Antibiotics prevent protein synthesis in bacteria by targeting various stages of the process at the bacterial ribosome, thereby disrupting the production of essential proteins.
Here's a breakdown of how different classes of antibiotics achieve this:
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Targeting the Ribosome: Bacterial ribosomes differ structurally from eukaryotic ribosomes. This difference allows antibiotics to selectively target bacterial ribosomes without significantly affecting protein synthesis in the host cell. Most antibiotics that inhibit protein synthesis act by binding to either the 30S or 50S ribosomal subunit.
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Specific Mechanisms:
- Aminoglycosides (e.g., Gentamicin, Streptomycin): These bind to the 30S ribosomal subunit and interfere with the initiation of protein synthesis, cause misreading of mRNA, and block translocation. This can lead to the incorporation of incorrect amino acids into the growing polypeptide chain or premature termination of protein synthesis.
- Tetracyclines (e.g., Tetracycline, Doxycycline): These antibiotics bind to the 30S subunit and block the attachment of aminoacyl-tRNA to the A (acceptor) site on the ribosome. This prevents the addition of new amino acids to the growing polypeptide chain.
- Macrolides (e.g., Erythromycin, Azithromycin, Clarithromycin): Macrolides bind to the 50S ribosomal subunit and inhibit translocation. They block the exit tunnel through which the growing polypeptide chain passes, physically preventing the polypeptide from elongating and exiting the ribosome.
- Chloramphenicol: This antibiotic also binds to the 50S ribosomal subunit and inhibits peptidyl transferase activity, the enzyme responsible for forming peptide bonds between amino acids.
- Lincosamides (e.g., Clindamycin): Similar to macrolides, lincosamides bind to the 50S ribosomal subunit and interfere with translocation, effectively stopping protein synthesis.
- Oxazolidinones (e.g., Linezolid): These bind to the 23S rRNA of the 50S ribosomal subunit and prevent the formation of the initiation complex (the complex of mRNA, ribosome, and initiator tRNA), thereby inhibiting the initiation of protein synthesis.
- Streptogramins (e.g., Quinupristin/Dalfopristin): These antibiotics, used in combination, bind to different sites on the 50S ribosomal subunit. Quinupristin interferes with chain elongation and causes premature termination, while Dalfopristin blocks peptide bond formation. They act synergistically to inhibit protein synthesis.
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Consequences of Inhibition: By disrupting protein synthesis, these antibiotics prevent bacteria from producing the essential proteins required for growth, metabolism, and replication. This ultimately leads to bacteriostatic (growth-inhibiting) or bactericidal (bacteria-killing) effects, depending on the antibiotic concentration and the bacterial species.
In summary, antibiotics disrupt bacterial protein synthesis by targeting the 30S or 50S ribosomal subunits and interfering with different steps such as initiation, elongation, translocation, or peptide bond formation, effectively halting the production of essential proteins required for bacterial survival and replication.
