q2 - lynna

[ArchivalUser]

An immigrant family from rural Mexico brings their 3-month-old child to the emergency room because of whistling inspiration (stridor) and high fever. The child's physician is perplexed because the throat examination shows a gray membrane almost occluding the larynx. A senior physician recognizes diphtheria, now rare in immunized populations. The child is intubated, antitoxin is administered, and antibiotic therapy is initiated. Diphtheria toxin is often lethal in unimmunized persons because it
A. Inhibits initiation of protein synthesis by preventing the binding of GTP to the 40S ribosomal subunit
B. Binds to the signal recognition particle receptor on the cytoplasmic face of the endoplasmic reticulum receptor
C. Shuts off signal peptidase
D. Blocks elongation of proteins by inactivating elongation factor 2 (EF-2, or translocase)
E. Causes deletions of amino acid by speeding up the movement of peptidyl-tRNA from the A site to the P site

[ArchivalUser]

D: block EF2 by ADP-ribosylation.

[ArchivalUser]

ANS:2

[ArchivalUser]

The answer is: D

The gene that produces the deadly toxin of Corynebacterium diphtheriae comes from a lysogenic phage that grows in the bacteria. Prior to immunization, diphtheria was the primary cause of death in children. The protein toxin produced by this bacterium inhibits protein synthesis by inactivating elongation factor 2 (EF-2, or translocase). Diphtheria toxin is a single protein composed of two portions (A and B). The B portion enables the A portion to translocate across a cell membrane into the cytoplasm. The A portion catalyzes the transfer of the adenosine diphosphate ribose unit of NAD1 to a nitrogen atom of the diphthamide ring of EF-2, thereby blocking translocation. Diphthamide is an unusual amino acid residue of EF-2.