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MCB 250: Aminoacyl tRNA Synthetases

Instructor Information

  • Instructor: Dr. James M. Slauch

  • Department: Microbiology

  • Vcast: 40

The Second Genetic Code

  • The ribosome's primary concern is whether the codon can base pair with the anticodon on tRNA.

  • However, the true specificity during translation is determined by the correct charging of each tRNA with its corresponding amino acid.

  • Aminoacyl tRNA Synthetases:

    • Each amino acid has its own specific synthetase (total of 20 for the standard 20 amino acids).

    • Each synthetase can handle multiple tRNAs corresponding to that amino acid.

    • Accuracy of charging: approximately 1 mistake per 1000 charges.

Overview of Aminoacyl-tRNA Synthetases

  • Generally, there is one synthetase for each amino acid, resulting in most cells having 20 different synthetases.

  • Synthetases are required to recognize:

    • A specific amino acid.

    • Multiple tRNAs for certain amino acids.

  • Synthetases implement strategies to avoid errors in charging, such as:

    • The presence of a second active site that can remove incorrectly charged amino acids (mischarged amino acids).

  • The accuracy of the amino acid charging process significantly influences the overall fidelity of translation.

  • Important note: The ribosome only detects the codon-anticodon interaction; it does not recognize the specific amino acid attached to the tRNA.

Mechanism of Aminoacyl-tRNA Synthetases

  • Synthetases "charge" tRNAs with the correct amino acid to ensure proper translation.

  • Reference figures from Watson et al. (2014) illustrate the mechanism as outlined in Molecular Biology of the Gene.

Class I Aminoacyl-tRNA Synthetases

  • Associated with the formation of Aminoacyl-AMP, which is an activated amino acid that forms a part of the charging process.

Distinguishing Amino Acids

  • Examples of amino acids and their synthetases:

    • Phenylalanine (Phe)

    • Isoleucine (Ile)

    • Valine (Val)

    • Tyrosine (Tyr)

    • Hydrogen bonding is crucial in distinguishing between tyrosine (Tyr) and phenylalanine (Phe).

    • Valine synthetase effectively excludes isoleucine (Ile), while the isoleucine synthetase faces challenges in excluding valine (Val).

The Double Sieve Model

  • A strategy employed by aminoacyl-tRNA synthetases to achieve high specificity during amino acid recognition.

  • Components of the model:

    • Each synthetase has two sites:

    • Proofreading site: ensures that incorrect amino acids are hydrolyzed and removed.

    • Acylation site: where the correct amino acid is attached to the tRNA.

  • Example Synthetases:

    • Val-tRNA synthetase

    • Ile-tRNA synthetase

  • The model explains how the activation of amino acids (creating aminoacyl-AMP) occurs:

    • Mixtures of “wrong” aminoacyl-AMPs are hydrolyzed, while the correct amino-acylated complexes (e.g., Ile-AMP) are transferred to their respective tRNAs.

Conclusion

  • The efficiency and accuracy of aminoacyl tRNA synthetases are essential in ensuring that proteins are synthesized accurately and reflect the genetic code present in mRNA, maintaining the fidelity of gene expression and cellular functions.