AAMC Practice Exam Q. 49 and 50

[squash_fan]

49) I'm not understanding why the stabilization is due to Aspartate rather than arginine. I initially thought the positive charge on arginine could be responsible for stabilizing the negative charge on OH groups on pantothenate.

50) For this question, the ADP is stabilized by positive charges on His and Arg. Why is this stabilization due to positive charges instead of the negative charges like in Q. 49? Am I misunderstanding something?

Please help. Thank you!

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[catie_jane]

I don't remember these questions exactly (took the practice test over 1 month ago) but based on what you've written..

1) Aspartate (or aspartic acid) is polar like the -OH group on pantothenate and polar likes to surround itself with like charges/polarity.

2) ADP is stabilized by positive charges because the phosphate groups we know are negatively charged, therefore they need a positive charge to stabilize.

I believe the difference between the two questions is that the -OH group in the first question is not actually deprotonated (therefore no negative charge), so the polar group will be attracted to similar polar groups