Case study- Pneumonia

Case details

A 54-year-old man develops a productive cough and a low- grade fever. He visits his family physician who suspects mycoplasma pneumonia. The physician tries to treat the patient empirically with Erythromycin. Which of the following describes the mechanism of Erythromycin?

A. Binds to 5′ cap of RNA

B. Inhibits the 30S ribosomal RNA

C. Binds to the Shine Dalgarno sequence

D. Inhibits the initiation factor IF-1

E. Inhibits the 50 S ribosomal subunit.

Answer- E-Inhibits the 50 S ribosomal subunit.

Ribosomes are large complexes of protein and r RNA. They consist of two subunits one large (Heavy) and one small (Light) (Figure-1) whose relative sizes are generally given in terms of their sedimentation coefficients or S (Svedberg) values. The S values are determined by the shape as well as by the molecular mass; their numerical values are strictly not additive. The prokaryotic 50S and 30S ribosomal subunits together form a ribosome with an S value of 70.

Prokaryotic ribosome

Figure-1- Prokaryotic ribosome

Mechanism of action of antibiotics

Clindamycin and Erythromycin bind irreversibly to a site on the 50 s subunit of the bacterial ribosome thus inhibit translocation (figure-2)

The tetracyclines (tetracycline, doxycycline, demeclocycline, minocycline, etc.) block bacterial translation by binding reversibly to the 30S subunit and distorting it in such a way that the anticodon of the charged tRNAs cannot align properly with the codons of the mRNA (figure-2)

Puromycin structurally binds to the amino acyl t RNA and becomes incorporated into the growing peptide chain thus causing inhibition of the further elongation.

Chloramphenicol inhibits prokaryotic Peptidyl Transferase, inhibits the formation of peptide bond (figure-2)

Streptomycin and gentamicin also inhibit 30 S ribosomal subunit (figure-2)

Diphtheria toxin inactivates the eukaryotic elongation factors thus prevent translocation

moa of antibiotics

Figure-2- Inhibitors of protein synthesis acting as antibiotics

As regards other options-

Ribosomes bind to 5’cap of eukaryotic m RNA, there is no Shine Dalgarno sequence as found in prokaryotes.

Initiation factors IF-1, 2 and 3 are found in prokaryotes. They are not the targets of Antibiotics.

Thus the right answer is- Erythromycin inhibits the 50 S ribosomal subunit of ribosome.

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