Wk 17&18 Lecture 2: Amoni acids and tRNA

Introduction to Transfer RNA (tRNA)

  • Role: Crucial in decoding genetic information and linking appropriate amino acids to polypeptide chains during translation.

  • Primary Structure: Individual tRNA consists of chains of 70-80 nucleotides.

Structural Features of tRNA

  • Secondary Structure: tRNA achieves a cloverleaf configuration due to four regions:

    • D loop

    • T SiC loop

    • Accepting arm (CCA)

    • Anticodon loop

  • Base Pairing:

    • Mix of Watson-Crick and non-Watson-Crick base pairs contributes to 3D structure.

    • Phosphorylation at the 5' end, usually at a guanine nucleotide.

    • The 3' end always ends with the CCA sequence, which is where amino acids attach.

  • 3D Structure: Not a perfect cloverleaf; adapted into an L-shaped structure through intramolecular hydrogen bonding. Each region interacts through multiple bonding types, allowing extensive looping and stability.

Major Structural Components

  • Acceptor Stem: Formed at the 5' and 3' ends. The 3' end presents the CCA sequence crucial for amino acid attachment.

  • Anticodon Loop: Responsible for recognizing codons on mRNA. Bases protrude for effective pairing during translation.

  • Loop Modifications: Structures may contain various modifications like methylation and deamination that influence tRNA function.

Amino Acid Attachment

  • Charging of tRNA:

    • Accomplished through Adenylation, activating amino acids by forming high-energy bonds essential for subsequent polypeptide synthesis.

    • Aminoacyl-tRNA Synthetases: Enzymes that attach the correct amino acids to their cognate tRNA.

  • Types of Synthetases:

    • Class I: Attaches to the 2' hydroxyl group.

    • Class II: Attaches to the 3' hydroxyl group.

  • Mechanism of Action: Involves two-step reactions where ATP is used to create an aminoacyl AMP intermediate, leading to the charged tRNA.

Detailed Mechanism of Amino Acid Activation

  • Step 1: Amino acid reacts with ATP, forming aminoacyl AMP and releasing pyrophosphate (PPi) which is quickly hydrolyzed.

  • Step 2: The aminoacyl AMP interacts with tRNA to form aminoacyl-tRNA and AMP.

    • High-energy bond formation drives peptide bond synthesis at the ribosome.

    • Ensures accuracy of amino acid incorporation during translation.

tRNA Nomenclature

  • Naming Convention: tRNA named based on its anticodon sequence, reflecting the amino acid it carries.

    • Example:

      • Alanine tRNA becomes Alany tRNA.

  • Mischarged tRNA: If a tRNA is incorrectly charged, it’s named after both attached amino acids (e.g., Cystine tRNA Alanine).

Fidelity of tRNA Synthetases

  • Editing Mechanism: Some synthetases possess proofreading capabilities to hydrolyze incorrectly charged tRNAs, preventing mutations.

  • Double Sieve Mechanism: The structure of the active site discriminates between amino acids based on size and structure, preventing mischarging.

  • Structural Recognition: Synthetases identify specific bases and conformational features of tRNAs to ensure correct attachment of amino acids.

Conclusion and Next Steps

  • Importance of tRNAs: They serve as the bridge between nucleic acids and proteins, facilitating accurate protein translation, which is essential for cellular function.

  • Next Lecture: Upcoming focus on the role of tRNAs in translation and detailed procedures involved in protein synthesis.