NBME 22 Protein structure (NBME Answers)

Retired NBME 22 Answers

nbme22/Block 1/Question#50 (reveal difficulty score)
A 9-year-old boy is brought to the physician ...
Protein structure 🔍 / 📺 / 🌳 / 📖
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+6 upvote downvote
submitted by organicmechanic(2)

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Increased sweat and Na+ concentration should point to cystic fibrosis (CF). The problem with CF is not that the gene is being transcribed less, but that the protein that the gene codes for is altered, which leads to the CF channel being degraded due to mis-folding --> less CF receptors on cell surface --> phenotypic CF.

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ls3076 why not membrane receptor? +8

a1913 delF508 is a 3 base pair deletion of phenylalanine at amino acid position 508. Mutation causes impaired post-translational processing of CFTR (improper folding) which rough ER detects. Sends mutant misfolded CFTR to the proteasome for degradation, preventing it from reaching cell surface. So problem is not malfunctioning CFTR channels in the surface; problem is complete absence of CFTR on cell surface (since they keep getting misfolded and sent to proteasome to be trashed). Source of primary problem: error in protein structure +8

angelaq11 @Is3076 because the CFTR is a channel not a receptor. +18

rainlad FA 2019 p. 60 +

dysdiadochokinesia @a1913 is correct- as for @angelaq11, you can still have a receptor that also functions as a channel as they are not mutually exclusive. An example of this is the nAChR found on postsynaptic NMJ neurons. This is a non-selective, ligand-gated, ionotropic receptor that functions as a channel once its ligand (i.e., ACh) has bound to the active site to induce conformational change. Similarly on the same realm: CFTR is an ionotropic receptor that concurrently functions as a Cl- channel once its ligands (ie. 2 ATP) is bound to open the channel and enable Cl- flux. This question in particular is asking for the underlying pathophysiologic mechanism for cystic fibrosis, which boils down to an issue with the primary structure of a protein resulting in its misfolding and subsequent sequestration/degradation. +1

+4 upvote downvote
submitted by ∗paperbackwriter(161)

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FYI this is nth* question I've seen that wants to see if you know that the CFTR mutation results in a misfolded protein that gets stuck in the rER.

Where n = a very big number

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castlblack I knew that but I thought the chaperone proteins counted as protein regulation :( +1

+3 upvote downvote
submitted by a1913(16)

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delF508 is a 3 base pair deletion of phenylalanine at amino acid position 508. Mutation causes impaired post-translational processing of CFTR (improper folding) which rough ER detects. Sends mutant misfolded CFTR to the proteasome for degradation, preventing it from reaching cell surface. So problem is not malfunctioning CFTR channels in the surface; problem is complete absence of CFTR on cell surface (since they keep getting misfolded and sent to proteasome to be trashed). Source of primary problem: error in protein structure

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