1. Chloramphenicol is a “broad-spectrum” antibiotic which inhibits protein synthesis in prokaryotes. Given below are a few statements regarding the mode of action of chloramphenicol.
A. Chloramphenicol inhibits the peptidyl-transferase activity of ribosomes.
B. Chloramphenicol can be used to treat moderate to severe infections, because mitochondrial ribosomes are not sensitive to chloramphenicol.
C. Chloramphenicol binds to one of the domains of 23S rRNA
D. Chloramphenicol competes for binding with the E- sitetRNA
Which of the following options describes correctly the mechanism of action of chloramphenicol?
(1) B and D only       (2) A and C only
(3) A, C and D          (4) B, C and D

Chloramphenicol is a classic broad-spectrum antibiotic, valued for its ability to inhibit a wide range of bacterial pathogens. Its unique mechanism of action, targeting the protein synthesis machinery of prokaryotes, has made it both a powerful therapeutic tool and a subject of intense study. In this article, we will explore the detailed mode of action of chloramphenicol, clarify common misconceptions, and answer a key question about its mechanism based on current scientific understanding.

What is Chloramphenicol?

Chloramphenicol is a neutral nitrobenzene derivative and one of the earliest antibiotics discovered with broad-spectrum activity. It is effective against both Gram-positive and Gram-negative bacteria and is primarily used in situations where other antibiotics are ineffective or contraindicated. However, due to its potential for serious side effects, such as blood dyscrasias and Gray syndrome in newborns, its clinical use is now limited and closely monitored1.

Mechanism of Action: Targeting Protein Synthesis

The primary antibacterial effect of chloramphenicol is the inhibition of protein synthesis in prokaryotic cells. This is achieved through a highly specific interaction with the bacterial ribosome.

1. Inhibition of Peptidyl Transferase Activity

Chloramphenicol binds to the 50S subunit of the 70S bacterial ribosome, specifically targeting the peptidyl transferase center. By doing so, it inhibits the activity of peptidyl transferase, an essential enzyme responsible for catalyzing peptide bond formation between amino acids during protein synthesis. The result is a blockade in the elongation of the nascent polypeptide chain, effectively halting bacterial growth12.

2. Binding to 23S rRNA Domains

The binding site of chloramphenicol has been mapped to specific domains of the 23S ribosomal RNA (rRNA) within the 50S subunit. Studies using chemical modification and UV crosslinking have identified that chloramphenicol interacts with nucleotides in domains II and V of the 23S rRNA, particularly at positions such as C2611 and C2612 in Escherichia coli. These sites are located at the entrance to the peptide channel, forming a hydrophobic crevice that accommodates the antibiotic32. This interaction is crucial for its inhibitory effect.

3. Competition with Aminoacyl-tRNA

Chloramphenicol competes with the aminoacyl-tRNA for binding at the peptidyl transferase center, specifically interfering with the A-site (aminoacyl site) of the ribosome. This competitive inhibition prevents the proper positioning of the incoming tRNA, further blocking peptide bond formation12. However, chloramphenicol does not compete for the E-site (exit site) tRNA binding, which is a common misconception.

Clinical Implications and Mitochondrial Sensitivity

One important consideration in the use of chloramphenicol is its effect on mitochondrial ribosomes. Mitochondria, the energy-producing organelles in eukaryotic cells, possess ribosomes that are structurally similar to those of bacteria. Contrary to the belief that mitochondrial ribosomes are not sensitive to chloramphenicol, recent research has shown that chloramphenicol can indeed inhibit mitochondrial protein synthesis, leading to potentially serious side effects in humans4. This is particularly relevant in tissues with high metabolic activity, such as bone marrow, explaining the drug’s hematological toxicity.

Evaluating the Statements: Which Are Correct?

Given the following statements regarding the mode of action of chloramphenicol:

• A. Chloramphenicol inhibits the peptidyl-transferase activity of ribosomes.

• B. Chloramphenicol can be used to treat moderate to severe infections, because mitochondrial ribosomes are not sensitive to chloramphenicol.

• C. Chloramphenicol binds to one of the domains of 23S rRNA.

• D. Chloramphenicol competes for binding with the E-site tRNA.

Let’s analyze each:

• Statement A: Correct. Chloramphenicol directly inhibits the peptidyl-transferase activity on the bacterial ribosome, blocking peptide bond formation12.

• Statement B: Incorrect. Mitochondrial ribosomes are sensitive to chloramphenicol, and its use can result in inhibition of mitochondrial protein synthesis and serious side effects4.

• Statement C: Correct. Chloramphenicol binds specifically to domains in the 23S rRNA of the 50S ribosomal subunit3.

• Statement D: Incorrect. Chloramphenicol competes with aminoacyl-tRNA for the A-site, not the E-site, of the ribosome12.

Correct Answer

The correct option is:

(2) A and C onl