A child presents with pharyngitis, a grey adherent tonsillar membrane, and cervical lymphadenopathy. The causative organism secretes a toxin that ADP-ribosylates elongation factor 2 using NAD as the ADP-ribose donor. What is the direct consequence for protein synthesis?

• A Inability of aminoacyl-tRNA to bind the A site
• B Freeze-locking of the translocated ribosome, halting elongation ✓
• C Premature dissociation of the 60S and 40S subunits
• D Misreading of codons during elongation

Explanation

Diphtheria toxin ADP-ribosylates a modified histidine residue called diphthamide on elongation factor 2. Modified EF-2 cannot catalyse translocation of the nascent peptide from the A to the P site, so the ribosome freezes on the mRNA and all elongation stops; a single toxin molecule can kill a cell. Misreading of codons is caused by aminoglycosides, which act on the 30S subunit, not on EF-2.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.