Macrolides inhibit bacterial protein synthesis by binding to which ribosomal subunit?

Explanation

## Macrolide Mechanism of Action **Key Point:** Macrolides bind to the **50S ribosomal subunit** and inhibit bacterial protein synthesis by blocking peptide translocation. ### Bacterial Ribosome Structure Bacterial ribosomes are 70S (composed of 30S + 50S subunits), distinct from eukaryotic 80S ribosomes (40S + 60S subunits). This structural difference allows selective toxicity. ### Mechanism of Macrolide Action 1. **Binding site:** 50S ribosomal subunit (specifically the peptidyl transferase center) 2. **Effect:** Blocks translocation of peptides from A site to P site 3. **Result:** Premature termination of translation and bacteriostatic effect 4. **Selectivity:** Minimal effect on eukaryotic 80S ribosomes (hence low toxicity) ### Classification of Protein Synthesis Inhibitors by Ribosomal Target | Ribosomal Target | Antibiotics | Mechanism | |------------------|-------------|----------| | **30S subunit** | Aminoglycosides, tetracyclines, spectinomycin | Cause misreading or prevent initiation | | **50S subunit** | **Macrolides**, chloramphenicol, linezolid, clindamycin | Block translocation or peptide bond formation | | **70S (whole)** | Fusidic acid | Inhibits EF-G | **High-Yield:** 50S-binding agents (macrolides, chloramphenicol, clindamycin) are bacteriostatic; 30S-binding aminoglycosides are bactericidal. **Mnemonic:** **MACL** = **M**acrolides, **A**minoglycosides, **C**hloramphenicol, **L**inezolid → **M**acrolides and **C**hloramphenicol hit **50S**; **A**minoglycosides hit **30S**. **Clinical Pearl:** Macrolides are effective against intracellular pathogens (Legionella, Mycoplasma, Chlamydia) because they achieve high intracellular concentrations and penetrate cell membranes well. [cite:Robbins 10e Ch 8]