Antibiotics Inhibiting Protein Synthesis: Mechanisms and Classification and Clinical Application
Biological Foundations of Protein Synthesis Inhibition
Antibiotics that target protein synthesis focus on the ribosome and the process of translation within the cell.
For an antibiotic to be effective and safe, there must be a biological difference between the target (generally a prokaryotic cell) and the host (the eukaryotic organism). This concept is known as selective toxicity.
While some targets are entirely unique to prokaryotes, such as the cell wall, other components like the ribosome are shared by both cell types. In these cases, exploitation of the structural differences between prokaryotic and eukaryotic ribosomes allows the antibiotic to be detrimental to the bacteria while remaining significantly less harmful to the host.
Selective Toxicity and Ribosomal Architecture
Prokaryotic ribosomes are identified as ribosomes, whereas eukaryotic ribosomes are identified as ribosomes.
Substantial structural differences exist between the two, even though they perform the same function:
The prokaryotic ribosome consists of a larger subunit and a smaller subunit ().
The smaller subunit in prokaryotes contains the RNA.
The eukaryotic ribosome uses an RNA in its smaller subunit and incorporates a higher number of proteins, making it more complex.
There are distinct differences in the types of RNAs and the total number of proteins present in the larger subunits of each type.
To inhibit the ribosome, antibiotics must successfully penetrate the cell and reach the cytoplasm. These drugs do not attack the exterior of the bacteria but must function internally.
Classification of Protein Synthesis Inhibitors
Antibiotics targeting the ribosome are categorized based on which subunit they attack:
The subunit (small subunit) is targeted by Aminoglycosides and Tetracyclines.
The subunit (large subunit) is targeted by Macrolides and Chloramphenicols.
Aminoglycosides: Irreversible 30S Subunit Inhibitors
Representative compounds in this group include streptomycin, dentomycin, and neomycin.
Nomenclature and Origin:
If the name ends in "MYCIN," the antibiotic was derived from a Streptomyces Genesis bacteria.
If the name ends in "MICIN," the antibiotic was generated by micro monospora.
Both parent organisms are Gram-positive, spore-forming, generally aerobic, and exhibit branching patterns similar to fungal mycelium.
Mechanism of Action:
Aminoglycosides bind irreversibly to the ribosomal subunit.
The binding occurs near the bottom where the mRNA and tRNA meet, specifically at the A site, though the molecule can also shift toward the P site.
Biological Consequences:
Prevention of Initiation: Binding can stop the translation process from starting entirely.
Misreading at the A Site: If translation proceeds, the antibiotic interferes with the selection of the correct transfer RNA (tRNA). For example, if the mRNA codon is (which normally codes for arginine), the presence of the antibiotic may cause a different, incorrect tRNA to bind despite the mismatch.
Mutations: This interference leads to mutations in the growing peptide chain, which can be silent, missense, or nonsense mutations, depending on which tRNA is incorrectly incorporated.
Damage and Lethality:
If the mutated proteins are essential membrane-bound proteins involved in fluidity or transport, the cell membrane can be disrupted.
Membrane disruption is often lethal to the cell.
Aminoglycosides are considered concentration-dependent bactericides. Because they bind irreversibly, every antibiotic molecule effectively removes one ribosome from the functional pool.
At low levels, they may act as bacteriostatic agents by only disabling a portion of the ribosomes, thereby slowing growth.
Spectrum of Activity:
These are broad-spectrum antibiotics because all prokaryotes possess ribosomes. However, Gram-negative bacteria are particularly susceptible because their active transport mechanisms can be exploited to bring the antibiotic into the cell.
Tetracyclines: Reversible 30S Subunit Inhibitors
Representative compounds include tetracycline and doxycycline.
Structural Characteristics and Origin:
The name reflects a four-cycle compound structure (tetra meaning ).
These are produced by Streptomyces type bacteria.
Mechanism of Action:
Tetracyclines bind reversibly to the ribosomal subunit at the A site.
Unlike aminoglycosides that cause misreading, tetracyclines physically block the next tRNA from entering the A site, effectively halting translation.
Biological Consequences:
They are bacteriostatic. Because the binding is reversible, the antibiotic molecules can pop on and off the ribosome and potentially be removed from the cell.
To maintain the inhibition of protein synthesis, a steady and continuous supply of the antibiotic is required. If the supply is removed, the bacteria can resume growth.
Spectrum of Activity:
They are broad-spectrum, effective against both Gram-positive and Gram-negative bacteria because they can penetrate the cell walls of both types with equal efficiency.
Macrolides: Large Subunit (50S) Peptidyl Transferase Inhibitors
Erythromycin is a prominent early example of this class.
Structural Characteristics:
Macrolides have a large, complex ring-like structure.
Mechanism of Action:
These antibiotics bind reversibly to the ribosomal subunit.
They specifically interfere with peptidyl transferase, the enzyme responsible for catalyzing the formation of peptide bonds (a process that involves the removal of a water molecule).
The antibiotic binds between the P site and the A site, preventing the transfer of the growing polypeptide chain from the P site to the new amino acid arriving at the A site.
Biological Consequences:
They are generally bacteriostatic. If the antibiotic is discontinued, the bacteria can return to normal growth. Slowing down bacterial growth allows the host immune system to overmatch and clear the infection.
However, at higher concentrations, during periods of rapid bacterial growth, or when bacterial density is low, macrolides may become bactericidal.
Spectrum of Activity and Clinical Use:
Due to their large and complex structure, they have difficulty passing through multiple membranes. This makes them more effective against Gram-positive bacteria (one membrane) than Gram-negative bacteria (two membranes).
Macrolides are frequently prescribed as an alternative for patients who have an allergy to penicillin.
Chloramphenicol: Broad-Spectrum 50S Inhibitors with Clinical Caveats
This antibiotic is produced by soil bacteria within the Streptomyces genus and contains chlorine atoms in its structure.
Mechanism of Action:
Similar to macrolides, chloramphenicol inhibits the subunit by blocking peptidyl transferase.
It acts as a physical block, preventing the formation of peptide bonds and stopping the transfer of the peptide chain from the P site to the A site.
Biological Consequences:
It is bacteriostatic and binds reversibly to the ribosome.
Spectrum of Activity and Clinical Status:
It is effective against both Gram-positive and Gram-negative bacteria. Its molecular structure allows it to enter cells more easily than the larger macrolide compounds.
Usage has declined significantly due to unintended side effects, most notably its negative impact on the production of red blood cells in humans.
Currently, it is rarely used in human medicine except perhaps as a small secondary antibiotic, but it remains utilized in veterinary medicine.
Summary of Interaction Sites
The central goal of these antibiotics is to target the ribosome to inhibit translation, either by blocking components of the subunit or the subunit.
Subunit Targets: Aminoglycosides and Tetracyclines.
Subunit Targets: Macrolides and Chloramphenicol.
The efficacy of these drugs ultimately relies on the host's immune system to clear the bacteria once their growth has been halted or significantly slowed.