Lecture 5

Central Dogma of Molecular Biology

  • Information Flow
    • Parental DNA is copied to daughter DNA with high fidelity via DNA replication.
    • RNA is synthesized using DNA as a template during RNA transcription.
    • Proteins are synthesized based on the information stored in messenger RNA during protein translation.

What is Protein Metabolism?

  • Encompasses:
    • Protein synthesis
    • Protein folding
    • Protein targeting
    • Protein processing
    • Protein degradation

Ribosome Characteristics

  • Key Fact: Ribosomes make up 25% of the dry weight of bacteria (Example: Escherichia coli has around 15,000 ribosomes per cell).
  • Subunits:
    • Bacterial ribosomes are called 70S, consisting of 30S and 50S sub-particles.
    • Eukaryotic ribosomes are 80S, consisting of 40S and 60S sub-particles.
  • Ribosomal Composition:
    • Bacterial ribosomes consist of roughly 65% rRNA and 35% protein.
    • Ribonucleic acid (rRNA) forms compact cores in ribosomal subunits, decorated by multiple ribosomal proteins.

Structure of Ribosomes

  • Two Unequal Separable Subunits:
    • Small Subunit: Initiates translation by decoding the mRNA.
    • Large Subunit: Catalyzes peptide bond formation.
  • Ribosomes can form compact cores by self-folding specific rRNA molecules.

Key Functional Centers of the Ribosome

  • Binding sites for tRNA: A (Aminoacyl), P (Peptidyl), E (Exit).
  • Functions of ribosomes:
    • Decoding function (A site).
    • Catalytic function (P site).
    • Translocation of ribosome along the mRNA prevents interruption in protein synthesis.

Stages of Protein Synthesis

  1. Activation of Amino Acids: Requires aminoacyl-tRNA synthetases.
  2. Initiation: In bacteria involves binding of mRNA and initiation factors (IF-1, IF-2, IF-3) to the 30S subunit.
  3. Elongation: Involves the formation of ternary complex (Aminoacyl-tRNA + EF-Tu + GTP) and subsequent peptide bond formation.
  4. Termination and Recycling: Signaled by a stop codon, releasing the polypeptide.
  5. Post-Translational Modifications: Such as phosphorylation and glycosylation will occur.

Post-Translational Modifications of Proteins

  • Modifications include:
    • Enzymatic removal of formyl group from first residue.
    • Addition of carbohydrate moieties (glycosylation).
    • Methylation of certain residues.

Antibiotic Targets in Bacteria

  • >50% of clinically used antibiotics directly target ribosomes.
  • Important antibiotic classes include:
    • Aminoglycosides (e.g., Streptomycin) - cause miscoding.
    • Tetracyclines - block A-site of ribosome.
    • Macrolides - inhibit the peptidyl transferase reaction in the 50S subunit.

Antibiotic Resistance

  • Major healthcare threat causing significant morbidity and mortality.
  • Resistance to antibiotics occurs through various mechanisms, including ribosomal modifications that affect drug binding.
  • High demand for new antimicrobial agents due to rising resistance rates.

Summary of Protein Synthesis

  • Ribosomes play a crucial role, with distinct 70S in bacteria and 80S in eukaryotes.
  • The synthesis pathway includes stages from activation of amino acids to protein folding and modifications.
  • Post-translation modifications are vital for protein activity and interactions with the cellular environment.