AAMC Practice Test 2 C/P #19

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xoxo111

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Could someone please explain to me why the answer is D?

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So it's asking which one will be most likely to achieve the same results. So we are looking at an amino acid sequence, which means we're automatically going to try to find the answer choice that is similar in charge and size to the original. The passage says spHM is phosphorylated sHM, which means the FLGFTY has the T phosphorylated. Phosphorylating something gives it a negative charge. Since all the answers are the same except the threonine is substituted with another AA, we can start there and see if there are any answer choices that give us a substitution with a negative charge.

A substitutes T with an alanine. This is an AA with no net charge at pH 7.2, so it's out.
B substitutes T with a glutamine. Don't get tripped up here. Glutamine has no net charge at pH 7.2, so it's out.
C substitutes T with a glycine. This is the same as the previous two answers.

So, we're left with D, which substitutes T with a glutamate. Glutamate has a net negative charge at pH 7.2, and so it would most closely mimic the properties of the original spHM.
 
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