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Free STEP1 Exam Braindumps (page: 81)

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Loss of the hepatic protein hepcidin can lead to severe iron overload with symptoms resembling those of hemochromatosis. Which of the following functions of hepcidin accounts for the iron overload when the protein is deficient?

  1. activates the expression of the ironresponse element-binding protein that regulates transferring receptor and ferritin mRNA translation
  2. decreases the level of intestinal membrane iron transporters, resulting in reduced iron uptake
  3. facilitation of the interaction of transferring with the transferrin receptor
  4. forms a complex with ferritin allowing for higher intracellular storage
  5. promotes the formation of hemosiderin, thus detoxifying iron

Answer(s): B

Explanation:

Hepcidin is a hepatically synthesized iron regulatory protein that functions by inhibiting the presentation of one or more of the iron transporters [e.g., DMT1 and Ireg1 (ferroportin)] in intestinal membranes. With a high iron diet, the level of hepcidin mRNA increases and conversely its levels decrease when dietary iron is low. This is occurring simultaneous to reciprocal changes in the levels of the transporters proteins themselves. Loss of hepcidin activity would then lead to unregulated iron uptake from the intestines leading to iron overload. In fact, it is now considered that defects in hepcidin function contribute to the development of hemochromatosis. Hepcidin does not activate expression of iron-response elementbinding protein (choice A), facilitate transferring interaction with the transferring receptor (choice C), form a complex with ferritin (choice D), nor promote the formation of hemosiderin (choice E).



In an enzyme with a critical Glu residue in the active site, which of the following amino acid substitutions would be expected to have the least effect on enzyme activity?

  1. Arg
  2. Asp
  3. Lys
  4. Ser
  5. Tyr

Answer(s): B

Explanation:

Glutamic acid is an acidic amino acid at physiologic pH and therefore, substitution for another acidic amino acid, such as aspartic acid, would be expected to have minimal effect on the activity of the enzyme.
Arginine (choice A) and lysine (choice C) are both basic amino acids at physiologic pH and would not be able to substitute for an acidic amino acid. Serine (choice D) and tyrosine (choice E) both have ionizable hydroxyl groups but the pKa values of those hydroxyl groups would not favor substitution for an acidic amino acid.



Many proteins undergo modifications during and/or following translation. The ATPdependent polyubiquitination of proteins is a signal for which of the following events?

  1. addition of the oligosaccharide core to the N-linkage site adjacent to the ubiquitin
  2. recognition by the toxin of diphtheria
  3. recycling of the protein back to the endoplasmic reticulum (ER) from the cis-Golgi to allow proper folding
  4. targeting the protein for degradation in the proteosome
  5. translocation of the protein into the nucleus

Answer(s): D

Explanation:

The ATP-dependent poly-ubiquitination of proteins is a signal that the protein is misfolded and thus, needs to be targeted by the proteosome for degradation. Mono-ubiquitination is known to initiate cell signaling by allowing other proteins that contain ubiquitin-binding domains to interact with the mono- ubiquitinated substrate. Mono-ubiquitination has also been associated with targeting of membrane proteins to the lysosome. None of the other examples (choices AC, E) are associated with addition of ubiquitin.



Some viruses, for example, poliovirus, contain a protease that cleaves one of the eukaryotic initiation factors allowing for cap-independent translational initiation of viral RNAs at internal ribosome entry site (IRES). Which of the following factors is the target of these viral proteases?

  1. eIF-2
  2. eIF-2B
  3. eIF-4A
  4. eIF-4E
  5. eIF-4G

Answer(s): E

Explanation:

Eukaryotic viral RNAs are not capped and the host 40S ribosome interacts with these viral RNAs at an IRES. Capped eukaryotic mRNAs are recognized by the complex of eIF- 4E and eIF-4G, which in turn allows for interaction of the 40S ribosome with the mRNA (see below figure).

Initiation factor eIF-4E physically binds the cap structure in eukaryotic mRNAs and this function is facilitated by interaction of eIF-4E with eIF-4G. Thus, loss of the interaction of eIF-4E with the protease cleaved eIF- 4G results in loss of translational initiation from capped mRNAs. However, the binding of 40S ribosomes to viral RNAs does not require eIF-4E, only eIF-4G. The protease encoded by the poliovirus cleaves the eIF- 4E binding site on eIF-4G. None of the other translation factors (choices AC, E) are targets for poliovirus protease.






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