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NBME 20 Answers

nbme20/Block 2/Question#41

Purified serum antibodies elicited by immunization ...

Proteins X and Y express the same epitopes

Purified serum antibodies elicited by immunization ...

Proteins X and Y express the same epitopes

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eacv
omg YES!! thanks Uworld I got it correct! exactly this qx asked the exact opposite thing! Hahaha I loved it !!
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pg32
Even after reading the UWorld explanation, I am still not sure how the answer that reads, "Protein Y expresses all of the epitopes expressed by protein X, but protein X does not..." is incorrect. Based on the graph, I don't see a way we can rule out that answer choice and it sounds more likely than both X and Y having the EXACT SAME epitopes. Can anyone explain? What would the graph look like if the quoted answer choice was correct?
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69_nbme_420
If you make up an example with numbers, it really helps! “Protein Y expresses all of the epitopes expressed by X, but protein X does not express all of epitopes expressed by Protein Y.”
If we say protein Y has epitopes 1, 2, and 3. Then Protein X has epitopes 1 and 3. Then we can clearly see the relationship the AMOUNT of Y added relative to X bound would NOT be linear. Stated another way – we need an exponentially more amount of Y to COMPLETELY unbind X and therefore there would not be a one to one depiction in the graph
Similar logic applies for the answer choice that states "protein X expresses all of the epitopes expressed by protein Y, but protein Y does not express all of the epitopes expressed by protein X.
E.g. If protein Y has epitopes 1 and 2. And protein X has epitopes 1, 2, and 3. Here again, we have satisfied the answer choices condition, and no matter how much we increase protein Y, protein X will still have epitope 3 bound in this case.
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69_nbme_420
Just to clarify for the first scenario: We have 3 epitopes on Y, and 2 epitopes on X. That means, assuming the epitopes are all present in equal amounts, if I add 300 grams of protein Y to the solution - only 200 grams will bind protein X. AND ONLY 200 grams of protein X can be unbound. Hope the numbers help!
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so how would the graph look for protein x expresses all of the epitopes expressed by protein y, but protein y does not express all of the epitopes expressed by protein x?

drw
then some anti-X cannot relocate to Y even Y is added at whatever high dose. at this condition, the line can never touch the axis-Y.
on the contrary, if Y express all epitopes on the X, but X does not express all epitopes on the Y, that means some Y epitopes are not seen on X. at this condition, I don't know what will be the line looked like.
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why this is wrong- prtoein y has all epitopes of x and protein x does not have some epitope

The graph actually did not touch axis-y when dose of Y is low And at some point, the bounded X started to drop How to explain the bounded X did not drop before that Y dose level is reached?

I found this, I think it gives kinda of an explanation: If the regression of log ((A'/A) -1) on -log B is linear with a slope of -1, then this indicates that the antagonism is competitive and by definition the agonist and antagonist act at the ** SAME recognition sites**. So basically the more you add Y, the less X is bound, which means they have a same structural component (epitope) and must act on the same site? Don't know if this makes sense...

http://facpub.stjohns.edu/~yoburnb/pages/dictimages/schild1.html

**MAYBE** they are competing for the same binding spot/epitope due to this relationship (epitope= antibody binding site)
??????????

your guess is as good as mine.....................................................................

submitted by usmile1(29), 2019-08-04T20:04:43Z

If you look at Uworld question ID 12299 it has a wonderful explanation for this. If they share the same epitopes, it will have a downward slope. If they share none of the same epitopes, the line will be horizontal across the graph (indicating no change as the amount of Y added increases)