Sample Test PS Passage 9, Question 49

[delods0]

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Hi all,

Sorry that my first post has to be me asking a question but I would like some help understanding the correct answer for Q49 on the PS section of the AAMC sample test.

For those of you that do not have the question:
The stabilization of pantothenate in PanK3 is most likely due to an active site:
A. Arginine
B. Asparagine
C. Aspartate
D. Glutamine

The correct answer is Aspartate because "the choices listed as response options, only aspartate has the necessary carboxylate side chain"

I picked Arginine... From what Pantothenate looked like, I thought that at a ~pH 7, the Pantothenate would have a net negative charge thus a R residue being positive would stabilize it more... Could someone explain to me why the right answer is C? Am I completely mistaken and negative charge is stabilized better by negatively charged amino acids???!

 

[RH8448]

I think you thought too deeply for this particular question. You're right in your thought process with amino acids interacting with each other in that way. That's really good to understand. But you need to apply that knowledge only when you're certain that is what is going on. This mechanism is not given and is probably more complicated (the mechanism is not even described). But, they do say that it is stabilized by a carboxylate group. That is the key info. Many side chains of opposing charges are present everywhere in every mechanism. When they describe specific binding sites then you can apply that process that you described. In other words, we need more info because we cannot assume that group is involved in the mechanism.