8-Translation

Translation Overview

• Location: Translation occurs in the cytosol.

• Mechanism: mRNA base pair sequences are translated into amino acid sequences.

• Codon Reading: Nucleotides are read in groups of three, forming codons.

  • There are 64 codons (4 different bases x 3 bases per code = 4³).

  • Each codon specifies a start, stop, or specific amino acid.

• Genetic Code: The relationship between a codon and its corresponding translation is known as genetic code.

tRNA Molecule

• Function: tRNA acts as the link between codons and their corresponding amino acids.

• Structure: tRNA is approximately 80 nucleotides in size with an anticodon region that binds to the mRNA codon.

tRNA Synthesis and Modifications

• Synthesis: tRNA is synthesized by RNA Polymerase III as large transcripts which are subsequently trimmed. —> trimmed at ends and in middle

  • Some bases undergo chemical modifications to yield unique non-traditional bases (e.g., Ψ, Im, Gm, and others).

• Conformation: These modifications define distinct final shapes of the tRNA molecules.

Aminoacyl Synthetase

• Function: Enzyme that links an amino acid (example: tryptophan) to its corresponding tRNA.

• Process: The attachment of an amino acid to tRNA involves the formation of a high-energy bond, utilizing ATP to convert into AMP and inorganic phosphates (2 Pi).

• High energy bond will be used later to form peptide bond . The peptide bond formed will be lower energy, more stable .

  

tRNA Editing Mechanism

• Editing Site: tRNA synthetase includes an editing site to remove incorrectly bound amino acids to ensure fidelity during synthesis.

• Hydrolytic Editing: in some tRNA synthetase, mischarged amino acids can be hydrolyzed and removed, helping maintain accuracy in tRNA-amino acid pairing.

Ribosome Structure and Function

• Composition: Ribosome is a large ribonucleoprotein complex made of two major subunits containing rRNA and over 50 proteins.

  • Eukaryotic Assembly: rRNA components are transcribed as pre-rRNA in the nucleolus and exported into the cytosol.

  • Millions of ribosomes exist in each eukaryotic cell.

Ribosome Organization

• Small Subunit: Contains the mRNA binding site and tRNA binding sites.

• Large Subunit: Catalyzes the formation of peptide bonds between amino acids.

Eukaryotic Initiation of Translation

• Initiator tRNA: The initiator aminoacyl tRNA loads onto the small ribosomal subunit (SRS).

• Initiation Factors: Additional eukaryotic initiation factors (eIFs) bind to SRS, recognizing the 5' cap of mRNA and scanning for AUG start codon, powered by ATP hydrolysis.

Eukaryotic Start of Translation

• Start Codon: Upon encountering AUG, eIFs dissociate; the large ribosomal subunit binds.

  • However : Early AUGs may sometimes be skipped in favor of a downstream AUG, a process known as 'leaky scanning.'

  • First AUG encountered reads as “start”

    

Bacterial Translation Start

• Consensus Sequences: Bacterial mRNAs lack a 5' cap; instead, they use consensus sequence signals for ribosome binding and scanning for their start codons (AUG).

  • Bacterial mRNA can be polycistronic, facilitating the synthesis of multiple proteins from a single mRNA strand.

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Translation Elongation Steps

1. Step 1: The growing polypeptide chain elongates as incoming aminoacyl tRNAs enter the A site of the ribosome.

2. Step 2: A high-energy bond in the first amino acid is broken to form a new bond; this energy is used to catalytically form the peptide bond between amino acids.

3. Step 3: The large subunit shifts while the small subunit and tRNA remain stationary; the unloaded tRNA is then displaced to the E site.

Translation Termination

• Stop Codons: UAA, UAG, and UGA act as signals for termination.

• Release Factors: When stop codons are reached, release factors bind to the A site, adding water instead of an amino acid, thereby releasing the polypeptide from the tRNA and concluding synthesis.

Polyribosomes (Polysomes)

• Definition: Many ribosomes translate the same mRNA strand simultaneously, enhancing protein synthesis efficiency.

• Polyribosome Function: In bacteria, translation can start even before transcription completes; this is not possible in eukaryotic cells.

Efficiency of Translation

• Trade-offs: Cellular processes like protein synthesis have a balance between accuracy and speed, generally favoring accuracy. High-energy bonds for each peptide bond formed reflect this trade-off.

Protein Folding

• Folding Mechanism: Secondary and tertiary folding occurs as protein synthesis progresses.

• Role of Chaperones: Molecular chaperones assist and sometimes accelerate tertiary protein folding, correcting minor flaws.

Types of Molecular Chaperones

• hsp70 and hsp60: Major eukaryotic chaperones.

  • hsp70 binds to hydrophobic regions and, utilizing ATP, compels improperly folded proteins to unfold, allowing for proper refolding.

  • hsp60 forms a barrel-shaped structure, creating an isolated space for mis-folded proteins to refold correctly.

Quality Control and Proteasomes

• Misfolded Proteins: Those proteins that cannot re-fold correctly by chaperones are targeted for degradation by proteasomes.

• Ubiquitin Tags: A special class of protease degrades large peptides into smaller peptides; additionally, misfolded proteins receive ubiquitin attachments as a degradation signal.