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.