theonlytycrane

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From the passage, the ligands tend to be hydrophobic and anionic.

So we want hydrophobic and/or positively charged amino acids in the binding site.
 

NextStepTutor_2

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For this question, why is it better to have amino acids that are hydrophobic and charged? Why not have two amino acids that are both charged?
This is a classic example of a very simple MCAT question just waiting for students to outsmart themselves.

The 1st paragraph explicitly stats that the ligands are characterized by hydrophobic regions, anionic (i.e. -) charge, and generally being electronegative (i.e. will be electron dense). In fact you even highlighted the relevant text. Though it looks like you highlight entirely too much and is likely why you may have missed this crucial information when needed.

Now what kind of AA residues would bind to hydrophobic, negative or near-negative ligands? Cationic or hydrophobic residues would do nicely. Only one answer choice satisfies these criteria. Choice a shows R (positively charged) and L (hydrophobic).

Choice B shows a - charged glutamic acid - OUT
Choice C shows 2 negatively charged AAs - OUT
Choice D shows a - charged aspartic acid - OUT

You could have 2 AAS that are + charged, but that was NOT given as an answer choice.

This is why you MUST NOT rely on memory alone when doing passage-based Qs. The passage is there for a reason and your brain has enough work to do already. The MCAT is a dumb exam, do not outsmart yourself.

Hope this helps, good luck!
 
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