Turns out this blog is still up and running :-)
so here is a new one: Sample GRE biochem test, question 55 with P+ of 31% (from 2016 edition)
[*P+ is percentage of test takers who gave correct answer.]
Show answer
An amino acid transporter protein is responsible for the transport of a specific amino acid across a membrane. The KI values of several competitive inhibitors of the amino acid transporter are shown above. Based on these data, which of the following is most likely the amino acid transported by this protein?
Show answer
Correct answer is E
And here is why.
First, let's unwrap some of the language. "The inhibitor constant, Ki, is an indication of how potent an inhibitor is; it is the concentration required to produce half maximum inhibition." (here)
So, the higher the Ki, more inhibitor concentration is necessary to prevent our transporter from moving amino-acid X through the membrane.
It is also mentioned in the question that inhibitors are competitive. That means, to some degree, that inhibitors are similar in structure to that of the amino-acid X. Most similar inhibitor will then have lowest Ki, is est will occupy transporter's "slot" very nicely preventing amino-acid X from binding. In other words, inhibitor with lowest Ki will have most similar structure.
Lowest Ki belongs to competitive inhibitor #3 (calledethyl benzene, by the way). It might be useful to notice second-best inhibitor as well, #5 (isoamyl alcohol).
Both of these compounds are relatively not charged.
Now let's look at structures of all possible answers:
As you can see, only tyrosine (E) carries benzyl group and also has less charge than other compounds. hence the answer is E
(A) 
(B) 
(C)
(D)
(E)
As you can see, only tyrosine (E) carries benzyl group and also has less charge than other compounds. hence the answer is E
