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CASE FILES: BIOCHEMISTRY
[9.4]
The 6-year-old son of a migrant worker is brought to a clinic with
chills, headache, nausea, vomiting, and sore throat. The examining
physician notes a persistent grayish colored membrane near the tonsils.
History reveals that the patient has not been immunized against diph-
theria. Diphtheria toxin is potentially lethal in this unimmunized
patient because it causes which of the following?
A. Inactivates an elongation factor required for translocation in pro-
tein synthesis
B. Binds to the ribosome and prevents peptide bond formation
C. Prevents binding of mRNA to the 60S ribosomal subunit
D. Inactivates an initiation factor
E. Inhibits the synthesis of aminoacyl-charged tRNA
[9.5]
Replication of a particular DNA sequence is noted to be under inhibitory
control usually. However, when substance “A” is added, it binds to a
repressor, rendering the repressor inactive and allowing transcription to
occur. Which of the following terms describes agent “A”?
A. Histone
B. Operon
C. Polymerase
D. Transcriber
E. Inducer
A nsw ers
[9.1]
B. Chloramphenicol inhibits protein synthesis by inhibiting peptidyl
transferase. This peptidyl group cannot be transferred to the aminoacyl-
tRNA in the A-site.
[9.2]
J. Tetracyclines bind to the A-site of the prokaryotic ribosome and
prevent aminoacyl-tRNAs from binding. Thus protein synthesis is
halted because new amino acids cannot be added to the growing protein.
[9.3]
C. Erythromycin and other macrolide antibiotics bind the 50S subunit
near the P-site and cause conformational changes that inhibit the
translocation of peptidyl tRNA from the A-site to the P-site.
[9.4]
A. Diphtheria toxin has two subunits. The B subunit binds to a cell
surface receptor and facilitates the entry of the A subunit into the cell.
The A subunit then catalyzes the ADP-ribosylation of elongation
factor 2 (EF2). EF2 is thus inhibited from participating in the translo-
cation process of protein synthesis; hence, protein synthesis stops.
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