Question

Which of the following describes the mode of action of Tetracycline as an antibiotic?

• A. Catalyzes the ADP-ribosylation of eEF-2 in mammalian cells
• B. Binds to 23S rRNA
• C. Prevents the binding of aminoacyl-tRNAs to the A site
• D. Inhibits the peptidyltransferase in the 60S ribosomal subunit

Hint:

Tetracycline targets the small (30S) ribosomal subunit, whereas Chloramphenicol and Erythromycin target the large (50S) subunit. All three selectively inhibit bacterial translation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Tetracyclines are a class of broad-spectrum bacteriostatic antibiotics used to treat various bacterial infections.
They work by selectively inhibiting protein synthesis in bacteria without affecting eukaryotic cells.

Step 2: Detailed Explanation:
Bacterial translation occurs on the 70S ribosome, which consists of a small 30S subunit and a large 50S subunit.
The ribosome has three tRNA binding sites: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site.
Tetracycline molecules cross the outer membrane of Gram-negative bacteria through porin channels and accumulate in the cytoplasm via active transport.
Once inside, tetracycline binds reversibly to the 16S rRNA in the 30S subunit of the bacterial ribosome.
This binding physically blocks the aminoacyl-tRNA from accessing the A site of the ribosome.
Because the incoming aminoacyl-tRNA cannot bind to the A site, the ribosome cannot add new amino acids to the growing polypeptide chain, halting protein synthesis.
Let us evaluate the other options:
- Option A describes the mechanism of Diphtheria toxin, which inhibits eukaryotic elongation factor 2 (eEF-2).
- Option B describes the mechanism of macrolides, lincosamides, and streptogramins, which bind to the 23S rRNA of the 50S subunit.
- Option D describes the mechanism of chloramphenicol, which inhibits peptidyltransferase in the large ribosomal subunit.

Step 3: Final Answer:
Tetracycline acts by preventing the binding of aminoacyl-tRNAs to the ribosomal A site.