site of the ribosome. It binds to the charged tRNA in a GTP-dependent manner.

  • EFTs (Elongation Factor thermo stable): Functions as a guanine nucleotide exchange factor, recycling EFTu by exchanging its bound GDP for a new GTP molecule, making EFTu ready for another round of tRNA delivery.

  • EFG (Elongation Factor G): Binds to the ribosome and, utilizing GTP hydrolysis, promotes the translocation of the ribosome along the mRNA, moving the peptidyl-tRNA from the A-site to the P-site and the deacylated tRNA from the P-site to the E-site.

• In Eukaryotes:

  • eEF1 (eukaryotic Elongation Factor 1): Performs a similar function to bacterial EFTu, delivering charged tRNAs to the A-site of the eukaryotic ribosome.

  • eEF1$ext{α}$ (eukaryotic Elongation Factor 1 alpha): Functions as a guanine nucleotide exchange factor, recycling eEF1 by facilitating the exchange of bound GDP for GTP.

  • eEF2$ext{βγ}$ (eukaryotic Elongation Factor 2 beta-gamma): Orchestrates the translocation of the eukaryotic ribosome along the mRNA, analogous to bacterial EFG.

Translation Termination

• Mechanism: The process by which polypeptide synthesis ends when the ribosome encounters a stop codon on the mRNA.

• Stop Codons: There are three stop codons: UAA, UAG, and UGA. Unlike sense codons, stop codons do not code for any amino acid and do not have corresponding tRNAs.

• Mediated by: This process is facilitated by protein factors known as Release Factors (RFs).

• Release Factors (RFs) in Bacteria:

  • RF1: Specifically recognizes the stop codons UAG and UAA.

  • RF2: Specifically recognizes the stop codons UAA and UGA. Note that UAA is recognized by both RF1 and RF2.

  • RF3: A GTPase that enhances the actions of RF1 and RF2 by promoting their dissociation from the ribosome after peptide release.

• Release Factors (RFs) in Eukaryotes:

  • eRF1: A single eukaryotic release factor that recognizes all three stop codons (UAG, UAA, and UGA). This is a significant difference from bacteria, which use two distinct factors for recognition.

  • eRF3: Functions as a GTPase, similar to bacterial RF3, enhancing the activity of eRF1 and facilitating its dissociation from the ribosome.

Summary of Translational Factors and Their Functions